Lecture 1 & 2

Steel StructuresSteel structures are assembly of structural steel shapes joined together by means of riveted, bolted or welded connections.Concrete structures are easily joined together by means of monolithic construction.Steel construction is used for every type of structure.There are two main categories of steel structures.i) Framework/skeletal systems ii) Shell systemsFramework/skeletal systems: Main load carrying elements are one dimensional or line elements forming two dimensional or three dimensional frames.e .g: industrial buildings, railways bridges , towers , poles ,multi-storey buildings, all trusses and rigidly connected components etc.Shell systems: Main load carrying elements are sheets and plates besides some skeletal members etc.e.g: gas tanks , tanks , reservoirs , bin and bunkers , blast furnaces , air heaters , large diameters pipes and all other plate and shell structures etc.Features of steel structures: Freedom of expression, creativity , easy extension , easy fixing of facade , easy and efficient fabrication , express function , large span , no limit of architectural design , recycling is possible , slender columns , more space , transparency , visible connections , weather independent construction. Merits of Steel Constructions:1-Reliability: (i) Consistency and uniformity in properties (ii) better quality control being factory manufactured. (iii) large ductility and elasticity (iv) because of the truly homogeneous and elastic material. (v) steel satisfies most of the assumptions involved in the derivation of the analysis and design formulas and hence the results obtained are reliable.2-Industrial Behavior: (i) Rolled steel section are manufactured in the factory. (ii) members may be cut and prepared for assembly in the factory and therefore adaption to prefabrication is possible. (iii) only joining of the components is carried out at the site by installing rivets , bolts or welds.3-Lesser Construction Time: (i) Due to industrial nature , the progress of work is very fast making the structure economical. (ii) The structures can be put to use earlier. (iii) The reduction of labour cost and overhead charges and the benefits obtained from early use of the building contribute to the economy.4-High Strength and Light Weight Nature: (i) High strength per unit weight. (ii) Lesser dear loads. (iii) When dead load reduces, the underneath members become still smaller due to lesser load acting on them.(iv) Important for long spans bridges , tall buildings and structures having poor foundation conditions.Factor (C) is defined as the ratio of the density (r) of the material to the stress (f) it can carry and is one of the least for steel. Aluminium 1.1x10-4,Steel 2.3x10-4, Wood 4.5x10-4 ,Concrete24x10-4.5-Uniformity,Durability and Performance: (i) Long life of the structure. (ii) Homogeneous and durable material. (iii) It satisfies the basic assumptions of most of the analysis and design formulas. (iv) If properly maintained by painting ect; the properties of steel do not change appreciably with time. (v) Hence steel structures are more durable.6-Elasticity: (i) It follows Hooks law up to high stresses. (ii) The stress-strain diagram remains a straight line. (iii) The steel sections do not crack or tear before ultimate load and hence the moment of inertia can be definitely calculated.7-Ductility and Warning Befor Failure: (i) The property of a material by which it can withstand extensive deformation without failure under hight stresses is said to be its ductility. (ii) Mild steel is a very ductile material (25 to 30% elongation in standard tension test specimen after fracture). (iii) Visible deflection in case of overloads.(iv) Extra loads may be removed from the structure to prevent the collapse. (v)Even if the collapse occur, time is available for the occupants to vacate the building.8-Addition to Existing Structures: (i) Additions to existing structures are very easy to made.(ii) Connections between existing and new structures can be employed very effectively. 9-Possible Reuse: Steel sections can be reused after a structure is disassembled.10-Scrape Value: Steel has a scrap value even though it is not reusable in its existing form.11-Water Tight and Air Tight Construction: (i)Steel structures provide completely impervious construction. (ii) Reservoirs , gas pipes , oil pipes etc. are preferable made from structural steel.12-Long Span Construction: (i) High rise buildings, long span bridges and transmission towers are made from structural steel. (ii) Industrial buildings up to a span of 90m can be designed by plate girders or trusses. (iii) Bridges span up to 260m are made with plate girders. (iv) For through truss bridges, spans of 300m have been used.13-Temporary Construction: (i) For temporary structures, steel construction is always preferred. (ii) Army constructions during war are mostly made of structural steel. (iii)The structures may be disassembled by opening few bolts, component parts are taken to the new places and the structure is easily reassembled.Demerits of Steel Construction1-High Maintenance Cost and Corrosion:(i)Periodic paints are required to avoid from corrosion. (ii) This require extra cost and special care. (iii)The use of weathering steel, in suitable design conditions tends to eliminate this cost. (iv) If not properly cared thickness loss may be 1 to 1.5mm per year. (v)35% weight loss during specified life and failure under extra loads.2-High Fireproofing Costs: (i) Although steel members are incombustible,their strength is tremendously reduced at temperatures prevailing in fires. (ii)At about 400 oC, creep becomes more pronounced. (iii) Creep is defined as plastic deformations under a constant load for a long period of time.(iv) Steel is an excellent conductor of heat and may transmit heat from a burning compartment of a building to start fire in the other part of the building.(v) Extra cost is required to properly fire proof the building.3-Susceptibility to Buckling: (i) Buckling is the type of collapse of the members due to sudden large bending caused by a critical compressive load. (ii) Steel sections consist of a combination of thin plates. (iii) Overall steel members dimensions are smaller than reinforced concrete members. (iv) If these slender members are subjected to compression, there are greater chances of buckling. (v) Sometimes steel , when used for columns , is not very economical because considerable material has to be used merely to stiffen the column against buckling.4-High Initial Cost/Less Availability: (i) In few countries like Pakistan, steel is not available in abundance.(ii)Its initial cost is very high as compared with other structural members. (iii) These factors has resulted in the decline of steel structure in Pakistan.5-Aesthetics: (i) For certain buildings, the steel form is architecturally preferred. (ii)For the residential and office buildings, the steel structure without the use of false ceiling and cladding are considered to have poor aesthetic appearance. (iii)A considerable cost is to be spent on such structures to improve the appearance.Cladding is the covering of metal , concrete plastic or timber put on the surface of a structural member to completely encase it.Specifications: The adequacy of a structural member is determined by a set of design rules called specifications.

Types of Loads1-Dead Load: It retains the magnitude and point of application throughout the life of the structure and is denoted by D.It is estimated by multiplying the volume of the structure with the standard density of the material. It constitutes a bigger part of the total load on the structure.2-Live Load: It is the load of the persons occupying the structure and their belongings. It is denoted by L.Its magnitude and point of application changes with time .The typical values for common occupancy types are given below;

3-Self Load: It is the type of the dead load , which is the self weight of the member to be designed.4-Imposed/Superimposed Load: This is the term used for all external loads , leaving the selfload , acting on the member to be designed . This includes live load , wind load , earthquake load etc. Part of dead load may also act as imposed load.5:Service Load:The maximum intensity of load expected during the life of the structure depending upon the certain probability of occurrence is called as service load.6-Factored Load:Service load increased by some factor of safety or overload factor is known as factored load.Mechanism of Load Transfer1-The gravity load passes from top to bottom through all the members of the structure until it reaches the underneath soil.2- The load acts at floor finish , goes to the underneath slab and transfers to beams and walls.3- This is then accumulated in the columns , moves to the foundations and dissipates in the soil.4- This terms members and supports are defined relative to each other.5- For roof slabs , beams and walls are supports.6- For the beams, columns are acting like supports and for the columns , foundations are acting as supports.7- Similarly , the underneath soil acts as support for foundations.8- The load path is only in one direction.9- The loads of supports can not act on the structural members.