Elastic or Allowable Stress Methods


Allowable stress, or elastic methods, are used in older codes, and in modified form in Japanese practice [1]. The idea is to make the structure strong enough and/or stiff enough so that it will respond elastically, or that stresses would remain under some allowable level (usually the nominal yield strength for steel) for the design earthquake forces. There are several problems with attempting to design a structure without explicitly considering nonlinear behavior.

First of all, it is difficult to quantify how large the forces will be, due to the uncertainty involved in predicting gound motions. Code predictions of forces have been unconservative in some cases, and overconservative in others.

Also, the failure mode of the structure is not considered. Individual members are sized to resist loads from the equivalent static load distribution (shears, moments, etc) without regard to the relative amount of overstrength in each member. For instance, suppose a column has an overstrength of 1.3 in flexure and an overstrength of 1.1 in shear. It will fail in shear if overloaded and give very undesirable performance even though the nominal strengths of the members satisfied the allowable stress requirements. Therefore, unexpected brittle failures can occur and there is no method to insure ductile system behavior.

Lastly, if the objective is to make the structure very strong and stiff, costs can be prohibitive.