4th Ishihara lecture: Soil–foundation–structure systems beyond conventional seismic failure thresholds

• A new paradigm in performance-based seismic design of soil–foundation–structure systems.
• The method “invites” the creation of two simultaneous “failure” mechanisms: substantial foundation uplifting and ultimate-bearing-capacity slippage.
• Numerical studies and experiments demonstrate that the consequences of such daring foundation design would likely be quite beneficial to bridge piers, building frames, and simple frames retrofitted with a shear wall.

A new paradigm has now emerged in performance-based seismic design of soil–foundation–structure systems. Instead of imposing strict safety limits on forces and moments transmitted from the foundation onto the soil (aiming at avoiding pseudo-static failure), the new dynamic approach “invites” the creation of two simultaneous “failure” mechanisms: substantial foundation uplifting and ultimate-bearing-capacity slippage, while ensuring that peak and residual deformations are acceptable. The paper shows that allowing the foundation to work at such extreme conditions may not only lead to system collapse, but it would help protect (save) the structure from seismic damage. A potential price to pay: residual settlement and rotation, which could be abated with a number of foundation and soil improvements. Numerical studies and experiments demonstrate that the consequences of such daring foundation design would likely be quite beneficial to bridge piers, building frames, and simple frames retrofitted with a shear wall. It is shown that system collapse could be avoided even under seismic shaking far beyond the design ground motion. Three key phenomena are identified as the prime sources of the success; they are illustrated for a bridge–pier: (i) the constraining of the transmitted accelerations by the reduced ultimate moment capacity of the foundation, to levels of about one-half of those developing in a conventional design; (ii) the beneficial action of the static vertical load of the structure which pushes down to “re-center” the leaning (due to uplifting and soil yielding) footing, instead of further distressing the plastic hinge of the column of the conventional design; and (iii) the substantial increase of the fundamental natural period of the system as uplifting takes place, which brings the structure beyond the significant period range of a ground motion, and hence leads to the abatement of its severe shaking.