Specifying Fixed-Base Columns or Pinned-Base Columns in Computer Analysis and Design of Steel Frames

Abstract

This paper addresses the application of support fixity condition of column bases in computer structural analysis and design of steel frames. All structural analysis software require input data to model the frames. Supports and connecting points are identified by joint or node numbers, members are identified by member numbers, and so on. The column bases are identified as support joints, and the degrees of freedom of all support joints must be specified. Most programs provide a default support condition as fixed, meaning fixed against all six degrees of freedom. The six degrees are displacement and rotation in the x-, y-, and z-directions. A release command is used to release the restraint in any specified degree of freedom. It has been found that some engineers in some sectors of industry have been using incorrect column base support conditions for major steel frames, specifying pinned supports instead of the fixed supports in all their designs for years. This is equivalent to assuming that plastic hinges (full plastic moment) had already formed at the bottom of all columns before the frames were even applied with any loads. The results were safe design, but grossly overdesigned because the entire elastic range of moment resistance at all column bases were ignored. This paper describes the fixed and pinned supports and demonstrates with typical drawings and pictures of fixed and pinned supports commonly seen. Force distribution at typical anchor steel base plate (fixed support) is given with equation for applied moment. Test results for moment resistance of anchor base plates (fixed supports) are referred to. Several examples of actual major structures that were designed using a fixed-base connection and their life-long integrity under actual sustained service loads are given. Understanding of this paper should lead to a safe and economical design of steel structures.