Effects of Vertical Ground Motion on Seismic Performance of Reinforced Concrete Flat-Plate Buildings

Nonlinear time-history analyses are conducted on a flat-plate building to explore the damaging effects of vertical ground acceleration on punching shear failure of slab–column connections, which have been exposed by field observations. The structure is modeled three-dimensionally using grid beam elements for the floor slabs. Twenty unscaled near-source ground motions recorded during the 1979 Imperial Valley, 1994 Northridge, 2010 Darfield, and 2011 Christchurch earthquakes are considered. Both the punching failure criterion recommended by a standard design code and one developed based on a critical crack width theory are employed to identify punching shear failures. The numerical simulations indicate that, although the vertical ground-motion component has negligible effects on interstory drift demand, it can lead to punching failure that would otherwise not occur under horizontal ground motion only. On average, the vertical ground motion can reduce the lateral drift capacity at punching failure by 23%. Moreover, an extraordinarily strong vertical ground motion alone can cause a punching shear failure. The two considered punching failure criteria result in similar predicted failure drifts. The extra gravity load approach suggested by a standard design code to indirectly account for vertical ground-motion effects is found appropriate if the vertical spectral acceleration at the fundamental period of floor slab is less than $1.4g$. The simulations also indicate slab-column frames can contribute around 15% of lateral stiffness and strength.