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[ATHS]

Hello,

I have come across a bridge that has diagonal cracks on the pier diaphragm as shown in the image. The bridge is a prestressed girder bridge. I know there are a number of reasons that could cause this but I have heard this is pretty common on prestressed girder bridges.
Can someone list out a few reasons why this could happen? Girder end rotation is one of the reasons, but I am having a hard time understanding why girder ends would rotate in the first place. Thermal movements are accounted for by the slots in the bearing.

 

[jayrod12]

bending rotating at the supports. Yes horizontal thermal movements are accounted for, but not the true rotation of the beam from bending.

 

[bridgebuster]

The beam is going to rotate under self-weight, when the deck and parapets are placed, and under live load.

I don't see any cracks; just a spall. The spall is probably due to the detailing. From the photo it appears that the diaphragm is only reinforced with mesh. A crack likely developed where the diaphragm changes section about the bottom flange of the beam.

 

[ATHS]

@jayrod12 and bridgebuster, Thank you for responding, what is typically done to address the rotation ?Is it the bearing pad ? See below the image for interface b/w pier diaphragm and pier cap. I have also attached the bearing pad schedule. In my opinion, the bearing pads are too thin for the loads and that is what is restricting the movement and causing the spalling and cracking of diaphragm. What do you guys think ?

 

[hokie66]

Because of the location of the spall near the bottom of the beam, I think that longitudinal shortening of the beam due to shrinkage and temperature variation caused the spall. Probably inevitable due to the beam and diaphragm not being tied together. The spalled part has just adhered to the beam side and gone for the ride. Spalling due to rotation would be expected at the top.

 

[bridgebuster]

I agree with hokie66's assessment as to the cause of the spall. Although a quick check - using AASHTO Standard Specs - reveals the bearings are undersized. The compressive area is about 2/3 of what it should be; the thickness is about right.

 

[ATHS]

@hokie66 - I have come across a few bridges where this is a common phenomenon. Isn't the beam shortening accounted for in the bearing pad? Could the stiff joint filler material at the interface b/w cap and diaphragm cause this ? What kind of practice is used to repair and avoid this in future ?

 

[hokie66]

Yes, the bearings are supposed to accommodate longitudinal movement. But that doesn't allow for interaction between the beam and diaphragm. In the picture we are looking at, it is apparent that the diaphragm was cast directly against the beam, with no attempt to separate the two. Perhaps a sheet of plastic would have helped, perhaps not. As I am not a bridge engineer per se, I haven't studied the problem.