Shao, Gu, Jia, Ge and Taguchi 2020
The authors evaluated a brace-type shear fuse (BSF) in order to test its seismic performance in comparison with conventional buckling-restrained braces (BRBs) and slit steel dampers (SSDs). Quasi-static tests were performed on five small-scale BSFs while a two-dimensional, six-story steel frame was subjected to a time history analysis.
System Concept
The BSF was proposed to avoid limitations typically associated with BRBs or SSD, including the coupled strength and stiffness and the core plate encasings which make damage status after a seismic event impossible. In order to accomplish this, a BSF has two middle bridging plates that are welded to the middle of two perforated flat plates, having chamfered corners to avoid strain concentration, to improve out-of-plane stability and transfer forced among steel strips. Two side bridging plates are welded to the sides of the perforate flat plates to transfer axial force between the neighboring shear units. Each perforated flat plate has two shear units, each of which has four pairs of steel strips.
Analytical Study
Theoretical analysis was also performed on the proposed BSFs. Formulas exist to predict various mechanical properties of the BSF, but they had to be adjusted due to the geometry. An internal force diagram was also made using the Displacement Method and the Superposition Principle while the Principle of Virtual Work was used to obtain the initial stiffness. This theoretical analysis suggested lower yield and ultimate strengths for each of the specimens than was actual measured by the experiment. The stiffness of each specimen, however, was well predicted using the theoretical analysis.
Experimental Study, Results, and Discussion
Five small scale BSF specimen had varying dimensions to investigate the effect of design parameters on seismic performance; in this study the main focus was on the height (H) and width (B) of the steel strips. The specimen were evaluated in an MTS testing system that had a load capacity of 500kN and a deformation capacity of 75 mm. The bottom ends of the specimen were fixed to bottom loading head by clamping the middle bridging plate to it, while the top end was clamped to the top loading head which allowed for movement in the longitudinal direction.
All five of the specimen failed due to rupture of a steel strip in the core plate during tensile half cycles. However, at the moment of crack initiation there was no apparent decrease in load-carrying capacity. Over time the cracks propagated along the steel strips which decreased the load capacity gradually. In some specimen rupture in one steel strip occurred, followed by rupture in the other strips. In other specimen force redistribution followed the rupture. Due to the sensitive nature of fracture, the cracking led to asymmetric deformation of the specimen. As the cracks propagated global bending and torsional deformation increased.
Comparing the reactions of the specimen with unique dimensions enabled authors to make connections between such dimensions and mechanical properties. As H increased the ultimate load of the BSF decreased but the deformation capacity of the BSF greatly increased. Opposingly, as B increased the ultimate load increased while the deformation capacity decreased. There was also a positive correlation between B and the initial stiffness, as well as the yield load. Out-of-plane displacement increased with the increased load on the BSF and after crack initiation. Cumulative inelastic ductility of the BSF increased as H increased, which also resulted in the deformation mechanism becoming more and more bending dominant. Despite the decrease in initial stiffness and yield load that resulted from a decrease in H, there was a significant improvement of deformation capacity and energy dissipation capacity. An increase in B also positively affected the energy dissipation capacity of the BSF. As the ratio between B and H, , decreases the cumulative inelastic ductility significantly increases. The stress and strain conditions became more flexural dominant as increased. A large value was associated with a shear dominant deformation mechanism that is accompanied by a poor deformation capacity. The equivalent viscous damping ratio of the specimen, used to index energy dissipation, increased as the axial deformation increased.
The two-dimensional, six-story, steel frame was employed to evaluate the seismic performance of structures with BSFs. It was compared to an unbraced frame to quantitatively analyze its performance. After eight earthquakes at a peak ground acceleration (PGA) of 4.00 m/s2, the BSF frame had an average maximum drift ratio of 1.12% while that of the unbrace frame was 38%. The average residual drift ratio for the BSF frame and the unbraced frame were 0.08% and 0.30% respectively. The same eight earthquakes at a PGA of 1.96 m/s2, resulted in an average maximum drift ratio of 0.45% for the BSF frame and 37% for the unbraced frame. The average residual drift ratio for each was 0.02% and 0.06% respectively. Overall, the BSF greatly reduced the maximum inter-story drifts and the residual story drifts of the frame, indicating it would function well during moderate and major seismic activity.
Reference
Shao, F., Gu, T., Jia, L.-J., Ge, H., and Taguchi, M. (2020). “Experimental Study on Damage Detectable Brace-Type Shear Fuses.” Engineering Structures, 225, 1–17.