Damage evaluation considering input order of strain amplitude for buckling restrained braces with cross-shaped core and welded box-section member

In recent years, buildings in Japan have been required to have high earthquake resistance that can be used continuously after the large-scale earthquakes occurred. In order to achieve this, the damage control design that allows to prevent the damage of main structure by absorbing the seismic energy in damping devices is effective. In addition, it is important to judge appropriately the damage state of the damping devices after the earthquake occurs. The Miner's rule has been widely used as the damage evaluation for the random waveform, including the plastic region. On the other hand, the damage evaluation method using the quantity of total energy absorption is considered to be a simple method in comparison with the Miner's rule. Therefore, the authors proposed the average amplitude method as one of the energy evaluation method that can evaluate equivalent to Miner's rule. In this method, the random waveform is replaced by a constant average amplitude waveform so that the total energy absorption becomes equal. However, it was confirmed that there was some difference compared with the damage degree calculated by the Miner's rule. In this paper, it aims to clarify the influence on the damage degree by the input order of the magnitude of strain amplitude and the difference in the calculation method of the average strain amplitude. The buckling restrained brace (BRB) to be evaluated is composed of a cross-shaped axial member using a low yield strength steel and restraint member of the welded box-section. First, the basic model specimens which exclude the influence of the buckling restraining capacity by shortening the length of the plasticized region of the axial member and by increasing plate thickness of the restraint member is prepared. Then, the multistage amplitude loading tests in which the input order is switched and the dynamic loading tests by the response waveform in which only time axis is reversed are carried out. Next, using the real-size specimens, the continuous two-stage amplitude loading tests in which large and small strain amplitude are alternately repeated and the dynamic loading tests in which the input order of magnitude of strain amplitude is exchanged are conducted. Furthermore, it is evaluated that the influence on the strain behavior of the BRB by the input order of large and small amplitudes using FEM analysis. The main results in this paper are summarized as follows: 1) The ratio of the damage degree obtained by the energy evaluation method (average amplitude method using logarithmic average strain amplitude) to the damage degree obtained by Miner's law is decreased by about 15% more when receiving the large amplitude first. 2) The order of the input amplitude including both the increase and the decrease like a seismic wave has little influence on the above damage degree ratio. 3) The damage degree by the average amplitude method using the cumulative average strain amplitude is calculated to be smaller than the damage degree calculated using the logarithmic mean strain amplitude. 4) As a result of FEM analysis, it is considered that the local strain concentration by small amplitude input and the residual deformation by large amplitude input influence the change of the restoring force characteristic after that.