Differential temperature and creep effects on prestressed bulb-tee girder anchorage zones
Kristam, Rama Pranav
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Provisions of AASHTO LRFD Bridge Design Specifications for pretensioned anchorage zones do not take into account concrete stresses due to temperature changes during curing, and storing of pretensioned bulb-tee girders. Due to high prestressing forces, anchorage zones of deep bulb tee girders frequently crack at prestress transfer. These cracks are observed to increase in size after de-tensioning is complete, during storing of girders. Stresses created by the differential cooling of web and flanges of a bulb tee after curing may be the reason for increased cracking, in addition to creep and shrinkage of concrete. This study investigated the impact of differential temperature and creep for bulb-tee cross section on anchorage zone crack propagation using nonlinear finite element modeling. Nonlinear material properties in tension and compression were used to accurately reflect the cracking behavior of concrete. Temperature stresses were simulated using a coupled temperature displacement analysis with applied prestress loads. Differential cooling of web and flanges were replicated in the models by using temperature data from literature during casting, and during and shortly after prestress release. Finite element models were validated using the anchorage zone concrete and reinforcing bar strain data from literature. Creep strains were modeled for a period of three months on the girders. Comparisons between anchorage zone stresses with and without temperature effects and with and without creep strains were reported using the validated finite element models. A parametric study was conducted to understand the impact of thermal properties of concrete on anchorage zone stresses. Recommendations were provided for incorporating differential temperature changes and creep in anchorage zone design of AASHTO LRFD Bridge Design Specifications.