steel cross frame stresses in a skewed bridge!

[Noopur]

I need some intelligent advice for a bridge rehabilitation project.
The exisitng bridge has an skew of about 30 degrees.
It need to be raised up to get required vertical clearnace.
hence we r raising the existing beams up/providing a new deck above.

My question is.....will there be any stresses in cross frames if I raise the beams up by different amount at different supports?

If so ....how can I take care of that!

 

[bridgebuster]

Typically, cross-frames are considered secondary members and are not designed for the same loads as the girders or stringers. When raising the structure, you should maintain the existing cross-slope to prevent damage/distortion to the cross-frame connections and introducing unwanted secondary stresses in both the frames and girders. Also, you should evaluate the cross-frame connection for fatigue. Some connections are highly prone to fatigue. If you are unsure of the procedure AISC offers an excellent manual written by Dr. John Fisher. If you need to retrofit the connections, this is the perfect time.

 

[Noopur]

Bridgebuster,
I already checked that the main members (beams) got some stresses due to this .......upward settlement/raising but those are within the allowable limits.
Also I can check the cross frames for fatigue.
But.....my main concern is that .....
because the bridge structure has a high skew....although the main members are not stressed ....the cross frames may get high stresses due to differential settlement/raising.
...because ......in a particular direction parallel to a cross frames....different beams connected to it are raised by different amount.
The beams are arranged somewhat like this.....

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and cross frames are perpendicular to them.
I hope I am making my point clear.

So...How can I make sure that strsses in cross frames is within limits? Is there any way?
Also....can you please tell me where I can find that manual from AISC?
please help!

 

[bridgebuster]

Noopur,

I recently completed the design of two steel bridges with non-parallel abutments and skew angles between 55 and 58 degrees. The individual girder lengths vary from 125' to 155'. One bridge has six girders and the other has eight.

We did a 3-D analysis using MDX and the maximum performance ratio in the X frames was 65%. The X frames consist of two angles 4" x4" x 5/8" Fy = 36 ksi.

To order the fatigue manual go to

http://www.aisc.org

The manual is published by the National Steel Bridge Alliance group of AISC.

I hope this helps.

 

[TpaRAF]

One method of eliminating the secondary stresses is to use a "manifold jack" to raise all the main girders simultaneously. A series of pancake jacks are connected by hydraulics, and are typically used for bearing replacements. Simplex is at least one supplier of this product.

This shifts the loads from the x-frames to the abutment or pier cap -- so it will have to take the forces caused by the jacks lifting the structure. Also, traffic above will have to be re-routed or greatly slowed down. This will allow you to install a permanent shim or bearing assembly under the beam to raise it up.