Clay
Naito, Ph.D., P.E. and Jonathan D. Kovach, MSSE
National Center for Engineering Research on Advanced Technology for Large Structural Systems
The research of this report investigates the horizontal shear stress of composite concrete beams without horizontal shear ties. Typically, in composite bridge and building construction shear ties are placed across the web-slab interface to help maintain monolithic behavior of the section once the bond or cohesion is lost between the concrete surfaces. The current standards almost always require that these shear ties are present in composite construction and give very little consideration to the horizontal shear resistance provided by the concrete interaction alone. Therefore, the current requirements prescribed by ACI and AASHTO provide a conservative estimate to the shear capacity of composite concrete sections without horizontal shear ties. This research program examines the feasibility of increasing the allowable horizontal shear capacity between a precast, prestressed concrete web and a cast-in-place concrete slab without interface reinforcement.
A series of structural tests were conducted on composite prestressed beams without horizontal shear ties. The beams were designed and fabricated to represent sections which are typical for composite concrete construction. The contribution to the horizontal shear capacity provided by the roughness of the interface surface finish and the compressive strength of the slab concrete were investigated. Several specimen of each combination of the research variables were fabricated and tested in order to achieve repeatable results.
The horizontal shear stresses achieved from the tests ranged from 475 psi to 1000 psi which is considerably greater than the recommended value of 80 psi presented by the code for composite sections without interface reinforcement. It was concluded from these experiments that the interface roughness had a pronounced effect on the horizontal shear capacity of the composite section. The effect of the slab concrete compressive strength was found to be inconclusive. It was also found that when a relatively large time period occurred between the placement of the concrete slab and the precast web, differential shrinkage will occur which may initiate delamination between the pieces and decrease the composite action. In the end, recommended horizontal shear capacities of 435, 465, and 570 psi were made for composite concrete sections with a broom, as-placed, and rake surface finishes, respectively.
References
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National Center for Engineering Research on Advanced Technology for Large Structural Systems
The research of this report investigates the horizontal shear stress of composite concrete beams without horizontal shear ties. Typically, in composite bridge and building construction shear ties are placed across the web-slab interface to help maintain monolithic behavior of the section once the bond or cohesion is lost between the concrete surfaces. The current standards almost always require that these shear ties are present in composite construction and give very little consideration to the horizontal shear resistance provided by the concrete interaction alone. Therefore, the current requirements prescribed by ACI and AASHTO provide a conservative estimate to the shear capacity of composite concrete sections without horizontal shear ties. This research program examines the feasibility of increasing the allowable horizontal shear capacity between a precast, prestressed concrete web and a cast-in-place concrete slab without interface reinforcement.
A series of structural tests were conducted on composite prestressed beams without horizontal shear ties. The beams were designed and fabricated to represent sections which are typical for composite concrete construction. The contribution to the horizontal shear capacity provided by the roughness of the interface surface finish and the compressive strength of the slab concrete were investigated. Several specimen of each combination of the research variables were fabricated and tested in order to achieve repeatable results.
The horizontal shear stresses achieved from the tests ranged from 475 psi to 1000 psi which is considerably greater than the recommended value of 80 psi presented by the code for composite sections without interface reinforcement. It was concluded from these experiments that the interface roughness had a pronounced effect on the horizontal shear capacity of the composite section. The effect of the slab concrete compressive strength was found to be inconclusive. It was also found that when a relatively large time period occurred between the placement of the concrete slab and the precast web, differential shrinkage will occur which may initiate delamination between the pieces and decrease the composite action. In the end, recommended horizontal shear capacities of 435, 465, and 570 psi were made for composite concrete sections with a broom, as-placed, and rake surface finishes, respectively.
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