Field inspection and analyses of the performance of structures during earthquake shaking of their foundations have clearly shown that building design which blindly follows seismic code regulations does not guarantee safety against collapse or serious damage. The reasons are discussed in detail in References 12 and 13 and can be summarized as follows. First, there are large uncertainties in many of the aspects involved in the numerical design of structures, particularly in establishing the design earthquake shaking and in estimating the demands and predicting the supplies of the real three-dimensional soil-foundation-building (superstructure) system; second, the performance of the system depends on its state when the earthquake strikes - thus construction and maintenance, which includes repair, retrofitting and/or modifications, must also be considered in addition to the design aspects [12, 13].
IMPORTANCE OF CONSTRUCTION & MAINTENANCE
Design and construction of a structure are intimately related and the achievement of good workmanship depends, to a large degree, on the simplicity of detailing of the members and of their connections and supports. For example, in the case of a reinforced concrete structure, although it is possible to detail complex reinforcement on paper and even to realize it in laboratory specimens so that seismic behavior is improved, in the field such design details may not be economically feasible. A design is only effective if it can be constructed and maintained .
NEED FOR A COMPREHENSIVE APPROACH TO EARTHQUAKE-RESISTANT CONSTRUCTION
The need for a more comprehensive approach to the earthquake-resistant construction problem than that covered by existing seismic code procedures has been discussed by the author in several publications and has been summarized in Reference 13. In a comprehensive approach to the design of a structure it is first necessary to establish the design criteria, that is, behavior of the structure - serviceability, damageability, and safety against collapse. Once the design criteria are established, depending on the limit state contolling the design, the selection of the design earthquake(s) should be done according to the comprehensive approaches summarized in Reference 13. In this comprehensive attempt to overcome the uncertainties involved in modeling the real three-dimensional soil-foundation-superstructure system and in the estimation of the demands and supplies, usually derived from numerical analysis, the design cannot be based on a single deterministic analysis of a single selected model. The designer should consider several models, based on possible ranges over which the parameters governing the behavior of the real system can vary.
IMPORTANCE OF CONCEPTUAL DESIGN
As discussed in several publications, the author believes that in order to overcome or decrease the uncertainties to which the values of most of the parameters in the estimation of the demands and supplies are subjected in any current seismic-resistant design procedure, it is necessary to pay more attention to conceptual design [12, 13]. Conceptual design is defined as the avoidance or minimization of problems created by the effects of seismic excitation by applying an understanding of the behavior rather than using numerical computations. Examples of conceptual design are given in Reference 13. From the analysis of the basic design equations and the general equation for predicting response [12, 13], it becomes clear that to overcome detrimental effects of the uncertainties in many of the factors in these equations the following philosophy can be applied: (1) control or decrease the demands as much as possible, and (2) be generous in the supply, particularly by providing large ductility with stable hysteretic behavior (toughness).
CONTROL OR DECREASE OF DEMANDS
Because of the uncertainties regarding the dynamic characteristics of future earthquake ground motions and their modifications as a result of the interaction of the soil with the foundation-superstructure system response, the conceptual idea would be to control the input to the structure foundation. One promising method is through the use of base isolation techniques including energy absorbing devices in the system . In the case of buildings, a decrease in demand can be achieved by a proper selection of the configuration of the building and its structural layout and by the proper proportioning and detailing of the structural and non-structural components, that is, by following the basic principles or guideline for achieving efficient seismic-resistant construction.