Seismic Evaluation of Contemporary Steel MRF with Design and Construction Defects: Case Study of Double IPE Sections and Defects in Groove Welds

There are various design and construction deficiencies that exist in the contemporary moment resisting frames in Iran. This, of course, greatly increases the vulnerability of such structures. The most common defects observed in a real structure are as follows: cracks in welds due to the lack of complete penetration in groove welds and/or incomplete fusion, use of latticed built-up sections instead of standard wide-flange sections as columns, and misplacement of continuity plates at the column-girder joints. These defects have a negative impact on the behavior of connections and hence resulting in an unpredicted structural response. The rate of impact of such defects on structures can be estimated in a probabilistic analysis of the behavior of these frames. In this study, to investigate the probabilistic behavior of constructed structures in Iran, two existing frames with three and five stories having the above-mentioned deficiencies are considered. The mean annual loss and failure probability of these frames are obtained and compared with that of similar structures with no such defects. At first, the moment-curvature of different elements of the defected structure is estimated by an analytical method. The standard and defected connections are modeled in finite element software, and the hysteresis behavior of these connections is estimated using the standard SAC loading protocol. Based on these results, the backbone curve of connections is determined and employed for modeling of the frames. Then, the non-linear probabilistic behavior of the frames are evaluated by performing nonlinear dynamic analysis, when the frames are subjected to several earthquake records of soil type 2 in far- and near-field pulses. Thereby, the seismic fragility function of frames is estimated by applying the results of the probabilistic analysis. Using the seismic hazard curve of the location of the structure, the probability of failure of the studied frame in different damage states is estimated. In addition, the mean annual loss of the frames is calculated and compared. The results indicate that the presence of defects in connections cause a decrease in yielding and ultimate moment capacity of connections by 34 and 17.1 percent, respectively. Moreover, in three and five-story frames, the failure probabilities of the defected frames are 1.7 and 4.12 times of perfect frames, correspondingly. Similarly, the mean annual loss of defected three- and five-story frames are 1.76 and 2.36 times of perfect frames, respectively. It can be concluded that, first, the safety of constructed structures with defects in connections is significantly lower than that of code’s ideal frames. Second, the mean annual loss of defected frame is more than two times that of code’s complied frames; this demonstrates the significant vulnerability of existing structures. Third, the effect of deficiency on the reliability of mid-rise frames is higher than that of low-rise frames. This higher vulnerability of mid-rise frame indicates that particular attention should be paid to the construction of such structures. Besides, results have shown that the reliability of constructed steel frames with deficiencies is significantly lower than that of frames, which are constructed according to the requirements of the code, especially for high-rise buildings.