Brock D. Hedegaard

Instrumentation and Modeling of I-35W St. Anthony Falls Bridge

The I-35W St. Anthony Falls Bridge was constructed to replace the steel truss bridge that collapsed on August 1, 2007. The design of the replacement bridge featured a smart-bridge system. The smart-bridge concept included instrumentation for long-term monitoring of the structural behavior of the bridge. Truck-load tests were conducted prior to opening the bridge and 26 months later to measure the response of the structure under controlled loading. The measurements were used to validate a FEM model of the bridge constructed to further investigate the behavior of the structure. The correlation between computed and measured results was found to be good. This paper describes the bridge, the instrumentation installed within the bridge, the FEM model, and validation of the model with respect to the truck-load tests. Recommendations are provided for static instrumentation plans of concrete box-girder structures.

Investigation of Thermal Gradient Effects in the I-35W St. Anthony Falls Bridge

Thermal gradients were measured through the section of the I-35W St. Anthony Falls Bridge, a posttensioned concrete box girder bridge in Minneapolis, Minnesota, over the course of 3 years. The magnitudes and shapes of the measured thermal gradients were compared with various design gradients, and a fifth-order curve was found to best approximate the shape of the gradients. The responses of the structure to the largest measured thermal gradients were compared with stresses and deformations predicted by finite-element modeling given applied design gradients. The measured structural response was found to be best predicted when the finite-element model of the bridge was subjected to a fifth-order design thermal gradient scaled to match maximum top surface temperature values proposed by AASHTO LRFD Bridge Design Specifications for the region. Stresses and deformations predicted by finite-element modeling using the AASHTO LRFD bilinear design gradients were found to be considerably lower than those derived from measured results. Recommendations for design thermal gradients are proposed.

Time-Dependent Monitoring and Modeling of I-35W St. Anthony Falls Bridge. I: Analysis of Monitoring Data

AbstractFor the successful implementation of long-term monitoring strategies of prestressed concrete structures, the expected behavior of the structure must be accurately quantified before anomalou...

Effects of Cyclic Temperature on the Time-Dependent Behavior of Posttensioned Concrete Bridges

AbstractTypically, measurement and analysis of concrete time-dependent behaviors such as creep and shrinkage are performed under the assumption of constant temperature conditions. However, many structures in the field are subject to variable seasonal and daily temperatures. This paper explores how changes in temperature affect the time-dependent behavior of concrete structures, with a particular focus on posttensioned concrete bridges. Temperature-dependent creep, shrinkage, aging, and relaxation models were incorporated into structural finite-element analyses examining a posttensioned concrete beam under variable thermal loading. The impacts of uniform temperature changes on the time-dependent deflections, strains, and stresses were accounted for using an Arrhenius-adjusted age based on the structure temperature. However, nonuniform temperature changes, such as thermal gradients through the depth of the cross-section, caused time-dependent behavior that could not be accounted for using the Arrhenius-adju...

Long-Term Monitoring Strategy for Time-Dependent Deflections of Posttensioned Concrete Bridges

AbstractA monitoring strategy for investigating the long-term longitudinal deflections at the expansion joints of the I-35W St. Anthony Falls Bridge was developed as a means of inferring the integr...

Smeared-Bar Model for Viscoelastic Analysis of Uncracked Reinforced Concrete Structures

This paper presents a technique for computing the time-dependent behavior of reinforced and prestressed concrete as a composite material. This technique, when implemented in a finite-element model and assuming that the concrete and steel reinforcement act as a composite material, is computationally advantageous compared with explicitly modeling the concrete and steel materials separately. The method is developed assuming linear viscoelasticity and uncracked sections. The approach starts by first approximating the creep compliance functions for the viscoelastic concrete as a Kelvin chain model. This approximation allows the viscoelastic behavior to be framed as a rate-type creep law, which converts the analysis to an equivalent elastic problem, simplifying the computations. This approach, originally developed for plain concrete, is extended in this paper to account for the effects of linear elastic reinforcement. Several implementation examples are provided documenting the viability of the method for problems of uniaxial, multiaxial, and bending behaviors. For the presented cases, the composite method is shown to provide similar results compared with models containing explicitly modeled reinforcement. The paper concludes with a discussion regarding how to extend the methodology to the general case with linear viscoelastic reinforcement and matrix materials.

Modeling and Prediction of Time-Dependent Deformations of the I-35W St. Anthony Falls Bridge

To properly identify anomalous data in a structural monitoring system, the expected behavior of the structure under changing environmental conditions must be accurately determined. The St. Anthony Falls Bridge, a post-tensioned concrete box girder bridge, was examined as a case study for testing a data normalization scheme for temperature- and time-dependent deformations of an in situ structure. A methodology based on linear regression was developed to extract time-dependent behavior from the measured data. An Arrhenius-adjusted time was proposed to correct for the slowing and accelerating creep and shrinkage rates during the winter and summer, respectively. The extracted time-dependent deformations were compared to estimates using the finite element method. Bayesian regression was used to update the finite element model predictions using the measured data, substantially narrowing the expected bounds and facilitating anomaly detection.

I35W collapse, rebuild, and structural health monitoring - challenges associated with structural health monitoring of bridge systems

During evening rush hour traffic on August 1, 2007, the major interstate highway bridge carrying I35W over the Mississippi River in Minneapolis catastrophically failed, tragically taking the lives of thirteen people and injuring many more. The steel truss bridge, constructed in 1967, was undergoing deck reconstruction during the collapse, and was estimated to carry more than 140,000 vehicles daily. This tragedy generated great interest in employment of structural health monitoring systems. The I35W St. Anthony Falls Bridge, a post‐tensioned concrete box bridge constructed to replace the collapsed steel truss bridge, contains over 500 instruments to monitor the structural behavior. Numerical models of the bridge are being developed and calibrated to the collected data obtained from truck load tests and thermal effects. The data obtained over the first few years of monitoring are being correlated with the calibrated models and used to develop the baseline bridge behavior. This information is being used to develop a system to monitor and interpret the long‐term behavior of the bridge. This paper describes the instrumentation, preliminary results from the data and model calibration, the plan for developing long‐term monitoring capabilities, and the challenges associated with structural health monitoring of bridge systems. In addition, opportunities and directions for future research required to fully realize the objectives of structural health monitoring are described.During evening rush hour traffic on August 1, 2007, the major interstate highway bridge carrying I35W over the Mississippi River in Minneapolis catastrophically failed, tragically taking the lives of thirteen people and injuring many more. The steel truss bridge, constructed in 1967, was undergoing deck reconstruction during the collapse, and was estimated to carry more than 140,000 vehicles daily. This tragedy generated great interest in employment of structural health monitoring systems. The I35W St. Anthony Falls Bridge, a post‐tensioned concrete box bridge constructed to replace the collapsed steel truss bridge, contains over 500 instruments to monitor the structural behavior. Numerical models of the bridge are being developed and calibrated to the collected data obtained from truck load tests and thermal effects. The data obtained over the first few years of monitoring are being correlated with the calibrated models and used to develop the baseline bridge behavior. This information is being used to d...