Nov. 11, 2020

Specifying Columns
in Computer Analysis + Design of Steel Frames

Muangsangop Seniwongse, Ph.D., P.E., CPEng, M.ASCE
Lead Technical Marine Specialist

Expert Insight

Most structural analysis software provides a default support condition for steel frame column bases as fixed, meaning fixed against all six degrees of freedom. A release command can be used to release the restraint in any specified degree of freedom. However, some engineers have been using incorrect column base support conditions for major steel frames for years, specifying pinned supports instead of the fixed supports in all their designs. This is equivalent to assuming that plastic hinges (full plastic moment) have already formed at the bottom of the columns before any loads are applied. The result is a safe but grossly overdesigned project, due to the entire elastic range of moment resistance at the column base being ignored.

Steel portal frames supporting an eight-story building of a major hospital in Boston, Massachusetts, showing fixed base columns.

A new thought-provoking whitepaper by Muangsangop Seniwongse, Ph.D., P.E., CPEng, M.ASCE, Lead Technical Marine Specialist at McLaren has been published by ASCE rethinking the specification of fixed-base and/or pinned-base columns in computer analysis and the design of steel frames.

The research in Mr. Seniwongse’s paper, “Specifying Fixed-Base Columns or Pinned-Base Columns in Computer Analysis and Design of Steel Frames,” demonstrates fixed and pinned supports commonly seen, sites actual test results and uses examples of major structures that were designed with a fixed-base connection and examining their life-long integrity under actual sustained service loads.

Abstract

Showing pinned base column at a major international airport terminal building.

SPECIFYING FIXED-BASE COLUMNS OR PINNED-BASE COLUMNS IN COMPUTER ANALYSIS AND DESIGN OF STEEL FRAMES

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 xx-, yy-, and zz-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.