Foundation, Concrete and Earthquake Engineering


A minimum specified concrete strength f΄c of 3000 psi and a maximum specified reinforcement yield strength fy of 60,000 psi are mandated. These limits are imposed as reasonable bounds on the variation of material properties, particularly with respect to their unfavorable effects on the sectional ductilities of members in which they are used. A decrease in the concrete strength and an increase in the yield strength of the tensile reinforcement tend to decrease the ultimate curvature and hence the sectional ductility of a member subjected to flexure. Also, an increase in yield strength of reinforcement is generally accompanied by a decrease in the ductility – as measured by the maximum deformation –of the material itself.

There is evidence suggesting that lightweight concrete ranging in strength up to 12,500 psi can attain adequate ultimate strain capacities. Testing to examine the behavior of high strength, lightweight concrete under high intensity,cyclic shear loads, including a critical study of bond characteristics,has not been extensive in the past. However, there are test data showing that properly designed lightweight concrete columns, with concrete strength ranging up to 6200 psi, maintained ductility and strength when subjected to large inelastic deformations from load reversals. It was felt by committee 318 that a limit of 4000psi on the strength of lightweight concrete was advisable, pending further testing of high strength lightweight concrete members under reversed cyclic loading. Note that lightweight concrete with a higher design compressive strength is allowed if it can be demonstrated by experimental evidence that structural members made with that lightweight concrete posses strength and toughness equal to or exceeding those of comparable members made with normal weight concrete of the same strength.

However, ASTM A 615 billet steel bars of Grade 40 or 60 may be used in these members if the following two conditions are satisfied :

Actual fy ≤ specified fy +18,000 psi
(actual ultimate tensile stress)/( actual fy) ≥12.5.
The first requirement helps to limit the magnitude of the actual shears that can develop in a flexural member above that computed on the basis of the specified yield value when plastic hinges from at the ends of a beam. The second requirement is intended to ensure steel with a sufficient long yield plateau.

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