Conservatism in Reinforced Concrete Frame Theory

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Title: Conservatism in Reinforced Concrete Frame Theory

Author(s): D.H. Clyde

Publication: Special Publication

Volume: 12

Issue:

Appears on pages(s): 383-403

Keywords:

Date: 1/1/1965

Abstract:
With discussion by M. Z. Cohn and D.H. Clyde. Existing design code requirements of English-speaking countries permit ultimate strength design. This method replaces the traditional stress analysis criteria of brittle behavior at stress level by brittle behavior at the level of moment capacity, possibly because limit design has been ruled out as unsuitable for rigorous design in reinforced concrete due to the limited ductility of concrete. Nevertheless, ultimate load methods have been proposed which allow limited redistribution by taking advantage of whatever ductility is available at moment level and checking against a deformation criterion. Design methods may be checked for conservatism by reference to the yield criterion (or interaction diagram for reinforced concrete cross sections) and to the theorems of limit design, particularly the lower bound theorem. This provides a necessary but not sufficient check on safety where there is a deformation criterion as well as a stress limit. It is shown that: 1. All methods which use an asymmetrical yield envelope and alternative loading systems can lead to unsafe designs; 2. the ultimate load method can lead to designs which satisfy the limit design uniqueness principle and, hence, violate certain assumptions of the method; and 3. the optimum limit design method, in solutions published by the proposers, violate the lower bound theorem of limit design. The correction of the deficiencies is straightforward in terms of the principles used to examine them but further development of the theories is necessary.