Fatigue load models for girder bridges.

Abstract

Bridge components experience millions of stress cycles which can cause the development and propagation of microscopic cracks in critical components. Fatigue damage, cumulative over time, has resulted in structurally deficient highway bridges. Current fatigue load models focus on single fatigue vehicles that simplify the bridge load spectra. Fatigue load models are developed herein that accurately describe complex bridge live load spectra. Weigh-in-motion (WIM) equipment, which collects axle weights and spacing, and strain data acquisition equipment is utilized in the field phase of this study. Seven bridges in Southeast Michigan were selected for study to represent a broad variation of span lengths, ADTT, and distribution of gross vehicle weight (GVW). A total of 22,000 trucks were processed for span lengths of 20 feet to 200 feet to calculate the maximum static lane moment. Statistics of the GVW, axle weight, and maximum static lane moments are presented in the form of cumulative distribution functions (CDF's). The results strongly demonstrate that bridge live load is site specific and a function of weigh station proximity and industrial areas. Stress spectra of each bridge girder and diaphragms have been obtained through field measurements of the dynamic stresses experienced by the selected bridges. Time histories were reduced using the rainflow method of cycle counting to enable construction of stress CDF's. Dynamic response of the bridge, load distribution characteristics, and indications of component importance for the structural system reliability analytical model are obtained. The results also indicate a significant variation in stress spectra between girders which provides a focus for inspection efforts. Correlation of the dynamic stress with the static lane moment and GVW is incorporated in the development of a new dual truck fatigue load model. The WIM 22,000 truck data base was processed to determine the most damaging vehicle types and corresponding axle configuration to formulate the fatigue truck models. Fatigue damage is calculated for spans of 20 to 200 feet based on the proposed model, actual stress histories, and the American Association of State Highway Transportation Officials (AASHTO) design criteria as a means of calibration for the proposed model.