SEISMIC EXCITATION WITH LARGE OVERTURNING MOMENTS - TENSILE CAPACITY, PROJECTING BASE MAT OR LIFTING-OFF

For severe earthquakes large overturning moments arise which may lead to tension in part of the area of contact of the base mat of the structure and of the soil, according to a calculation based on a linear theory. If no tensile capacity (prestressed anchors, piles) is provided, two different solution procedures are commonly used. The first method consists of enlarging the base mat using projections to avoid tension. In the second, the structure is allowed to lift off and is designed to resist the nonlinear dynamic response. On the basis of a parametric study, it is concluded that allowing the structure to lift off generally leads to a reduction of the total accelerations, of the stress resultants and of the relative displacements within the structure, while the total displacements, the vertical response and sometimes the response spectra in the higher frequency range are increased, compared to the linear elastic solution with the original base mat. The response using projections is non-uniform when compared to that with lifting-off. It is beneficial to achieve lifting-off to make use of the reduction in the strength requirements. Projecting base mats should be avoided. © 1978.