Steel Buildings in Europe

Part 2: Concept Design 2 - 30 4.4 Columns Columns in multi-s torey steel f rames are generally H s ections, predominantly carrying axial load. When the stability of the structure is pro vided by cores, o r discreet vertical bracing, the beams are generally designed as simply supported. The generally accep ted design m odel is that nom inally p inned connections produce nominal moments in the column, calculated by assuming that the beam reaction is 100 mm from the face of the colum n. If the reactions on the opposite side of the column are equal, there is no net m oment. Columns on the perimeter of the structure will have an applied moment, due to the connection being on one side only. The design of columns is cov ered in detail in Multi- storey steel buildings. Part 4: Detailed design [6] . For preliminary design, it is appropriate to base the choic e of column section on axial load alone, but ensure that the column is only working at 90% of its capacity, to allow for the subsequent inclusion of the nominal moments. Typical column sizes are given in Table 4.1. Table 4.1 Typical column sizes (for medium span composite floors) Number of floors supported by column section typical column size ( h ) 1 150 2 – 4 200 3 – 8 250 5 – 12 300 10 – 40 350 Although small column sections may be preferred for architectural reasons, the practical issues of connections to the floor beams should be considered. It can be difficult and costly to provide connection into the minor axis of a very small column section. For ease of constru ction, colum ns are us ually erec ted in two, or sometim es three-storey sections (i.e. approxim ately 8 m to 12 m i n length). Colum n sections are joined with splices, ty pically 300 mm to 600 mm above the floor level. It is common to vary the column size within the height of the building, to make efficient use of the steelwork. Although it may be convenient to align the columns on a single c entroidal axis, it m ay be pref erable to maintain the same external face, so that all edge details, and supports for cl adding, are sim ilar. The f loor beams will be slightly dif ferent lengths, and the additional mom ent induced by offsetting the upper column section will need to be accounted for in design. Typical splice details are shown in Figure 4.5, when a change in section has been accommodated by a division plate between the sections.