Bearing Replacement

[HighPanda]

I am going to replace the bearings of a prestressed bridge (by jacking up the deck).

Does any bridge guys give me some clues how much should the deck be jacked up for 1) installing the new bearings and 2) installing the new bearings + cleaning bearing shelves?

If the bridge is simply supported, don't I have to worry about secondary effects (i.e. additional forces) due to jacking?

Last but no least, how long should it take to replace ONE bearing?

 

[Qshake]

Why would you want to replace the bearings? Even back in the old days when prestressed girders were being produced, they were mainly set on elastomeric bearing pads. Is this not the case with your bridge?

Generally speaking jacking up the bridge should not be done under live load, but that the calculations for jacking should include live load. Wherein possible, jacking is usually done simultaneously for all girders to elminate differential displacements from one girder to the next. In most cases this is for steel and their diaphragms. However for concrete girder the diaphragms are very heavy but nonetheless susceptible to 'x' cracking if not properly accounted for. Another feature to remember is that you are jacking against the prestressed configuration unless you have a simple span and jack from both ends simultaneously. If you jack from one end only you will likely have some cantilever behavior to contend with.

The jacking should only be 1/8" greater than that required to remove the bearing and replace the new bearing.

As to the time, if you can jack all at once and everything goes well (not likely to happen with a leaky hydraulic system or manifold) then I would say 4 hours each location. That's not 4 hours per girder, but location such as each endbent or intermediate bent.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Zambo]

I think more information is required. For example is it a bridge constructed from prestressed beams or is it a segmental or CIP box section?

I assume it is a box section and that due to the high load the bearings are not elastomeric pads but pot bearings.

If the bearings are pot bearings are they fixed or sliding bearings?

Why are the bearings being replaced, what are the signs of failure?

 

[Umassengr]

HighPanda

How old is you bridge and what is the condition of the deck? This is a concern with concrete girders and no diaphragms between beams at the bridge ends. I have seen bridge decks of this that were in poor condition to begin with come apart under the jacking loads(jacking ht over 2'). In your case it doe not sound like you will need to raise the bridge all that much, and secondary effects would be minimal.
It also depends on the lifting system your contractor has selected to raise the bridge. There are computer controlled systems (read: $$$$) that can lift be used to limit or eliminate the secondary stresses due to the precision and control that allow during the lifting. The manual systems that are more common will not give you that kind of control.

The actual lifting of the bridge takes a few hours. The most important aspect as Qshake said is live load. If you can close the bridge to traffic and do this work that is ideal, and will take tha least amount of time. If you have to jack under traffic, you will need to be able to shut the road down or pace traffic for the time it takes to lift the bridge. This usually done in 2-3" increments and then place shims...2"-3" more, shim, etc. Preparation to lift the bridge is what usually takes the most time. The contractor will be working to get rigging in place and set up the jacks. This can take several days depending on what methods are required to lift the structure.
I would also suggest contacting a local contractor that specializes in this work. They could give you an idea of what they need to lift the bridge.

 

[kelowna]

Just to add to the very good comments already posted.

Jacking points need to be studied carefully. Modern bridges should have provisions for bearing replacement in terms of jacking points already designed to take the loading. I do not know if that is your situation.

Jacking clearances of 5 mm are good. 10 mm are great for contractors doing the work. Carefull if the work is done with live load, and carefull with other elements, for example, expansion joints.

You might have to provide horizontal restraint (depending on bridge configuration) since it might be taken by the bearings at present.

I would recommend that the whole pier or abutment is jacked up even if only one bearing is replaced.

Carefull with the hydraulic/jacking system. If all jacks are connected to the same line you might have equal pressure (and therefore equal load), not equal displacement, and that could create problems.

You donot want to relay on hydraulics to hold the bridge. Horse shoe shims or locking head jacks should be used.

Last, and most important, get somebody who knows about this and has done this job before. Comments from this forum can help, but if you do not know about this subject and you are relying on us to serve as your only source of information, you have a problem

Good luck

 

[HighPanda]

Thanks for you guys' comments!!

The bridge is 27m long and carries edestrian loads over a highway.

The bridge is pretty small in cross section (900mm wide x 1000mm deep with a 530mm diameter hole and a 600mm long wide flange/cantilever slab on either side). The deck is simply supported by fixed bearings at one end and free bearings at the other end.