Shear Strength of Deep Hollow-Core Slabs

Hollow-core (HC) slabs are the most commonly used and economical precast/prestressed concrete flooring system. HC slabs produced in the U.S. market typically have untopped depth ranging from 6 to 12 in. (152 to 305 mm). Recently, deeper HC slabs (for example, 16 in. [406 mm]) have been produced to satisfy the growing need for longer spans and/or heavier loads. ACI 318-14, Section 7.6.3.1, requires a minimum shear reinforcement to be provided where ultimate shear force is greater than 50% of the factored concrete web shear strength (φVcw) for untopped HC slabs deeper than 12.5 in. (318 mm). The 50% modification factor was introduced in 2008 based on a limited testing conducted by HC suppliers on deep HC slabs. This paper briefly summarizes parameters that affect concrete web shear strength of deep HC slabs and presents the results of shear and flexure testing of 16 in. (406 mm) HC slabs. Ten different shear strength provisions adopted by ACI 318-14, AASHTO LRFD 2014, CSA A23.3-04, JSCE 2007, fib MC 2010, AS 3600-2009, EN 1168, and Yang's method were compared using a database of 51 web shear tests (12 conducted by the authors and 39 obtained from the literature) of deep HC slabs. Comparison results indicated that the 10 shear strength provisions vary significantly with respect to the accuracy and consistency of their predictions and, therefore, different modification factors need to be used.