Improved culvert load rating through an evaluation of the influence of cover soil depth, demand model sophistication, and live load attenuation method

Date

2015-12

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Abstract

This dissertation evaluates the influence of three factors – cover soil depth, demand model sophistication, and live load attenuation method – on the load rating of cast-in-place (CIP), reinforced-concrete (RC) box culverts. Concrete box culvert load rating appears simple but is quite complex. The governing federal policy, analysis principles, challenges, and the disconnect between load rating calculations and field inspection observations are discussed in detail. Cover soil depth above the culvert directly influences culvert load rating results in non-linear ways. A population of Texas Department of Transportation CIP RC culvert standard designs developed between 1930 and 1980 were load rated using AASHTO policy guidance and a direct-stiffness demand model for a full range of cover soil depths. Three typical rating vs. depth relationships are illustrated and described in detail. The distribution of characteristic rating vs. depth relationships based on culvert geometry, design cover soil depth, and design era are explored. Cover soil depth is shown to be a critical parameter that must be explicitly considered for the intelligent load rating and design of reinforced concrete box culverts. Demand model sophistication influences the accuracy and precision of culvert load rating calculations. Two production-simplified culvert load rating demand models were analyzed using live load test data from three instrumented reinforced concrete box culverts under four cover soil depths. The demand models were a structural-frame model and a soil-structure interaction model. As expected, increased sophistication in the soil-structure interaction model as compared to the structural-frame model resulted in higher precision and accuracy for predicted moments. Variations in predicted moment accuracy and precision were not uniform but are a function of the critical section location in the culvert structure. The soil-structure interaction model requires an out-of-plane, live load attenuation method; this method directly affects the accuracy and precision of the culvert load rating calculation. A new method, called the depth-calibrated method, attenuates out-of-plane live load to the critical section depths in a culvert. The depth-calibrated method improves current practice by increasing the accuracy and precision of live load demand predictions, particularly in culvert walls and bottom slabs. Use of the depth-calibrated method helps close the disconnect between calculated load rating and observed structural performance by more accurately predicting both the location of the weakest critical section and the live load magnitude. The effectiveness of the depth-calibrated method was evaluated by comparing predicted live load moments to measured live load moments obtained from published datasets from full-scale culvert load tests. A load rating example shows the improved alignment between load rating and observed performance. Understanding the influence of cover soil depth, modeling sophistication, and live load attenuation allows for more accurate and precise load rating of cast-in-place, reinforced-concrete, box culverts and better correspondence between load rating calculations and field inspection observations. This dissertation advances the state of production-simplified load rating practice and knowledge.

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Keywords

Culvert, load rating, reinforced concrete

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