Comparison of Seismic Fragility of Steel Moment Frames with and without Beam Web Opening

Steel moment frames have been known as one of the most reliable structural systems for many years. These structures were being designed and used as an alternative to masonry structures. It was assumed that these structures are very robust and will not collapse. However, some historic events such as the 1994 Northridge earthquake and subsequent tests proved otherwise. As a result, researchers tried to find ways to improve the seismic behavior of these structures. One of these methods is creating an opening in the beam web to move the plastic hinge to a region far from the connection welds. The purpose of this research is to analyze and investigate the dynamic behavior of steel moment frames with openings in the beam web and secondly to compare their behavior with the conventional moment frames. To achieve this goal, two four-story special moment frames with and without beam web openings have been modeled in OpenSees software, and nonlinear dynamic analysis has been performed on them. Finally, IDA curves, fragility curves and drift distribution in the height of structures are presented and compared. The results showed that the creation of beam web opening, increases the seismic collapse capacity of the steel moment frames and reduces their probability of failure during a seismic event. On average, the collapse capacity of a moment frame with beam web openings is almost 25% higher, and its collapse probability at different acceleration levels is 25% to 30% lower than the frame beam web openings.