Rocking isolation of low-rise frame structures founded on isolated footings

This paper explores the effectiveness of a new approach to foundation seismic design. Instead of the present practice of over-design, the foundations are intentionally under-dimensioned so as to uplift and mobilize the strength of the supporting (stiff) soil, in the hope that they will thus act as a rocking-isolation mechanism, limiting the inertia transmitted to the superstructure, and guiding plastic 'hinging' into soil and the foundation-soil interface. An idealized simple but realistic one-bay two-story reinforced concrete moment resisting frame serves as an example to compare the two alternatives. The problem is analyzed employing the finite element method, taking account of material (soil and superstructure) and geometric (uplifting and P-? effects) nonlinearities. The response is first investigated through static pushover analysis. It is shown that the axial forces N acting on the footings and the moment to shear (M/Q) ratio fluctuate substantially during shaking, leading to significant changes in footing moment-rotation response. The seismic performance is explored through dynamic time history analyses, using a wide range of unscaled seismic records as excitation. It is shown that although the performance of both alternatives is acceptable for moderate seismic shaking, for very strong seismic shaking exceeding the design, the performance of the rocking-isolated system is advantageous: it survives with no damage to the columns, sustaining non-negligible but repairable damage to its beams and non-structural elements (infill walls, etc.).