Pushover and seismic response of foundations on stiff clay; analysis with P-delta effects

Finite-element analyses are performed for the response to lateral monotonic, slow-cyclic, and seismic loading of rigid footings carrying tall slender structures and supported on stiff clay. The response involves mainly footing rotation under the action of overturning moments from the horizontal external force on - or the developing inertia at - the mass of the structure, as well as from the aggravating contribution of its weight (P-delta effect). Emphasis is given to the conditions for collapse of the soil-foundation-structure system. Two interconnected mechanisms of nonlinearity are considered: detachment from the soil with subsequent uplifting of the foundation (geometric nonlinearity) and formation of bearing-capacity failure surfaces (material inelasticity). The relation between monotonic behavior (static "pushover"), slow-cyclic behavior, and seismic response is explored parametrically. We show that with "light" structures uplifting is the dominant mechanism that may lead to collapse by dynamic instability (overturning), whereas "very heavy" structures mobilize soil failure mechanisms, leading to accumulation of settlement, residual rotation, and ultimately collapse.