Sliding shear defined by the United States Army Corps of Engineers (USACE) and shear friction defined by the American Concrete Institute (ACI), the Precast/Prestressed Concrete Institute (PCI), and the American Association of State Highway and Transportation Officials (AASHTO) all refer to the same failure mode that consists of a sliding plane developing within a reinforced concrete structure that may occur at a lift line/construction joint, at geometric/material discontinuities, or at other stress riser locations or weakened planes where it is deemed appropriate to postulate a cracked interface.
The equations to calculate sliding shear resistance all start with the traditional shear friction model which was first described and documented in numerous publications in the 1960s. This model states that as slip occurs at the interface, separation will also occur due to roughness and irregularity of the crack, creating tension in the rebar crossing the shear plane and a resulting clamping force that may be additive to any net compression normal to the plane. This total normal force is multiplied by a friction factor that considers surface preparation/condition.
ACI and PCI guidance both use this model but differ on how the friction factor and upper limits on capacity are determined. AASHTO uses a slightly modified version that also includes the effects of cohesion across the interface. The USACE equation for seismic loading is identical to that in ACI; however, it adds a 0.25 coefficient on the area of steel crossing the shear plane, effectively reducing the steel clamping effects by a factor of 4. This has potentially significant ramifications on the calculated shear friction capacity and resulting conclusions made about the structure’s performance and probability of failure. USACE guidance is widely used for the design and evaluation of reinforced concrete hydraulic structures (RCHS), but there is very little clarity on the origins and reasoning behind the inclusion of this 0.25 coefficient.
This paper takes a deep dive into referenced literature that forms the basis for the equations set forth in these guidance documents, including tracking down details on the 0.25 coefficient. Based on the information reviewed in this study, suggestions are made to engineers-of-record, owners, and regulators for consideration when evaluating the shear friction failure mode in RCHS for code-based, deterministic or risk analyses, as well as design.
