Temp sliding bearing design 1

[jmen68]

I need help with a temporary fix. I need to design a sliding bearing to shift a bridge a few inches (like 2in max). 5 girders max vertical reaction at each girder will be about 50 kips. My plan is to jack the bridge up using hydraulic rams, place a PTFE sheet sandwich between the bottom flange and bearing plate, then use some hydraulic rams to shift the bridge parallel to the bent. The PTFE sandwich design and considerations is where I am looking for some help... I plan to use some "off the shelf" PTFE sheets from Mcmaster, I don't think I should need and special reinforced sheets, unless told otherwise by you guys? Any input on thickness? My planned sizes of sheets will be 20" x 20" so 400 sq-in, bearing load will be 150 psi. Would you lubricate also, my reading tells me that it shouldn't be necessary. Thanks for any input.

 

[BridgeSmith]

What type are the permanent bearings?

I don't think I'd bother with temporary bearings. I'd use greased plates on top of the jacks and just slide it on those. You should be able to a achieve a coefficient of around 0.3, so a 100 kip lateral force should move it.

 

[jmen68]

Perm bearings are elastomeric. The main focus is to get the girders back centered on the sole plates, the bridge is still under construction and there isnt a deck. The jacks we will be using will be a bit squirrly to slide on top of them, which is why were looking to lower it back down on the PTFE, then slide it. Would your initial thoughts with those constraints still go to greasing steel plates and sliding on that?

 

[Lomarandil]

Steel plates with dish soap will get the sliding coefficient well below 0.3, maybe as low as 0.1. Don't bother with PTFE at your scale.

 

[BridgeSmith]

You should check the force required to deform the pads the 2" required. It may not be that significant. You may be able to push it over the 2" and then lift it, align everything, and set it back down.

 

[jmen68]

bridgsmith, excellent suggestion, but we're locked in that direction with the anchor bolts. Slots in the sole plate are the opposite direction of intended sliding.

 

[SlideRuleEra]

Two layers of 30# roofing felt was used at the expansion end of bridge slabs in the 1930's. As a bridge contractor we widened several of these bridges in the 1970's. The roofing felt was still working.

 

[BridgeSmith]

Ah, I think I get it now. The bearings and sole plates are in place in proper alignment, but the girders were set off-center on the sole plates? I think I was thrown off because on our bridges the sole plate is shop-welded to the girder.

I think Lomarandil's idea of lubricating the surfaces with dish soap is good. Stick wax is another option that will work similarly. The wax shouldn't cause any issues with welding; not sure about the dish soap. If you need very low resistance, you could look at using steel shotblasting shot between the girders and sole plates.