چكيده به لاتين
This study is carried out aimed to evaluate the performance of rehabilitated frames with buckling restrained knee brace (BRKB). The BRKB system under severe earthquakes can exhibit stable and symmetrical inelastic behavior. BRKBs can tolerate plastic deformation and absorb earthquake energy with stable and symmetrical behavior during successive cycles with tensile yield. Therefore, six frames (1, 2, and 3-story) were selected among the schools that needed to be rehabilitated. The beam-to-column joints of these frames are rigid. Opensees software was used to model these frames and relevant analyzes. The buckling restrained knee brace-to-frame joint was assumed as a hinged joint. In order to obtain an optimum angle of the buckling restrained knee brace, the frames were analyzed at different angles, and it was observed that at the angle of 45 degrees, the maximum value of inter-story drift of the frames was minimal.
Then, in order to obtain the optimum length of the buckling restrained knee brace, the frames were analyzed with buckling restrained knee brace with different length. Increasing the length reduced inter-story drift. It was observed that as the length of the buckling restrained knee brace is increased, the ratio of the reduced inter-story to the steel used is increase, but the slope of this increase is decreasing. By inserting a coefficient to observe the amount of space occupied by each brace and its architectural effects, it was found that the optimal length is between 70 and 100 centimeters. Then, the difference in the distribution of braces was compared in terms of inter-story drift and the uniform distribution, which showed that in the low-rise structures, the difference between these two distributions is negligible.
The frames were analyzed in terms of time history under 20 Near-Field Earthquake accelerometers and the maximum inter-story drift was obtained between the frames. These accelerometers were scaled so that to be equal with earthquake with the hazard level 1. The soil on which the frames are installed are assumed to be type-2.
Using the results of time history analysis and fragility analysis, various percentages of probability of reaching and passing from the life-safety limit state were obtained. Then, for each of these scenarios, a nonlinear static analysis was performed and the stiffness and strength values of the frames were obtained in each case from the pushover chart. At the end, a graph is depicted that, for any stiffness and strength of the frame, shows the possibility to exceed it from the life-safety limit state. This diagram is called fragility contours. Since this chart shows the probability of passing the life-safety limit state under an earthquake with a hazard level 1, it can be used for the frames basic rehabilitation.
Keywords: Buckling Restrained Knee Brace, Low-rise Structures, Near-Field Earthquake, Fragility Analysis, Fragility Contours, Basic Rehabilitation
