SUMMARY In recent earthquakes, many buildings have been damaged due to the soft‐storey mechanism failure. The seismic design codes for buildings do not contain enough criteria to predict the real displacement of such buildings. This paper focuses on evaluating the nonlinear displacement of buildings that fail in soft‐storey mechanism form. Results show that the nonlinear static procedure with coefficient method, which is described in Chapter 3 of ASCE/SEI 41‐06, does not have sufficient accuracy for estimation of structure displacement demand in such buildings. In this paper, the coefficient methodology is used for evaluating the target displacement for 5‐storey, 8‐storey and 15‐storey special moment resisting steel frames. For this purpose, dynamic nonlinear time‐history analysis has been applied for the mentioned structures having a soft‐storey mechanism failure form. The numerical results of storey displacement and interstorey drift were compared with those values obtained from the coefficient method described in Chapter 3 of ASCE/SEI 41‐06. Copyright © 2012 John Wiley & Sons, Ltd.

Most structures with masonry infills that are continuous along their height, which are interrupted in the lowest storey, are damaged by earthquakes. These structures are anticipated to collapse due to the undesirable soft-storey mechanism formed by lateral stiffness of masonry infills in other storeys. The seismic design criteria of UBC97 code for special moment-resisting steel frame (SMRSF) are reviewed. In this paper, a new criterion for seismic design of such structures is presented. The proposed criteria are used to design three SMRSFs: 5, 8 and 15 storeys. Nonlinear time-history dynamic analyses are applied for the designed SMRSFs based on the proposed criteria. Displacements and storey drifts, which are obtained by the proposed method, are compared with nonlinear time-history dynamic analysis results, finally.Step 1: SMRSF structure is designed according to the criteria of common seismic design codes, e.g.UBC97, using ductile-compact sections/ignoring the frame-infill interaction.Since the proposed method is an iterative method, it is necessary to assume the initial size of the members. The best sections for the initial design are those obtained according to the criteria of seismic design codes. For the three moment-resisting steel frames 5SSB, 8SSB and 15SSB, the initial profiles of the members are those in 5GM, 8GM and 15GM moment-resisting steel frames that are shown in Figure 2.Step 2: Analytical natural period related to the first vibration mode of the structure is calculated. This period is based on the mathematical model of beam and column sections determined in the previous section as well as modelling the lateral behaviour of masonry infills.Considering the basic differences of SMRSF designed based on general failure mechanism with soft-storey failure mechanism in the lowest storey, common seismic design codes could not be used for calculating the natural period of the structure. A mathematical model should be used in which all affecting parameters on lateral stiffness of structure are considered. Figure 2. Member sizing 5GM, 8GM and 15GM.