Design Implications of Structural Irregularity

Abstract

Irregular building designs present special problems to the structural engineer due to their uneven distributions of mass, stiffness, and strength. Because of these factors, irregular structures may have significantly different dynamic performance than a regular structure, which can lead to unanticipated force concentrations, deflections, and subsequent stresses on building members. Irregular building designs, while often more visually and architecturally interesting, are significantly more challenging to engineer for seismic loads. Discontinuities and irregularities in mass, configuration, and form can create many unwanted and unexpected effects when a structure is subjected to seismic forces. The Uniform Building Code (UBC) 1997 edition has addressed this concern by requiring dynamic analysis of irregular building designs greater than five stories in areas with greater seismic activity (seismic zones 3 and 4). The UBC’s requirement of a dynamic lateral force analysis, along with the requirement of a higher base shear force for irregular building designs (regular buildings are given a 10% base shear reduction bonus when dynamic analysis is performed), has made irregular building designs unattractive to structural engineers. Some structural engineers may question whether the UBC provisions are unnecessarily punitive to irregular building analysis, particularly for smaller buildings. To test this hypothesis, this study compares the results of using much simpler static seismic loading analysis with the results obtained from a dynamic analysis on two steel-frame six-story irregular building designs. The first building is irregular due to a type 3 vertical geometric irregularity (specifically a 3-story tower asymmetrically located above the remaining 3 stories). The second building is irregular due to a plan structural irregularity (a large central courtyard which creates diaphragm discontinuities in the top three stories). Both buildings are considered to be located in seismic zone 4, with a forcing input based on the 1997 UBC figure 16-3 used for the dynamic analysis. This study aims to present the design implications of structural irregularity. It seeks to investigate the differences in the calculated seismic forces, deflections, and stresses due to the two different methods of analysis.