Horizontal shear forces are developed in the web post in order to transfer the incremental tension force to the bottom tee. The model shown in Figure 2 shows the forces and moments acting on the web post between adjacent openings. The precise distribution of shear between the top and bottom tees is determined from the analysis procedure and must be compatible with the vierendeel bending capacity of the tees. The shear resistance of a cellular beam takes account of the contribution of the web and flange for rolled sections and the concrete slab. In beams with closely spaced openings the web post will often govern the load carrying capacity.

In bending the utilisation of the bottom flange of the section will depend on the ability of the web posts to transfer horizontal shear. The global bending capacity and the vertical shear capacity are rarely the critical modes of failure in cellular beams. A composite beam with large openings tends to resist bending predominantly by tension in the bottom tee and compression in the concrete.

The global bending capacity of the beam is calculation at the centre of each opening and is based on a reduced cross sectional area. The modes of failure are described briefly below, for a fuller explanation of the structural behaviour reference may be made to Lawson et al (1) or P068 (2). The structural behaviour of beams with web openings is relatively complex and involves five main failure modes which are included in the design model for ULS design at room temperature and in fire conditions. The use of asymmetric sections is also common, as this allows the weight of the beam to be reduced by removing steel from the top of the section where it is less effective. Figure 6: Relative values of material properties of steel with temperature

Castellated beam design software