Discrepancies in AISC Elastic and Plastic Section Moduli For Unsymmetrical Section

Geometric properties i.e., elastic and plastic section moduli of unsymmetrical sections is determined through a method used by the American Institute of Steel Construction, AISC. This method is based on the assumption that neutral axis always remains parallel to the legs of sections, however, in reality when an unsymmetrical section is used as a flexural member, and load is applied about an axis parallel to one of the legs of the section, biaxial bending is produced in the section which adjusts the placement of neutral axis to a state where the neutral axis does not necessarily continue to be parallel to the legs of section. AISC uses the same procedure for the design of unsymmetrical steel sections as that for symmetrical sections. As design is usually based upon their geometric properties like elastic and plastic section moduli, moment of inertia, etc., so AISC method yields unsafe and uneconomical results. Therefore, unsymmetrical sections should be designed using the author’s recommendations based upon their actual analysis. This paper deliberates the incorrect use of values of the AISC in the given circumstances and presents an alternative procedure for the easy and correct design of unsymmetrical sections. This involves the use of equivalent section moduli, Sx, and Zx including the effect of lateral bending.