Effective Strength Coefficients of Concrete Strut

The reinforcement required by above table is related to the tensile force in the concrete due to the spreading of the strut. Reinforcement placed to resist the splitting force restrains crack widths, allows the strut to resist more axial force, and permits some redistribution of force.

Above Figure shows two layers of reinforcement crossing a cracked strut. This reinforcement will help control cracking in a bottle-shaped strut and result in a larger strut capacity than if this distributed reinforcement was not included.

Often, this distributed reinforcement is difficult to place in structures such as pile caps. If this reinforcement is not provided, the value of β_s=0.60λ should be used.

Reference

1. American Concrete Institute (2014) Building Code Requirements for Structural Concrete (ACI 318M-14) and Commentary(ACI 318RM-14), Farmington Hills, Michigan, USA.