Analytical, Numerical and Experimental Analysis of the Helperzoom Post-Tensioned T-Girders

Abstract

The Helperzoom bridge is one of few existing prestressed slab-between-girder bridges that is being managed by Rijkswaterstaat. These bridges are built before 1980 and do not comply with the current standards. To evaluate the structural safety and capacity these bridges are being assessed. A total of four girders were taken from the Helperzoom bridge, named HPZ1 through HPZ4, and have been destructively tested at TU Delft to determine the shear capacity and failure mode. The first two experiments, HPZ1 and HPZ2, have been considered in this research. Shear-tension failure is one of the failure modes that was being looked for and where the experiments were based on. The most important results of HPZ1 and HPZ2 have been combined and summarized in the experimental analysis, such as the failure load, the failure mechanism, the load at which the first inclined crack occurred, the cracking angles of the observed cracks, the calculated inclinations of the compression field from the LVDT measurements, the contribution of the shear reinforcement and the contribution of the prestressing cables to the shear capacity. Remarkably, the outcome of the test resulted in shear-compression/flexure-shear failure instead of the predicted shear-tension failure. The purpose of this thesis is to check whether the observed shear capacity and failure mode from HPZ1 and HPZ2 can be validated with existing models that are currently used by (inter)national standards and numerical programs. The Euler-Bernoulli beam theory and five (inter)national codes: ACI 318-14 (American Concrete Institute), AASHTO 8th Edition (American Association of State Highway and Transportation Officials), NEN-EN 1992-1-1:2011 (Eurocode 2), RBK 1.1:2013 (Richtlijnen Bestaande Kunstwerken) and EC2 draft 2018 (prEN 1992-1-1:2018 D3), have been used to calculate the shear capacity and if possible the failure mode of the Helperzoom girders. Next to the nominal shear resistance, failure of the compression field is considered for three codes: AASHTO, EC2 and EC2 draft 2018. For the two Eurocodes the value of the inclination of the compression field can be chosen by the user: the EC2 has a lower limit of theta = 21.8°, the lower limit of the EC2 draft 2018 (based on state of strains) is reached when theta is chosen such that failure of the compression field occurs simultaneously with yielding of the stirrups. For the EC2 draft 2018 different ways have been used to determine the longitudinal strain: with the expression given in the code, analytically determined with a sectional analysis and with the results of the LVDTs in the two experiments. In addition to the analytical calculations, a non-linear finite element program Response-2010 has been used, with which a cross-sectional analysis was made with two values for the prestressing force. With this program it is easy to determine the maximum shear capacity, the failure mechanism and many other results per loading step, such as the inclination theta, acting and maximum compressive- and tensile stresses, strains, etc. The maximum shear capacity is divided into components: the contribution of the uncracked concrete, cracked concrete, transverse reinforcement and a vertical prestressing force. Four different shear capacities have been calculated: the inclined cracking load, the nominal shear resistance according to the general procedures of the design codes, failure of the compression field and the moment that failure of the compression field occurs simultaneously with yielding of the stirrups. The Euler-Bernoulli beam theory, ACI (flexure-shear) and Response-2010 show that the load for the first inclined crack are close to the results of HPZ1 and HPZ2. For the nominal shear capacity, none of the codes nor Response-2010 are in line with the results of the two tests: all shear capacities are underestimated. The AASHTO overestimates the shear capacity for failure of the compression field,
because the inclination theta is not included in the expression. The EC2 overestimates failure of the compression field due to limitation of theta = 21.8°. The EC2 draft 2018 allows for lower values for theta, when the longitudinal strains are based on the state of strains with theta lower than 19.08°. The results for the shear capacity and inclination of the compression field of the latter design code are much more in line with the observed failure loads and inclinations calculated from the LVDTs. The lower limit for the shear capacity is when failure of the compression field occurs simultaneously with yielding of the stirrups. With this limit, the shear capacity is underestimated and the inclination of the compression field is lower than the inclination calculated from the LVDTs.
The results between the three analyses have been compared with each other and it was concluded that there are major differences between the analyses for the calculated shear capacities, internal lever arms, critical positions and the inclinations of the compression field. None of the results from the codes match the observed failure load in the experiments. Considering the expressions given for failure of the compression field, the EC draft 2018 based on the state of strains were closest to the test results for both the shear capacity and the calculated inclinations of the compression field from the LVDTs. The moment that yielding of the stirrups occurs simultaneously with failure of the compression field, the EC2 draft 2018 gave too low values for the shear capacity. For the numerical results, Response-2010 gave a representative shear force for the first inclined crack and failure mechanism, but not for the ultimate shear capacity. After the onset of the first inclined crack, the girder did not build up extra capacity as was observed in the experiments. The critical position was also not in line with the results from the test. It can be concluded that the results should always be looked at from a critical point of view and that no clear answer can be given as to which analysis is the "best" to determine the shear capacity of (prestressed) concrete T-girders. For the Helperzoom girders the EC2 draft 2018 based on the state of strains with theta lower than 19.08° gave the best results for the shear capacity.