Field testing and finite element analysis of steel bridge retrofits for distortion-induced fatigue

Fatigue-crack formation in the web-gap region of multi-girder steel bridges has been a common occurrence due to differential deflections between girders resulting in diaphragm forces that subject the web-gap to out-of-plane distortion. However, bridge design specifications do not directly deal with distortion induced stresses. The objective of this paper is to examine the behavior of web-gap distortion of a skewed multi-girder steel bridge through field testing and finite element studies. The field test involved measurements of strains for eight load cases by using strain gages placed in the web-gap region and in the cross bracing adjacent to the web-gap. The results of the field test were used as a basis for the verification of developed finite element models. Critical spots in the web-gap were examined utilizing the finite element analysis. Retrofit methods that include the provision of a connection plate between the stiffener and the girder top flange, loosening of the bolts connecting the cross-bracing to the stiffener, and supplementing a stiffener plate opposite to the original stiffener side were investigated. The results indicated that the connection plate addition and loosening of bolts alternatives were effective in reducing induced strains and stresses in the web-gap region. The finite element study on the impact of web-gap height variation indicated an inverse relationship between web-gap height and induced strains and stresses with the shortest web-gap height resulting in the highest strains due to increased bending by diaphragm forces in the web. Influence surfaces for strains, diaphragm force and relative out-of-plane displacement provide insight into the behavior of web-gap distortion, and can be used to provide quick estimate of the effect of truck load positioning. Expressions developed that relate vertical stresses and relative out-of-plane displacements combined with measurements of out-of-plane displacements by transducers can be utilized for prediction of induced stresses at other critical web-gap regions of the bridge and at critical locations in the web gaps of similar bridges.