Title: Over-Reinforced Cross-sections In AS3600 And The Effects Of High Strength Concrete

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Publication: CIA

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Date: 2/13/2011

Abstract:
When high percentages of reinforcement are present in a cross-section subjected to flexure, it is possible that the reinforcement does not yield and failure occurs through compressive failure of the concrete. This type of failure is considered non-ductile and undesirable. In AS 3600, a cross-section is considered to be over-reinforced if the neutral axis depth ratio (ku) is less than a prescribed amount. To account for this nonductile behaviour which is associated with compressive failure of the concrete, AS 3600 reduces the global capacity reduction factor (φ) below that determined for the ductile behaviour which is governed by tensile behaviour of the steel. The magnitude of the reduction in the capacity reduction factor is related to the reduced ultimate strength (Mud). Current Australian design practice tends to avoid the design of over-reinforced cross-sections through the introduction of additional compressive reinforcement. This compressive reinforcement reduces the value of ku to below the prescribed limit, thus avoiding the significant reductions in design strength through the reduction in the capacity factor from 0.8 to as low as 0.6. This paper investigates current design practice in various concrete standards for the determination of the capacity reduction factors.