Evaluation of Reliability Index for Long Span Steel Moment Frames Subjected to Vertical Earthquake Component

Records in the near-fault areas indicate the strong vertical ground motion. Near-fault earthquakes are different from far-fault earthquakes due to the adverse effect of their vertical component, which can be very destructive for long span frames. The Vertical Component of earthquake is different from the horizontal component. Vertical component of earthquake occurs when the compression P waves separate. Whereas the horizontal component occurs as the shear S waves separate. The frequency content of vertical component of earthquake is higher than the horizontal component. The natural frequency of structures in the vertical direction is more than that of the horizontal direction. These factors may cause the resonance in the structures. The first step in the examination of the effects of vertical component of an earthquake on structures is identifying the stimulating source of vertical component of the earthquake, response spectrum of the vertical component and affecting parameters on the vertical component. In this research, the effect of horizontal and vertical components of earthquakes has been investigated in long span building frames by modeling two types of such structures with various openings sizes in a powerful finite element software. For this purpose, in the first phase of the study, the structures have first been evaluated under the effect of horizontal component of earthquake alone, and then, the vertical component of earthquakes has been added and performance of the structures has been evaluated again. To evaluate the performance of structures, both linear and nonlinear as well as static and dynamic analyses according to the seismic rehabilitation of structures guidelines have been performed. In the second phase of the study, for more confident evaluation, the reliability indices as suitable tool, recommended in recent researches, have been used. For this purpose, the amounts of moment, shear force, axial force of columns, and deflection of the long span beams have been calculated, and the corresponding reliability indices have been obtained by using an appropriate statistical software.