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Title: Effect of Flexural Reinforcement Ratio, Effective Depth and Slenderness Ratio on Punching Strength or Spread Footings

Author(s): Neil M. Hawkins and Carlos E. Ospina

Publication: Symposium Paper

Volume: 353


Appears on pages(s): 50-72

Keywords: Spread Footings; Footings; Foundations; Punching Shear; Shear Design; Size Effect

DOI: 10.14359/51737111

Date: 7/1/2022


The 2019 Edition of ACI 318 introduced several key changes to the one-way and two-way shear design of slabs. Chapter 22 introduced a size effect for the one-way and two-way shear strength of concrete for slabs either without or with shear reinforcement. ACI 318-19 also introduced a revised version of the Vc equation for one-way shear that included the effect of the member flexural reinforcement ratio. Chapter 8 of ACI 318-19 also introduced new minimum flexural reinforcement requirements to prevent premature punching failure of lightly-reinforced above grade two-way slabs and to promote instead a flexure-driven type of response. In ACI 318-19, recognition of the size effect is required for above grade slabs only and not for shallow footings or foundation mats. The Code is silent on whether the new Chapter 8 minimum flexural reinforcement requirements for slabs apply to spread footings.

This paper examines the reasons for the differences in size effect requirements between slabs and footings. It also evaluates the applicability and the impact that the new Vc equation for one-way shear has on the shear strength of isolated or continuous footings and whether the new flexural reinforcement requirements of Chapter 8 can be extended to spread footings. It also suggests a design recommendation to acknowledge the beneficial effect that compact footing dimensions can have on punching strength.