Nonstructural Elements in Performance-Based Seismic Design of Wood Frame Structures

Abstract

This paper presents the method and analysis of a study whose objective was to numerically examine the effect that nonstructural finishes and nonstructural partition walls would have on the performance-based seismic design of a wood frame structure at the system level. The incremental dynamic analysis (IDA) approach was modified slightly, termed incremental mass analysis (IMA), and used as an investigative tool in order to determine what seismic mass numerically produces a prescribed performance level for a wood frame structure or subassembly. An illustrative example for a single wood shearwall using the IMA method that utilized an existing suite of 20 earthquakes at three different seismic hazard levels produced 60 curves. These curves for weight per unit length versus resulting transient drift from nonlinear dynamic time history analysis were analyzed using basic statistics to produce 16th, 50th, and 84th percentile IMA curves for the wood shearwalls. A one-story wood frame house was designed based on standard wood frame design concepts including windows and doors. Then, using the IMA approach presented for the wood shearwall, but with a single earthquake record, the weight at roof level versus transient drift curve was developed for three different seismic hazard levels. In order to investigate the effect of nonstructural elements numerically, analyses were performed for four versions of the wood frame structure: (1) the exterior wood shearwalls with only oriented strand board (OSB) and no finishes; (2) the exterior wood shearwalls with OSB and gypsum wall board (GWB); (3) the exterior wood shearwalls with OSB and GWB and a nonstructural partition wall; and (4) the finished structure with stucco included. Based on the analysis presented in this study, it is concluded that the allowable seismic mass can be significantly increased when non-structural elements are included in the design, in some cases more than 70 percent.