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Title: Strength of Interface Shear Reinforcement with Limited Development Length

Author(s): Regina N. Waweru, Guillermo Palacios, and Shih-Ho Chao

Publication: Structural Journal

Volume: 115

Issue: 4

Appears on pages(s): 983-996

Keywords: box beam; composite beam; development length; embedded length; interface shear; pullout test; push-off test; shear friction; slab beam

DOI: 10.14359/51702061

Date: 7/1/2018

Abstract:
The ability of a composite beam to act as a monolithic member relies heavily on the bond between the precast beam and the cast-in-place (CIP) slab. Insufficient anchorage of the interface reinforcement could lead to direct pullout due to localized fracture of the surrounding concrete, thereby failing to develop the reinforcement’s yield stress. The short embedded length of approximately 2 in. (50.8 mm) used for interface shear reinforcement is a common practice for the box and slab bridge beams in 70% of the states in the United States. In this study, 18 push-off specimens were tested to evaluate the effect of clamping action of reinforcement on the friction component of the interface (or “horizontal”) shear capacity. In addition, 24 pullout specimens were tested to evaluate the effect of width and bend angle on the pullout strength of interface shear reinforcement. The tests conducted in this research indicated that the American Association of State Highway and Transportation Officials (AASHTO) equation used to account for interface shear in composite concrete beams can overestimate the contribution of the interface shear reinforcement to the friction force for composite beams with a short embedded length less than 4 in. (101.6 mm). Experimental results indicate that, while the clamping force is considerably lower for short embedded length, the presence of the reinforcement effectively engages the concrete in the CIP slab, which enhances the interlocking action and, hence, the overall shear resistance. A modification to the current AASHTO equation is proposed to provide more accurate estimation of the friction force from interface shear reinforcement with a short embedded length.