Response modification factor evaluation for vertical irregular MRF buildings

Most design codes assume the nonlinear seismic performance of structures using response reduction/modification factor (R). The R factor is sensitive to a variety of factors in terms of overall ductility and over-strength. This research assesses the actual R factor for vertical irregularity cases for RC bare buildings with moment-resisting frames (MRF) systems. Also, this research derives a significant relationship between R values and identified vertical irregularity index calculated from relative stiffness between adjacent stories. Three-dimensional numerical models are carried out for the soft story and setback irregularity scenarios using ETABS. Modal pushover analysis (MPA) is selected to obtain the inelastic seismic capacity. The obtained results demonstrate that vertical irregular buildings have weak inelastic seismic capacities compared to regular one. So, the response modification factor (R) should be scaled down before the design stage by 15% to 40% for single and combined vertical irregularity scenarios. Structures with a combined asymmetric setback with a soft ground story experience the worst R factor. Also, R factors are sensitive to the identified vertical irregularity index (Vtm) that has 80% regression percent. So, it may be used to specify the allowable vertical irregularity ratio, location, and combination for each seismic zone.