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STEEL STRUCTURES—STRUCTURAL ENGINEERING
to carry their loads in the most efficient manner. Standard framing arrangements have been developed for all types of buildings. 1.8.2 Structural idealization Structures support loads and enclose space and are of three-dimensional construction. Idealization means breaking the complete structure down into single elements (beams, columns, trusses, braced or rigid frames) for which loads carried are estimated and analysis and design made. It is rarely possible to consider the three-dimensional structure in its entirety. Examples of idealization are as follows. 1. Three-dimensional structures are treated as a series of plane frames in each direction. The division is made by vertical planes. For example, a multistorey building (Figure 1.6 (a)) readily divides into transverse rigid frames and longitudinal braced frames. The tower structure in Figure 1.6(b) could be analysed, using software, as a space frame. Alternatively it is more commonly treated as a series of plane frames to resist horizontal loads and torsion due to wind. 2. The structure is divided into vertically separated parts by horizontal planes into say, roof, walls, and foundations. These parts are designed separately. The reactions from one part are applied as loads to the next part. The horizontal division of a truss and stanchion frame is shown in Figure 1.6(c), while a portal frame is treated as a complete unit. A domed-roof stadium similarly divided is shown in Figure 1.6(d). The domed roof may be designed as a three-dimensional unit or, in the case of specially framed domes, as a series of arched ribs. 1.8.3 Modelling Another idealization method is modelling for analysis. In this case the structure is changed so that the analysis of a different form of structure that is more convenient to carry out can be made. This often means modelling the structure for analysis using a plane frame program. Two examples shown in Figure 1.7(a) and (b) are: • a ribbed deck idealized as a grid; • connected concrete shear walls modelled as a plane frame. These examples can also be modelled for more accurate finite element analysis. The modelling problem with steel structures also often includes composite action of steel and concrete elements. Examples are as follows. • In situ concrete slab and steel beams in a steel rigid frame. For analysis the elastic transformed section can be used for the composite beams (Figure 1.7(c)). Design is based on plastic theory. • Concrete shear wall in a rigid steel frame (Figure 1.7(d)). If there is compression over the whole section the slab can be transformed into equivalent steel. If compression occurs over part of the section, use the transformed section in bending including reinforcement in tension. The shear wall is to be connected to steel sections. • A three-dimensional steel rigid frame building with concrete core (Figure 1.7(e)). This can be modelled for plane frame analysis for horizontal wind loads by linking frames and replacing the shear wall by stiff vertical and horizontal members. Alternatively the structure can be analysed as a space frame. Great care is needed in interpreting results for design. Finite element analysis will give more accurate results.
1.9 DRAWINGS, SPECIFICATIONS AND QUANTITIES 1.9.1 Steelwork drawings Steelwork drawings show the detail for fabrication and arrangement of the structure for erection. They are also used for taking off the materials list and preparing the bill of quantities and estimates of cost. It is essential that drawings are presented