Design of Indirectly Loaded Large Footings for One-Way Shear

To better understand the shear behavior of large footings under indirect loading, where the support reaction does not apply a concentrated compressive force to the top or bottom surface of the member, five indirectly loaded large footing specimens are tested under the combined action of uniform loads that simulate the soil pressure and indirectly applied point loads. Comparison of these test results to previously tested companion specimens, which were directly loaded, showed that the shear response of indirectly loaded specimens to test variables is similar to the response of their directly loaded companions; however, indirectly loaded footings consistently failed at lower shear stresses. For short footings, this difference in shear capacity is observed to be less than what is suggested by current codes. Experimental results have shown that there is a distinct strut action also under indirect loading, which provides adequate shear strength even for thick, indirectly loaded footings. A strut-and-tie model is suggested and shown to give reasonable shear-strength predictions of indirectly loaded short footings. However, analytical studies have shown that as the shear slenderness of indirect footings increase, shear-strength predictions of long indirectly loaded specimens may become unsafe because ACI design procedures do not account for the size effect in shear. It is suggested that ACI minimum shear reinforcement exemption for footings be limited to indirectly loaded footings with shear slenderness ratios less than 2.5.