| A shear wall supported by a shallow foundation system is a commonly used seismic-force-resisting structural building system, yet there are many unresolved issues regarding their design and performance. One of the major changes in the traditional seismic design procedures adopted in the 1997 Federal Emergency Management Agency's (FEMA) Building Retrofit NEHRP Guidelines was to allow mobilization of the ultimate capacity and rocking behavior of shallow foundations to reduce the ductility demands on structures. However, the uncertainty in soil properties, the absence of practical reliable foundation modeling techniques, and the resulting permanent settlement beneath the footing due to foundation rocking have hindered the use of nonlinear soil-foundation-structure interaction as a mechanism for reducing demands on the structure in practice.; About sixty footings, representative of footings for building shear walls, were tested in a centrifuge under cyclic vertical (V), horizontal (H), and moment (M) loading; some were tested in slow cyclic loading while others were shaken using the shaking table on the centrifuge. Footing dimensions, depth of embedment, shear wall weight, soil strength, initial static vertical factor of safety (FSV), and loading paths were systematically varied. Experimental findings suggest that footings with higher FSV are capable of dissipating energy (approximately 20% of critical damping for FS V ≈ 10) through foundation rocking without causing excessive permanent settlement. Moment capacity of the foundation does not degrade significantly with increasing cyclic rotation for the types of soils tested in the experiments.; A new "contact interface model" has been developed to provide coupled nonlinear constitutive relations between cyclic loads (V-H-M) and displacements (settlement, sliding, and rotation) of the footing-soil system. The footing and the soil beneath the footing, considered as a macro-element, were modeled with the introduction of a new parameter, critical contact length ratio (Lc/L), where Lc is the minimum length of the footing required to be in contact with the soil to satisfy equilibrium. The contact interface model is able to predict load capacities, stiffness degradation, permanent and cyclic displacements, and hysteretic energy dissipation as measured in the centrifuge experiments. The contact interface model is implemented and tested in OpenSEES finite element framework. |
