Cyclic Lateral Load Test for RC Moment Frames Retrofitted with External Steel Frames

A seismic retrofit method using external steel frames was studied for old RC buildings with nonseismic details. To verify the external retrofit method, 2-story frame specimens were tested under cyclic lateral loading. The test results indicated that the external steel frame successfully enhanced the stiffness and strength of deficient RC moment frames without deteriorating the deformation capacity. The energy-dissipation capacity was also improved by restraining early failure of the RC beam-column joints. The failure mode of the retrofitted frames was governed by the flexural mechanism of the first-story RC columns, with partial yielding of the upper structure. The RC columns were subjected to additional axial forces that were transferred from the external steel frame, in which the steel columns were not extended to the foundation. The strengths of retrofitted frames were evaluated by two approaches based on the frame plastic hinge mechanism: (1) the sum of the strengths of independent RC and steel moment frames, and (2) the strength assuming full composite section of concrete and steel members. On the basis of the test results, design considerations for the external steel frame-retrofitting method are recommended.