Finite Element Modeling of Pretensioned Concrete Girders: A Methodological Approach with Applications in Large Strands and End Zone Cracking Open Access
Two issues pertaining to the current state of pretensioning industry are investigated in this research: spacing of 0.7-in. diameter strands and effectiveness of end zone reinforcing details. Given the current economic constraints and the limitation of testing facilities, there is a great need for reliable and unified methodological approaches for simulation of the behavior of pretensioned concrete members to complement experimental investigations. Therefore, two finite element techniques are evaluated: extrusion and embedment techniques. Once the techniques are verified and validated against closed form solutions and experimental data, respectively, they are utilized for analytical purposes of this research. Extrusion technique is utilized to show that 0.7-in. diameter strands can be potentially spaced at reduced spacing of 2-in. on center under certain conditions including minimum concrete compressive strength of 10,000 psi at the time of release. Embedment technique is utilized for comparative investigation of four end zone reinforcing details: AASHTO LRFD , NCHRP Report 654, Illinois and combined AASHTO-Illinois details. The research indicates that NCHRP Report 654 offers an optimum end zone remedial scheme for typical AASHTO/PCI bulb-tee girders. Without requiring any additional reinforcement in comparison to AASHTO LRFD, it results in fabrication-friendly rebar spacing. Finally, eight Washington WF100 Super-girders are instrumented and investigated for potential end zone cracking as part of the Alaskan Viaduct Project in the state of Washington. Each girder is 100 in. deep and over 200 ft long among the largest precast girders in North America at the time of this research. The instrumentation set-up is intended for on-site collection of strain data at the end zone reinforcing bars and the 0.6-in. diameter strands. The experimental observations indicate that the response of the super-girders at release is similar to other I-girders commonly used in practice. In addition, all eight girders experienced different levels of end zone cracking most severely along the web-bottom flange interface. A new closed-form solution is proposed and validated against the experimental results, based on the shear-friction theory to estimate the tensile cracking at the web-bottom flange interface of precast I-girders immediately after the release of pretensioning.
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