Global-Local Finite Element Modeling in Skewed Multi-girder Bridges

Steel plate girder and multi-girder bridges are commonly used in the United States. There are approximately 9000 such bridges in Virginia alone. Of these, approximately 2600 are skewed bridges, which have their cross-members connecting adjacent girders at the piers and abutments.
Fatigue of welded steel bridges has become a significant problem in the United States in recent years. A number of different fatigue mechanisms have been identified. In particular, many steel plate girder bridges have been found to be vulnerable to out of plane web distortion, which can cause fatigue cracking where cross-frame members or diaphragms are connected to girders. The connection plates at these joints have been designed with a web gap at the top and bottom of the plate and it is common, but not universal, practice to not weld the connection plate to the tension flange. This gap can lead to web distortion that causes fatigue cracks. Out of plane web distortion fatigue cracking was observed in a web along a skewed cross-frame located at pier-10 in the Carter-Glass Bridge.
This thesis investigates the use of global-local finite element models to evaluate the fatigue potential of cross-frame to girder connections, with particular reference to the Carter-Glass Bridge, which has experienced a significant fracture at a skewed cross-frame connection. A global model of this bridge was created previously. The objective of this thesis is to develop a local model that can identify the magnitude of stress in a common welded cross-frame to girder connection and to apply that model to the Carter-Glass Bridge. The local model was developed to be versatile enough to act as a template for various skew angles and girder dimensions. The exterior skew connection of the Carter-Glass Bridge was the main finite element local model studied. This skewed connection occurs in a region of negative bending. The boundary conditions for a global-local model and a partial-local model were analyzed. The ultimate goal was to determine the stress intensity factor for this complex three-dimensional geometry.