Effectiveness of an external steel frame retrofit system for nonductile reinforced concrete school buildings

초록

Existing reinforced concrete (RC) school building structures with seismically-deficient details have been retrofitted with an external steel frame to mitigate their seismic vulnerabilities. This study aims to estimate the retrofit effects of an external steel frame with bolt-connected prefabricated concrete-filled tube columns for a seismically-deficient RC frame through quasi-static loading tests, and propose a linear-elastic design process maximizing the retrofit effects. By comparing the test results between as-built and retrofitted specimens, the effectiveness of the retrofit system was quantified in terms of hysteresis behavior, strain-based bond-slip effects, and damage mechanisms. As compared to the as-built test frame, the retrofit system enhanced lateral resisting capacities and ensured target performance objectives: (1) no strength reduction in a code-defined performance limit (life safety level), and (2) two times the maximum strength and three times the energy dissipation. However, the ductility capacity of the retrofitted test frame was reduced by about 58 % due to the column shear failure. The installation of the retrofit system shifted the damage concentration areas from the beam-column joints for the as-built specimen to the columns for the retrofitted specimen. Using the proposed design process, a stiffness ratio (= initial stiffness of as-built to retrofitted columns) was varied from 0.23 (initial design) to 0.74 (proposed design) until preventing the column shear failure. The proposed process was demonstrated from the nonlinear pushover analyses for existing RC school building structures with the retrofit details maximizing the structural performance.

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