عنوان مقاله [English]
The effect of fatigue on the behavior of steel structures has long been the focus of researchers. But with the widespread damage of steel structures after the Northridge earthquake, it is important to investigate the issue of low cycle fatigue. The bylaws attempted to address the weaknesses in the connections and introduced prequalified connections. In the present study, the behavior of two prequalified welded unreinforced flange-welded web and reduce beam section under low cycl fatigue is investigated. The behavior of high cyclic fatigue has been the focus of many researchers, While there is only a limited number experimental data available for LCF. In this study, the S-N-curve was used to obtain the Nastar method and available experimental data. This curve has been developed using the available experimental results for the low cycle region. 4 buildings with different heights have been analyzed by linear time history analysis and their behavior under low cycle fatigue is considered. Then, the cumulative fatigue damage was investigated by the Palmgren-Miner fatigue analysis method for the above two connections. Rinflow method is also used for counting cycles. By defining the cumulative fatigue index, for the critical beam of structures, the maximum value of this index is observed at the wuf-w, which is obtained for 0.059 3-storey structure and increases by 0.24 with increasing structural height. The index for the critical column in the wuf-w junction in the 15-storey structure was 0.042, while in the other structures the maximum value is up to 0.197. Also, the fatigue index values for WUF-W binding were higher than those obtained for RBS, indicating that the first connection performance was weaker. For columns this ratio is 1.43. The results indicate the need for more attention to the effect of low cycle fatigue on connections of steel moment frames, especially on the WUF-W.