Conventional steel constructions for the performance-based earthquake retrofit of low-rise school buildings

An on-going $1.5 billion project for the retrofit of schools in the province of British Columbia, Canada, considers both the seismic risk assessment and retrofit of more than 700 low-rise buildings for the next ten years. Conventional steel constructions such as eccentrically brace frames and moderately ductile steel moment-resistance frames have been considered as feasible retrofit solutions. Guidelines have been published including a procedure for the determination of minimum required factored resistance values of a structural system. These resistance values are associated to an instability drift limit defined by a collapse performance objective under a 2500-year return period earthquake. Results presented in the guidelines are obtained from several nonlinear dynamic analyzes, including several sensitivity studies of parameters such as yield drift, strain hardening and the ratio tension to compression force of braced frames. This document summarizes some technical aspects of the retrofit guidelines and results obtained from the sensitivity study. (a) Prototype S-1 for concentrically braced steel frames with tension bracing only; (b) Prototype S-2 for concentrically braced steel frames with tension/compression bracing; (c) Prototype S-3 for eccentrically braced steel frames; (d) Prototype S-4 for moderately ductile steel moment-resisting frames; Unless a detailed analysis indicates otherwise, chevron braces are to be considered to have a negligible contribution in the assessment of risk. Table 1 shows a summary of the steel prototypes with their respective instability drift limit, ISDL, and over-strength factor, Ro, according to the code (NBCC 2005). The ISDL was defined for a life safety objective and the Ro factor was used to modify the resistance values obtained from the analysis. Table 1. Summary information of steel prototypes Prototype No. ISDL Ro Hysteretic Properties S-1 4.0% 1.3 Slip S-2 1-2.5% 1.3 Slip/Buckling S-3 4.0% 1.5 Elastic-Plastic S-4 4.0% 1.5 Elastic-Plastic