Question

Explain why the concept of shear ductility is revised through a maximum capacity to shear and why this is a function only of the strength of the concrete.

Answer 1

An experimental investigation was conducted to study the ductility of shear-predominant (shear-critical) reinforced lightweight concrete beams of normal as well as high strength concrete.

A total of 15 shear-critical reinforced concrete beams without and with shear (web) reinforcement were tested, in a ‘stiff’ testing facility, and complete load-mid span deflection curves including the post-peak portion were obtained. The experimental variables were the concrete compressive strength, shear span-depth ratio and the amount of shear reinforcement. Concrete strength (∝c) was varied between 30.5 MPa (4430 psi) and 89.3 MPa (12950 psi). The shear span-depth ratio () was varied between 1 and 4 and the shear reinforcement ratio (γyW) was varied between 0 and 0.784%

For the range of variables tested, the results indicate that for beams with or without shear reinforcement, the shear ductility index (μ) decreases with an increase in the concrete strength.

The effect is more pronounced for beams with of 3 as compared to beams with of 1. Normal strength concerete beams with of 3 exhibited a near plastic post-peak response, when the shear reinforcement provided was about five times that of the minimum amount required by Section 11.1.21 of the ACI 318-89 Code. Increasing the shear reinforcement ratio (γW) up to 0.51% has an insignificant effect on the shear ductility index of beams with of 1. However, for beams with of 2 and 3, the shear ductility index increases. For beams with of 3, increasing the shear reinforcement ratio from 0.51 to 0.65% increases the shear ductility index by 25%. Further increase in the shear reinforcement ratio does not increase the shear ductility index.