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This study evaluated the pure confinement effect of steel spirals embedded in concrete with varying compressive strengths. The experimental variables in the study were the compressive strength of concrete and the yield strength and volumetric ratio of the steel spirals. To estimate the pure confinement effect of the steel spirals, all the specimens were designed to exclude concrete cover and longitudinal reinforcement. The experimental results showed that the confinement effect of spirals increased with yield strength and volumetric ratio of the spirals, but greatly decreased as the concrete compressive strength increased. Using the relationship between axial and lateral strains at peak stress of spirally confined concrete, an analytical model was developed to predict the stress–strain relationship of confined concrete. The proposed model can be successfully used to predict the structural behaviour of concrete confined by normal-, high- and ultra-high-strength steel spirals, regardless of the compressive strength of concrete.

Monotonic shear loading tests were conducted on three half-scaled reinforced concrete deep beams with shear span-todepth ratios of 0.5 to 0.75. The obtained test results were investigated in detail based on the experimental measurements and finite element analysis. From these investigations, a new macro model for deep beams was established. This model is composed of two crooked main struts formed between both beam end sections and branched-off sub struts. The compressive force introduced to main struts balances the flexural compression and the external shear force. The bond stress of the longitudinal reinforcement and the tensile force of the stirrup produce the diagonal compression in the sub strut.Theoretically predicted shear strengths of tested deep beams showed good agreement with experimentally observed shear strengths, where the effective strength of concrete was assumed to be 75% of the cylinder strength.

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