Soft Story Limits for Safety of Framed Buildings

Abstract eng:
In past earthquakes in Peru and worldwide it has been observed that soft story irregularity in building structures represents a serious hazard for its integrity and stability. To prevent collapse to preserve occupant lives inside a structure is an objective in all modern design standards. Therefore to know from which first story height this irregularity becomes critical to determine the possibility of collapse due to soft story. This is important for both design of new structures as for assessment and rehabilitation of existing structures. Several worldwide codes were reviewed to learn how this irregularity is defined and a summary is presented. Framed buildings of three, five, nine and fifteen stories, with two spans in both directions were analyzed. Openings span is five meters, typical height is three meters and the first floor height varies from three to nine meters. These frames were previously designed according to Peruvian Building Regulations. For comparison, a regular pattern structure, both in plan and elevation model was developed. Cases representing irregular structures were defined by modifying the vertical distribution of stiffness, increasing the height of the first floor model pattern to identify the weakest structural elements to fail first. The goal was to find the limit of the irregularity to ensure the stability of the structure and prevent collapse. Thus it is ensured that the structure reaches a collapse mechanism chosen during the design stage (weak beams strong columns), evaluating interstory drift limits for various performance levels according to Vision 2000. The procedure followed was (a) define analytical models of buildings designed under Peruvian standards, (b) compute parameters of models for inelastic behavior of elements experiencing nonlinear behavior, (c) analyze nonlinear performance in these models (pushover), and finally, (d) evaluate results obtained from these nonlinear analysis methods. With this information, behavior of soft story was investigated and factors, causes and results from this irregularity are explained in detail. In all buildings studied (three, five nine and fifteen stories) for first story height varying from three to under seven meters (6.5m) drift is between immediate occupancy (IO) and life safety (LS) performance level. For first story height varying from seven to nine meters drift lies between life safety (LS) and collapse prevention (CP) performance levels. Therefore it can be concluded that maximum first story height of a framed building, for it not to present soft story and have acceptable performance, must be under seven meters. This means a height ratio (first story height to typical) of under 42%. This limit is much lower than percentage that declares soft story in Peruvian Standards E.030-2003 (75%) and also in International Standards. In 2016 version of Peruvian Standard, soft story is declared based on drift relations (first floor to typical), height relation becomes now (60%). Compared to the maximum height of the first floor for analyzed buildings (linear analysis), it could be said seismic Peruvian standard E.030 is safe, somehow conservative as in International Standards .

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