Importance of dimensional considerations to structural reliability

Abstract

Published results on the statistical distribution of structural failures show that lack of engineering design experience accounts for the major percentage of these events. In professional design activity, typical structures include sizes which can vary up to four orders of magnitude, a range outside the experience of many engineers. Thus, failures often result from incorrect extrapolation of previously used approaches in attempts to determine either environmental conditions different in character, or the response of structures considerably changed in size from those previously encountered. Discrepancies in the mathematical predictions of response for a full–size structure using results of small–scale model studies are addressed by the research community through studies on 'size effects'. However, in the design process a standard approach is often utilized, even for structures different in size by orders of magnitude. Cases are known whereby engineering procedures applied successfully in previous designs to seemingly similar structures led to failure due to significant size differences. Examples are presented to illustrate failures due to dimensional considerations in structural stability, joint design and construction, definition of safety margin as related to reduced redundancy and application of realistic dimensional tolerances. Building codes and standards do not, usually, address all intricacies related to the size of a structure. Therefore, it is left to the designer to identify those features and then incorporate them into the design process. For instance, the designer should be aware of the dimensional tolerances which can be expected to arise as a practical matter in the construction of unusually large structures, and should not uniformly accept code–specified values. Specifically, it is likely that excessively rigid dimensional limits used as a base for design calculations will be violated by the contractor, thus lowering the intended structural margin of safety. Deterministic design of structures is likely to remain the basic design approach whereby theoretical developments and practical experience are combined. Indiscriminate extrapolation of these methods beyond the range of past practical experience can lead to reduced structural safety through lowered redundancy of the system with simultaneous underprediction of environmental loading effects.