PROBABILITY THEORY FOR HIGHWAY BRIDGE FATIGUE STRESSES

THE RECENT CONCERN FOR FATIGUE OF HIGHWAY BRIDGE GIRDERS IS DUE TO HEAVIER AND MORE NUMEROUS TRUCKS, LIGHTER AND MORE FLEXIBLE BRIDGES, HIGH STRENGTH ALLOYS, WELDED AND CONTINUOUS SPAN SECTIONS, AND REPORTS OF FATIGUE CRACKS. ON THE OTHER HAND, MEASUREMENTS OF EXISTING BRIDGES USUALLY SHOW LOW STRESS LEVELS UNDER RANDOM TRAFFIC AND THUS THERE MUST PRESENTLY BE INCONSISTANT SAFETY LEVELS AGAINST FATIGUE LEVELS. THIS REPORT DESCRIBES AN ANALYTICAL PROJECT TO CALCULATE HISTOGRAMS OF HIGHWAY BRIDGE LOADINGS WHICH CAN BE USED TO PREDICT FATIGUE AND TO PROPERLY SIZE GIRDER. SECTIONS. A PROBABILISTIC LOAD MODEL IS DEVELOPED USING TRUCK VOLUME AND WEIGHT HISTOGRAM, HEADWAY SPACING DISTRIBUTIONS BETWEEN TRUCKS, IMPACT FACTORS AND DISTRIBUTION ANALYSIS. A RELIABILITY OF RISK APPROACH TO CHOOSING SAFETY FACTORS IS ALSO DESCRIBED. THE COMPUTER DATA FOR TRUCK LOADING AND FATIGUE DESIGN COMES FROM A LITERATURE STUDY REPORTED IN APPENDICES ON FATIGUE, TRUCK AND TRAFFIC CHARACTERISTICS AND DYNAMIC BRIDGE BEHAVIOR. THE CALCULATED LOAD HISTOGRAMS SHOW GOOD AGREEMENT WITH REPORTED MEASUREMENTS. THE METHODS ARE ILLUSTRATED WITH SINGLE SPAN BRIDGES OF DIFFERENT LENGTH, WELD CATEGORY, AND TRUCK RECORDS OF DIFFERENT STATES, PARALLEL AND OPPOSING FLOW BRIDGES AND THREE SPAN CONTINUOUS BRIDGES. /FHWA/