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Research Abstracts Online
January 2010 - March 2011

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University of Minnesota Twin Cities
College of Science and Engineering
Department of Civil Engineering

PI: Catherine E. French

I-35W Structural Monitoring and Investigation of Shear Distribution Factors in Prestressed Concrete Girder Bridges

These researchers are using MSI resources for two projects. In the first, they are monitoring the new I-35W bridge in Minneapolis. After the collapse of the old I-35W bridge, a new bridge was designed that includes instrumentation to understand the behavior of the new system and to compare it to assumed design models. The instrumentation consists of strain gauges in various cross sections of the bridge. These gauges measure strains, and the associated stresses can be computed for the instrumented cross sections. Additionally, thermistors have been placed to measure the thermal gradient through the depth of the box and through the thickness of the webs, bottom flange, and top flange at one of the sections. Accelerometers measure the dynamic bridge response to ambient and traffic forcing. The bridge is being modeled using ABAQUS. The measurements taken from the thermistors can be applied to the model to investigate the effects due to thermal gradients. Load tests were performed upon completion of the bridge construction. These load cases will also be applied to the model. A comparison of the measured data and model output can be used to investigate the validity of the assumptions used in design. It is anticipated that the actual behavior of the bridge will be different than that predicted by design. As a consequence, the model can be used to better interpret the implications of the measured data.

Bridges are also the focus of the second project. The Minnesota Department of Transportation (MNDOT) has found that a number of their prestressed concrete girder bridges rate low for shear. A better understanding of the distribution of shear in prestressed bridge girders is needed to more accurately assess load-rating capacity. As part of the project, finite element models will be constructed using ABAQUS. These models will be calibrated against bridges studied previously in the literature. After calibration, the models will be used to conduct a parametric study investigating the effect of girder spacing and size, end conditions, and type and location of diaphragms on shear live load distribution factors. This will aid in the development of recommendations for shear distribution factors that can be used by MNDOT. Investigation of these variables will typically take place when the bridge materials are undergoing elastic stress/strain, but inelastic behavior of a prestressed concrete girder bridge will also be studied. This model will require the most computing power.

Group Member

Ben Dymond, Graduate Student