Bridge Jacking Loads

[JEmH]

Lately it seems like I'm either involved with or hear about a lot of projects where it is necessary to jack a bridge superstructure off its bearings. This is generally to replace bearings or repair the substructure. So far, these have all been steel bridges, and the bearing types have varied.

I have noticed that sometimes the jacking load is very close to the reactions that were calculated. Other times, they are nowhere near.

I'm curious as to what other's experiences have been. How well, have you been able to predict both the force it takes to lift a bridge off its bearings, and the force that holds it there?

 

[Qshake]

A few points to make on your inquiry.

1. The most reliable load to estimate is the dead load. Of course that will depend on modifications such as wearing surfaces that are placed.

2. We will often use Live load in our calculations but then require that all jacking be done without live load. Obviously the live loads vary from older structures to what we're required to use today.

3. I find that we can generally get within 5 to 10% of the original plans. Typically I get disappointed as we approach 10% difference and begin to look for what might be the reason.

Regards,
Qshake

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

 

[PEinc]

The jacking load can also be greater than that calculated if you are trying to jack a beam next to another beam that is not being jacked simultaneously. Diaphragms can cause you to start picking up load from the adjacent beam(s).

http://www.PeirceEngineering.com

 

[PEinc]

Diaphragms AND bridge deck.

http://www.PeirceEngineering.com

 

[ajose]

JEmH,

When i design a steel or concrete bridge i normally left the provision for replace the pad bearings (Bigger Diaphragms). I place on drawings the place for each jack so the bridge does not rotate. The equipment for replacement is jacks plus some compressor, and it is no hard to get on site.

This addition to design it is not expensive, remember you need to voit overturning form seismic effects and it is going to be used something like once in 15 years.


If you do not do that it is not impossible perform the pad change. You have design a beam to place at girder bottom, perforate the slab for lifting cables and use a big capacity crane

Normally estimate dead load plus 5 to 10% it is enought.

Ajose