Design Interaction Equations for Steel Members

Abstract

Most of the steel design specifications have two equations for stability and strength check of structural members. This approach has two weaknesses. The first weakness is that if the length of a member is reduced to zero in the stability interaction equation, it does not always reduce to the strength interaction equation. As a result, the transition from long to short members is not smooth. The non‐smooth transition makes the design equations unconservative in the transition region and overly conservative in the rest of region. Secondly, the stability equation is based on the concept of moment amplification factor. This amplification factor, by definition, should not be less than unity. However, in order to combine properly the amplification factor with the strength equation, these specifications allow the moment amplification factor to be less than unity. To eliminate these weaknesses, a single design equation for both strength and stability check is desirable. This paper presents a logical method of deriving a single design equation to satisfy both strength and stability criteria.