Design approaches and criteria for earthquake-resistant shallow foundation systems

Current practice for earthquake-resistant shallow foundation design does not allow the foundation to fail in bearing, and does not consider the interaction of non-linear responses of the soil, foundation, and superstructure. Design tools exist that allow this interaction to be considered, and with the use of these tools, an alternative design approach that allows shallow foundations to yield during an earthquake becomes available. This paper uses examples to demonstrate the benefits of this alternative design approach. The approach enables the performance of the foundation to be balanced against that of the superstructure. Foundation and superstructure actions may be reduced significantly, whilst incurring only modest permanent foundation displacements. Broad suggestions of the type of criteria that might be required for the yielding foundation design approach are made. It is concluded that it may be preferable to develop criteria that describe performance aspects and system responses that should be considered, rather than prescribe firm quantitative criteria. The traditional design approach can result in foundations being significantly larger than would be required under static loading only, especially for tall slender structures that can impose large eccentric loadings on foundations. Larger foundations are stiffer than smaller ones, and have a shorter natural period. This affects the dynamic response of the soilfoundation-structure system. 2.2 No reserve of bearing strength during design earthquake (seismic factor of safety of unity) One alternative to the traditional design approach is to size the foundation so that bearing failure is only just prevented from occurring during the design earthquake. Under a LRFD framework, this would correspond to both the earthquake loading demands on the foundation and the available vertical bearing strength being unfactored (a seismic factor of safety of one). Possible advantages to this design approach are a smaller, presumably cheaper foundation, and a less stiff soil-foundation-structure system (which may have benefits for structural performance). A disadvantage to this design approach is that although in theory no yielding will occur during the design earthquake, there is no margin for error. Thus the approach may not be acceptable if it is critical to prevent bearing failure occurring, given the uncertainty that is usually present in assessing soil strength. However, as will be demonstrated, minor instances of bearing failure during an earthquake may not have significant consequences. 2.3 Design based on static LRFD requirements only (seismic yielding allowed) A second alternative to the traditional design approach, and the one that is the focus of this paper, is to size the foundation based only on static considerations (bearing capacity and settlement). Under this approach, brief instances of foundation yielding (bearing failure) might occur during an earthquake, resulting in the accumulation of permanent displacements. This is referred to herein as the ‘yielding foundation’ approach. A major advantage to this approach would be the ability to consider and balance the performance of the foundation and the superstructure, without having the prerequisite requirement that the foundation is not permitted to yield. In many cases, allowing the foundation to yield may have beneficial effects for the superstructure performance or for the system as a whole, and under this approach, these benefits could be obtained. Si ng le de gr ee of fre ed om su pe rs tru ct ur e m od el R ig id fo un da tio n