Formulating Earthquake Resistant Design Criteria

The primary function of design criteria in general, and earthquake-resistant design criteria in particular, is to restate a complex problem that has unknowns and uncertainties into an unambiguous, simplified form having no uncertainties. The design criteria should provide clearly stated guidelines for the designers. For example, when actually designing a structure, an engineer needs to know the forces and deformations that the structure should be able to resist. Some of these forces, such as dead loads imposed by gravity, are well known, but others that result from transient actions of nature or man, such as earthquake, wind or live loads, are not known. This lack of knowledge must somehow be circumvented and a precise, unambiguous statement of the design conditions must be given to the design engineer. This is accomplished by means of the design criteria. The designer also needs to know the properties of the materials and structural elements that will be used, but as these are not precisely known, mainly because of imperfections in materials and workmanship, the design criteria must also take this into account. In the preparation of the design criteria, allowance must be made for the uncertainties, and it is necessary to be cognizant of all the unknowns for which allowances must be made.