LIVE-LOAD STRESSES IN A STRAIGHT BOX-GIRDER BRIDGE

CURRENT AASHO DESIGN CRITERIA GOVERNING TRANSVERSE DISTRIBUTION OF LIVE LOADS IN STEEL BOX-GIRDER BRIDGES WERE DEVELOPED AS THE RESULT OF NUMEROUS ANALYTICAL AND EXPERIMENTAL STUDIES. HOWEVER, SINCE THESE STUDIES WERE CONFINED TO RIGHT SIMPLY SUPPORTED STRUCTURES, APPLICATION OF THESE DESIGN RELATIONSHIPS TO OTHER STRUCTURAL CONFIGURATIONS IS QUESTIONABLE. CONSEQUENTLY, A FIELD TESTING PROGRAM WAS INITIATED TO VERIFY THE APPLICABILITY OF THESE RELATIONSHIPS TO ATWO-SPAN CONTINUOUS TWIN BOX-GIRDER BRIDGE WITH SKEWED END SUPPORTS. EXPERIMENTAL TRANSVERSE INFLUENCE LINES FOR STRESS AND VERTICAL DEFLECTION WERE DEVELOPED AND EMPLOYED IN DETERMINING LIVE-LOAD DISTRIBUTON FACTORS FOR BOTH THE POSITIVE AND THE NEGATIVE BENDING MOMENT REGIONS. THE TEST RESULTS SHOW THAT FOR THIS STRUCTURE, THE EXPERIMENTAL DISTRIBUTION FACTORS, WHICH VARIED FROM 2.59 TO 3.08,ARE DEPENDENT UPON BOTH THE LONGITUDINAL AND TRANSVERSE POSTIION OF THE GIRDER CROSS-SECTION CONSIDERED. ALTHOUGH THESE FACTORS DIFFERED BY +5 TO -11 PERCENT OF THE DESIGN VALUE OF 2.933, EXISTING CRITERIA GOVERNING LATERAL DISTRIBUTION OF LIVE LOADS IN COMPOSITE BOX-GIRDER BRIDGES ARE CONSIDERED APPROPRIATE FOR THIS STRUCTURE. IN ADDITION, IN THE POSITIVE MOMENT REGIONS THE TOTAL EXPERIMENTAL LIVE-LOAD BENDING MOMENT AVERAGED ONLY 85 PERCENT OF THE DESIGN CONTINUOUS BEAM MOMENT. ALTHOUGH THE MAGNITUDE OF THIS REDUCTION IN MOMENT WILL DIFFER FOR STRUCTURES OF DIFFERENT GEOMETRIES, THE IMPLICATION IS THAT FOR SKEWED BOX-GIRDER BORIDGES IN GENERAL, A REDUCTION IN DESIGN LIVE-LOAD MOMENT WOULD BE APPROPRIATE FOR THE POSITIVE MOMENT REGIONS. /FHWA/