steel cross frames in skewed bridges?

[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!

 

[wiktor]

Noopur:
Of course you will have moments in the cross frames (diaphragms?).
As you will be replacing the deck at the same time try to do the rehab in stages and cut both deck and relase diaphragms at the same time.After lifing, when these are at the same, or almost same relative level, bolt it back together. This will work for typical steel stringer/RC deck bridge.
If the bridge is RC on RC/PC, situation is more complicated, but the method is the same. Longitudinal cut and jacking in sections are the only solution.

 

[Noopur]

Wiktor,
What exactly do you mean by " bolitng it back together"?
The beam and cross frames will be acting as one single unit.
I am going to raise the beam (yes steel beam) and cross
frames togehter.

 

[austim]

Noopur,

If you must raise ALL of the beams and cross frames together (and fully connected) but by differing amounts, then you are almost certain to generate considerable stresses in the cross frames.

The only condition that I can see that would give you no such stresses would be if you were to impose a genuinely mathematical 'rigid body' movement to the bridge.

To clarify what I mean, consider your move in three stages
(taking z coordinate as vertical) :
1. Lift every point by the same distance : dz=constant=A
2. Rotate the entire structure about one side : dz=B*y
3. Rotate the entire structure about one end : dz=C*x

Any combination of these three movements would give zero Xframe stresses. In other words, if the final movements that you are aiming for fit the equation dz = A + B*x + C*y at every beam/Xframe intersection (and at every stage of movement), then you need not worry about cross frame stresses.

If not, then you need to do what Wiktor says - 'loosen' every crossframe/beam connection before you start, and 're-connect' again after you have finished the move.

Hopefully, 'loosen' means simply slacken off sufficient connecting bolts (eg all bottom chord bolts). If your structure is fully welded, then you have a much more difficult task, and will have two choices - cut out sufficient welds to make 'hinges' at every beam/Xframe connection (probably cut out all bottom chord welds), or do a detailed structural analysis to see if the imposed stresses can be accepted.