EVALUATION OF THE APPLICABILITY OF VARIABLE R FACTORS IN PERU CONSIDERING LOW-CYCLE FATIGUE

The influence of using constant and variable values of the response modification factor R in obtaining the damage index proposed by Park-Ang was evaluated. The Park-Ang damage index (DI_PA) considers, in addition to the ductility demand, the accumulated hysteretic energy demand due to low-cycle fatigue (LCF). The constant values of R are equal to the target displacement ductility μ (=1.5, 2, 3, 4, 5, 6, 8, 10), while the variable values of R have been obtained from the simplified expressions proposed by other researchers in previous investigations. To obtain the DI_PA values, a set of 690 single-degree-of-freedom (SDOF) systems with vibration periods T distributed between 0.01 sec and 20 sec and with a 5% critical damping ratio were analyzed. Nonlinear behavior for reinforced concrete elements was implemented using five hysteretic models (HM): perfect elastoplastic, bilinear, modified Clough, modified Takeda, and gamma model. The two horizontal components of 58 seismic ground acceleration records measured on the Peruvian coast, applied unidirectionally, were used to obtain the DI_PA values. Based on the results obtained, it was observed that the use of constant values of R provides DI_PA values on average greater the unity regardless of the variables analyzed. While the use of variable values of R provides lower DI_PA values and, in many cases, close to unity. The latter depends on the dynamic properties and HM used. Likewise, it was observed that the effect of LCF on the DI_PA values is up to 80%. Finally, simple expressions for estimating the R factor have been proposed considering in its conception the effect of and LCF.