Shear in Prestressed Concrete Bridge Girders
Prestressed concrete I-beams are used extensively as the primary superstructure components of bridges in highways. This workshop intends to address one of the most troublesome problems in prestressed concrete beams: shear. Central to the problem is the lack of a rational model to predict the behavior of prestressed concrete structures under shear and the various modes of shear failures. Because of this deficiency, all existing shear design provisions, including those in the ACI Codes and ASSHTO Specifications, are empirical, complicated and have severe limitations. In this workshop we will present complementary theoretical and experimental studies, which are divided into two parts: 1. To establish the constitutive laws for prestressed concrete membrane elements and to develop an analytical model for predicting the shear behavior of such elements. 2. Several series of full-scale presetressed beams 25 ft long were tested to study their behavior in web shear and flexural shear failures.
The test results and the shear analysis of the prestressed beams were used to develop a new and simple equation for shear design of prestressed girders. In this equation, the shear capacities of prestressed beams are a function of the compressive strength of concrete and the a/d ratio of the beams. The amount of prestressing force and the angle of the failure planes were neglected because they were found to have insignificant effect on the ultimate shear capacity. The new equation was supported by test results of other prestressed beams available in literature. The predicted shear capacities of all the beams were then compared with the shear capacities calculated using the ACI and the AASHTO shear provisions and were found to be more reasonable.
Prestressed Concrete (PC) I-girders are used extensively as the primary superstructure components in Texas highway bridges. A simple semi-empirical equation was developed at the University of Houston (UH) to predict the shear strength of PC I-girders with normal strength concrete through the project TxDOT 0-4759. The UH-developed equation is a function of shear span to effective depth ratio, concrete strength, web area and amount of transverse steel. This report intends to (1) validate the UH-developed equation for high strength concrete by testing ten 25-feet long full-scale PC I-girders with different concrete strength and to (2) validate the UH-developed equation for different sizes of PC girders and studying the possibility of having premature failure due to local failure in end zone.
To extend the developed equations for shear design of prestessed beams to high strength concrete, ten Tx28 PC girders were tested. The girders were divided into three groups (namely Groups A, C and F) based on the concrete compressive strength. Group A consisted of two girders with a concrete compressive strength of 7000 psi. Group F had four girders with a concrete compressive strength of 13000 psi and Group C included four girders with a compressive strength 16000 psi. Girders in Group A were designed to have a balanced condition in shear. A pair of girders each belonging to Group F and Group C were designed to have a balanced condition while remaining girders were designed as over-reinforced sections. Each group of the PC girders was tested with different shear span to effective depth ratio so as to get two types of shear failure modes, i.e. web-shear and flexure-shear. The validity of the proposed UH-developed equation was ascertained using the girders test results. UH-developed equation was found to accurately predict the ultimate shear strength of PC girders having concrete strength up to 17,000 psi with sufficient ductility.
In addition, six PC girders of Tx-series with three different sizes were also tested. The girders were divided into three groups (namely Groups D, E and G) based on the girder depth. The test data shows that the PC girders of the new Tx-series has no cracks under service loads and can reach the maximum shear capacity without having a shear bond failure. Also, these girders’ test results ensured the validity of the UH-developed equations for PC girders with different sizes.
Simulation of Concrete Structures (SCS), a finite element program recently developed at UH, was used to predict the shear behavior of the tested girders. Analytical results proved the validity of SCS to predict the behavior of PC girders with different concrete strength up to 17,000 psi and with different depth up to 70 inches.