The Relation of Analysis to Structural Design

Confusion sometimes exists in structural design as to the use to be made of analyses. The designer soon realizes that precision is futile in some cases and important in others, and the experienced designer realizes fully that analysis of the conventional type is frequently a poor guide to proper proportions. That analysis shows a certain member to be overstressed commonly indicates that the member should be made larger; but the over-stress sometimes has little importance and may be disregarded. In some cases where the over-stress is serious, the best solution is not obvious; sometimes the structural layout should be changed entirely.

Critical study soon leads to recognition of important differences between load-carrying stresses and stresses which produce no appreciable resistance to the applied loads. The latter may be due either to external movement of abutments or to internal distortions, or they may be due to deformation induced in one part of a structure as a result of that in another part. The load-carrying stresses may also be divided into two groups. The distinction in this case, however, is based upon response to changes in design; these sub-groups are not always clearly distinguishable, but they have characteristics so widely different in certain cases as to force their differentiation.

A classification is presented herein, with the idea of suggesting a convenient arrangement of certain familiar characteristics rather than with any wish to define the groups formally. The designations suggested, therefore, are for convenience of reference only. The non-load-carrying stresses will be distinguished as Deformation Stresses and Participation Stresses; and the load-carrying stresses as those normal in their characteristics and those which are hybrid.

Deformation stresses are a consequence of strain and strain is a consequence of internal or external movements not due to stress in the structure, such as abutment movements, shrinkage, or the effects of temperature change. Participation stresses are similar to deformation stresses, but they are due to a quite different cause. They include what are known as “secondary stresses” in bridge trusses and “participation stresses” in bracing systems as special cases. The designation is used herein for want of a better one, but the term is used in a wider sense than usual.

The primary action of most structures is such that the stress in any one part is independent, or nearly independent, of that of the other parts. This is termed normal, structural action. The group indicated includes all structures statically determined and, for good reasons, it includes also most of the forms of indeterminate structure that experience has shown to be useful.

There is a type of structure in which one member cannot be designed separately but must be designed with due consideration for its effect on other members. Such action is referred to as “hybrid,” because it has some of the characteristics both of normal structural action and of participation action. The group of structures seems to be quite large and to have characteristics of great importance to designers.

Although the designations assigned herein may be new, the concepts involved are not new. What the writer wishes to do is to classify and arrange certain views of structural design which have an honored tradition in American practice. The paper is not intended to be quantitative, except in so far as quantitative statements may help in defining qualitative action. The classification proposed has some value in reconciling discordant views held by practical designers and those held by theoretical analysts, and seems, further, to have value in reconciling conflicting views held by theoretical students in the field. It is also of value in anticipating the characteristics of proposed structural types.