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

摘要

Reinforced concrete flat plate is a type of structural system widely used for office and residential buildings in many areas including those with high seismic risk. In the regions of high seismicity, moment frames or shear walls are employed to resist lateral loading while the flat-plate system is designed to primarily resist gravity loading. Flat-plate structures must maintain lateral deformation capacity as well as resist gravity loading under seismic loads. The deformation capacity of a slab-column connection is a function of vertical shear transferred from slab to column. The effects of vertical loading become even more exaggerated when the flat-plate structure is located near a fault. Near-source seismic events can cause large vertical-to-horizontal peak ground acceleration (V/H) ratios greater than unity with high velocity and frequencies which can have disastrous effects due to significant vertical accelerations.Flat-plate structure is prone to punching shear failure at slab-column connections which may lead to a catastrophic progressive collapse. Slabs of flat-plates functioning only as gravity system generally have a reinforcement ratio less than 1% for the tensile bars resisting negative bending moment. Large vertical accelerations combined with low percentage of steel reinforcement may increase the risk of punching failure in flat plates. To date little is known about the structural performance of flat plates subjected to strong vertical accelerations during a near source event.In this study a flat-plate structure taken as a prototype is subjected to eight seismic ground motions recorded in stations with a distance less than approximately 35km to the seismic epicenter and fault line. The ground motions were scaled to reflect the seismic risk corresponding to the maximum considered earthquake for design purposes. It is found from the numerical simulation that the addition of vertical ground motion increases lateral drift of the building by less than 3% when compared with applying ground motions in the horizontal directions. However, the vertical ground motion can significantly increase the slab deformation localized at the columns on average by as much as 30% and create an average slab rotation near 0.05 radians. The deflection at slab panel centers is also enhanced due to the addition of vertical ground motion by an average of 39%. Finally, vertical ground motion amplifies the total vertical shear transferred between the slab and columns; as compared with bi-directional ground motion, an average of 29% change in the axial force in the columns is identified.The potential of punching shear failure of flat-plate structures under combined effects of vertical and horizontal ground motion is examined based on the three design criteria given in ACI 318-14: eccentric shear stress model, story design drift limit for slab-column connections without shear reinforcement, and two-way shear strength for one side of a slab-column connection. The failure criterion suggested by Muttoni et al. (2008) is also used to evaluate punching failure potential. Results signify ACI 318 eccentric shear stress model and two-way shear strength approach may not be able to adequately predict the potential of punching shear failure in flat plates. Conversely ACI 318 drift limit approach and Muttoni’s punching failure criterion model predict similar risk of punching failure and a greater possibility of punching failure when vertical ground motion is incorporated.Finally, soil-structure interaction is incorporated into the prototype structure model for dynamic tri-axial loading considering stiff soil. Translational and rotational soil springs are included with spring stiffness based upon the suggestions by Gazetas (1991). The incorporation of soil-structure interaction produces little change in the dynamic response of a flat-plate structure if the soil is stiff. Insights gained from this study will create knowledge needed to improve design of flat-plate buildings subjected to vertical ground motions.