Reliability Analysis of Prestressed Concrete Bridges in Mexico: Assessment and Live Load Factors Proposal

Abstract

The methodologies to predict the safety levels of highway bridges must consider suitable live load models from real and critical traffic conditions. This research is focused on the reliability assessment of prestressed simply supported Mexican bridges, which are the most common structures for short and medium spans in Mexico. The safety levels of highway bridges are determined to show the impact generated by two different kinds of live models on using prestressed type AASHTO I beams in Mexico, with spans of 10, 15, 20, 25, 30 and 35 m. These safety levels are obtained using a probabilistic approach based on the reliability theory. Selected highway bridges are designed according those procedures proposed by the Mexican Institute of Transportation rules and AASHTO LRFD bridge specifications, which are commonly used by Mexican bridge designers. For all the cases, results of IMT-66.5 live load model achieve higher safety levels than T3-S2-R4 truck. Shear reliability analyses indicate high level overdesign using Mexico City design code, leading to high safety levels for all evaluated cases. For bending moment analysis, the bridges with spans higher than 20 m have reliability indices under 3.5 for both live load models and a traffic scenario with 35% trucks. Therefore, we propose new live load factors for bridges with spans longer than 20 m to achieve the target reliability index of 3.5. For traffic scenarios with 18% and 35% trucks, proposed live load factors for IMT 66.5 and T3-S2-R4 trucks have increments of 6.7% and 7.8%, and 16.7% and 16.8%, respectively. An overall methodology was also proposed to be applied in other places with similar truck overloading problems for evaluating new and current bridge infrastructure.