Chapter 7 - Design Criteria

Abstract

Real practical structures are seldom subjected to static-only or dynamic-only excitation. Usually, both kinds of excitation exist simultaneously. The structural design has to answer both kinds of loads. A good, optimal design is the one where the acting stresses are lower than the allowable stresses with a pre-determined factor of safety. When this factor of safety is higher than the one required, the design is too conservative, not optimal, and may be too expensive. The correct design criterion should never be based on the input, although inputs are usually the outcome of the product design specifications and are part of the contract between the customer and the designers. Usually, structures are designed according to input specifications. They also may be based on past experience of the designers or on data collected on previous models' designs. It is quite advisable that a new model design of an existing structural system should be based as much as possible on knowledge collected in the previous design, as well as feedback from field experience and tests of the previous design. It is essential that the design criteria set at the beginning of a project will be the best and as clear as possible. Erroneous criteria lead to bad designs. Bad design is not only the one that fails but also the one that is too conservative—too heavy, too expensive. The importance of pre-determination of design criteria is an essential part of the design.