This is the K-10 bridge over Kill Creek in DeSoto, Kansas. The following image is one of the six girders in that bridge. This bridge was constructed in 1975. When structural engineers inspected this bridge for the structural integrity in July 2022, they found the following issues in the bridge (As per NBI Report).
- Cracks on the Crossframe.
- Gap in between the girder and the concrete deck.
- Poor condition of the concrete deck with section loss.
Brief Explanation of each issues
Crack on the Crossframe
The crack on the crossframe did not look like it was a traditional ‘fatigue’ crack, which is usually from cyclical truck loading and differential settlements between girders. In our case, cracks may have originated due to longitudinal thermal contraction and concrete binding components that are not usually part of the traditional ‘fatigue’ crack.
As the original deck and girders of the bridge were not made composite with the use of headed shear studs, girder and slabs were able to move independently during thermal expansion, except for at the top of the abutment cross frame, which is embedded in the slab. A gap was visible in the picture in between the deck and the girder. This means that forces due to differential thermal expansion in the concrete deck and steel girders are transferred at the abutment cross frames. This cyclical thermal loading is the most likely cause of the large crack in the cross frame, but when you zoom in on the image at the top of the weld to the stiffener, a half inch or so long crack is formed in the weld as well.
Solution of the issue
- Replace the abutment cross frames and bearing stiffeners.
- Use bolted connection rather than welded connection.
- Use headed shear studs for making the bridge composite.
Brief explanation of solution
To address this in this bridge and other modern rolled beams and welded girder bridges, headed shear studs are used. The shear stress is evenly distributed along the length of the top flanges between the deck and the superstructure by making them composite using shear studs. This reduces the shear force in any one location and improves performance by increasing the effective depth of the flexural member by using the deck and reinforcement as additional top flange area.
We also avoided welding cross frames to stiffeners and used bolted connections which are less likely than welds to concentrate stresses. By also replacing the abutment bearing stiffeners we are ensuring that we aren’t missing cracks or warping in the existing stiffeners and welds that would continue to concentrate forces after we replace the frames.