Steel Buildings in Europe

Part 2: Concept Design 2 - 59 5.8 Asymmetric beams and deep decking 5.8.1 Description Asymmetric beams (ASBs) may be used to support composite slabs using deep decking. ASBs are hot rolled steel beam s with a wider bottom flange than top. The section m ay have e mbossments rolled into the top flange and acts compositely with the concrete encasement without the need for additional shear connectors. The decking spans between the bottom flanges of the bea ms and supports the loads during construction. Span arrangements are normally based on a 6 m to 9 m grid, with a slab depth of 280-350 mm. Decking requires propping during the construction stage for spans of more than 6 m. Reinforcing bars (16 – 25 mm diameter) placed in the ribs of the slab give sufficient fire resistance. ASB sections are generally approxim ately 300 mm deep. The sections m ay be rolled with rela tively thick webs (e qual to or thicker than the f langes), which offer a fire resis tance of 60 minutes without additional protection (for norm al office loading). Services can be integrated by for ming elongated openings in the webs of the beams, and by locating ducts between the rib s of the decking, as illus trated in Figure 5.19. Edge beams can be slim floor beams ut ilising a rectangular hollow section, or downstand beams. Ties, normally Tees with the leg cast in th e slab, are used to restrain th e colum ns interna lly in the direction at right angles to the m ain beams. Mesh reinforcem ent (A142 for 60 minutes fire resistance and A193 for 90 minutes) is placed in the slab ov er the ASB. If the top flange of the ASB is level with the surface of the concrete, the slabs e ach side of the ASB should be tied together to m eet robustness re quirements, norm ally by reinforcem ent (typically T12 bars @ 600 centres) passed through the web of the ASB. ASBs are norm ally designed as non-composite if the concrete cover over the top flange is less than 30 mm. Figure 5.19 Integration of services using asymmetric beams