Wind Loads on Bridges Cross Bracing (Frame)

[SHAHB]

I wonder if anyone knows how to calculate the wind force on Cross Bracing of a Bridge.
The wind force is determined (psf) and the forces on Bracings and Chords are questioned.
Please see the attached.
Thank You!

 

[jayrod12]

It seems to me that there may be more to that bracing then just wind load. It might be stability bracing for the bottom chord of the bridge girders as well.

To get the wind loading at each brace frame you would multiply the PSF load by the spacing between brace frames.

 

[SHAHB]

This was the SE question and they asked for the forces not stiffness or stability.
It a matter of wind load distribution and knowing how to find the forces (Analysis). I am not sure this is acting as truss since the girder's web are not members.
Thank You!

 

[bridgebuster]

I would neglect wind on the parapet and assume it is carried through the deck. You could analyze the cross frame as a truss but it's statically indeterminate (assuming that the girder webs are or aren't members doesn't change things). The down and dirty way would be to use 1/2 the wind force on the girder and analyze one joint of the cross frame as you would a truss.

Attached is a paper by the late Dennis Mertz, which offers some guidance on cross frame design.

 

[SHAHB]

Hi Bridgebuster:
I think we can't ignore the wind on the deck since the height is considerable and the test shows the height specifically.
On the other hand I agree we can consider the wind distribution like flex diaphragm.
Please see the attached and let me know if you know any solution.
Anyway, I truly appreciate your time.

 

[bridgebuster]

See attached sketch. To me that's a quick and reasonable solution.

 

[Lomarandil]

SHAHB -- I don't have AASHTO in front of me for a reference, but you should ignore the wind force for the entire area above the mid-height of the girder (for local bracing design/effects). Wind on the parapet carries to the deck (which is a wonderful diaphragm for lateral loads). Wind on the top half of the girder carries to the top flange (typically continuously braced and not controlling the design) then to the deck. I think it's in the earlier parts of 6.10 (mabybe commentary).

Saying that, I would consider the wind force acting on the area of half the girder height times the brace spacing, and apply that to the bottom node. Assuming typical bridge dimensions and without any other information on member sizes, I'd assume that the x-brace diagonals are an order of magnitude stiffer in tension than they are in compression. So I'd carry 90-100% of the force in the bottom chord (C) and tension diagonal.

If this is from the SE exam, that test is known to gauge your ability to discard irrelevant information and boil the problem down to the necessary parts. Sounds like they might have caught you on this one.

If this is from the SE exam, you are also in violation of your agreement not to discuss specific material from the test. Just FYI.



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The name is a long story -- just call me Lo.

 

[SHAHB]

Thank You Lo,
I'll double check the tributary area of the wind force in AASHTO.
I wonder if you know how to find the forces on the chord as I shown on previous attachment.
This was a SE multiple choice!
Regards,