Direct shear behavior of composite concrete girders with tall haunches




Rutenberg, Shay P.

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Composite bridges constructed using prestressed concrete (PSC) girders and concrete slabs may require haunches—a region of concrete that connects the bridge deck and girders—to maintain a uniform deck thickness despite camber, cross-slope, construction errors, and perhaps other reasons. The Texas Department of Transportation (TxDOT) provides structural design guidance for a limited range of haunch heights, though not for tall haunches that exceed 3.5 in. Gravity loads on composite bridges induce flexure in the concrete slab and PSC girders. The overall strength and stiffness of the bridge deck system depend upon the properties of the composite connection between the reinforced concrete (RC) deck and PSC girders, which relies on shear transfer at the interface between these components. Push-out tests idealize this shear transfer behavior by isolating a part of the deck and girders and then subjecting the interface to direct shear loading. The shear transfer parameters between the PSC girders and deck include reinforcement detailing, friction, and cohesion. Experimental tests at the Ferguson Structural Engineering Laboratory (FSEL) examine the effects of design variables such as haunch height, material properties, and reinforcement detailing on the shear capacity and failure modes of composite push-out test specimens. Parametric studies (validated with the experimental tests) examine the effects of a wider array of design variables than possible with experimental tests alone. The parametric studies conducted for this project rely on commercial finite element analysis (FEA) software to create three-dimensional, symmetric models of the experimental push-out test specimens. The results from this project show that the capacities of composite bridge decks with PSC girders and cast-in-place slabs depend on the haunch depth, concrete compressive strength, surface roughness, and reinforcement detailing at the shear transfer interface. Changes to the reinforcement spacing, size, and type show limited influence on the strength of these specimens but can influence their ductility. Tapering a specimen’s haunch increases its haunch width and capacity compared to a specimen without a tapered haunch. Specimen strength increases as a function of girder concrete compressive strength but is not significantly influenced by the haunch concrete compressive strength. Validation efforts indicate that PSC girder specimens' shear strengths and failure modes depend on the friction and cohesion properties between the concrete surfaces at the composite interface. Parametric studies demonstrate that the strength of concrete girder bridges with composite decks in direct shear are not sensitive to variations in haunch height, so the current TxDOT provisions reasonably neglect this variable. Composite concrete girder bridges may experience brittle debonding failure at the composite interface when subjected to direct shear, necessitating the use of a shear strength reduction factor. Future research can investigate ways to increase composite friction, cohesion, and haunch width as methods to improve the ductility and strength of composite concrete bridges.


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