ACI 318-02 Beam Strain Limit of 0.004 versus 318-99 75% Balanced Steel

Abstract

There are several changes in the American Concrete Institute’s 318-02 building code from 1999 that have had a significant impact upon beam analysis and design. On one side of the equation, the load factors are now based on ASCE 7-98 combinations. On the other side of the equation, strength reduction factors were “adjusted to be compatible with ASCE 7-98.” Concrete sections are now classified as either tension controlled or compression controlled with a transition region between. For beams with little or no axial force, there is a limit on strain in the tension steel that replaces the previous limit resulting from a maximum allowable tension of 75% balanced. This new strain limit has some interesting ramifications for the design of new beams or the capacity of existing beams that structural engineers will need to know. There are clarifications that will probably eventually be part of the 318-02 commentary or will appear in future reinforced-concrete texts. This paper shows, through several numerical examples, how the new strain provision may affect capacity.