Staggered cross frames in skewed bridges

[OSUCivlEng]

I recently designed a new steel bridge on a 45 degree skew with staggered cross frames. I received a comment from the DOT reviewer that this increases the possibility of fatigue cracks. I was puzzled by this comment, as everything I have read says that staggered cross frames reduces cross frame forces. If you reduce the connection force shouldn't the magnitude of the stress go down? I do understand that it increases the lateral bending in the flanges.

 

[ernestog84]

Refer to the FHWA Steel Bridge Design Handbook Vol 13 "Bracing System Design". Staggered cross frames are discussed and refutes the DOT Reviewer on the fatigue cracks.

https://www.fhwa.dot.gov/bridge/steel/pubs/hif16002/

If this fails, you should ask for a source for his statement.

 

[OSUCivlEng]

Thanks. I'm well aware of that document. Apparently they have had several plate girders with staggered cross frames develop some pretty large cracks. After seeing the evidence I am not really convinced it's the staggered bracing. They may be open to adding an additional stiffener on the opposite side of the web at cross frame connection locations.

 

[bridgebuster]

An additional stiffener was my thought also. Question: Are the cross frame connection plates welded to the tension flange or do you provide a tab plate?

 

[OSUCivlEng]

Yes they are welded to the tension flange. I have seen details for a tab plate, but I'm not aware of them ever being used in this state.

 

[bridgebuster]

Tab plates & bolting the stiffener to the tension flange with angles is common in the northeast.

 

[ernestog84]

Does the DOT have a preferred detail or preferred crossframe layout?