Probability Theory for Highway Bridge Fatigue Stresses

Probability Theory for Highway Bridge Fatigue Stresses - Phase II

F. Moses / A. P. Pavia

Recent years have seen an increased presence of factors which relate to the occurrence of fatigue cracks on steel highway bridge girders. The 1973 modifications in AASHTO fatigue specifications often caused fatigue limits to govern section sizes and in some cases causing changes in design usage of high strength steel and certain weld attachments. This study combined statistical data on truck loads, bridge behavior and measurements and laboratory tests of steel attachments to develop a model of fatigue life prediction. A fatigue design specification was derived from the study which permits truck volume and loadometer survey data to be considered in calculating the allowable design stresses. This can lead to more economical girder sections. To illustrate the proposed design procedure several single and multi-span bridges are analyzed and compared in detail using the proposed method and current AASHTO fatigue specifications including modification of the distribution factor. Based on this study it is possible to specify a load and design technique so that fatigue damage has a small chance of occurrence under proper detailing and inspection procedures. Economic optimization methods based on a risk model including damage consequences are also discussed and may be used for rating and evaluation of possible increased truck load limits.