The purpose of this study is to perform structural analysis and to determine suitable repairs for bridges experiencing cracking problem. Numerous cases of cracking on steel bridges have been observed by the KDOT (Kansas Department of Transportation) Special Inspection Team due to the past use of fatigue prone details. Development of cracks severely shortens the service life of the bridges. Much of the cracking now seen is due to details commonly used on welded steel structures designed between the 1950's and 1960's that have since been identified as fatigue prone details. These bridges have been in use for many years and are susceptible to fatigue failure due to repeated truck loading. For example, welded steel structures designed during that period did not require positive attachment between girder flanges and transverse connection plates, for the purpose of preventing in-plane fatigue cracking. However, unexpected out-of-plane fatigue cracks were found to have developed at small web gaps at girder flange, web, and stiffener plate connections due to the secondary stress induced distortion. Cyclic out-of-plane distortion induced stresses have resulted in relative rotation and displacement between longitudinal girders and transverse members framing into these girders, generally through transverse connection plates on the longitudinal members. These stresses have led to fatigue cracks.

Out-of-plane distortion composed the major cracking problem in KDOT welded steel bridges. Through consultation with KDOT Special Inspection Team, the following representative bridges were chosen for the research: the Hutchinson Bridge, the West Gate Bridge, the Winfield Bridge, and the Hump Yard Bridge. All four bridges developed fatigue cracks due to the out-of-plane bending. Two major types of crack details will be investigated. The first type is the well-known horseshoe cracking developed at girder-to-diaphragm small web gaps. Cracks were located either close to the girder top flange, as observed in the West Gate Bridge, or close to the girder bottom flange, as found in the Winfield and the Hump Yard Bridges. The second type is the horizontal cracking formed in the transverse connection plate, and is only identified in the Hutchinson Bridge. The flanges of the floor-beams are coped and blocked back on one side, then welded horizontally to the girder connection plates. Horizontal cracks developed in the connection plates along the floor-beam top coped flange to connection plate fillet welds.

This research is aimed at analyzing cracking behavior of fatigue prone details, evaluating different repair approaches, and recommending retrofit methods to extend bridge lives and to avoid costly bridge replacements. Finite element method (FEM) is adopted as the computational tool for fatigue stress analysis. Both 3-D bridge structures and crack details are modeled and examined by using ANSYS software package. HS20 fatigue truck is loaded per AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges. According to the stress results obtained from finite element procedure, fatigue life and crack growth behavior are then investigated based on fracture mechanics principles. Once the model has been calibrated and its behavior matches that seen in the field, the appropriate repair methods will be proposed to meet the fitness for service criteria. Fatigue life after bridge retrofitting shall be increased to a level that no more crack propagation will ever develop at the fixed details, thus allows continuing use of the structure. An evaluation procedure will also be established to determine the service life of steel bridges and to make design recommendations as well. No experimental testing is included in this project. However, recommendations for strain gauging on certain repaired details will be proposed depending on the finite element analysis results to verify the actual stress conditions.

Figure 2 - Horizontal Crack at Connection Plate
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