Incremental seismic retrofitting for essential facilities using performance objectives: A case study of the 780-PRE school buildings in Peru

Abstract

This paper presents a methodology to develop incremental seismic retrofitting alternatives for essential buildings through a nonlinear analysis. The retrofitting levels are defined in terms of the performance objectives such as immediate occupancy, life safety, code compliance and collapse prevention. Each of these is associated with a seismic hazard linked to a return period equivalent to a frequent, occasional, rare, and extreme event. The methodological approach conceives the non-linear behavior of the system through the definition of plastic hinges for structural components such as beams and columns. As soon as the model is properly defined, a nonlinear pushover analysis is performed to obtain the seismic response of the buildings. In addition, an incremental dynamic analysis is carried on for the equivalent single degree-of-freedom models of the same buildings, to estimate the critical seismic intensity at which collapse occurs; this allows corroborating the suitability of the proposed retrofitting alternatives. Once the methodology was well explained, a school building typology designated as a 780-PRE was chosen as an interesting case study for validation purposes. A total of 40,000 of its type were designed (under the national building design code RNC-77) and constructed between 1977 and 1997 throughout all the Peruvian territory. Typically, this system is characterized by reinforced concrete moment-resisting frames with unreinforced infill masonry walls. Despite its widespread use, this system typology has shown to be highly vulnerable under previous seismic events, depicting repetitive short-column failure mechanisms and partial collapses. This confirms a fragile failure contrary to the intended original design behavior for the as-built condition. Based on the previous understanding of the problem, several alternatives of incremental seismic retrofitting were considered in this study, supported on a benefit-cost evaluation to come up with feasible seismic vulnerability reduction strategies. After the interventions were selected and implemented, a noticeable improvement of the seismic response was perceived, depicted by a reduction in the expected damage under low and moderate seismic demands. Finally, the analyses were extended to several seismic zones in Peru.