Special braced stairs versus typical braced frames. New architectural-structural-seismic approach to stair design

Abstract

This paper presents a new approach to the project of steel buildings, mainly focused on the architectural, structural, and seismic design of stairs. The objective is to design a structural stair system capable of controlling seismic damage and contributing to the bracing system of the building. The article begins with a review of the seismic standard (ATC, FEMA, and EC8) on which the current design criteria for new buildings with stairs are based. The research is based on two spatial building models (A–B) with the same bracing elements but placed differently. Reference Model A follows classical design approaches. It means, stairs are considered nonstructural elements that do not influence the seismic behavior of the building. This structure corresponds to typical braced frames (IV-CBF and EBF) according to EC8. Model B includes a stair system designed to help control the effects of inter-story drifts and inertia forces. In this case, the same bracing elements of Model A were integrated into the stair structure of Model B. A comparative seismic behavior analysis of typically braced frames (A) versus specially braced stairs (B) is presented. The research was based on the static nonlinear (pushover) analysis and the capacity spectrum method (ATC-40) according to the seismic performance levels (FEMA) and damage limitation (EC8). Finally, the braced stairs was verified via nonlinear time-history analysis in order to better capture the structural safety of the evacuation routes and their influence on the behavior of the building. This deterministic analysis of the braced stairs verified satisfactory results compared to reference bracing systems.