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Re: [SEAOC] Concrete Dilema

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The method of the Working Stress Design assumes both the materials, steel
and concrete, to be linear elastic.  This is applicable for serviceability

The method of the Ultimate Strength Design considers the state of failure
of an under-reinforced beam.  The steel has yielded and hence it is no more
linear elastic.  The concrete has reached the ultimate strain of 0.003 and
its stress-
strain curve cannot be approximated by a linear elastic relationship.  The
Whitney's stress block is commonly used, for simplification, to substitute
the actual stress- strain curve at failure.  The expression is thus
applicable at failure of an under-reinforced beam only.

Amlan K. Sengupta
Dept. of Civil Engg.
University of Missouri-Rolla