Stress Evaluation in Web Gap Details Prone to Distortion Induced Fatigue

Abstract

The most common fatigue problem in steel structures is fatigue cracking caused by distortion. Although methods of repair for structures that suffer from distortion induced fatigue do exist, our ability to assess the resistance of existing structures is lacking. Consequently, it is difficult to assess whether structures with details that are known to be prone to distortion induced fatigue should be rehabilitated or not; costly repairs are often executed without knowing whether these rehabilitations are necessary. An investigation of prediction methods for web gap stresses indicated that the governing web gap stress is a function of several parameters (e.g. lateral stiffness of the flange, stiffness of the stiffener, magnitude of the differential displacement, and several other factors that have been identified to affect the stresses developed in the web gap) and a simple method of analysis that incorporates all these parameters does not exist. In this study the web gap stress prediction equation was built upon the knowledge gained in earlier projects and finite element investigation to develop an accurate method of assessing resistance to distortion induced fatigue and to develop a model that can assess the stress in details subjected to distortion induced fatigue. Dimensional analysis approach is used to obtain the equation to calculate the magnitude of stress in these details as a function of geometrical variables and applied loads. The results of this research will be used to assess the fatigue resistance of these details in order to extend the service life of aging bridge steel structures without compromising their level of safety.Although the proposed empirical equations are analytically more complicated than the previously used equations to calculate the stress, they predict the stress in the web gap within 20 percent and cover a wide range of parameters which almost includes all the steel multi-girder bridges with bend plate diaphragm. Considering all the complexity in the web gap details, this is an acceptable accuracy for the stress calculation. Linear elastic fracture mechanics approach, used in this study to investigate fatigue behavior of the web gap detail, proved that even though the fracture in the detail is a mixed-mode fracture, Mode I is the governing mode for these details.