Seismic Assessment of Steel Moment Frames with Irregularity in Mass and Stiffness

Abstract

Irregularity, consider to be one of the main reasons of buildings collapse in recent earthquakes. Irregularity also affects the seismic behavior and maximum capacity of structures. The effect of mass and stiffness irregularity was evaluated in this research using static and dynamic analysis. Three frames with 5, 10 and 15 stories with a 20% and 50% increase in mass of the middle stories and a 20% and 50% decrease in the ground level and middle stories were investigated separately. Maximum drift, first mode period, mass participation coefficient, and base shear force were evaluated using a developed program in MATLAB and SAP2000 based on finite element method. The results showed that changes in mass and stiffness causes a maximum increase of shear force by 14% and 5% in short and tall frames respectively. Maximum drift and the longest period in short frame occurred when the stiffness of ground level was decreased by 50 percent. In addition, such irregularity causes around 85% increase in mass participation coefficient in both short and tall frames.

Keywords

• irregular structure
• stiffness
• response spectrum analysis
• time history analysis