Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 16 Category B: Frames that fall outside of Category A (See Figure 3.8), but excluding tied portals. For frames that fall outside of Category A, first-order analysis may be used if all the applied loads are amplified by:        cr 1 1 1,1  or          est cr, 1 1 1,1  if the axial force in the rafter was found to be significant. 1 2 L L L 1 1 2 (>> ) 3 1 Asymmetric 2 Sloping site 3 Multi-span with unequal spans Figure 3.8 Examples of Category B frames 3.4 Base stiffness Analysis should take account of the rotational stiffness of the bases. The following simple rules in this section are recommended. These recommendations might not be accepted in certain countries; the relevant National Annex and the local regulatory authorities should be consulted. It is important to distinguish between column base resistance and column base stiffness. Column base resistance is only relevant to elastic-plastic or rigid-plastic calculations of frame resistance, not to deflections. Column base stiffness is relevant to elastic-plastic or elastic frame analysis for both resistance and deflection. If any base stiffness is assumed in ULS design, the base details and foundation must be designed to have sufficient resistance to sustain the calculated moments and forces. In many general analysis computer programmes, these base stiffnesses are most conveniently modelled by the introduction of a dummy member, as shown in Figure 3.9.