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Posted 17 January 2013
Two-way slabs are unique to reinforced concrete (RC) construction. The most common type, due to its ease of forming and speed of construction, is the flat plate, a slab of uniform thickness supported by columns without beams, drop panels, or capitals. Flat plates are common in building construction and can also be found as deck components in waterfront piers and wharves.
The design of RC flat plates is generally governed by serviceability limits on deflection, or by the punching shear capacity of the slab at the slab-to-column interface or at locations of concentrated loads. Punching may occur before (brittle) or after (ductile) a yield line mechanism has formed in the slab around the column. Brittle punching is undesirable because there is little warning of the impending failure.
American Concrete Institute 318 (ACI 318) has used the same equation to calculate nominal concentric punching shear capacity of two-way RC slabs for nearly 50 years. However, with the increasing use of higher strength steel reinforcement and concrete, the equation is facing increasing scrutiny from researchers and practitioners, especially in the case of lightly reinforced thick slabs.
In the January issue of STRUCTURE magazine, “Addressing Punching Failures” thoroughly examines the design philosophy behind the ACI 318 punching shear design provisions for two-way slabs, discusses perceived shortcomings, and suggests ways to improve the existing code provisions. The article is part of a larger effort undertaken by Carlos E Ospina, BergerABAM senior project manager, and Neil Hawkins, emeritus professor, University of Illinois, aimed at improving this ACI 318 code section.