Dynamic collapse test on eccentric reinforced concrete structures with and without seismic retrofit

Abstract

Reconnaissance reports on structures damaged or collapsed by severe earthquakes have revealed several common characteristics in their structural members and systems, such as insufficient reinforcement details in beam-column joints and transverse confinements, low aspect ratios, soft and or weak stories, and eccentric plans. Dynamic tests were carried out to investigate the collapse process of reinforced concrete structures that had seismically deficient reinforcement details (light transverse reinforcement) and seismic systems (soft/weak stories and eccentric plans). A comparison of collapse behaviors with and without seismic retrofits also verified the effectiveness of the SRF (super reinforced with flexibility) strengthening method, which was developed to prevent the loss of axial load carrying capacity even at excessive lateral deformation. The columns of one specimen were strengthened with polyester fiber belts and its shear walls with polyester fiber sheets, while the members of the other were not. Each specimen was designed following old (1970s) reinforcement detail practice in Japan, and is a one-third-scale reinforced concrete structure with considerable stiffness and strength eccentricity in the first story. The specimens were composed of independent column frame and shear wall frame. Torsional response resulting from the eccentricity in the 1st story induced a displacement concentration on the weak frame, and eventually the independent columns of the RC specimen failed in shear and lost their axial load carrying capacity. On the other hand, the SRF specimen survived not only an identical earthquake load to the one that caused the RC specimen to fail, but also three additional earthquake loads, although significant strength deterioration and considerable lateral and vertical deformation were generated at the end of the test. The following conclusions were drawn from the comparison of the two specimens’ responses: axial column collapse cannot be predicted from vertical responses since the vertical behavior of bare RC columns was not discernibly different from that of SRF columns until axial collapse was initiated, and the SRF strengthening method is effective in confining the column and preventing the cracking progress, thus modifying the failure mode of the RC columns from brittle shear failure after flexural yield to flexural dominant behavior.