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Staggered cross frames in skewed bridges

Staggered cross frames in skewed bridges

Staggered cross frames in skewed bridges

(OP)
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.

RE: Staggered cross frames in skewed bridges

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/hif1600...

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

RE: Staggered cross frames in skewed bridges

(OP)
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.

RE: Staggered cross frames in skewed bridges

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?

RE: Staggered cross frames in skewed bridges

(OP)
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.

RE: Staggered cross frames in skewed bridges

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

RE: Staggered cross frames in skewed bridges

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

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