Steel Buildings in Europe

Part 2: Concept Design 2 - 48 The size an d spacing o f the openings can be restricted by the fabrication method. However, the full range of secti on sizes is available from which to chose the sizes of the top and bottom chords. For com posite design, the top chord is generally chosen as a lighter section than the bottom chord Cellular beams can be arranged as long- span secondary beams, supporting the floor slab d irectly, or in some cases , as long-span prim ary beams supporting other cellular beams or I section secondary beams. Figure 5.8 Long-span secondary cellular beams with regular circular openings 5.4.2 Beam spans and design criteria Secondary beam s should be spaced at 3 m to 3,75 m to avoid temporary propping of the decking during constructi on. Opening sizes are typically 60 to 80% of the beam depth. Stiffeners m ay be required for elongated openings. Large (elongated or rectangular) openings should be located in areas of low shear, e.g. in the middle third of span for uniformly loaded beams. A structural arrangement for corner of a floor around an atrium is illustrated in Figure 5.9. In this case, the floor grid is 15 m  7,5 m, in which the cellular beams are 15 m span and 670 mm deep. Internal beam s are elim inated except around the service cores. Shear or buckling of the web posts m ay occur between openi ngs, particularly near high point loads or adjacent to elongated openings. In this case, the spacing of the openings should be increased or heavier sections used. Typical chord sizes for 12 m to 18 m span cellular secondary beams at 3 m spacing are presented in Table 5.4.