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Load bearing walls have the advantage that they can also act as the external envelope and as internal partitions whereas using a structural framing system means that the external envelope and internal partitions must be built as separate building elements.

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• External walls are more commonly built using cavity construction although a solid wall with a cladding applied to the outside can sometimes suffice. Basically there are three main types of construction used for masonry walls. • Solid walls: These walls are normally associated with internal use. The walls increase in thickness in modules, usually 112 brick thick (102.5mm), 1 brick thick (215mm), 1½ brick thick, etc. The required thickness is brick wall dictated by the load that will be applied to the wall, the height of the wall and the horizontal distance between buttressing in the wall such as piers and columns • Cavity walls: The cavity originally being installed to prevent moisture passing from the outer, wet skin, to the inner dry skin. The two skins are coupled for strength using wall ties set at regular intervals in the wall. The 5Omm wide cavity can be varied, but as wall ties are manufactured for a standard cavity of 5Omm such variations are difficult.

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Monadnock Building In 1884, the owners of the property along the west side of Dearborn Street between Jackson and Van Buren Streets discovered that Chicago's City Council was contemplating an ordinance that would restrict buildings to 13 stories in height. Burnham & Root were commissioned to quickly file a building plan for a sixteen-story structure with the Building Department. John Root's design reflected an austerity of detail prescient of future skyscrapers. Yet the building plan also was a product of its time, crafted before steel skeleton construction was perfected. John Root died of influenza in 1891 at the age of 41, and later that year the building he designed seven years earlier was finally built on the southwest corner of Jackson and Dearborn Streets. Named the Monadnock Building, its soaring height and unornamented, gently tapered and curving facade pointed toward a thrilling architectural future. At 16 stories and 215 in height, it was the world's tallest building when completed, and even today is the tallest ever built with weight-bearing masonry walls. Architect Louis Sullivan wrote of the Monadnock that it was "an amazing cliff of brickwork, rising sheer and stark, with a subtlety of line and surface, a direct singleness of purpose, that gave one the thrill of romance." The Monadnock was more a product of Root's artistic vision than Burnham's engineering sense, and it underscored the loss to architecture that Root's untimely death represented. Daniel Burnham wrote at the time, "John's death has left a hole into which not one, but several strong men must be flung."

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Masonry construction can and has been used to produce buildings of 10–15 storeys (for example, the Madanock building in Boston USA). However, to do this masonry walls have to be of substantial thickness towards the lower levels of the building. However, internal space is very cellular in nature which reduces the flexibility of the space, particularly in the case of office buildings This is because the loads imposed from the upper storeys are substantial and masonry walls have a tendency towards flexure and buckling under heavy loading. Masonry walls have little ability to cope with the magnitude of forces that are imposed by tall buildings because of the lack of flexural strength in horizontal joints ie. the mortar beds. The only way to build in sufficient strength in buckling in masonry walls is to increase their thickness. The increase in the mass of the walls greatly increases the 'gravity' loads that must be supported by the foundations. Another side effect of this strategy is that the internal space is very cellular in nature which reduces the flexibility of the space, particularly in the case of office buildings. In general masonry construction can become uneconomic beyond four or five storeys.

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Timber is a very versatile and environmentally friendly construction material. In engineering terms it has favourable properties when compared with steel and concrete. However, the use of timber frames (apart from in low rise domestic buildings) is unusual. We may find it used in relatively low rise or long span commercial buildings, but it’s use in a typical commercial building as the sole load bearing structural material is rare. At the present time, it will not be considered as a major structural frame material. So, we are left with Steel, or Concrete as our primary choices.

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As a very general guide, the optimum span for beams in commercial buildings, is 7.5m. This is not always achievable, or desirable. So, as a rough guide, you should be aiming for main beams with spans between 5.0m and 10.0m. Clearly, the main beams span between columns, so this is really a part of the column arrangement process. The span of beams can be varied in different parts of a building.

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Alternative steel frame approaches A cantilever frame is one where beams span continuously over or past columns so that they cantilever out beyond columns and extend the floors of the building outside of the grid lines of the columns themselves. A key point here is that the beams must be continuous, which affects the detailing at the connection. When the beams span continuously over the columns, the cantilever effect on one side of the column will, to an extent, balance the load that is exerted in the centre of the span between the columns. The beam will hogg over its points of support at the columns. This means that the overall bending moment, the stress that the beam must accommodate, will be reduced. An additional advantage of the cantilever frame is that it removes the columns from the perimeter of the floor slab, which may have aesthetic or functional benefits for the building. For example, a glass cladding system may be attached to the floor structure and can be free from any aesthetic interference that would be caused by perimeter columns. functionality, it could be that the building needs to be extended over a pavement, sidewalk or a road at its 1st or second storey. This would provide a greater floor area in the upper levels of the building than at its ground floor, meaning that the building could offer more rentable space than would otherwise be possible. An example of an engineering reason could be that the building is being constructed immediately adjacent to an existing building that makes it difficult or undesirable to place foundations at the very perimeter of the building site, which would be necessary if a simple cage frame were used. Or, it could be that the soil is of poorer bearing capacity at the perimeter of the site.

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A propped cantilever frame is one where the extent of the cantilevered sections is such that it is more economic to offer some support to the ends of the cantilevers with lightweight props. In this regard, the cantilevers no longer act as pure cantilevers. There are occasions where it may be desirable or necessary to cantilever beams out from supporting columns to greater distances than 1/4 or 1/5 of the main span. In fact we might want the cantilever to extend a greater distance than the centre span. This might happen where we need uninterrupted space in the main usable building areas, but have to have a central corridor down through the spine of the building. We could use the walls of the main corridor to conceal the main supporting columns and then cantilever the beams out beyond to support the usable floor areas.

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Precast hollow core planks are manufactured by numerous suppliers and you should refer to manufacturers that are referred to at the end of this unit. Different depths of plank can be supplied depending on span and load conditions. Typical depth sizes would be 150mm, 200mm and 250mm. Various widths of planks can also be supplied. Different manufacturers have different guidance on how the planks are secured to UB’s or UKB’s. Some manufacturers suggest small fixing plates be fixed to the underside of beam flanges. This then allows a bolted connection between the underside of the plank and the beam. Alternatively the plank may be secured purely by gravity bearing. In this case the bearing distance is critical.

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Different manufacturers have different guidance on how the planks are secured to UB’s or UKB’s. Some manufacturers suggest small fixing plates be fixed to the underside of beam flanges. This then allows a bolted connection between the underside of the plank and the beam. Alternatively the plank may be secured purely by gravity bearing. In this case the bearing distance is critical. Precast planks used in non-composite upper floors are very common and allow a very quick and efficient construction process. They also offer the advantage of an even, flat soffit if the underside of the slab is visible. However, because the floor does not contribute to the ‘whole’ strength of the frame it does not offer lateral restraint to beams or transmit horizontal loads between columns, shear walls or cores. Because it does not stabilise the frame or its components, the structural steel sections need to be larger so that the overall strength of the frame is robust enough to cope with all imposed forces. This means that approaches like this, although efficient from a construction perspective, are not greatly efficient from a structural perspective. This approach is suitable for spans between 4m and 8m. Non-Composite hollowcore floor planks means that the connection between the structural steel beams and the floor is not robust enough to allow composite action between floor and frame, so the frame and the floor are still separate structural elements.

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Construction Laborer Dies after Falling off Collapsed Precast Concrete Floor Slab

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In the case of a composite floor, the planks are robustly connected to the beams. There are various ways of doing this, but the most common approaches involve welding shear studs to the top flange of the steel beams and then embedding these OR the top flanges of beams within a structural concrete topping that is cast on top of the precast concrete planks. Where shear studs are used it would be typical for these to be welded to the top flanges of steel beams during off site fabrication. It is also typical to see reinforcement tie bars spanning from plank to plank above the top flanges of beams. The metal deck has 2 functions. First it acts as permanent formwork, supporting the concrete until it has cured. In this respect, the metal deck is often used in conjunction with proprietary metal 'Deck Stops' that act as formwork at the perimeter of the floor slab, preventing concrete from flowing over the edge and replacing traditional timber deck stopping. The second function is that the profiled metal deck contributes to the structural reinforcement of the concrete slab. Where the deck fulfils this purpose, you will notice raised, angled grooves pressed into the surface of the deck to allow a good mechanical key or grip between concrete and steel. This is just the same as the deformations found in rebar to allow a robust connection between the two materials.

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The alternative approaches to in-situ concrete frames we will now consider are:- Flat Slab frames (sometimes called 'plate' floors, Ribbed Slabs (with beams or band beams), Troughed slabs with integral beams, Solid slabs with drops, Waffle slabs (plain, with beams and with integral beams), and Tunnel Frames. For Flat slabs, the beams are removed from slabs and slabs are supported directly by columns. The structural action that would normally be provided by the beams is now performed within the slab. The soffit or underside of the slab is now completely flat with no interruptions or downstands from beams. This means that we can use larger, simpler, proprietary sections of formwork and falsework, for example, table forms and flying forms. These can be craned in very quickly and efficiently allowing a speedier construction process. They can also be used repetitively from floor to floor reducing waste. Shortcomings of flatslabs First, the distance that can be spanned between columns by a flat slab is less than the distance that could be achieved if a alternative approaches are used. Spans of up to 11.0m are achievable economically using a flat slab approach. If a flat slab is post-tensioned (discussed later), its economic span can be increased to approximately 14.0m. Second, there is a problem of 'punching shear'. The shear forces imposed on floors by columns would ordinarily be carried by beams. However, without the beams it is easy to understand that columns could, in theory, punch through a relatively thin floor slab quite easily. This means that additional shear reinforcement is required in flat slabs at column heads to prevent this 'punching' behaviour.

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Slabs with drops The drops also stiffen the slab slightly, meaning that it will suffer slightly less deflection than will be the case in a straightforward flat slab. Although the construction process is slowed by the need to form the drops in the slab, many of the advantages of the flat slab as already discussed are retained and the approach represents a highly economic solution in heavily loaded buildings where a flat slab may not be strong enough but where the other alternatives that could cope with the higher loads would be slower to construct or would produce an overall floor depth that would be greater than can be achieved using a solid slab with drops. Ribbed Slabs (with beams or band beams) In this approach the floor slab is supported off beams that span between columns, but the floor is now ribbed in profile. Two alternative beam types can be used. Conventional beams that tend to be relatively narrow but deep in section size, or band beams which tend to be very wide (up to 2.4m) but shallow in section size. The choice of band beams allows a shallower overall depth of construction for the structural floor, while still permitting reasonably long spans between columns to be achieved. The advantages of a ribbed slab is that its profile is inherently strong when compared to the amount of material (ie. concrete) that is required to achieve it. This means that it has a more advantageous strength to weight ratio when it is compared to solid slabs suitable for spanning the same distances. So, for a given span, less material and therefore less weight is required compared to solid slabs. This in turn increases structural efficiency and reduces cost (before we consider construction issues!). Depending on the ground conditions, less weight in the structure may simplify the design of foundations and lower the construction costs associated with groundworks. A further associated advantage of this approach is that the thermal mass of the concrete in the slab can be used beneficially in the services and HVAC design in the building to improve energy efficiency. A ribbed slab presents more surface area than a flat slab in this respect. However, these advantages must be balanced with some disadvantages in construction process. Clearly a ribbed slab with beams or band beams will require significantly more complex formwork than, say, a flat slab. Table forms can be used but obviously additional moulds must be placed to allow ribs to be formed. This will slow the construction process and will add cost to the construction process. Spans of up to 8.0m are achievable using a ribbed slab supported by beams; however, this can be increased to 16.0m if band beams are adopted as the support for the slabs.

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Waffle Slabs (of various types) There are various approaches in the provision of waffle slabs in concrete frames. These include:- * Waffle slabs on beams. * Waffle slabs with integral beams; and * Waffle slabs without beams. Each approach is illustrated below. A waffle floor is a grid floor that could be thought of as a grid of interlocking rib beams that are covered by a relatively thin concrete topping which provides the floor surface. Standard sizes are available for the grid, eg. 600mm and 900mm, however, bespoke grids can be created as necessary. The waffle grid is created using a series of moulds known as waffle pans. These may be manufactured using GRP, Polypropylene or Polystyrene and come in standard sizes for the appropriate grid size and come in standard depths of 225mm, 325mm & 425mm, the depth to be used being dependent on the span and load conditions. The width of waffle beams would typically be 125 - 150mm. The minimum depth of the floor topping would be 50-90mm depending on the type of waffle floor used. The advantage offered by waffle floors is that they provide a very robust, strong floor which is capable of supporting relatively high loads, for relatively shallow depths of overall floor construction. Once again they offer a highly efficient structural solution in terms of the strength to weight ratio that can be achieved. As with ribbed floors (which span in only one direction when compared with the waffle floors 2 way span) the soffit of a waffle floor may be used in terms of its thermal capacity so that a more efficient services solution can be employed. Additionally, depending on taste, some view the soffit of a waffle floor to be aesthetically appealing and if this is the case it requires no additional internal finishes or suspended ceilings. Again, the disadvantages with waffle floors are that they are relatively slow to construct as each waffle pan has to be secured. Reinforcement in grid beams is also difficult to fix between waffle pans. This slows the construction process and this offsets the cost advantages achieved by having less weight of material by adding costs as a result of longer and more complicated on site construction processes. Waffle slabs on beams can be used to achieve spans up to 16m . Waffle slabs with integral beams can be used to achieve spans up to 10m. Waffle slabs without beams can be used to achieve economic spans up to 12m

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The construction process is very efficient and makes use of 'tunnel' formwork. This is essentially a series prefabricated formwork panels in the shape of inverted 'L's'. These form tunnels that allow floor slabs and walls to be poured very quickly. The repetitive nature of the floor plans of these types of buildings allow the quick and efficient construction of tunnel or box frames.

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In the event of fire breaking out in a building walls have an important role to play. The internal walls of a building are normally required to inhibit the spread from room to room of flames, smoke and gases. External walls while needing to fulfil the same functions as the internal wall, must also contain the fire within the building for prescribed period of time, thus limiting spread to adjacent buildings. Materials are classified as being either combustible or non-combustible, this being a measure of their ability to fuel a fire. Combustibility has no unit of measure. Fire spread on the other hand has units of measurement, namely Classes 0, 1, 2, 3 and 4. Class 0 materials not aiding the spread of fire across its surface, Class 1 materials have a surface with very low fire spread, whereas Class 4 materials have a surface with a rapid fire spread.

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It is important not to base a selection based on speed of construction alone. This is a very common approach. The general assumption is that steel is always faster than concrete. There is only some truth in this, however, in order to properly differentiate between steel and concrete it is essential to consider the total construction time, not just the structural erection time. Steel frames need to be largely fabricated off site and are often complex. Depending on the market conditions, the lead time for steel frames can be very significant. Moreover, steel frames once constructed need to be protected from fire. Thus the total build time can actually be relatively long. On the other hand the lead time for concrete frames is generally very short and fire protection is in built. The disadvantage is the need for formwork and falsework and for curing time. However, as we will see, modern in-situ construction techniques can vastly reduce the construction times for in-situ concrete frames so that the total build time can rival that of steel.

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Defining the skyline of a city. It is apparent that within the human psyche or ego there has always been a desire to build big or to build tall. We can consider the pyramids and early obelisks built by the Ancient Egyptians and other cultures, as early examples of this desire.

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We need to consider that a buildings cost will increase significantly with its height. As the desire to build taller and taller buildings grew in the 20th Century, the demand for more and more efficient frames also grew. For example, suppose it takes 100 tonnes of steel to produce a structural frame for a 5 storey building. If we can find a way to rearrange or redesign that frame so that only 90 tonnes of steel would support the 5 storey building just as safely as in the 100 tonne scenario, the cost of providing the 5 storey building decreases.

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Steel continues to have one very significant advantage when compared with concrete and that advantage is speed of construction. However, concrete technology has made significant advances in this area as well and the use of optimised formwork and falsework systems is now commonplace in almost all insitu concrete framed buildings, regardless of height. Developments and advances in ‘slip form’ construction, which is particularly suitable for forming internal concrete cores designed to resist the significant lateral loads found in tall buildings, have also made construction in concrete a more feasible proposition. Additionally, the development of High Performance Concrete (HPC) aids considerably in speeding the construction of concrete construction. HPC offers significant reductions in curing times for concrete when compared to traditional concrete mixes, but according to Ali [10] it also offers increased strength, enhanced durability and ductility, improved density and mixture stability and lastly better chemical resistance.

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