Seismic Design and Tests of a Full-scale 2-story RC Frame with Buckling Restrained Braces
Date Issued
2015
Date
2015
Author(s)
Yang, Hsun-Horng
Abstract
Buckling-restrained braces (BRBs) have been widely used nowadays in steel structures as it can provide high stiffness, strength and ductility, thereby effectively absorbing seismic input energy. Researches on effectively using BRBs for seismic retrofit of existing RC buildings have been reported. It has been found the construction of BRB and RC member interfaces are often difficult, mostly due to the tensile and shear strengths of post-installed anchors in concrete are limited, the size and effectiveness of the BRBs are restricted. In addition, researches on applying BRBs for new RC building constructions are somewhat limited. This research investigates the seismic design and analysis methods for using the proposed I-shape steel embedment as the interface for the BRB and RC members. Steel embedment must be designed to transfer the BRB normal and shear forces in order to improve the seismic performance of the RC frame buildings. In this study, a full-scaled two-story RC frame with BRBs (BRB-RCF) is tested with four hybrid tests and cyclic loading test. A36 steel BRBs are arranged in zigzag configuration. The design of gussets incorporates the BRB axial and RC frame actions, while the beam and column members comply with ACI318 seismic design provisions. The tasks of this study include: (a) analyze the frame action effect on gussets using the equivalent strut model; (b) develop the design method for the D-region; (c) develop the design and construction methods for the proposed steel embedment; (d) develop the simplified and refined analysis procedures for the BRB-RCF; (e) develop the complete design procedures for the BRB-RCF using the proposed steel embedment. This study is in close cooperation with Ms. Jie-Lun Huang. Details of the specimen response predictions and simulations can be found in Ms. Huang’s thesis. Under the 50/50 hazard level earthquake, the maximum 2nd story’s inter-story drift ratio (IDR) was 0.23%, while all members remained elastic. During the 10/50 level earthquake, BRBs’ and beam ends’ yielding occurred. In the 2/50 earthquake, the maximum 2nd IDR was 2.5%. After three hybrid tests, the specimen’s lateral force vs. deformation responses still remained very stable and the residual IDR was 0.47%. After the same 2nd 2/50 earthquake was applied as an aftershock, the specimen’s stiffness and strength remained pretty much the same, suggesting the BRB-RCF specimen have performed very well under the four, from small to very large, earthquake load effects. During the subsequent cyclic loading test, plastic hinge formed at the 1st-story column base when the IDR reached 1.4%. At this IDR level, all BRBs and RC members yielding have occurred and the sequence agreed well with the predictions. When both two stories reached an IDR of 3.5%, the lateral force vs. deformation response of the specimen was still very stable. In the 3rd IDR=3.5% cycle, because of all bottom bars in the two top beam ends have fractured, the 2nd story shear reduced by 15 %. Up to the 1st IDR=4.5% cycle, 1st story top gusset buckled first, leading to the subsequent flexural buckling of the 1st story BRB to occur also. This is consistent with the predicted results when the gusset’s effective length factor K is assumed 2.0, DCR is 1.05; and steel casing’s DCR is 0.95 for the 1st story BRB. In the 50/50 earthquake, the ratios of peak BRB shear and BRB-RCF shear are 52% and 70% for the 1st and 2nd story, respectively. Similar ratios can be obtained if a factor 0.7 is applied on the gross moment of inertia for RC members in the ETABS elastic model. In the 10/50 and 2/50 events, the ratios become about 60% and 70% for the 1st and 2nd story, respectively. These indicate that BRBs can provide a high lateral stiffness. At the end of 3rd 2.75% IDR cycle, the cumulative plastic deformation CPDs were 476 and 680 for 1st- and 2nd-story BRBs, respectively. The hysteresis energy dissipated ratio in the four hybrid tests are ranging between 60-94% for the 1st and 2nd stories, confirming that BRBs can effectively dissipate seismic input energy. This study demonstrates that the proposed design and construction methods for the steel embedment are effective. No failure of the steel embedment or gusset is observed in the tests. Test results confirmed that the ACI provisions and the softened strut-and-tie model can be effectively applied to prevent the D-region failure. Test results confirm that the gusset force demands are consistent with the predictions. This study proposed the construction and design procedures of the BRB-RCF using the proposed BRB-to-RC member connections.
Subjects
reinforced concrete frame
buckling restrained brace
steel embedment
softened strut-and-tied model
hybrid test
gusset plate buckling
frame action effect
discontinuous region in RC member
Type
thesis
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