Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 65 common to provide a truss which is two bays deep, rather than a truss in the end bay only. 13.3 Stability The majority of multi-span portal frames have slender internal columns. When a horizontal load is applied to these frames, there is only a small bending moment induced in these slender internal columns, because the external columns are much stiffer. A typical bending moment diagram is shown in Figure 13.2. This difference in bending moment distribution and the associated reduction in internal column stiffness has a significant impact on frame behaviour. At the Ultimate Limit State, the frame is likely to be operating at 20 to 30% of its overall elastic critical load. With the spread of plasticity from the critical hinge position, the effective critical load will reduce, increasing the effective critical load ratio further. This effect is addressed by appropriate second order, elastic / plastic software. H Figure 13.2 Bending moments in a typical two-span frame under horizontal loading The frame in Figure 13.2 can be considered as two sub-frames, each comprising an external column and a rafter pair, as shown in Figure 13.3. For multi-span frames in general, the two external sub-frames provide the majority of the stiffness, so the same model of a pair of sub-frames could be used for hand calculations. Where the stiffness of the internal columns is to be included, it is preferable to use software for the analysis of the entire frame. H Figure 13.3 Sub-frames for a typical two-span frame