The Role of Building Foundations in Seismic Retrofit

Existing buildings which do not satisfy modern seismic design requirements may be rehabilitated economically by the addition of steel bracing or reinforced concrete shear walls. However, steel bracing and concrete shear walls can place severe demands on the building foundation, and the existing foundations are likely to be inadequate. Strengthening of existing foundations may be exceedingly expensive, and the cost of the foundation work may dominate the seismic retrofit effort. Several foundation retrofit strategies are described and discussed. These strategies are invariably expensive, and the engineer may be tempted to place the major investment into strengthening or stiffening the structure with minimal work on the foundation. This concept may lead to increased potential for uplift and temporary overload of footings. In some cases this practice may have beneficial effect regarding the overall seismic response of the structure. It may reduce both seismic forces and maximum displacements experienced by the structure. The reduced foundation retrofit is unlikely to increase the collapse potential or life-safety risks of the building, while the structural retrofit should significantly improve the life-safety concerns. However, the combination of structural rehabilitation combined with minimal foundation rehabilitation are likely to increase the potential damage and economic loss during an earthquake.