Multi-Level Behavior Factors for Steel Moment Frame Accompanied with RC Slit Shear Wall System

Earthquake loads induce significant damages and cause widespread failures into buildings. Having appropriate system against seismic loads is a minimum necessary requirement for a structure. Moment Resisting Frame Systems (MRFS) are one of the common seismic resisting systems against lateral seismic loads. Ductility is the most important properties of these kinds of systems; but increase in ductility leads to decrease stiffness and increase lateral deflections and hence induces damages to nonstructural components. Although stiffness can be magnified through increasing section sizes of members, but it would not be economical. To compensate this deficiency, the combination of these systems with reinforced concrete (RC) shear walls may be useful. Although in general, this combination (RC shear walls and MRFS) decreases the section size and increase stiffness; but in low rise structures using this combined system cause decrease in ductility and dissipation of energy under moderate/strong earthquakes.This deficiency can be improved by using vertical slits in RC shear walls of low to moderate height. These slits invert shear behavior of RC shear wall into flexural behavior of several columns and are able to increase ductility. So, for the first time in this paper, a study was conducted on introducing behavior factor (R) for Steel Moment Frame (SMF) with reinforced concrete slit shear wall system at two levels of demand and supply.
In view of existing concerns about precise of behavior factors in seismic design codes, due to developing these factors based on engineering judgment from observing seismic performance of structures subjected to past earthquakes besides the lake of these information in current seismic design codes causes the seismic design of RC slit shear wall system needs more research works. The behavior factors are used to reduce the linear elastic design spectrum to account for the energy dissipation capacity, over-strength and redundancy of the structure. The most distinctive feature of this study respecting to similar studies is multi-level definition of behavior factors and their extraction with respect to seismic intensity, and accepted damage level as expected performance levels in designing RC slit shear wall structural system. Hence, the demand/supply behavior factors are determined with a more accurate attitude involving the effective parameters such as ductility, over-strength, redundancy, seismic hazard level, performance levels, etc.
In this study, to determine the appropriate behavior factor, static pushover analysis along with Incremental Dynamic Analysis (IDA), are used. The behavior factors in two levels of demand and supply are obtained with two procedures: At the first, the pushover analysis was applied on case study structures and then relationship for SDOF system of Newmark and Hall, Nassar and Krawinkler, and Miranda to evaluate behavior factor for MDOF structures were used. At the second stage both pushover and incremental dynamic analysis were used to achieve directly the behavior factor for MDOF structures.
In this paper, two 5 and 10-story steel moment resisting frame with RC slit and ordinary shear wall systems were designed by ETABS software. These structures were designed in which their behavior factors were the same values. Then the pushover and IDA were conducted on sample structures using nonlinear analysis software PERFORM. Results show that, although initial elastic stiffness has not been considerably changed in slit RC shear wall systems, but they show higher behavior factor relative to regular RC shear wall systems. Converting the shear behavior of RC ordinary shear wall to ductile flexural behavior of a series of wall pieces as columns by providing slits in shear wall may be considered as the reason for achieving more ductility and dissipating high seismic energy in this innovative systems.