Enhancing code compliance of non-engineered and pre-engineered buildings in urban Nepal through retrofitting interventions in RC construction

Abstract

Nepal’s earthquake risk is intensified by rapid urbanization, substandard construction practices, limited preparedness, and vulnerable reinforced (RC) buildings. Many RC structures are either non-engineered or built using outdated Mandatory Rules of Thumb (MRT) (NBC 205:1994), termed “pre-engineered”, which no longer meet current standards. The Gorkha earthquake exposed severe vulnerabilities, highlighting the need for retrofitting interventions to enhance seismic resilience. While previous studies assessed existing low- to mid-rise RC buildings, significant gaps remain in the application of conventional methods, open-source software, and cost-effective retrofitting schemes that incorporate both traditional and innovative techniques across diverse building typologies. This study selected six typical buildings including symmetrical, plan-irregular, and vertically irregular structures of three- and four-stories. The seismic vulnerability of non-engineered and pre-engineered buildings within these typologies was assessed through non-linear static and dynamic analyses performed in OpenSees. Retrofitting methods, including RC jacketing, Steel jacketing, and Fiber-reinforced cementitious matrix (FRCM) including Glass FRCM and Hemp FRCM, were applied to upgrade these structures to meet the latest seismic design code NBC 105:2020. Incremental dynamic analysis (IDA) was conducted, and fragility functions were derived for each building model before and after retrofitting. The findings indicated that retrofitted models exhibited higher ductility and lateral load-carrying capacity, reduced inter-story drifts, and delayed damage onset under increasing seismic loads, thereby meeting current code requirements. At 0.4 PGA, the probability of exceeding the life-safety limit state decreased from 100% to 45% in non-engineered buildings and from 90% to 38% in pre-engineered buildings. Glass-FRCM was the most effective, while Hemp-FRCM was least effective. Irregular structures required more extensive retrofitting than symmetrical ones.