Evaluation of seismic torsional provisions in uniform building code

The main change in the torsional provisions of the 1991 edition of the Uniform Building Code (UBC) from the 1979 UBC edition is the need to amplify the accidental eccentricity for torsionally flexible structural systems. This paper evaluates the consequences of the change on the strength design requirement and the ductility demand on the edge lateral force-resisting elements in torsionally unbalanced structural systems, which are designed based on the two editions of the UBC code. The major improvement of the 1991 UBC is its ability to limit the additional ductility demand on the stiff-edge element of a torsionally unbalanced system to an acceptable level, even when the system is torsionally flexible.