Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 18 Plastic global analysis: Any base moment capacity between zero and the plastic moment capacity of the column may be assumed, provided that the foundation is designed to resist a moment equal to this assumed moment capacity, together with the forces obtained from the analysis. Elastic - plastic global analysis: The assumed base stiffness must be consistent with the assumed base moment capacity, but should not exceed the stiffness of the column. 3.4.3 Nominally semi-rigid column bases A nominal base stiffness of up to 20 % of the column may be assumed in elastic global analysis, provided that the foundation is designed for the moments and forces obtained from this analysis. 3.4.4 Nominally pinned bases If a column is nominally pin – connected to a foundation that is designed assuming that the base moment is zero, the base should be assumed to be pinned when using elastic global analysis to calculate the other moments and forces in the frame under Ultimate Limit State loading. The stiffness of the base may be assumed to be equal to the following proportion of the column stiffness:  10% when calculating  cr or  cr,est  20% when calculating deflections under serviceability loads. Column base plates with a relatively thin base plate and four bolts outside the profile of the column section are considered in some countries as nominally pinned if they have sufficient deformation capacity, although in fact they will exhibit semi-rigid behaviour. Such bases have the additional practical advantage that they provide sufficient base stiffness to enable the column to be free-standing during erection, and assist in the aligning of the column. 3.5 Design summary Analysis for the Ultimate Limit State:  may be carried out either by elastic analysis or by plastic analysis  should take account of second order ( P-  ) effects, when  cr or  cr,est is less an 10 (elastic analysis) or 15 (plastic analysis)  if necessary, second order effects can be accounted for either directly (using a second order analysis) or by the use of a modified first order analysis with an amplification factor. For most structures, greatest economy (and ease of analysis and design) will be achieved by the use of software that:  is based on elastic/perfectly plastic moment/rotation behaviour  takes direct account of second order ( P-  ) effects.