Frame optimization and column design

Abstract

Abstract This paper examines the meaning of the term effective length. The failings of the relevant literature to satisfy the needs of present and future automated design techniques are highlighted. The different approaches currently available for evaluating effective lengths are reviewed. The eigenvalue method which establishes an overall structure instability criteria is considered. The remainder of the methods all involve expressing the effective lengths of a member in terms of the member stiffness and the stiffnesses provided by the end supports. The importance of obtaining accurate stiffness values representing the member end conditions becomes clear from the formulations obtained and the examples presented in the paper. A method of estimating suitable stiffness values during a typical optimization or automated design procedure is suggested. One of the approaches for evaluating effective lengths is extended to take advantage of the information available from the analysis and to allow for partial restraint against sidesway. The various methods are compared by testing them on a series of typical frames. Comments on the results are strongly influenced by the consideration that the values obtained should yield structures with the required safety factor.