Abstract

One of the best alternatives for bridges which need accurate load rating, is through field testing. Presented in this dissertation are two studies on evaluating the performance of bridges based on the results of a diagnostic load test. The first topic is focused on load rating bridges without plans using the results of a diagnostic test. The second topic is focused on utilizing measured displacements from a field test to evaluate the bridge performance.

Many short-span reinforced concrete bridges, particular slab bridges, exist throughout the United States, which were constructed decades ago. Although little or no design documentation exists for some of these bridges, many are still in service and performing well. Analytical load-rating methods for these bridges tend to be very conservative. Two procedures, the "Steel area method" and the "Simplified method" have previously been developed at the University of Delaware, to assist in load-rating bridges without plans. In this dissertation, the steel area method is extended to more general load cases, like that used in a diagnostic load test. The method is also verified in the laboratory using four large reinforced concrete beams. A procedure for load rating bridges without plans based on the steel area method is proposed. The results of a field test that was conducted on a concrete slab, for which plans are available, show the average percent error for the estimated steel area are -31% and 38% by virtue of the measured strain and displacement data, respectively. In addition, load-rating bridge without plans based on the simplified method was also illustrated.

In this dissertation, three methodologies were developed to determine the experimental displacement influence line (DIL) of a bridge from semi-static diagnostic load tests. The first is referred as the Continuous DIL Method, and is based on basic beam theory and least square theory. The second is referred to as the Discrete DIL Method, and is based on the least square theory and interpolation. The third is referred to as the Decomposition Method, and is based on the principle of superposition. All three methods have been successfully verified by field tests. The results show that the experimental DILs can be used to predict the deflection responses under various trucks.

Application of the experimental DIL is also investigated. A displacement-based load-rating equation, which is equivalent to the force-based equation in the elastic range, is established; results of the experimental DIL can be utilized in this equation very easily. Field verification shows that the differences between the load rating factors from the displacement-based load-rating equation and from the LRFR approach are within 12%. A method for deriving the stiffness matrix of the bridge, using experimental DILs is presented. The results from the field tests are consistent and comparable. Finally, a load-rating equation based on the deflection criteria is proposed. The numerator is specified in the AASHTO Specifications or other codes. The denominator is the live load deflection plus dynamic effect, which can be either theoretically calculated or calculated based on the experimental DILs. The experimental verifications on Bridge 1-911S and 2-063 using the load rating equation based on deflection limits show that the real stiffnesses of these two bridges are greater than predicted by theory.