Steel Buildings in Europe

Part 2: Concept Design 2 - 32 Floors spanning onto th e steel beams will nor mally be either pre cast concrete units, or composite floors. The supporti ng beams may be below the floor, with the floor bearing on the top flange (often known as “downstand” beams), or the beams may share the sam e zone with the floor construction, to reduce the overall depth of the zone. The availa ble construction zone is often the determining factor when choosing a floor solution. Beams within the floor zone are known as slim floor beam s, or integrated beams. Beams may be non-composite, or composite. In composite construction shear connectors are welded to the top fl ange of the beam , transferring load to the concrete floor. Shear connecto rs are of ten welded on s ite to the top f lange of the beam which has been left unpainted, through the steel decking (known as “through-deck” welding). Despite exte nsive testing and research that demonstrates the adequacy of through-d eck welding, some authorities prefer that the studs are welded off site, and the deck must therefore be single span, or must be punctured to fit over the sh ear connectors. Alternatively, shear connectors can be mechanically fixed (often shot fired) through the decking to the beam. Precast concrete units m ay be used for low rise fram es, but com posite floors are common in both low rise and high rise structures. Table 4.2 Typical floor solutions Form of construction Typical solution Low rise, modest spans, no restriction on construction depth downstand beams precast units or composite floors Modest spans (less than 9 m), restricted construction depth integrated solutions – precast or composite floors Low rise, long span (e.g. 15 m) downstand beams in the façade precast concrete units (15 m), composite floors with secondary steel beams spanning 15 m Medium and high rise, modest spans, no restriction on construction depth downstand beams, composite construction Medium and high rise, long spans (to 18 m) restricted construction depth composite floors with cellular long span secondary steel beams 4.5.2 Composite beam arrangements Composite beams support composite slabs, which span between the beams. For design of orthogonal grids, two generic beam arrangements may be considered:  Long span secondary beams, supported by shorter span primary beams (see Figure 4.7). In this case, the beam sizes can be sele cted so that the prim ary and secondary beams are of approximately equal depth.  Long span primary beams, supporting shorter span secondary beams (see Figure 4.8). In this case, the primary beams are relatively deep. Cellular beams are m ore efficient when used for long span secondary beam s, whereas fabricated beam s are m ore efficient for long span prim ary beam s, where shear forces are higher. It is al so possible to elim inate secondary beams