This text is intended to give an introduction to construction techniques,
considerations and general materials for civil engineering students, although it
may prove useful to students of other engineering disciplines, surveyors and
architects; and practicing structural engineers who require an everyday
reference or introduction to the topic.
The entire module will be subdivided into 5 smaller sub-sections, with construction
techniques being one of these smaller sections. The other components are
engineering contracts, sustainability, health and safety and contract law.
Construction techniques and technology is an ever-growing area of research
and great care should be taken by students and practitioners to maintain a solid
understanding of the subject through constant CPD.
It is by no means exhaustive, as the topic of constructability is a vast area of study
and changes as new materials and processes are determined and defined.
Instead the student should use this as a springing point for their learning about
construction design and technologies.
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Module Requirements
Engineering construction is a two semester, 20 credit module taken by all year 1
civil engineers. This means that students are expected to devote 200 hours of
study over two semesters.
The aims of the module are:
To develop knowledge and understanding in the field of civil engineering construction.
The learning outcomes of the module are:
Upon successful completion of the module, students should be able to:
• Understand the formation, nature and types of contract used in civil engineering practice.
• Understand the principles of specification of construction works. • Understand the nature, purpose, field of operation, legal requirements and
use of accounting. • Prepare and read; a balance sheet, a profit and loss account, a cash flow
statement. • Understand costing and budgeting theory. • Embrace a culture of sustainable development. • Be aware of different techniques of constructing a structure. • Be familiar with items of plant and their capability. • Be aware of health and safety issues • Understand the role of the client, engineer and contractor in conventional,
DBFO and partnering
The learning modes are:
Students will use lectures and case study appraisal, supported by tutorials and group work. Visual aids and demonstrations will be used as appropriate. This module handbook contains essential theory and will be cross referenced with blackboard resources which will include other relevant material and tutorials.
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Face-to-face lecturing will be the main vehicle of acquiring knowledge. Lectures will introduce each topic and explain the important concepts which underpin it. Tutorial sessions offer a chance to work through examples under guidance.
Worked Examples and real world examples may be used throughout the notes and lectures to assist in understanding of the theory and application to real life.
Self Assessment Exercises are provided at the end of each topic to allow students to judge whether they have understood the theory and application. Completed solutions are available to allow rating of understanding (these are posted on Blackboard).
The assessment regime:
The object of studying structures is to develop a useful capability in civil engineering construction topics. Since understanding how civil engineering structures are procured and constructed is a key requisite for being a civil engineer, it is also necessary to ensure that sufficient knowledge has been acquired by the student (to ensure the possibility of success at years 2 and 3), so the assessment regime constitutes one end-of-semester 2 examination and a component of the design project that is common across various modules in this year.
The Examination is a two and half hour opportunity to demonstrate how much has been learnt during the course. The results of the examination form 80% of the assessment.
The Design Exercise is continually assessed group work and constitutes a comprehensive design process. The results of the coursework form 20% of the assessment for this specific module.
Students must pass all assessments to pass the module. A student who does not attempt the last element of assessment of a module (in this case the examination) must be expelled from the course under the University’s current rules.
The University of Salford provides students with access to the Blackboard Virtual Learning Environment (vle.salford.ac.uk). This is an on-line aid to learning where you will find this handbook, answers to tutorial questions, email access and discussion boards. You are encouraged to use the Discussion Board to ask any questions you have about the course material, this way all students who have the same question can see the answer provided.
Students are also provided with an ATHENS login and password, which can be used to access the websites of many technical publishing houses and
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www.info4education.com where electronic copies of many publications can be found.
All civil engineering students are encouraged to join The Institution of Civil Engineers and Institution of Structural Engineers in year 1. Student membership is free of charge and gives access to regular magazine publications, the Institution libraries and evening meetings. Join on-line (web addresses are in the References section of this handbook).
All students are granted access to www.info4education.co.uk where you will be able to log in using your Athens log in details that are provided for you by the library. There are various guides available from the CIS component of the website that you can use to support your learning. You are expected to read beyond this handbook and your lecture notes to promote deep learning, the Clifford Whitworth library also has various texts relating to construction that you will find useful. You should also consider attending ICE and IStructE seminars in the evenings, specifically those that talk about case study projects. They are free to everyone and you will find them good opportunities to meet experienced engineers in the area.
Recommended Reading
In order to acquire an understanding of construction design and technology you must read wider than just this module handbook. This is because you will need to experience a diversity of opinion and methods to fully understand the subject. The Clifford Whitworth library (first floor) retains a large amount of high quality material on this subject. Students should refer to texts listed in the references section at the end of this handbook, many of which are held in the Clifford Whitworth Library, there are also likely to be various blogs, wikis and online guidance documents available, but care should be taken with regards verifying the appropriateness of these sources.
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Contents 1.1 Blackboard. 6 1.2 Weighting of the assignment 6 1.3 Key dates 6
2 Construction Techniques. 7 2.1 Order of construction 7 2.2 Permanent Works 8 2.3 Temporary Works 8 2.4 Types of activity 8 2.5 Temporary propping 9 2.6 Adjacent foundations 9 2.7 Highway construction 10 2.8 Basement construction. 11
3 What is superstructure? 12 4 What is substructure? 14
4.1 Substructure 14 5 Steel 17
5.1.1 What is steel? 17 5.1.2 Process 20 5.1.3 Simple Frames. 21 5.1.4 Portal frames 22 5.1.5 Trusses 22 5.1.6 Long span floors 23 5.1.7 Composite decking 24 5.1.8 Bi-steel 25 5.1.9 Modular construction 25 5.1.10 Example 27
5.2 Cranage 29 5.3 Lots 29 5.4 Phases 30
6 Concrete 31 6.1.1 What is concrete? 31 6.1.2 Types of concrete? 31 6.1.3 Designated 32 6.1.4 Designed 33 6.1.5 Self compacting 33 6.1.6 Super high strength 34 6.1.7 Pour sequence 34 6.1.8 Identity testing 35 6.1.9 Jointing 36
8 Masonry 62 8.1 What is masonry? 62 8.2 How are they made? 63 8.3 Common units. 64 8.4 Clay bricks 64 8.5 Facing bricks 65 8.6 Calcium Silicate Brick 65 8.7 Concrete block 65 8.8 Fair finished block 67 8.9 Terminology 67 8.10 Ties 68 8.11 Bond 69 8.12 How is it built? 70 8.13 Common defects 70
9 What is timber? 72 9.1 Forms of timber. 73 9.2 Origins of timber. 73 9.3 Softwoods 73 9.4 Considerations. 74
9.4.1 Hygroscopy 74 9.4.2 Hygroscopy 74 9.4.3 Propensity to creep under sustained load 75
Table of figures. Figure 2-1 Temporary propping to a concrete slab. 9 Figure 2-2 Section through an indicative highway. 10 Figure 2-3 Excavation with batter along the right hand edge. 11 Figure 3-1 Bridge superstructure 12 Figure 3-2 Skyscraper height comparison including Burj Khalifa 13 Figure 4-1 Leaning Tower of Pisa. 14 Figure 4-2 Cofferdam being used to construct bridge pier. 16 Figure 5-1 Steel sections 18 Figure 5-2 Plate Girder 18 Figure 5-3 Compound sections 19 Figure 5-4 Cell beams 19 Figure 5-5 Castellated Curved Rafters 20 Figure 5-6 A truss for No 1 Deansgate. 22 Figure 5-7 Weight restriction sign for a weak bridge. 23 Figure 5-8 Composite beam with fire protection. 24 Figure 5-9 Cross-section through a bi-steel panel. 25 Figure 5-10 Photograph of MoHo in Castlefield. 27 Figure 5-11 Mobile crane. 29 Figure 5-12 Eiffel tower construction sequence. 29 Figure 6-1 Reinforcement placement drawing from design and detailed. 38 Figure 6-2 Example of Shape Code 13 for RC Detailing. 39 Figure 6-3 Steel reinforcement being concreted. 41 Figure 6-4 Forwork for a column. 43 Figure 6-5 Formwork 45 Figure 6-6 Backpropping load distribution. 46 Figure 6-7 Carboard Formwork 47 Figure 6-8 Tremie pipe used in piled foundations. 49 Figure 6-9 Concrete pumping truck. 50 Figure 6-10 Concrete skip used on tall buildings. 51 Figure 6-11 Pier Luigi Nervi hangar roof using downstand beams. 52 Figure 6-12 Stability core at Media City formed using slip form. 55 Figure 7-1 Bubbledeck 59 Figure 7-2 Formwork for a viaduct. 60 Figure 8-1 Normalised block strengths from a manufacturer. 63 Figure 8-2 Coursing dimensions for brickwork. 64 Figure 8-3 Types of masonry units. 67 Figure 8-4 Forms of masonry wall construction. 68 Figure 8-5 Common Wall Ties 68 Figure 8-6 Common UK bond patterns. 69 Figure 9-1 Deflection of a timber beam. 76 Figure 9-2 Common sawing patterns. 77 Figure 9-3 Timber gridshell using engineered timber 78 Figure 9-4 Structure of ply. 79
Each section of the course will be co-ordinated with Blackboard and will require you to regularly log in and co-ordinate with the reading programme. Within BlackBoard there will be a series of on-going self-tests and self-based learning activities. These may form part of your overall mark and will require you to achieve a minimum mark in each section.
These tests will be taken within controlled conditions and each test will be randomly taken from a pool of questions maintained within the Blackboard database.
Once the self-based tests have been completed for all students, the same pool of questions will be thrown open to allow you to study and revise for your exams.
1.2 Weighting of the assignment
The coursework for this module will be integrated into the larger design project for all first years and will form 20% of your overall mark for this module. You should ensure that sustainability, construction technology, construction sequencing, financial matters, legal and contracts are specifically identified within your design reports.
1.3 Key dates
You will be examined on this subject at the end of semester 2.
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Civil Engineering Construction
2 Construction Techniques.
One of the key requirements often overlooked by engineers when designing a structure or a piece of civil engineering infrastructure is how these components will be built, maintained and even operated. This section of the module is intended to take the student through some of the common techniques utilised within construction within the UK.
This handbook will make reference to various articles linked into the corresponding area on blackboard and the student will need to ensure that these are also read in conjunction with reading this text, the associated texts, attending lectures and tutorials.
2.1 Order of construction
Construction is an ordered and logical process if undertaken correctly. For example, it’s often difficult to put a roof on the building until you’ve constructed the supporting frame - and to build the supporting frame you need to first construct the corresponding foundations.
There are various permutations on these sequences though and unusual techniques, which whilst not frequently used in industry, do have a position and can be the most optimum solution for given criteria.
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2.2 Permanent Works
When a structure is designed it will be designed to support a given load, for a set of deflection criteria that will result in the structure safely supporting the loads and ultimately fulfilling the design brief.
This analogy has been simplified greatly, but ultimately the end result of the design process is a structure that is safe, economical and fulfils the brief.
This structure is said to be the permanent works, ultimately being the end result of the design process. As the designers if this structure deflects too much, or perhaps is unable to support the full design load then the designer will be liable for these deficiencies.
2.3 Temporary Works
One of the key concepts to understand when designing structures is that frequently they are unstable during construction and require additional support to allow them to be built safely.
This additional support could be simple propping, or formwork for a concrete beam perhaps, but the key element to consider is that without the use of these temporary propping sequences and requirements is that the structure could not be built.
As designers you will have a requirement to ensure that your structure is not only safe in its final state, but also that it can be constructed safely.
2.4 Types of activity
Civil engineering is a very broad and wide ranging topic, which encompasses a large number of activities. Within this module you will be given an overview of different forms of common construction methods, including how they can be applied to various different forms of civil engineering structures.
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This handbook is sub-divided into two key components of construction, the substructure and superstructure.
There is also a section relating to site organisation and planning.
2.5 Temporary propping
Temporary propping is frequently used to aid with the stability of a structure during construction. Whilst the structure should be strong enough to support itself and any imposed loadings once completed, it may require additional structure to enable it being constructed. Common examples include the use of scaffold, concrete formwork, temporary steel bracing and wailings.
Figure 2-1 Temporary propping to a concrete slab.
2.6 Adjacent foundations
One of the key considerations when excavating is will the excavation be stable and will it influence any adjacent areas. This is particularly important when other buildings, structures or properties are along adjacent boundaries.
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These properties are protected by a robust piece of legislation called the Party Wall Act 19961.
The Party Wall Act provides a framework for preventing and resolving disputes in relation to party walls, boundary walls and excavations near neighbouring buildings. It is based on some tried and tested provisions of the London Building Acts, which applied in inner London for many decades before the Act came into force.
Anyone intending to carry out work (anywhere in England and Wales) of the kinds described in the Act must give Adjoining Owners notice of their intentions.
2.7 Highway construction
The UK has an estimated 225,000 miles of road all of which must be maintained and relief roads constructed, this doesn’t include other types of highway and hard standing construction such as dock yards, runways and private areas of hard standing at large distribution warehouses.
Figure 2-2 Section through an indicative highway.
Highways must be constructed systematically, firstly, by being excavated down to suitable strata; secondly, the sub base and
corresponding levels are laid, rolled, and compacted using suitable stone; thirdly, the finer upper layers are constructed; finally, the wearing course or blacktop is installed on the top.
2.8 Basement construction.
When constructing a basement there will commonly be a need to integrate some temporary support around the perimeter whilst the hole is being excavated. This can be provided by carefully designing the permanent works to support the earth during the temporary condition, through the provision of additional support by using sheet piles, or where space permits by digging the perimeter of the excavation with an appropriate batter (angle) around the perimeter.
Figure 2-3 Excavation with batter along the right hand edge.
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Superstructure
3 What is superstructure?
Superstructure is a term used in civil engineering to identify a specific area of a building or development. The phrase superstructure is used in many other disciplines and is not exclusive to engineering, ranging from psychology through to describing parts of Marxist theory. They all share a common notion though in that it is used to describe a thing that is supported by another.
In simple buildings, the superstructure can simply be thought of the area of the building that sits above the foundations or basement level. For bridges the superstructure is classed as the section that sits upon the piers and abutments.
Figure 3-1 Bridge superstructure
Superstructure can be built a wide variety of forms, materials and types and the purpose of this module to identify various techniques and processes involved when constructing different types of structure.
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One of the key challenges for an engineer is to identify the boundaries where the superstructure begins and how this can affect the design of the building.
For a skyscraper the superstructure can change material at various points in the building. The Burj Khalifa for example the lower stories are constructed from a combination of insitu concrete and post-tensioned concrete floors, but several of the upper floors are constructed from steel. Whilst the steel component of the building is clearly supported by the lower concrete stories, it could perhaps be argued that this is the reason that the steel elements are classed as superstructure, but in reality both the concrete and steel elements all belong to the skyscraper as it’s entirety. The whole of the skyscraper frame above ground is then classed as superstructure, with the foundations being classed as substructure, which will be the subject of our next chapter.
Figure 3-2 Skyscraper height comparison including Burj Khalifa
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Substructure
4 What is substructure?
Substructure is a term used in civil engineering to identify a specific area of a building or development. In its simplest form the substructure can be imagined as something that provides support to something above and is thus a critical component.
A common example of substructure is the humble foundation, the first component typically constructed on a building site and frequently one given the least amount of acknowledgement when looking at large exotic structures, but without solid foundations, buildings would sink into the ground or dangerously topple.
Figure 4-1 Leaning Tower of Pisa.
4.1 Substructure
The substructure is a key component within civil engineering, it is typically the element of works that either sits in or beneath the
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ground and provides primarily a supporting function to the primary structure.
There are lots of different types of foundations for example, including:-
But each of these types of foundations can be used as a permanent foundation to hold up a structure long term, or equally they could be used to provide support in the temporary condition.
Some substructure works is specifically targeted to enable other works to proceed, for example, a piling mat is commonly required to support the heavy weights of the piling rigs. A piling rig can be of the order of 45 tonnes and if it were to become unstable it could easily topple whilst the auger is at its highest position. To combat this a piling mat is formed using stone, sometimes including geotextiles, which allow the weight of the rig to be evenly dispersed over a wider area and reduce the risk of the rig toppling over from the soft spots.
Another example of temporary works used to enable construction is that of a cofferdam. A cofferdam is typically a steel box that is constructed to exclude water; either in a river or stream; the introduction of a dam completely excludes water and allows for construction to proceed in a safe and efficient manner.
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Figure 4-2 Cofferdam being used to construct bridge pier.
Sometimes, there may be large amounts of groundwater present and a cofferdam may not work as the groundwater can still come up through the floor. Where this is a concern, then it is common to require additional pumps to continually remove water from the excavation during construction until the permanent water exclusion measure is constructed. Even so, the effects of buoyancy are critical as once the water has returned to the ground, if there is not enough mass or ballast within the new structure it can be susceptible to floating and thus buoyancy checks will need to be carried out during the design process.
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Steel
5 Steel
5.1.1 What is steel?
In its simplest form steel is an alloy between iron and varying amounts of carbon, anywhere between 0.2% and 2.1% typically. Other elements can be added to the composition to give differing mechanical and chemically resistant properties, creating variations such as stainless steel.
Structural steel is commonly provided within the industry at various common grades, with the two most commonly used grades in the UK being S275 and S355 although higher grades are available for specialist uses.
The naming convention for these steels is logical with S275 steels commonly having a minimum strength of 275N/mm2 and S355 steels having a minimum strength of 355N/mm2.
Question?
What is the minimum diameter of S355 steel wire that would be needed to support a male African Elephant weighing 6 tonnes?
Answer: 14.53mm
Steel frames are common in industry, being one of the two common construction materials in the UK, with the other being reinforced concrete.
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There are fundamentally two different types of steel sections used to construct bridges and buildings, the first being pre-rolled steel sections such as those supplied by CORUS which are manufactured to high tolerances for a variety of shapes.
Figure 5-1 Steel sections
Sometimes though, the sections provided through the standard tables may not be large enough for the loading regime being specified and a customised section may need to fabricated. These sections can either be fabricated from stock size steel plates, or through the amalgamation or adaptation of existing steel sections.
It is not unusual for plates to be welded into steel sections for high loading regimes, or for long spans, such as bridges or transfer beams in tall buildings. These plates can be welded into a variety of shapes including box-girders, plate girders or fish belly girders for example.
Figure 5-2 Plate Girder
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Where a single standard steel section perhaps doesn’t have the required section properties, but a plated section would prove too expensive, then the steel sections can be modified to increase the geometrical properties. This can be accomplished through fixing too sections together either with welds or bolts, these types of sections are called compound sections.
Figure 5-3 Compound sections
Other common adaptations of standard steel sections include the splitting of the section along its length in a “saw tooth” pattern and then re-welding the section to create a castellated beam.
Figure 5-4 Cell beams
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These can be quite labour intensive, but there are various companies that create equivalent standardised stock sections such as Westok and Fabsec.
Figure 5-5 Castellated Curved Rafters
Castellated beams and cellular beams have distinct advantages when compared to stock sections in that building services can be fitted to run through the openings and typically there can be a weight saving within the design. The downside though is that these beams will require a deeper overall structural zone to sit within the floor zone and this cannot always be accommodated within the building as it may affect the overall building height.
5.1.2 Process
As with all elements of design, the design cannot start properly until a brief has been determined. The brief can take different forms depending on the nature of the design problem, for a simple beam to form an aperture through a masonry wall this could simply be the performance criteria for the beam (i.e. its span, limiting deflection and load it must support).
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For more complex buildings this could be simply how many square metres of floor are required and that certain areas are not allowed to have internal columns. It is at this point that the brief will start to drive the potential solutions and start to attract costs for the design of the scheme.
From the conception of the structural form, it’s critical that the designers start to consider how these structures can and will be constructed. Many schemes have never made it through to construction stage due to them being impractical or the associated costs of temporary propping or the shear number of cranes required to lift them.
There are numerous families of structural forms that can be constructed in steel and below are a selection of some of the more common forms that engineers will be expected to understand, identify and present the construction sequence for.
5.1.3 Simple Frames.
A simple frame in the context of steel construction is a common multi-storey frame as used in office buildings, hotels, student accommodation etc. The building maintains stability either through the use of bracing or shear walls. These stability elements are typically contained in blank walls, around lift shafts or stair cores and prevent the building from falling over on windy days…
The floor beams in this form of construction are often simply supported beams that span between columns or other beams to form a grillage. This grillage is in turn held up by columns that support the floors, because these columns don’t resist the moments from the end of the beams, they are often referred to as simple columns within the Eurocodes… although they need to be designed to resist bending moments due to the eccentricity of the beam loads placed upon them from the end reactions. [This is something that you will be taught how to design in future years of your degree]
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5.1.4 Portal frames
Portal frames are used widely in construction, particularly in the construction of large commercial units. If you’ve ever been to a supermarket or a warehouse then the chances are that you’ve been within a portal frame. The frames themselves provide the stability in one direction, but typically have bracing along the longitudinal axis which means that the temporary stability of the building during construction needs to be considered quite carefully.
Question: What sort of building commonly use portal frames?
5.1.5 Trusses
Trusses are used to span long distances and are commonly used in large span structures such as exhibition arenas, concert halls, large lecture theatres and similar type buildings. As the trusses themselves are typically longer than a standard trailer can transport, they are brought to site in several sections and then bolted together to create a complete truss. The positions where the trusses are bolted back together are called splices and the number of sections that a truss is subdivided into can be as a result of many governing factors including the size of the fabrication shop, the length of the vehicles delivering the components, low bridges en route, etc…
Figure 5-6 A truss for No 1 Deansgate.
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Typically vehicles and their loads that are no wider than 2.9m and no longer than 18.75m require no special measures to be undertaken such as police escorts. However the planning of the route needs careful consideration so as to avoid low bridges and other sections of road that may not be suitable to support the axle load from the wagons.
Figure 5-7 Weight restriction sign for a weak bridge.
Clearly until the entire truss is bolted together it has no ability to span, it’s incomplete and has no structural integrity. This may mean that the truss is assembled on the floor and lifted into place as one piece, or that the truss is assembled insitu using some form of crash decking or temporary support decks.
Question: Where on a house would you see a truss?
Question: And how large are they and would they require special permission and/or access requirements to be delivered?
5.1.6 Long span floors
Long span floors are commonly used to span between steel beams to create open areas to increase the spacing between columns. This type of arrangement is beneficial when constructing column free environments, such as car parks for example.
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Typically these kinds of flooring systems will make use of prestressed concrete planks, which can create their own issues particularly during erection where they may not be stable. As a plank is added to one side of the beam it can create a twisting load which is called a torsion. If this torsional load is too great then the beam can rotate and in extreme cases this can result in the planks slipping from the top flange and several accidents have happened over the years from just this type of phenomenon.
Question: Why would you use a long span floor in a car park?
5.1.7 Composite decking
Composite decking is also used widely for creating steel buildings, with the composite decking acting as a permanent shutter for the concrete. By attaching the slab to the beam with a positive connection such as shear studs, it is possible to enhance the load carrying capacity of the beam.
There are several risks associated with the installation of composite decking installation as the decking must be affixed to the beam to prevent it from sliding off.
Figure 5-8 Composite beam with fire protection.
From the general arrangement drawings, the supplier of the composite decking is able to take the slab drawings and create a layout drawing which shows how many sheets of composite decking
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are needed and their accompanying layout including how large the sheets are, how many shear studs are needed and where additional support is required. This sequencing will need to be aligned with deliveries onto site and co-ordinated with the steel frame erection, the sequencing of the frame construction may well require that there are floors constructed to allow the cherry pickers to erect the steel frame.
Question: Do floors constructed from composite decking require temporary propping?
5.1.8 Bi-steel
Corus produced a product called Bi-steel which is essentially two sheets of steel that are connected together using metal studs to create a sandwiched panel.
Figure 5-9 Cross-section through a bi-steel panel.
The benefit of these panels is that they are comparatively light when compared to a concrete wall and can be erected quickly to form shear walls around lift cores and similar types of structures. Once they have been erected then they can be filled with concrete if necessary to increase their axial load carrying capacity.
5.1.9 Modular construction
Modular construction is a type of offsite construction, the modules are constructed in a factory and then delivered to site and simply stacked on top of each other to create a completed building. There are clearly limits on how many units can be stacked together
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whilst retaining structural stability from wind loads and accidental damage.
Modular construction can take a variety of forms from sub-components such as prefabricated bathroom pods that are delivered to site complete with showers, toilets, baths, tiled surfaces and other secondary fittings right down to including a toilet roll holder. Other modular forms include prison cells pods, which are designed to single stackable units formed entirely out of precast concrete. They have integral toilets, beds and washing facilities all cast from concrete to prevent inmates from damaging them and the doors are designed with hinges that can open both ways to prevent doors being barricaded in the event of a riot.
Larger modular units can be achieved; some are large enough to encompass entire flats. The key to an efficient design for a modular structure is repetition the more of a single type of unit that you can manufacture, the more efficient the factory becomes. There are a variety of manufacturers that make residential units and they all come complete with integrated kitchen appliances and services that are connected together on site as the modules are interconnected. Large developments are possible using this form of construction and in Manchester a large development was completed in Castlefield, called Moho which has a central steel framed skeleton that forms the stability core and the modular units are connected around the edges.
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Figure 5-10 Photograph of MoHo in Castlefield.
Question: Would modular construction require any special requirements at site?
5.1.10 Example
Think how each of these types of building might have been constructed, specifically identify the following:-
• What materials are used? • How was the material delivered to site? • How was it built? • Did it use temporary support, permanent support or both? • Draw a sequence of construction sketch to show the process.
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Building 1: Wembley Stadium.
Building 2: Steel framed shed.
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5.2 Cranage Another of the key considerations of constructing steel frames is the amount, positioning and location of any cranage that is necessary to lift the steel elements into position. As steel elements are delivered to site on the back of a trailer, they are lifted off and stored in sequence to allow the frame to be assembled in a logical and systematic fashion.
Figure 5-11 Mobile crane.
5.3 Lots The order that the steelwork is delivered is referred to as lots. Each batch (or lot) of steel that is delivered to site is carefully planned so that it maintains stability for itself whilst the rest of the frame is erected and is frequently co-ordinated with the access for cranage and other site access requirements. For taller buildings though, the allocation of lots is determined by the logical construction sequence.
Figure 5-12 Eiffel tower construction sequence.
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5.4 Phases Larger steel frames are further sub-divided into phases or regions, with each phase being an independent frame. Examples of this include large shopping centres such as the Trafford Centre or the Lowry Outlet Mall, the latter of which is subdivided into three sub-regions or phases.
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Concrete
6 Concrete
6.1.1 What is concrete?
Concrete in its simplest form is a combination between cement, coarse aggregate, fine aggregate and water. By varying the different forms and ratios of these elements allows for concrete with differing properties to be manufactured. Further amendments to the mix such as the inclusion of fly ash, use of recycled aggregate or various chemical admixtures can also reduce the amount of carbon required to manufacture concrete.
The design requirements of concrete will vary dramatically depending on its intended end use, the environment to which it is intended, the method of construction, placement and climatic conditions to name a few.
The following sections will cover some of the key criteria used when designing and constructing in concrete and are intended to aid the student develop an understanding of modern concrete construction. It is fully intended that the student however will also research the ideas contained within under their own initiative and research the topic and construction techniques further.
6.1.2 Types of concrete?
Concrete can be provided in a variety of forms, ranging from historical lime based mixes through to highly technical and complex
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ultra high strength mixes. The intention of this section is not to teach teach you about the finer points of concrete and mix design, but to instruct you on some of the basic principles associated with concrete specification and construction to provide you with information to aid with your future designs.
6.1.3 Designated
Designated mixes are typically the most common type of concrete used in modern building and civil engineering construction. The methodology behind designated mixes is that they are standardised both in terms of their specification but also in their production and provision from concrete suppliers.
In theory this should mean that each batch of concrete is uniform and has been through the same quality control procedure and thus will have the same mechanical properties.
Each batch of concrete will have a ticket when it leaves the factory and should the concrete be amended in any way (including the addition of water) between it leaving the factory and being placed then the driver should note this on the batch ticket before the concrete is placed. If the batch has been amended then the customer does not necessarily have to accept delivery of the concrete.
Not recording any alterations to the mix on the ticket can often result in the person responsible being dismissed such is the risk to the mechanical and chemical properties2 of the concrete mix.
2 One of the reasons toilets are often set up at regular intervals through the floors of tall
buildings to prevent workers urinating in the formwork. There have been cases where the ammonia from the urine has adversely affected the performance of the concrete.
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Consequently it is advisable wherever possible to use standardised designated mixes to enable economical designs and to increase the quality control of the overall construction.
6.1.4 Designed
Every building is designed to respond to a certain set of criteria and occasionally this will require the specification of a concrete mix that is beyond the capability of a standard designated mix.
This may be because the client requires the inclusion of a specific admixture, such as a colour to the concrete or a specific high quality finish. These are outside the scope of a generic concrete specification and mix design and will require the mix for the concrete to be designed specifically.
High strength concretes above 60 N/mm² often require the use of designed mixes.
6.1.5 Self compacting
Self compacting concrete has the ability to self compact with no vibrating of the formwork required after the initial pour. This can be valuable where complex forms and intricate shapes are required within the structural element that would prevent the use of a vibrating poker to aid the compaction of the concrete.
Self compacting concrete is very different from high slump concrete and the two should not be confused or exchanged during the construction process.
High slump concrete whilst more workable than standard mixes does not possess the same level of self-compaction as self compacting concrete and if used instead will frequently leaving voids within the concrete depending on the complexity of the forms.
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This has been noted on the exchange between the two mixes when constructing hybrid structures such as twin wall, where voids have been uncovered at the ends of panels.
6.1.6 Super high strength
Typically concrete within civil and structural engineering are used between a strength range of between 20 N/mm² and 60 N/mm² depending on their location and intended design life. However, given the ongoing race for taller and taller buildings there are developments of much stronger concretes with the use of 100 N/mm² and 120 N/mm² concretes becoming more commonplace within skyscraper design and construction.
The use of these new concretes can be outside the scope of the design standards though and special consideration should be given to the design and construction when using high strength concretes.
This is due to the high amount of energy and heat released when these high strength concretes are placed as they can result in the element being subjected to significant shrinkages and thermal strains.
The increased use of stronger concretes is also gaining momentum in the use of precast construction concrete design where the turn around time on the forms is often reduced to allow the production rate of the factory to be increased.
6.1.7 Pour sequence
When concrete is placed on a project the contractor will make a note of where the concrete was placed on a drawing including the details of that particular batch. The pour sequence for the concrete is an important part of the construction process, this will determine the short and long term behaviour of the structure and consequently the contractor should provide details of the pour sequence ideally
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as part of the tender process, but certainly no later than 2 weeks from commencement on site.
The size and nature of the concrete pours can significantly affect the programme and this information should be defined by the design team at the earliest opportunity and confirmed with the contractor as part of the negotiation and tender award process.
In order for the contractor to physically place the concrete using a pump there may be a need for a plasticiser to be added to the mix or for the aggregate size to be reduced to allow the pumps to run at a reduced pressure or to increase the distance which concrete can be pumped.
This may require a departure from the specified mix (designated or designed) and will require the contractor to submit proposals for the engineers approval.
6.1.8 Identity testing
Whilst the use of designated mixes in theory is intended to remove the need for contractors to undertake additional mechanical testing of their concrete cubes, it is still sometimes advisable to take additional cubes for identity testing during the construction works.
These tests can allow you to identify areas where the concrete is non-compliant with the construction specification, such as it could be slow in attaining the required 7 or 28 day design strength during the curing and placement process.
By having these additional tests, the engineer can correlate them with the pour sequence drawings to identify which areas may be affected by the non-compliant concrete.
Additional identity testing of the concrete is often resisted by the contractors, who will note that identity testing is not required as the concrete is being supplied by an accredited supplier and the necessary paperwork will be provided with the delivery.
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Experience has shown that the additional testing often provides an additional element of re-assurance to the clients that the concrete supplied is of appropriate quality and issues with batches are typically identified early in the construction process which allow for remediation strategies to be employed whilst still cost-effective and also prevent any shortcomings from re-occurring.
Testing can be done insitu, but will typically require cores to be taken from the existing structure to allow these tests to be calibrated and compared between the various parts of the structure. It should be noted that these tests are expensive, time consuming and are no replacement for good quality record keeping.
6.1.9 Jointing
Where large areas of slab are required, these cannot be readily be poured in a single pour for several reasons. Primarily there will be a logistic limitation as to how much concrete can be delivered to site and continually poured before the site must close for the day and this must be factored into the construction sequence. Where pours stop and start for the day are frequently called “day joints”.
Even if the amount of concrete required to pour a large floor slab could all be delivered and placed in one day, the amount of concrete involved may result in high quantities of heat being generated within the concrete which can have an adverse affect on its curing times and sequences. This is because concrete is exothermic and consequently gives up large amounts of heat as it cures, this effect is exacerbated for deep and large volume pours such as mass concrete retaining walls and dams, where the amount of heat generated can be quite considerable.
As concrete cures; it slowly gives up a portion of its water content as it dries out and this giving up of water results in shrinkage of the concrete. This can be problematic for large areas of slabs, depending on their restraint conditions near large stability cores and other stiff elements. The shrinkage of concrete slabs can be a
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significant limitation on the amount of concrete poured in a day and one method to prevent these early thermal shrinkage effects from limiting the structure is to integrate a control strip around stiff elements such as cores, with the control strips being poured much later than the rest of the slab, ideally once the early thermal shrinkage has occurred.
6.2 Process
The design, detailing and construction of insitu concrete buildings passes through several key stages during the process. Some of the key stages are outlined below but do not make reference to the early conceptual design stages, merely in the production of construction information through to the physical construction of the building.
6.2.1 Design
The design of the structural element to be constructed will determine the shape, form, strength of concrete, amount and arrangement of the reinforcement. A good designer will always be developing how the element can be constructed safely and economically as well as ensuring that the design is safe and will not deflect excessively in the permanent condition.
6.2.2 Drawings
To identify the overall shape of the element and its location within the building a well drawn set of plans, sections, elevations and details are likely to be required. These drawings will be produced by the engineer at the appropriate design stage and will typically be co-ordinated with the other disciplines such as the architect and building services engineers to allow for a co-ordinated design to be produced.
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6.2.3 RC Placement drawings
Reinforcement placement drawings differ from general arrangement drawings as they are not intended to show how the building is arranged, but instead show where and in what configuration individual or groups of steel reinforcement should be position within the structural elements.
Figure 6-1 Reinforcement placement drawing from design and detailed.
These drawings will give each piece of steel reinforcement their own unique number for the project, this number is called the “bar mark” and will be accompanied with a bending schedule.
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6.2.4 Bar bending schedules
Whilst reinforcing bar can be bent into any practical shape within the limitations of its diameter and associated diameter, it is typically bent into predefined standardised shapes that are governed by a British Standard3.
These standardised shapes are called “shape codes” and enable reinforcement suppliers to automate their factories and also enable the creation of simple repeatable patterns of reinforcement that are governed by a common set of rules and parameters.
Figure 6-2 Example of Shape Code 13 for RC Detailing.
These rules also prevent reinforcing bars being bent to radii that are too tight and that might otherwise affect the strength of the reinforcement through cold working of the steel reinforcement.
6.2.5 Placement
The reinforcement is placed on the formwork as described in previous sections. Reinforcement should be placed on clean, dry formwork within a temperature range as identified within the engineering specification. The reinforcement within a beam or column element is tied together using the shear links to create a
3 BS8666
BS 8666:2005
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Table 3 — Standard shapes, their method of measurement and calculation of length
Shape code
Shape Total length of bar, L measured along centre line
00 A
01 AStock lengths.
See Note 4.
11 A + (B) – 0.5r – d
Neither A nor B shall be less than P in Table 2
12 A + (B) – 0.43R – 1.2d
Neither A nor B shall be less than P in Table 2 nor less than (R + 6d)
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Key1 Semi-circular
A + 0.57B + (C) – 1.6d
B shall not be less than 2(r + d). Neither A nor C shall be less than P in Table 2 nor less than (B/2 + 5d). See Note 3.
14 A + (C) – 4d
Neither A nor (C) shall be less than P in Table 2. See Note 1.
cage that can be lifted up into place and position directly into the formwork.
Reinforcing bar lengths are typically provided in either 6m or 12m lengths, depending on the diameter and the designer should give consideration as to how these bars will be physically picked up and positioned on site whilst they are preparing their designs. If a reinforcing bar cannot be position by the labourers then this can frequently tie up valuable resources and crane time to lift and position the reinforcing bar into place, plus it may not present a safe and economical solution.
If a structural element is longer than the typical length of a reinforcing bar, then this will require a lap in the reinforcement, this lap can be formed by lapping the reinforcing bars next to each other for a minimum length as described within the relevant standards, by the inclusion of mechanical couplers or by positioning the lap in a location where the reinforcement is no longer required to form a beam design utilising lapless construction. Mechanical couplers are frequently avoided in commercial applications given the increase in costs and standard laps instead are prepared, in nuclear construction though the bar diameters are typically large and congested and in these instances couplers provide an economical alternative to lapped reinforcement.
All debris should be removed from the bottom of the formwork prior to concrete being placed, there is often clippings of tying wire around the bottom of the formwork that can cause unsightly staining to the underside of the concrete slab and any foreign matter left on the bottom of the formwork can result in the cover to the reinforcement being compromised.
6.2.6 Vibrating
Once the reinforcement is in place within the formwork and all debris has been removed from the bottom of the forms, then the concrete can be placed.
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The concrete can be placed in a variety of methods, from being hand barrowed into place or it can be pumped or skipped into place. Frequently contractors are using concrete pumps to place concrete into position with new pumps being able to pump concrete over appreciable distances and heights before running into difficulties.
Figure 6-3 Steel reinforcement being concreted.
Careful design and consideration should be given when designing the concrete itself so as to prevent the mix from segregating during the placement process as this can adversely affect the quality and workmanship of the concrete.
Unless self compacting concrete is being used however, there will be a need for the labourers to use a vibrating poker to vibrate out any air pockets within the concrete so as to increase the consistency of the concrete within the forms and to prevent the inclusion of any voids whilst the concrete cures.
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6.3 Common defects
Within the construction of concrete structures there may be several defects that are encountered whilst they are being built, some of which are listed below:-
• Ill fitted formwork allowing the wets from the concrete to bleed out at the joints.
• Poor compaction of the concrete. • Incorrect spacers being used, providing too much or too little
cover. • The use of paving slabs as spacers for the reinforcement. • Lack of containment on the reinforcement. • Hydrostatic pressure causing the formwork to burst and fail.
Each of these defects may require different forms of remediation depending on the types of concrete elements being constructed, but the best form of control is to ensure that a pre-pour inspection is undertaken by the engineer prior to construction beginning.
6.4 Formwork
Formwork is a key component of construction using concrete and can take a variety of forms from the traditional timber formwork, through to the introduction of permanent formwork through the use of composite steel decking.
The costs associated with formwork and the complexity involved is often overlooked by engineers during the design process, but ultimately if you can’t make the formwork, then you can’t make the structure.
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Figure 6-4 Forwork for a column.
Various materials can be used to make the more complex shapes seen in concrete construction including GRP, Polystyrene and even inflatable formwork has been used to make domes and other historically expensive forms.
The purpose of formwork is to provide a clean, rigid mould that allows the wet concrete to develop its long term strength and subsequently become a concrete frame. To achieve this the formwork must be designed to resist the wet weight of the concrete and the associated hydrostatic head that the wet concrete develops whilst in the concrete.
If the formwork is not strong enough or if the formwork deflects beyond acceptable limits then this can lead to the concrete element not being within allowable limits and subsequently being condemned. Once an element is condemned then it may either be remediated or in extreme cases demolished and rebuilt which may
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have financial and time costs to the person or persons responsible for the creation of the defect.
6.4.1 Timber shuttering
Traditional formwork is constructed from timber and carpenters will frequently specialise in formwork joinery to allow them to construct formwork for construction sites.
Timber formwork design is both a skill and an art within its design and construction, with the ability of the formwork being able to resist the hydrostatic pressures and self weight of the concrete whilst not excessively deflecting or the joints prising apart and leaking being fundamental to the success of the project and the element design.
Timber formwork is used widely on construction sites and within precast factories, although through the use of more sophisticated construction processes the use of timber formwork is not as widely used as it was perhaps 20 years ago.
For highly complex areas of construction where intricate forms are required the use of timber formwork still possesses various advantages as it can be adapted and adjusted on site with relative ease and can have high performance materials and mouldings integrated into its design to increase its performance.
One of the significant advantages of timber formwork is the ability for it to adopt highly complex and organic shapes and consequently it was the formwork material of choice of one of the great engineers Heinz Isler who was famous for creating complex concrete shell structures in a pre-computer age.
6.4.2 Table forms
Where flat soffited slabs are required, the use of table forms can accelerate the construction of slabs and also allow for safer construction with higher quality finishes.
Table forms particularly used in multi-storey buildings, where there are high levels of repeatability within the floor plates. Effectively the
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table element of the formwork is constructed from high quality steel or timber with adjustable trestles or supports beneath it.
Figure 6-5 Formwork
The formwork is propped on the floor below (which may be subsequently propped to preceding floors) to allow the weight of the wet concrete to be supported. The number of floors able to contribute to the support of the wet floor is defined within various industry guides but for ease of reference the load distribution tbale below is replicated from BCA document Early Striking and improved backpropping. (ISBN 07210 1556 5)
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Figure 6-6 Backpropping load distribution.
As table forms will typically be brought out of the sides of tall buildings to then be lifted up to floors above, it is often beneficial to not include edge beams and to use flat slabs where possible to simplify the removal of the formwork at the edges.
Modern table forms will often project beyond the edges of the building to provide a safe working platform around the perimeter of the slabs, these projections will have integral hand railing. This additional working space is essential for post-tensioned slabs where space is required around the perimeter to allow the tendons to be jacked4.
6.4.3 Steel forms
Steel forms are expensive to produce and are frequently utilised where repetition of common elements have been identified. Through their continual re-use through the building the cost of the formwork can realise significant savings through their repeated use.
4 Visit blackboard to see a video of tendons being jacked around the perimeter of the slab.
2
IntroductionThe use, installation and striking offalsework and backpropping* is a vitallyimportant part of the safe and economicconstruction of in-situ concretestructures. In order to strike a slab, theconcrete must be strong enough toavoid failure or undue cracking anddeformation of the slab. Recent researchduring the construction of the EuropeanConcrete Building Project (ECBP) in-situbuilding at Cardington has led to a newunderstanding of the constructionprocess. It is now possible to makesavings in construction time and ontemporary works equipment whilemaintaining site safety and theperformance of the constructed flat slabframe. This Guide summarises the newrecommendations for formwork striking and slab backpropping. For a detailedanalysis and a worked example see Guide to flat slab formwork and falsework (Ref. 1).
Planning the strikingsequenceThe striking of formwork and falseworkfrom a slab needs careful consideration.The constructor is responsible forcarrying out the striking process safely.It is the responsibility of the TemporaryWorks Co-ordinator (TWC) to managethe risks in early striking. This willinclude preparing detailed proceduresand method statements, which shouldbe approved by the safety officer andPermanent Works Designer (PWD).
The PWD’s role is to provide designinformation so that the constructionmethods do not adversely affect theperformance of the structure. All technical and managerial staff must be fully aware of the implicationsof the methods and procedures adopted.Flow charts for striking and back-propping procedures are given in Ref. 1.
Construction loadconsiderationsIn multi-storey in-situ concreteconstruction, the critical loadingcondition for a slab is not necessarilywhen it is struck and becomes self-
Table 2: Sequence of striking flat slabs
Figure 2: Diagrammatic representation of backpropping
Table 3: Load distribution by backpropping
Notes
1. Assumes lower and supporting floors have been struck, have taken up their deflectedshape and are carrying their self-weight
2. Floor loading from imposed loads and self-weight is not considered
3. The strength of particular slabs to carry applied loads will have to be considered separately
4. All floors are suspended floors
5. Figure 2 gives location of loads wp, wb1, and wb2
*Backpropping is defined aspropping installed at levels belowthe slab supporting the falsework.
It is done to distribute the loadapplied to the uppermost slab to suitable supports, such as lower slabs or foundations
•Obtain approval to strike
•Check the safety precautions
•Strike according to guidance given below and as illustrated.
◊ Strike reinforced concrete slabs in two stages:
1. Ease all the supports
2. Starting at midspan, remove supportsworking towards columns and walls.
◊ Post-tensioned slabs will tend to lift off the formwork on tensioning, but use method for rc above.
◊ On large slab areas, comprising internal and edge panels, strike internal bays first, followed by edge and corner bays.
◊ Where soffit form is part of cantilever, start removal from tip and work towards wall, beam or columns.
◊ Post-tensioned cantilever slabs may havespecial striking requirements due to deflection of adjacent spans.
Location Load No backprops One level of Two levels ofbackprops backprops
On slab On slab In props On slab In props
New slab being cast total 100% 100% 100%
Falsework/formwork wp 100% 100% 100%
On supporting slab (1) 100% wp 70% wp 65% wp
In backprops wb1 30% wp 35% wp
On lower slab (2) 30% wp 23% wp
In backprops wb2 12% wp
On lower slab (3) 12% wp
One level Two levelsof propping Load of propping
Slab to be cast
Falsework wp
Supporting slab (1)
Backprops wb1
Lower slab (2)
Backprops (when fitted) wb2
Lower slab (3)
REINFORCEDCONCRETE SLAB
POST-TENSIONEDCONCRETE SLAB
REINFORCEDCONCRETECANTILEVER
POST-TENSIONEDCONCRETECANTILEVER
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Also worth noting is that the finished quality of the concrete and accuracy of the formwork can be increased when compared to timber formwork and steel forms are frequently used for columns that are exposed and consist of a visual element of concrete.
Steel shuttering may also be more economical for tall complex pours where large hydrostatic pressures are anticipated due to their increased strength compared to timber forms.
6.4.4 Cardboard
The use of modern engineered cardboard formwork is a common sight on UK construction sites due to its robustness and economical benefits. Disposable cardboard formwork is bought in predefined sizes and is more commonly used for columns where it can be bought for circular, rectangular and square column profiles. The formwork can only be used once and is designed to be cut from the columns with a knife once the formwork has been struck.
Figure 6-7 Carboard Formwork
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The inside of the cardboard tubes is lined with high quality polished cardboard which enables the formwork to give a high quality surface finish with even standard designated mixes.
Whilst the formwork itself allows for high quality construction to be completed, the nature that the tube will slide over the bars can sometimes result in the inside of the formwork becoming snagged and ripping on the reinforcement of the spacers.
To overcome this, the column is designed with a slightly greater cover to accommodate tolerances within the cage construction and special spacers can be used that spin to facilitate the placement of the concrete tubes over the top of the reinforcement.
Another benefit is that once the column has been struck and inspected, then the cardboard formwork can be placed back onto the column and taped back on to offer some element of protection to the concrete columns during the rest of the construction stages.
6.5 Tremie pipe
All methods of placing concrete under water are designed to prevent cement washout and the consequent formation of weakly cemented sand and gravel pockets. In the tremie process, concrete is placed through a vertical steel pipe with an open, funnel-shaped upper end. The lower end of the tremie is kept immersed in plastic concrete so that freshly placed concrete doesn't come into contact with the water. This process is not just reserved for pouring concrete in rivers, lakes, and streams and is frequently used for concreting piles where groundwater is present.
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Figure 6-8 Tremie pipe used in piled foundations.
Tremie pipe diameter usually ranges from 200-300mm. End plates or plugs are used when a dry pipe technique is employed for starting the tremie pour. As the pipe is lowered to rest on the bottom, water pressure seals the gasket and the pipe is kept dry. In very deep placements, an open-ended pipe can be set and a go-devil or traveling plug inserted to keep water from penetrating the first concrete placed in the pipe. All vertical movements of the tremie pipe must be done slowly and carefully to prevent a loss of seal. If loss of seal does occur, the tremie must be brought back to the surface, the end plate must be replaced, and flow restarted. A go-devil must not be used when restarting a tremie after a loss of seal. Water pushed out by the go-devil will wash cement out of the previously placed concrete. Concrete placement should be as continuous as possible through each tremie. Longer delays must be treated by removing, resealing and restarting the tremie.
6.6 Concrete pump.
As concrete is a fluid, it is possible to pump it from a discrete point to another point under pressure. This is typically achieved by using a truck mounted pump with a large mounted boom arm attached.
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Figure 6-9 Concrete pumping truck.
To allow the concrete to be pumped large distances, the concrete mix will require some form of modification, typically through the introduction of plasticisers which help the concrete to remain more fluid like for longer periods of time. Some of the larger more powerful pumps are able to pump and place concrete up to a maximum distance of 70 metres. Clearly due to the inclusion of plasticisers to the concrete mix, this can increase the curing time required for the concrete to achieve its 28 day strength.
6.7 Tall buildings.
Tall buildings present their own unique challenges compared to their shorter; vertically challenged brethren. As the height of the building increases, so do the logistical challenges of moving materials to the construction interface of the building. Similarly as the weight of the building above increases, so does the load within the columns and this can cause the columns to compress and shorten. This process is called axial shortening and can be significant when designing skyscrapers as it can cause cladding to not fit as the distance between floors shortens, this results in the floor to floor height for the
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lower floors being smaller than the upper floors due to the increase in axial load
As the construction work reaches higher and higher levels, it will extend beyond the reach of a traditional concrete pump and a different approach will be needed. Typically the concrete is lifted in a skip via the tower cranes up to higher floors and either deposited into a hopper for pouring or placed directly onto the relevant floor slab.
Figure 6-10 Concrete skip used on tall buildings.
6.7.1 Table forms.
Table forms are so called because they resemble large tables and because of this they are typically used to cast large areas of flat soffited concrete such as slabs.
The legs on the table forms are designed to enable them to be folded underneath them, and this is how they are collapsed and removed from the building once the concrete has achieved adequate strength and is able to support itself. There is division between contractors with regards the integration of edge beams and the use of table forms. Most contractors prefer that downstand beams are used in the design, as this allows for the concrete to be poured in a single pour because upstands require additional formwork and are typically poured after the slab has gained
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strength. An upstand sits above the top of a slab, like a small wall whereas a downstand sits below the slab like a beam.
Figure 6-11 Pier Luigi Nervi hangar roof using downstand beams.
6.7.2 Cantilevers
Cantilevers require special attention when assessing their constructability, particularly in concrete, as concrete has no inherent strength until it has cured for sufficient time. The formwork therefore, provides all of the support to the cantilever until the strength requirements have been met and it is this requirement that can introduce great cost and complexity to the construction of significant cantilevers, rather than just the fixed lengths of the cantilever in the permanent condition.
6.7.3 Jump forming
Generally, jump form systems comprise the formwork and working platforms for cleaning/fixing of the formwork, steel fixing and concreting. The formwork supports itself on the concrete cast earlier so does not rely on support or access from other parts of the building or permanent works.
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Jump form, here taken to include systems often described as climbing form, is suitable for construction of multi-storey vertical concrete elements in high-rise structures, such as:
• Shear walls
• Core walls
• Lift shafts
• Stair shafts
• Bridge pylons
These are constructed in a staged process. It is a highly productive system designed to increase speed and efficiency while minimising labour and crane time. Systems are normally modular and can be joined to form long lengths to suit varying construction geometries. Three types of jump forms are in general use:
• Normal jump/climbing form – units are individually lifted off the structure and relocated at the next construction level using a crane. Crane availability is crucial.
• Guided-climbing jump form – also uses a crane but offers greater safety and control during lifting as units remain anchored/guided by the structure.
• Self-climbing jump form – does not require a crane as it climbs on rails up the building by means of hydraulic jacks.
Jump forming offers various benefits over traditional shuttering, including:
• Fast construction can be achieved by careful planning of the construction process.
• Self-climbing formwork cuts down the requirement for crane time considerably. By allowing the crane to be used for other construction work this may reduce the total number of cranes needed on site.
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• The formwork is independently supported, so the shear walls and core walls can be completed ahead of the rest of the main building structure.
• High quality surface finishes can be achieved.
• Climbing forms can be designed to operate in high winds.
• Highly engineered nature of jump form systems allows quick and precise adjustment of the formwork in all planes.
• Some formwork systems can be used at an inclined angle.
• A small but skilled workforce is required on site.
• It is easier to plan construction activities due to the repetitive nature of the work.
As the formwork system is self-enclosed, it offers a wide range of safety benefits, including:
• Working platforms, guard rails, and ladders are built into the completed units of market-leading formwork systems.
• Self-climbing formwork systems are provided with integral free-fall breaking devices.
• The completed formwork assembly is robust.
• The reduced use of scaffolding and temporary work platforms results in less congestion on site.
• The setting rate of concrete in those parts of the structure supporting the form is critical in determining the rate at which construction can safely proceed.
• The repetitive nature of the works means site operatives are quickly familiar with health and safety aspects of their job.
Jump form is typically used on buildings of five storeys or more; fully self-climbing systems are generally used on structures with more than 20 floor levels.
Trailing and suspended platforms are used for concrete finishing and retrieving cast-in anchor components from previous pours.
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6.7.4 Slip forming
Slip form is similar in nature and application to jump form, but the formwork is raised vertically in a continuous process. It is a method of vertically extruding a reinforced concrete section and is suitable for construction of core walls in high-rise structures – lift shafts, stair shafts, towers, etc. It is a self-contained formwork system and can require little crane time during construction.
Figure 6-12 Stability core at Media City formed using slip form.
This is a formwork system which can be used to form any regular shape or core. The formwork rises continuously, at a rate of about 300mm per hour, supporting itself on the core and not relying on
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support or access from other parts of the building or permanent works.
Commonly, the formwork has three platforms. The upper platform acts as a storage and distribution area while the middle platform, which is the main working platform, is at the top of the poured concrete level. The lower platform provides access for concrete finishing.
Benefits
• Careful planning of construction process can achieve high production rates
• Slip form does not require the crane to move upwards, minimising crane use.
• Since the formwork operates independently, formation of the core in advance of the rest of the structure takes it off the critical path – enhancing main structure stability.
• Availability of the different working platforms in the formwork system allows the exposed concrete at the bottom of the rising formwork to be finished, making it an integral part of the construction process.
• Certain formwork systems permit construction of tapered cores and towers.
• Slip form systems require a small but highly skilled workforce on site.
Safety
• Working platforms, guard rails, ladders and wind shields are normally built into the completed system.
• Less congested construction site due to minimal scaffolding and temporary works.
• Completed formwork assembly is robust.
• Strength of concrete in the wall below must be closely controlled to achieve stability during operation.
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• Site operatives can quickly become familiar with health and safety aspects of their job
• High levels of planning and control mean that health and safety are normally addressed from the beginning of the work.
Other considerations
• This formwork is more economical for buildings more than seven storeys high.
• Little flexibility for change once continuous concreting has begun therefore extensive planning and special detailing are needed.
• Setting rate of the concrete had to be constantly monitored to ensure that it is matched with the speed at which the forms are raised.
• The structure being slipformed should have significant dimensions in both major axes to ensure stability of the system.
Standby plant and equipment should be available though cold jointing may occasionally be necessary.
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Concrete (Alternate forms)
7 Concrete (Alternate forms)
Hybrid forms of construction are becoming increasingly common in industry with the development of thin high strength concrete being used to form hollow tubular structures that can be filled with insitu concrete to give them their final load carrying capacity.
Various different forms of hybrid structure are available for forming walls within buildings including TwinWall5 systems, with each system being a different manufacturers variation on a common theme.
The ability for these systems to have integral formwork are wide ranging and include:-
• high quality finish, • accurate tolerances, • precast window apertures, • integrated insulation, • lighter weights for craning into position • increased speed of construction.
Similar hybrid systems (such as Omnia Deck6) exist for concrete slabs and follow a similar principle with a thin high strength concrete
5 http://www.precaststructures.com/TwinWall.asp
6 http://www.heidelbergcement.com
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biscuit providing the load carrying capacity within the slab to allow the support of the structural element in the temporary condition with the additional weight of the wet concrete, but once the concrete has cured then the slab will attain its permanent load carrying ability. This method typically requires no additional reinforcement to be included as longitudinal steel is included within the concrete biscuit, however additional reinforcement can be included where required on an ad hoc basis.
The weights of these panels can be considerably reduced through the introduction of biaxial void formers such as Cobiax7 or Bubbledeck8.
Figure 7-1 Bubbledeck
7 http://www.cobiax.ch/en
8 http://www.bubbledeck.com
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7.1 Edge protection
The formalisation of the CDM regulations and the education of the values of Health and Safety through the construction industry has boosted the development of various technologies and innovations in the design and construction of buildings.
One of these is the increasing availability that formwork has to integrate temporary edge protection around its perimeter during construction and for this philosophy to be extended through cast in edge protection systems once the formwork has been struck.
Figure 7-2 Formwork for a viaduct.
7.2 Debonding agent
As the formwork is being prepared, the faces that are to be in contact with the wet concrete are first painted with a de-bonding agent that aid with the striking of the concrete. This prevents the concrete from bonding itself to the formwork and can help create a higher quality finish on the concrete.
If de-bonding agent isn’t used, then as the formwork is struck then pieces of concrete can come away with the formwork that leave unsightly pocks and marks on the face of the concrete.
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7.3 Sealant
Concrete structures can produce large quantities of dust, even after the concrete itself has long since cured and attained its 28 day strength. This can be problematic in areas that require dust free environments or even in car parks where the slabs are frequently trafficked by vehicles.
To limit the amount of dust produced from the building, dust sealant can be applied to the concrete elements in the form of a clear paint. This paint suppresses the dust and prevents it leaving the concrete and is frequently used on areas of exposed concrete such as core walls and soffits of slabs.
Where additional toppings are to be applied to the tops of concrete slabs then it may be required to omit the dust sealant so as to provide a good key between the two surfaces. Each area should be carefully assessed by its intended use and proposed finishes to determine if dust sealant will be required. Frequently the sealant is specified by the architect and included within their finishes schedules.
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Masonry
8 Masonry
8.1 What is masonry?
Masonry is one of the oldest materials still used today and has been used in several of the iconic historic buildings still standing today including the Great Wall of China and the Great Pyramids.
Masonry is an encompassing name for a variety of units, which include stone, bricks and various man made blocks when it comes to structural design and it is not uncommon for different units to be mixed and matched. For example, on modern houses frequently the internal skin is constructed using blockwork due to the speed at which it can be erected, although traditional bricks are still commonly used for the external skin because of the increased visual quality that they present and also they perform well in external and exposed environments.
One of the key things to remember when designing and constructing masonry structures is that whilst it may have a small tensile component, it is generally advisable to construct masonry structures so that they work in compression. If as an engineer you are ever unsure if a material is suitable to design to resist tension forces, ask yourself a simple question…
“If I was in prison, would I make an escape rope from it?”
If the answer is no, then the chances are, that it’s a poor material in tension.
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8.2 How are they made?
Bricks traditionally were made through the placing of clay into moulds to create the brick shape and then these units were fired in an oven at high temperatures to create a stable unit. Modern methods also include the creation of long continuous columns which are then cut into smaller units using a wire cutting device. These sub units are then dried and then fired in a kiln to create bricks.
Figure 8-1 Normalised block strengths from a manufacturer.
Masonry units come in a variety of strengths and each different type of unit can behave very differently from another unit, however the strength of the unit is only one factor that can affect the overall strength and load carrying capacity of masonry structure. The mortar and quality of the construction will also greatly affect the overall load carrying capacity of a masonry structure.
The 6 key factors that will affect the compressive strength of masonry are listed below.
• The mortar strength • The unit strength • The relative values of unit and mortar strength • The aspect ratio of the units (ratio of height to least horizontal
dimension)
Unit strength (in N/mm2) to BS 5628-1
Normalised strengths (in N/mm2) for unit widths of:
75mm 90mm 100mm 140mm 190mm 215mm
2.9 4.1 4.1 4.0 3.8 3.5 3.4
3.6 5.1 5.0 5.0 4.7 4.3 4.2
7.3 10.4 10.2 10.1 9.5 8.8 8.5
10.4 14.9 14.6 14.4 13.5 12.5 12.1
17.5 25.0 24.5 24.2 22.8 21.0 20.3
22.5 32.2 31.5 31.1 29.3 27.0 26.1
30.0 42.9 42.0 41.4 39.0 36.0 34.8
40.0 57.2 56.0 55.2 52.0 48.0 46.4
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• The orientation of the units in relation to the direction of applied load
• The bed joint thickness
8.3 Common units.
There is an overwhelming range of masonry products available and given the amount of time that they’ve been used in the UK for construction there is no real standard size as frequently “specials” are used, especially for heritage and conservation work where perhaps the bricks were made by a local artisan originally.
However, for modern construction, commonly adopted brick dimensions include:
Figure 8-2 Coursing dimensions for brickwork.
8.4 Clay bricks
There are a wide range of clay bricks within the UK, each with subtly different finishes, colours and performance criteria that have been developed over the years to suit the various uses and intended locations of the bricks.
Clay bricks can be made by hand or within a factory and can have a density ranging from anywhere between 22.5 to 28kN/m3.
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5.2.1 BRICKSBricks are manufactured from a variety of materialssuch as clay, calcium silicate (lime and sand/flint),concrete and natural stone. Of these, clay bricksare by far the most commonly used variety in theUK.
Clay bricks are manufactured by shaping suit-able clays to units of standard size, normally takento be 215 ! 102.5 ! 65 mm (Fig. 5.5). Sand facingsand face textures may then be applied to the ‘green’clay. Alternatively, the clay units may be perforatedor frogged in order to reduce the self-weight of theunit. Thereafter, the clay units are fired in kilns toa temperature in the range 900–1500 °C in orderto produce a brick suitable for structural use. Thefiring process significantly increases both thestrength and durability of the units.
In design it is normal to refer to the coordinatingsize of bricks. This is usually taken to be 225 !112.5 ! 75 mm and is based on the actual or work
Fig. 5.3 Wall ties to BS EN 845-1: (a) butterfly tie; (b) double triangle tie; (c) vertical twist tie.
Fig. 5.6 Coordinating and work size of bricks.
size of the brick, i.e. 215 ! 102.5 ! 65 mm, plus anallowance of 10 mm for the mortar joint (Fig. 5.6).Clay bricks are also manufactured in metric modu-lar format having a coordinating size of 200 ! 100! 75 mm. Other cuboid and special shapes are alsoavailable (BS 4729).
Fig. 5.4 Damp proof courses: (a) lead, copper, polythene,bitumen polymer, mastic asphalt; (b) two courses of bondedslate; (c) two courses of d.p.c. bricks (based on Table 2,BS 5628: Part 3).
Fig. 5.5 Types of bricks: (a) solid; (b) perforated;(c) frogged.
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Clay bricks have a tendency to expand as a result of water absorption, whereas engineering bricks are much more stable with regards water absorption.
Locations where a high strength, high quality brick is required will usually benefit from the inclusion of engineering brick due to its excellent durability and structural stability.
• Class A Engineering Bricks - 70N/mm2; Water absorption ≤ 4.5% by mass.
• Class B Engineering Bricks - 50N/mm2; Water absorption ≤ 7.0% by mass.
There are typically 60 standard bricks per square metre of wall, with a 2:1 gang (2 bricklayers and 1 labourer) being able to lay approximately 1,000 bricks in a day.
8.5 Facing bricks
Facing bricks are typically not as strong as engineering bricks and are used to create a high quality visual appearance in areas of a building where a certain type of look is required.
8.6 Calcium Silicate Brick
These types of bricks are aimed at the mass produced, low cost/unit end of the market. They are typically formed from sand and slaked lime and are rarely used due to their tendency to shrink and crack which can make their integration into a building structure very difficult.
8.7 Concrete block
Concrete blocks are widely used in the industry for a variety of reasons. Whilst they may not be as visually appealing as bricks, their relative cheapness offers them a significant advantage.
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Similarly as the units themselves are quite a bit larger than bricks, the amount of wall that can be constructed in a day is much greater as a bricklayer can effectively replace multiple bricks with a single block.
There are a multitude of different types of concrete blocks, ranging from lightweight blocks with a density of 5kN/m3 through to dense blocks with a density of 20kN/m3.
Care should be taken when specifying the blocks, as it is possible to get the same strength of block at a variety of densities and depending on the use of the block this can affect how it performs with regards its durability.
For example, hanging large heavy components such as boilers off a wall constructed in lightweight block can sometimes cause difficulties as the wall anchors are being installed as they can cause lighter weight blocks to disintegrate and fall apart.
The solution isn’t to specify the heaviest block possible, there are limits on block weights that you can legally expect a brick layer to lift continually through the course of the day and these are contained within the CDM regulations.
Similarly, if the heaviest blocks are used, these can increase the dead load that a floor needs to support which can have associated ramifications on the cost of the building if larger floors, columns and foundations are needed to support the extra weight of the masonry.
Common9 strengths of concrete blocks range between 3.6 to 22.5N/mm2.
Blockwork walls typically will either have an external render or leaf of brickwork and be concealed.
For internal blockwork, the walls are typically plastered or lined but an alternative is the specification of fair faced blocks which will allow the wall to receive a coat of paint directly.
8.9 Terminology
Masonry units have a strange terminology which may not be apparent from the description given and engineers will need to be familiar with these terms so that they are able to appropriately communicate with contractors and anyone else that may need to use their drawings.
Figure 8-3 Types of masonry units.
Brick and blockwork is typically constructed in leafs, this can be either a single lead, cavity wall, or sometimes the leaves can be reinforced through the integration of piers to help increase either their vertical or lateral load carrying capacity of the wall as can be
Eurocode 6: Design of masonry structures
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A number of procedures for conditioning ofmasonry units prior to testing are outlined in EN772-1: Methods of test for masonry units: Determinationof compressive strength. This standard advises thatthe conditioning factor for air-dried units is 1.0.Masonry units manufactured to British Standardsare wet strengths in which case the recommendedvalue of the conditioning factor is 1.2. Values forthe shape factor, !, are given in Table A1 of EN772–1, to allow for the height and width of units.For example, for 102.5 mm " 65 mm bricks ! =0.85, and for 215 mm (height) " 100 mm (width/thickness) blocks, ! = 1.38.
10.8.3 COMPRESSIVE STRENGTH OF MORTARTable 10.4 shows the masonry mortar mixes recom-mended for use in the UK to achieve the appro-priate strength given in EC 6. As will be noted thechoice and designation of masonry mortars in EC6 and BS 5628 are identical and therefore for agiven application or exposure similar mortars maybe specified. According to clause 3.2.2 of EC 6,masonry mortars may be specified by compressivestrength, expressed as the letter M followed by thecompressive strength in Nmm#2, e.g. M4, or mixproportion, e.g. 1:1:6 signifies the cement-lime-sandproportions by volume. The latter has the advant-age, however, that it will produce mortars of knowndurability and should generally be used in practice.
calcium silicate units have between 25–55 per centvoids and aggregate concrete units between 25–60per cent voids. Table 10.3 gives further detailsof the requirements for unit groupings. All UKbricks currently manufactured to British Standards,including frogged and perforated bricks, fit intoGroup 1 unit specification although the authorunderstands some Group 2 units are becomingavailable. Cellular and hollow blocks fit intoGroup 1 or Group 2 unit specification, dependingon the void content. Masonry units which fall withinGroups 3 and 4 have not historically been used inthe UK. The manufacturer will normally declarethe group number appropriate to his unit.
10.8.2 NORMALISED COMPRESSIVE STRENGTHA difference in the size of units available through-out Europe and differences in test procedures hasmeant that the compressive strength of masonryunits had to be normalised.
The normalised compressive strength, fb, is thecompressive strength converted to the air driedcompressive strength of an equivalent 100 mm wide" 100 mm high unit of the same material. Thenormalised compressive strengths of masonry, fb,is given by
Vertically Perforated Unit Vertically Perforated Unit
Frogged Unit Vertically perforated Unit
Fig. 10.1 Examples of high density (HD) clay masonry units (Fig. 3, EN 771-1).
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seen in Figure 8-4 below.
Figure 8-4 Forms of masonry wall construction.
8.10 Ties
Masonry leaves are knitted together through the introduction of wall ties that can take a variety of shapes and formed. As building efficiencies increase and the requirement to increase the cavity to help raise the thermal performance becomes greater and greater, so the size of the cavity too will increase and this requires stronger and stronger wall ties.
Figure 8-5 Common Wall Ties
In the above figure type (a) are butterfly ties, (b) are double triangle ties and (c) are half twist ties which are typically used for the larger cavities.
Design in unreinforced masonry to BS 5628
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COMPRESSION:Gk characteristic dead loadQk characteristic imposed loadWk characteristic wind load!f partial safety factor for load!m partial safety factor for materialsfk characteristic compressive strength of
FLEXURE:fkx par characteristic flexural strength of masonry
with plane of failure parallel to bed jointfkx perp characteristic flexural strength of masonry
with plane of failure perpendicular to bedjoint
" bending moment coefficientµ orthogonal ratioM ultimate design momentMR design moment of resistanceMpar design moment with plane of failure
parallel to bed jointMperp design moment with plane of failure
perpendicular to bed jointMk par design moment of resistance with plane
of failure parallel to bed jointMk perp design moment of resistance with plane
of failure perpendicular to bed joint
5.5 Design of vertically loadedmasonry walls
In common with most modern codes of practicedealing with structural design, BS 5628: Code ofPractice for Use of Masonry is based on the limit
Fig. 5.8 Masonry walls.
The ultimate design load is a function of the actualloads bearing down on the wall. The design loadresistance is related to the design strength of themasonry wall. The following sub-sections discussthe procedures for estimating the:
1. ultimate design load,2. design strength of masonry walls and3. design load resistance of masonry walls.
5.5.1 ULTIMATE DESIGN LOADS, NAs discussed in section 2.3, the loads acting ona structure can principally be divided into threebasic types, namely dead loads, imposed (or live)loads and wind loads. Generally, the ultimate designload is obtained by multiplying the characteristic(dead/imposed/wind) loads (Fk) by the appropriatepartial safety factor for loads (!f)
N = !fFk (5.2)
state philosophy (Chapter 1). This code states thatthe primary aim of design is to ensure an adequatemargin of safety against the ultimate limit statebeing reached. In the case of vertically loaded wallsthis is achieved by ensuring that the ultimatedesign load (N ) does not exceed the design loadresistance of the wall (NR):
N # NR (5.1)
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5.2.1 BRICKSBricks are manufactured from a variety of materialssuch as clay, calcium silicate (lime and sand/flint),concrete and natural stone. Of these, clay bricksare by far the most commonly used variety in theUK.
Clay bricks are manufactured by shaping suit-able clays to units of standard size, normally takento be 215 ! 102.5 ! 65 mm (Fig. 5.5). Sand facingsand face textures may then be applied to the ‘green’clay. Alternatively, the clay units may be perforatedor frogged in order to reduce the self-weight of theunit. Thereafter, the clay units are fired in kilns toa temperature in the range 900–1500 °C in orderto produce a brick suitable for structural use. Thefiring process significantly increases both thestrength and durability of the units.
In design it is normal to refer to the coordinatingsize of bricks. This is usually taken to be 225 !112.5 ! 75 mm and is based on the actual or work
Fig. 5.3 Wall ties to BS EN 845-1: (a) butterfly tie; (b) double triangle tie; (c) vertical twist tie.
Fig. 5.6 Coordinating and work size of bricks.
size of the brick, i.e. 215 ! 102.5 ! 65 mm, plus anallowance of 10 mm for the mortar joint (Fig. 5.6).Clay bricks are also manufactured in metric modu-lar format having a coordinating size of 200 ! 100! 75 mm. Other cuboid and special shapes are alsoavailable (BS 4729).
Fig. 5.4 Damp proof courses: (a) lead, copper, polythene,bitumen polymer, mastic asphalt; (b) two courses of bondedslate; (c) two courses of d.p.c. bricks (based on Table 2,BS 5628: Part 3).
Fig. 5.5 Types of bricks: (a) solid; (b) perforated;(c) frogged.
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8.11 Bond
The pattern that the bricks are laid is called the bond pattern and in the UK there are five commonly used bond patterns, although many more patterns are available.
Figure 8-6 Common UK bond patterns.
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8.12 How is it built?
As the mortar is a key component of masonry construction, it can also be a limiting factor. The amount of masonry that can be constructed in a single day is not just dictated by the rate at which the bricklayer can construct the wall.
Indeed if too much masonry is constructed in a day, then the weight of the bricks above the wall can cause the mortar to squeeze out between the bricks like toothpaste and vastly reduce the quality of the wall.
Similarly if walls are constructed in temperatures that are too cold, this can affect how the mortar cures and can result in serious defects with the masonry resulting in the wall needing to be reconstructed at a later date.
Another key consideration is that whilst the mortar is still curing it will be very weak and present no appreciable strength characteristics and so the weather conditions will need to be considered along with the need for temporary propping. This could be to prevent the wind from blowing it over during construction, or just to support the load of the masonry itself before it has obtained the ability to support its own weight on something like a masonry dome.
8.13 Common defects
Common defects associated with the construction of masonry are commonly linked with geometric imperfections or material deficiencies. The geometric imperfections can take various forms including misalignment of the bricks resulting in the bricks not being coursed effectively, walls and columns may not necessarily be built truly vertical and the lean induced into the element can induce additional overturning or flexural components into the wall.
The capacity for tolerance and potential quality control is reflected in the large factors of safety typically included within masonry designs. Although surprisingly masonry is quite a resilient material, with the peak district being a prime example of this resilience with
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various walls damaged through vehicle impact, but still free standing.
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Timber
9 What is timber?
Timber is one of the oldest construction materials. It is biologically produced by nature and further processed by man.
Timber is a commonly used construction material, with the ability for taller and taller timber buildings continuing to develop with the recently completed 9 storey Stadthaus10.
If appropriately sourced, timber can be a sustainable material and there are a variety of forestry stewardship programmes that ensure that as trees are harvested that a minimum number of trees are planted for replacement.
There are numerous timber products available:
• Solid timber (traditional sawn timber)
• Glue Laminated timber (laminates planks of wood glued together)
• Plywood (panel product of thin laminates glued together)
• Particleboard (panel product formed by compacting chippings with resin)
9.2 Origins of timber.
Whilst timber comes from trees, there are two fundamental different types of timber that are used in construction, softwood and hardwood. Hardwood and softwood are broad biological terms used to describe species of wood. The terms have nothing to do with the physical hardness of the wood. Hardwoods come from broad-leaved trees and softwood species from coniferous, evergreen trees.
Timber is manufactured from the trunk part of the tree. When the tree is felled, it is taken to the timber mill where it is sawn into sections of the desired size and shape. Primarily, there are two sawing techniques adopted plane-sawing and quarter-sawing. Plane-sawing is quicker and easier to undertake, but more of the tree material is wasted. Quarter-sawing is more time consuming, but generates less waste.
9.3 Softwoods
Various softwoods are commonly used within the UK for construction purposes and for different purposes.
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Commonly used species include:-
• Douglas Fir • Scots Pine • European Spruce • Sitka Spruce • Pitch Pine • Parana Pine • Western Hemlock • Western Red Cedar • Hardwoods.
Various hardwoods are commonly used within the UK for construction purposes and for a different purposes.
Commonly used species include:-
• Beech • Iroko • Mahogony • Oak • Sapele • Teak
9.4 Considerations.
When compared with other structural materials, there are peculiarities to the use of timber in construction. These are:
9.4.1 Hygroscopy
Propensity to creep under sustained load
Anisotropy
9.4.2 Hygroscopy
Timber is a natural product derived from trees. Since trees need water to survive during their lifespan, timber, by its nature, is a hygroscopic material – it attracts water. The effect of hygroscopy is not just true for living timber but also for dead timber. If, for example, a structural timber element is taken from a dry timber merchant and
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applied in a damp, moist environment, it will attract and absorb water from the environment.
The significance of hygroscopy is that water influences timber at the microscopic level, to such an extent that the absorbed water affects the strength of the timber. The strength reduces as more water is absorbed into the microscopic structure.
9.4.3 Propensity to creep under sustained load
When loaded over a sustained period of time, timber experiences:
• A significant loss of strength
• A loss of stiffness (increase in deflection)
9.5 Strength
A piece of timber has an initial value of strength when a load is first applied to it. If the load is sustained over a considerable period of time it can be seen that the load carrying capability is significantly reduced. Its strength after this time period is a fraction of its initial strength value. Severe loss of strength could result in failure of the timber or creep rupture.
9.6 Deflection
When a load is applied to a piece of timber it has an initial elastic deflection. If the load is sustained the deflection steadily increases – even though there is not any increase in the magnitude of load. In other words, the piece of timber is subjected to the effect of creep. The deflection increases as time progresses until creep rupture occurs.
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Figure 9-1 Deflection of a timber beam.
9.7 Creep relationships.
The effects of creep are more exaggerated if a piece of timber has high moisture content.
Anisotropy
As stated previously, timber is a construction product derived from trees. A tree grows in a way which best suits the demands that are placed upon it throughout its life. Trees, and therefore structural timber have different strength properties in different directions. It is an anisotropic material, as shown below.
9.8 The anisotropic nature of timber strengths.
Eurocode 5 uses the limit state design philosophy. This means that the strength capacity of the timber to withstand the applied actions is checked at the ultimate limit state (ULS). Additionally, a check on the actual deflection under the applied actions is within an acceptable deflection limit is undertaken at the serviceability limit state (SLS).
9.9 Processing timber.
Timber is cut down from whole trunks and the direction of the grain aligns with the longitudinal axis of the trunk. Below is a diagram which shows some common forms of cutting timber down into useable planks and timber elements.
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uinst
ucreep
ufin
l
Fig. 2.6. Deformation.
Where it is relevant to show the direction of the grain of the timber it is defined bythe symbol used in Figure 2.5.
2.3.2 Serviceability limit states (EC5, 2.2.3)
In EC5 the deformation of a member or structure is required at two stages:
(i) When the loading is immediately applied; this is called the instantaneous defor-mation: uinst.
(ii) After all time-dependent displacement (i.e. creep deformation, ucreep) has takenplace; this is called the final deformation: ufin.
These deformations are shown diagrammatically in Figure 2.6 in relation to a simplysupported beam without any pre-camber.
Deformation is calculated in two different ways, depending on the creep behaviourof the structure:
(a) Structures comprising members, components and connections having the samecreep behaviourCreep behaviour in timber and wood-related products is a function of several fac-tors, and to simplify the design process the assumption is made in EC5 that whensubjected to a permanent load over the lifetime of a building, the instantaneousdeflection (uinst) and the creep deflection (ucreep) are related as follows,
ucreap = kdefuinst (2.27)
where kdef is a deformation factor whose value is dependent on the type ofmaterial being stressed as well as its moisture content. Values for the factor havebeen derived for timber and wood-based materials at defined environmentalconditions when subjected to constant loading at the SLS over the design life,and are given in EC5, Table 3.2. The environmental conditions are referred toas service class 1, 2 or 3 (discussed in 2.2.20) and values for kdef for timber andsome wood-related products at these conditions are given in Table 2.10.
For structures or members complying with the above conditions the finaldeformation, ufin, can then be written as
ufin = uinst + ucreep = uinst(1 + kdef) (2.28)
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Figure 9-2 Common sawing patterns.
9.10 Engineered Timbers.
The use of engineered timbers is commonplace in the UK and the development of high performance materials are continuing to push the boundaries within their use, the following descriptions are not intended to be exhaustive but give an indication as to some of the current uses of these materials.
9.11 Kerto
Deriving its strength from a homogeneous bonded structure Kerto is produced from 3mm rotary-peeled Spruce veneers glued together to form a continuous billet.
Available in 3 different types, Kerto-S, Kerto-Q and Kerto-T it is a product suited to a variety of applications.
Kerto-S: ideally suited to deliver long spans delivering excellent technical performance with minimal deflection. Suitable for all roof shapes as well as joists and lintels.
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(c) Typical sawing patterns
(a) Breakdown of a debarked log
Debarked log
Wings cut Centre cant (boxed heart)
Splits (pith on edge) Winged split
Through conversion with near quarter sawing
Through conversion(plain sawing)
Quarter sawing (two different radial cuts) –slow procedure requiring large logs
Through conversion(billet sawing)
Tangential sawing – conversionwith boxed heart
Radial sawing
Radial wedge
Tangential sawing
(b) Tangential and radial sawing
Fig. 1.3. Examples of log breakdown and cutting pattern.
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Kerto-Q: with roughly 20% of the veneers cross-bonded Kerto-Q is suited to applications where high shear strengths are a necessity. Perfect for large floor or roofing panels.
Kerto-T: made from lighter veneers, Kerto-T is ideally suited for use as a stud in both load-bearing and non load-bearing structures.
Figure 9-3 Timber gridshell using engineered timber
Kerto is commonly used to create advanced forms and geometries such as timber gridshells, because it is an engineered product it has a much greater reduced risk from concealed defects within the timber and has much greater load carrying capacity.
9.12 Plywood
Plywood is a wood product manufactured out of many sheets of veneer, or plies, pressed together and glued, with their grains going in opposite directions. Plywood tends to be extremely strong, though not very attractive, and is treated in many different ways depending upon its intended application. Because of the way in which plywood
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is constructed, it also resists cracking, bending, warping, and shrinkage, depending upon its thickness. Plywood is also referred to as an engineered wood, although it is made from a composite of wooden materials, and various forms of it have been made for thousands of years.
Figure 9-4 Structure of ply.
The plies that form plywood are generally cut on a rotary lathe, which cuts a continuous roll of wood while a log, called a peeler, is turned against it. Rotary lathing is rapid and makes efficient use of the wood while turning out veneers highly suitable for plywood. Some lathes are designed to expose more interesting parts of the wood grain, although they may be more wasteful of the wood.
Rotary lathed veneers tend to be dull in appearance, although perfectly functional. After the veneers are cut, they are overlaid with layers of glue and pressed together until dry to form a flat, even, tight piece of plywood. Plywood is sturdier than regular sheets or panels of wood, because the veneers are laid with their grains opposing, which also causes the wood product to resist warping because the grains pull each other tight.
The main types of Plywood Sheets are Shuttering, WBP, Softwood, Hardwood, Interior, Exterior and Marine. Within these different types there are then also different grades depending on the type of wood that is used for constructing the wood laminates and the quality of
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Face ply
Back ply
Cross ply (core)
Grain directions
(a) The structure of a three-ply plywood
(c) Three-ply blockboard
(d) Five-ply blockboard
(e) Laminboard
(b) Five-ply plywood
Fig. 1.9. Examples of plywood and wood core plywood.
1.7.2 Plywood
Plywood is a flat panel made by bonding together, and under pressure, a number ofthin layers of veneer, often referred to as plies (or laminates). Plywood was the firsttype of EWP to be invented. Logs are debarked and steamed or heated in hot water forabout 24 hours. They are then rotary-peeled into veneers of 2–4 mm in thickness andclipped into sheets of some 2 m wide. After kiln-drying and gluing, the veneers arelaid up with the grain perpendicular to one another and bonded under pressure in anodd number of laminates (at least three), as shown in Figure 1.9a. The outside plies,always made of veneer, are referred to as faces (face ply or back ply) and the innerlaminates, which could be made of either veneers or sliced/sawn wood, are called core.Examples of wood core plywood include blockboards and laminboards, as shown inFigures 1.9c–1.9e.
Plywood is produced in many countries from either softwood or hardwood or acombination of both. The structural grade plywoods that are commonly used in theUnited Kingdom are as follows:
! American construction and industrial plywood! Canadian softwood plywood and Douglas fir plywood! Finnish birch-faced (combi) plywood, Finnish birch plywood and Finnish coniferplywood! Swedish softwood plywood.
The plywood sheet sizes available sizes are 1200 mm ! 2400 mm or 1220 mm !2440 mm. The face veneer is generally oriented with the longer side of the sheet exceptfor Finnish made plywoods in which face veneers run parallel to the shorter side.Structural plywood and plywood for exterior use are generally made with waterproofadhesive that is suitable for severe exposure conditions.
The structural properties and strength of plywood depend mainly on the numberand thickness of each ply, the species and grade and the arrangement of the individual
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the face of the sheet – it can be smooth, sanded and with or without knots.
One of the common difficulties with the use of plywood on construction sites is that if it gets wet, it can cause the panel to disintegrate and become like wet cardboard. To overcome this the use of marine grade ply is commonly used where there is a risk of the timber being wetted.
9.13 Cross Laminated Timber (CLT)
Cross laminated timber is being used to create modular buildings that have the capacity to be prepared offsite, there are a variety of companies that manufacture different types of walls and floors from Cross laminated timber including Eurban11 and KLH12.
Cross laminated timber (CLT) panels are produced from mechanically dried spruce boards which are stacked together at right angles and glued over the entirety of their surface. Each CLT panel is produced is between three and seven boards thick depending on the amount of structural loading required.
Gluing at high pressure reduces the timbers expansion and shrinkage potential to a negligible level. The result is a rigid structural timber member that can be used both vertically and horizontally to construct a buildings frame.
By alternating the grain of the timber through both directions in this manner, very strong floor panels can be constructed that have bi-directional spanning capabilities that allow thin floor zones to be constructed.
11 http://www.eurban.co.uk/
12 http://www.klhuk.com/
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9.14 Drying of timber
Look at any felled tree and the first thing you notice is how wet it is. When it’s sawn and is still wet, it will not twist, warp or bend. When you dry timber, this problem is eliminated. If you manufacture anything from wet timber, there is a very good chance it will become distorted, shrink, split and generally be unusable if it dries in the wrong conditions.
By drying timber in a correct manner, tension within the timber, which can cause many defects, is relieved, and we create timber that is ready for the manufacturing process. There are several advantages to using dry timber. It is stronger and holds nails better than green timber; it is more stable and also minimises future warping; it is less subject to stain, decay and insect attack and is easier to paint and treat with preservatives.
9.15 Air Drying
This is carried out by stacking pieces of timber on top of each other, separated by lathes or sticks to allow air to circulate. Alternatively, it can be achieved by placing timbers at right angles to each other, creating gaps for air circulation. This method of drying can take some considerable time.
9.16 Kiln Drying
This is the method used to speed up the drying process using a kilning chamber or de-humidifier.
9.17 Grading of timber
The grading of timber considers the size, quality and condition of a piece of timber at the time of the original inspection.
It is necessary to distinguish between the two fundamental types of grading:
Mechanical grading is employed when we try to determine the strength characteristics of a piece of timber. This is usually done by
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measuring the stiffness of the timber when a load is applied to it by a strength grading machine.
The need to assess the strength of a piece of timber is usually required where timber is to be used in a constructional or load bearing capacity, such as the manufacture of roof trusses.
Visual grading is a judgement of the timber’s appearance and suitability for its end use, considering both the natural characteristics and manufacturing imperfections of each piece.
Visual grading can be and is used to determine grades for constructional use, although it is more usually used to determine the grade based on appearance.
Figure 9-5 Grading stamp.
Timber can have many defects, most of which will be identified within the grading process, but an awareness of some of the most common types of defect is important to allow the engineer to understand and recognise these defects on site.
9.18 Strength classes.
The strength class of the timber is dictated by the wide range of timber species that are available within the market.
The strength of a timber element is highly dependant upon the duration of the load and the environmental exposure of the element in consideration. These elements will need to be considered during the specific design life and assessment of how the element will behave, particularly if it’s envisaged that there is the potential for these elements to change and become more onerous than the original conditions that the element was designed for.
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Table 1.2 Softwood combinations of species and visual grades that satisfy therequirements for various strength classes!
Timber species Grade and related strength classes
British grown timberDouglas fir GS (C14), SS (C18)Larch GS (C16), SS (C24)British pine GS (C14), SS (C22)British spruce GS (C14), SS (C18)
Imported timberParana pine GS (C16), SS (C24)Caribbean pitch pine GS (C18), SS (C27)Redwood GS (C16), SS (C24)Whitewood GS (C16), SS (C24)Western red cedar GS (C14), SS (C18)
Douglas fir-larch (Canada and USA) GS (C16), SS (C24)Hem-fir (Canada and USA) GS (C16), SS (C24)Spruce-pine-fir (Canada and USA) GS (C16), SS (C24)Sitka spruce (Canada) GS (C14), SS (C18)Western white woods (USA) GS (C14), SS (C18)Southern pine (USA) GS (C18), SS (C24)
!Timber graded in accordance with BS 4978:1996; based on Table 1.2, BS 5268-2:2002.
the load to induce a known deflection) is then automatically measured and comparedwith pre-programmed criteria, which leads to the direct grading of the timber sectionand marking with the appropriate strength class. An example of the grading marking,based on the requirements of BS EN 14081-1:2005, is shown in Figure 1.7.
In general less material is rejected if machine graded; however, timber is also visuallyinspected during machine grading to ensure that major, strength-reducing, defects donot exist.
1.5.3 Strength classes
The concept of grouping timber into strength classes was introduced into the UnitedKingdom with BS 5268-2 in 1984. Strength classes offer a number of advantagesboth to the designer and the supplier of timber. The designer can undertake the designwithout the need to check on the availability and price of a large number of species andgrades that might be used. Suppliers can supply any of the species/grade combinations
PRODUCT
CODE
NBODY
DRY GRADED
M
C24
Key:PRODUCT: producer identificationCODE: Code number of documentationDRY GRADED: used if appropriateNBODY: identification of notified bodyM: machine gradedC24: strength class or grade and grading
Fig. 1.7. Example of grading marking.
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9.19 Sourcing timber.
The selection of materials is made simpler through various schemes currently in operation for numerous materials, one of the better known schemes which extends to sustainably sourced timber within the UK is the Forestry Stewardship Scheme (FSC13) which provides an audit trail for the production of timber, from the planting of the sapling through to its harvesting and subsequent processing. There are several alternative schemes available throughout Europe, including the PEFC14 and the SFI15 and whilst timber from other European countries will require more energy to be delivered to a UK site, the timber produced particularly from colder Scandinavian countries can be desirable as the colder climate forces the timber to grow slower, creating a denser stronger timber.
9.20 Defects
Timber can have many defects, most of which will be identified within the grading process, but an awareness of some of the most common types of defect is important to allow the engineer to understand and recognise these defects on site.
13 http://www.fsc-uk.org/ (Accessed 6th April 2011)
14 http://www.pefc.org/ (Accessed 6th April 2011)
15 http://www.sfiprogram.org/ (Accessed 6th April 2011)
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Figure 9-6 Timber seasoning defects.
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10 Structural Timber Design to Eurocode 5
Bowing
(b) Seasoning defects
Shake Knot Wane
Diagonal grain Cross grain Flat grain
End splitting Honeycombing
Springing Twisting
(a) Natural and conversion defects
Cupping
Fig. 1.6. Defects in timber.
Timber is described as being hygroscopic, which means that it attempts to attain anequilibrium moisture content with its surrounding environment, resulting in a variablemoisture content. This should always be considered when using timber, particularlysoftwoods, which are more susceptible to shrinkage than hardwoods.
As logs vary in cross-section along their length, usually tapering to one end, a boardthat is rectangular at one end of its length might not be so at the other end. Therectangular cross-section may intersect with the outside of the log, the wane of thelog, and consequently have a rounded edge. The effect of a wane is a reduction inthe cross-sectional area resulting in reduced strength properties. A wane is an exampleof a conversion defect and this, as well as other examples of conversion or naturaldefects, is shown in Figure 1.6a.
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Civil engineering structures
10 Self based exercise.
This chapter is all about self based learning, with a group of friends on your course, form teams to determine what the following structures are, what materials they may be constructed from and how they could be built… one thing to note is that there may be several variations on the materials and the construction techniques for each type of structure.
