Development of Pre-stressed CFRP Fatigue Retrofits for Common Steel Bridge Connections

Aging or deterioration of the nation’s bridge infrastructure is a significant issue that requires attention. Causes for much of this deterioration can be attributed to two main factors, 1) corrosion, and 2) metallic fatigue, both of which work together to reduce the strength and serviceability of bridge components over time. In many instances, strengthening of bridge components using localized retrofits offers an economical and fast solution for increasing the longevity of existing steel bridges; however, such retrofits must be resilient to further corrosion and fatigue damage. In this study, a localized retrofit is developed using pre-stressed Carbon Fiber Reinforced Polymer (CFRP) strips to strengthen fatigue sensitive details within existing steel bridges. Four stringer/multi-girder steel bridges are considered with varying construction types are analyzed using 3D finite element modeling techniques. Critical fatigue regions are identified for each bridge based on the stress history resulting from the passage of an HS 20-44 design truck. Pre-stress forces required to shift the steel component stress range from a state of finite to infinite fatigue life are determined using the Goodman constant life criterion. Results of the analyses showed that connection details near cross-frame configurations within skewed bridge geometries are more susceptible to fatigue damage than bridges with non-skewed geometries due to distortion induced fatigue in longitudinal girders during loading. Additionally, the developed retrofit successfully reduced the mean stress of a diaphragm connection detail during a laboratory test, indicating that the pre-stressed CFRP retrofit is capable of improving the fatigue performance of structural details. Equations and pre-stressing forces required for the CFRP retrofit are developed for several truck load levels (allowing consideration of increased truck traffic weights)