Collapse Modelling of Soft-Storey Buildings

Studies undertaken by the authors in recent years have indicated that the existing building stock at most risk of damage and collapse from earthquake excitation in lower seismicity regions such as Australia are unreinforced masonry buildings and soft-storey structures. Soft-storey buildings possess storeys that are significantly weaker or more flexible than adjacent storeys, and where deformations and damage tend to be concentrated. Soft-storeys commonly occur at the ground floor, where the functional requirements dictate a higher ceiling level or a more open configuration, such as for car parking or retail space, resulting in an inherently weaker and more fl exible level, as shown in figure 1. In high seismic regions, soft-storey structures and unreinforced masonry are banned, yet in regions of lower seismicity such building types and configurations are common, and are often occupied by organisations with a post-disaster function or house a significant number of people. This paper will address the performance of soft-storey buildings under earthquake excitations specifically. Research findings presented in this paper are directly relevant to low-moderate seismic regions worldwide such as Thailand, Vietnam, Hong Kong, China and Singapore, where similar soft-storey structures of limited ductility are commonly constructed.

Soft-storey buildings are considered to be particularly vulnerable because the rigid block at the upper levels has limited energy absorption and displacement capacity, thus leaving the columns in the soft-storey to defl ect and absorb the seismic energy. Collapse of the building is imminent when the energy absorption capacity or displacement capacity of the soft-storey columns is exceeded by the energy demand or the displacement demand. This concept is best illustrated using the Capacity Spectrum Method shown in figure 2, where the seismic demand is represented in the form of an acceleration-displacement response spectrum (ADRS diagram) and the structural capacity is estimated from a non-linear push-over analysis expressed in an ADR (as illustrated in Wilson & Lam, 2006).

The structure is considered to survive the design earthquake if the capacity curve intersects the demand curve, and collapse if the curves do not intersect. In regions of high seismicity, the maximum displacement demand could exceed 200-300 mm, which translates to a drift in the order of 5-10% in a soft-storey structure. Such drift demands are significantly greater than the drift capacity of soft-storey structures even if the columns have been detailed for ductility. This is the reason soft-storey structures have behaved poorly and collapsed in larger earthquake events around the world. In high seismic regions, buildings are configured and detailed so that in an extreme event a rational yielding mechanism develops to dissipate the energy throughout the structure and increase the displacement capacity of the building. Ductile detailing in reinforced concrete columns includes closely-spaced closed stirrups to confine the concrete, prevent longitudinal steel buckling and to increase the shear capacity of columns (Mander, 1988; Park, 1997; Paulay & Priestley, 1991; Watson et al, 1994; Priestley & Park, 1987; Bae et al, 2005, Priestley, 1994; Bayrak & Sheikh, 2001; Berry & Eberhard, 2005; Pujol et al, 2000; Saatcioglu & Ozcebe, 1992). The emphasis is on the prevention of brittle failure modes and the encouragement of ductile mechanisms through the formation of plastic hinges that can rotate without strength degradation to create the rational yielding mechanism.

Current detailing practice in the regions of lower seismicity typically allow widely spaced stirrups (typical stirrup spacing in the order of the minimum column dimension) resulting in concrete that is not effectively confined to prevent crushing and spalling, longitudinal steel that is not prevented from buckling, and columns that are weaker in shear. Design guidelines that have been developed in regions of high seismicity (ATC40, FEMA273) recommend a very low drift capacity for columns that have such a low level of detailing. The application of such standards in the context of low-moderate seismicity regions results in most soft-storey structures being deemed to fail when subject to the earthquake event consistent with a return period in the order of 500-1500 years.