Probabilistic Evaluation of the Fragility of Diagrid Structures with and without Internal Moment Frames and Various Perimeter Geometric Configurations

The evaluation of the fragility functions is an analytical approach that allows different ground motions to be used at varying intensity levels and represent various characteristics of low-intensity and high-intensity shakings. The fragility curves demonstrate the structure’s probability of collapse, or other limit states, as a function of some ground motion intensity measures (IM). The intensity measure is often quantified by spectral acceleration (Sa) or peak ground acceleration (PGA). Based on the statistical procedures, the parameters of the fragility functions are computed by assessing the results of nonlinear dynamic time history analyses. Therefore, the probability of failure associated with a prescribed criterion (e.g. the maximum inter-story drift) is estimated based on the probabilistic distribution relations.This paper evaluates the effects of internal flexural frames on the seismic performance of diagrid structures based on fragility curves. This evaluation is achieved by designing a group of 24-story studied diagrid models with various diagonal angles of 49, 67, and 74 according to the Iranian Standard No. 2800 (4th edition) and the Iranian National Building Code (Steel Structures-Issue 10). Then, some specific interior gravity frames of the studied diagrid models are replaced with bending frames. The seismic vulnerability of the studied diagrid structures with and without internal bending frames is assessed using nonlinear time history and incremental dynamic analyses (IDA) under near-field earthquake records containing different directivity effects. Finally, the fragility curves for the studied structures were obtained based on the lognormal probabilistic distribution function for the seismic performance limit states including IO, LS, CP, and global instability (GI). Moreover, the seismic performance levels of the studied structures were determined based on the FEMA 356.The results of performed nonlinear time history analyses indicate that the application of internal bending frames in diagrid structures would reduce the value of inter-story drift in upper floor levels, especially when the angles of exterior diagonal members are large. The results also show that the global instability of diagrid structures without internal bending frames can occur at a faster rate than the skeletal models with internal bents. Also, the contribution of the internal bending frames in improving the nonlinear behavior of diagrid structures depends on the perimeter triangular patterns. Due to this dependency, the increase in the angle of the inclined members in skeletal geometric configuration can increase the effectiveness of the internal bending frames in preventing the occurrence of global dynamic instability. The fragility curves of the studied diagrid structures illustrate that the internal bending frames reduce potentially excessive seismic performance levels. Furthermore, the internal bending frames amplify the seismic energy dissipation capability of the diagrid structures.