If you have to change 26 pads in 5 hours, thats one pad every 11.5 minutes.  Sounds pretty tight to me.  But I do not know what it takes to remove and install a new pad.
As for jacking the girders, make sure you dont crush them at your jack point.

With 26 girders, that means 52 bearings, so you have less than 6 minutes, on average! May wish to consider jacking and bearing replacment at one end in the first 5 hours, then the other end in a another shift - if the bridge structural system will permit such staging.

You have not stated if the girders are steel or concrete, so i am not sure what type of weight is required to be lifted. Also, you have not stated where you are located - in some locations there are a lot of expertise in bridge bearing replacement, so i would encourage you to speak to a specialised contractor in your area.

It would be prudent to have a "trial lift" before you elect to go for replacing 26 (or 52) bearings in one shift. Presumably you are going to specify hydrailic jacks for this purpose. I would state that the contractor to place and synchonize the jacks, and do a "trial lift" to see the duration it takes, and sort out some "bugs". Then upon success, in following shifts, replace the bearings.

FYI - the Kingston Bridge in Scotland in 1999 was lifted 15mm off its piers - the superstrcuture weighs 52,000 tonne!!! The lift was maintained for a duration of 9 months whilst major repairs were undertaken.

HTH

Well, presumably you will have sufficient manpower to switch out the bearings simultaneously (or nearly so)
In that case your project becomes closer to:

1.  Assemble all lifting jacks in position(s)
     (possibly several days involved)
2.  Shut down traffic (begin your timer)

3.  Lift girders (all personnel in safe positions)
      perhaps a half hour to a full hour - your contractor will be able to give a good idea.

4.  Personnel into position to change (all) bearings.
      perhaps another 1/2 hour

5.  Bearings changed.  Allow a generous time span for possible set-backs.  An hour?  Two?  You would know best on this.  Talk to the specific personnel that would perform the work.  

6.  Personnel moved back to safe positions - another 1/2 hour.

7.  Girders lowered.  Perhaps a 1/2 hour.  One hour?

(Timer stops)
8.  Release traffic again.

9.  Remove jacks. a few days.

Obviously, this is an over-simplified time-chart - but it should give you a good idea of the feasibility.  It certainly seems like you should be able to accomplish it, if you have sufficient personnel.  Obviously, the ease of change-over will be a big factor also - - if they can be 'slapped in' by one man in 5 minutes, you may only need a crew of a half-dozen or so.  If there is any reason to suspect it might take a three man crew two hours to replace a single bearing...well, then things will change, won't they?

Just some 'common sense' stuff from a project-engineer's viewpoint.

Curmudgeon

Theres a lot of good advice in the responses listed above.  From time to time I am involved in projects which swap bearings and can say from experience that 5 hours is optimistic at best.  Also, do to logistics of traffic, support systems and structural framing the whole structure is usually never lifted at once.  Jack hydraulics is another factor.  All girders to be lifted at one time requires synchronization of jacks and means a risky hydraulic system.

In general, the superstructure is lifted only a limited amount and the old bearing are extremely heavy.  This will require quite a man-effort.  The effort is intensified by significant height (piers vs. abutments).  Additionally space is limited because the temporary jacking towers are usually close required by design.  

Time must be taken to ensure that the bearings are set properly as well.

I would, if you haven't already, request to see some type of plan in writing or drawn up which details all aspects of the operation.

Dont forget - things NEVER go as planned.  If you plan on 12 minutes to replace a pad, count on 20.  There will always be a few pads that are more difficult to remove or place than the rest.

Alright, things sometimes go as planned.  How 'bout this - things RARELY go as planned.

5 hours sounds tight to me.  Let us kow what happens.

I hope this helps because I personally think 26 bearings in 5 hours is a tough call.---Way back when (much like once upon a time) I replaced 4 bearings and rockers on a railroad overcrossing in Los Angeles with a 5 man crew (concrete girders about 90 feet each, total of 8 to span the roadway).  After all the falsework was erected to the 20 foot or so elevation and all the jacking parafinalia was in place, we began by air-arching the base plates to about 80% complete. At that point we stopped the traffic flow under the bridge and cut the remaining 20%.  The jacking went without a hitch, and the old rocker was pulled out and the new installed (VERY HEAVY WORK) the alighnment complete, the jacks removed and welding to about 20% complete in roughly 4 hours with another 4 hours to complete the welding.  It took about a day to move the falsework to the next pier, to a total of 4 moves and a timeframe of about 10 days start to finish.
All work performed by union Ironworkers from Los Angeles.
Some problems with welding and heat treating the T-1 steel resulted in some degree of panic from time to time, a broken tap in one of the hold downs was another, but by and large it went smoothly well within the required time limit.

I realize this does nothing toward answering your question or solving your problem.  I do hope, however, it will give you a little insight as to the difficulty I perceive in your project.


Rod

Good info Rod!

Good info all...Rod and Q right on target with my experience with synchronous jacks.  Tough to do, but do-able with enough up-front planning.  Be careful that you don't have people falling all over each other.  Rod's small crew, high experience, good planning is the way to go, but 5 hours is optimistic.

I would suggest oversizing the hydraulics as my experience with synchronized jacks is that the sum of the parts doesn't usually seem to equal the whole!

Thank you to all that took time out to reply to my question!
Some additional information is that we will be changing only 26 bearing pads at one time.  The other side will be left for changing at another time.  We think that we will jack the bridge five separate times at five different locations to change out all of the pads.  In my limited experiance, it took us four hours to change out 10 pads on another project. For your info the task that took the longest to do is removing the old pad and cleaning the surface.  Primarilly the contact cement used to hold the pad in place.  Why do they use that anyway?  The forces exceed the strength of the glue and it just makes it more dificult to remove later.  However, the way I look at this is, albeit a simplistic view, is that if it took us 4 hours to remove 10 pads 26 can be remove within the same time frame providing the number of crews are increased proportionally.  While it is true that removal of the pads math wise will only allow about five minutes per pad but 26 crews will allow 5 hours.  I think the solution is providing the number of crews somewhere between 1 and 26 and like someone stated have proper planning.  I know the time frame is extremely tight, hence my question to this forum.  Again, thanks to all.

KSF---

I have never tried to coordinate 26 separate crews but I did run a job with 5 crews, about 50 men, in a refinery.  It was an all consuming effort and not at all easy.  If it can go wrong, it WILL go wrong (Murphy # 1 law of probability).

The more people you get in a confined work area, the less work accomplished, overall.  BUT---IF YOU THROW ENOUGH MONEY AT IT---(Ronald Reagan # 1 law of economics).

I do truly wish you the best of luck.


Rod

KSF - it sounds like you have it in-hand, and that you are aware of the tightness of your constraints.  As a matter of curiosity - let us know how the project goes, and what areas create(d) problems.
As words of advice - - do a dry-run walk-through or two with the actual crewmen you intend to use.  If they will be working in teams (as it sounds) - try to have at least one experienced man on each team, and at least one experienced 'floater' to come to the aid of that one team that always seems to have unexpected problems.
Also, sitting down with the crew-leaders and writing up a step-by-step flow-chart can be quite revealing for all parties.  This was a life-saver for myself and my crew when we had a major project loom up in another state, where we had limited or poor help from the on-site locals.  Myself, my draftsman, and the three main crew-leaders sat down for two different two-hour sessions and flow-charted and brain-stormed a project, that we all originally thought would be three weeks, into a long week for the main crew, plus one more week for the electricians and 'hook-up' crew.  Every single person at the table managed to save the crews at least 2 man-days by their suggestions.
My draftsman and I heard from just about every person on the crews that went, and from a half-dozen of the locals from the site, that they had never seen a project of that size go so quickly, and with so few snafus.  ummm.... not that there were NO snafus, you understand.  But there was actually spare time available to address them when they popped up.
 Just like they teach you in school - - Prior Planning Prevents Poor Performance.

Curmudgeon

As a PM for a few large GC's I have had my share of these projects.  My suggestion is that the contractor be allowed 5 hour closures, over the beam lines being modified.  I am guessing that your bridge is 120 plus feet wide, carrying several driving lanes.  Moving the traffic around on top will give plenty of time for the replacements.  Typical problems that I have experienced, are missed tack welds at the bearing plates, a weldment sticking up a bit and hanging up the bearing as you are trying to slide it out, and the general under estimating of the weight of the bearings in place.  What I picture as a possible scenario is, a crew of 5 or 6, working 8 straight hours, setting up the traffic control, placing three separate jacks under the closed beams, jacking to a 1/4 inch clearance, using a come along to pull out the existing bearings, ditto to position the new, lowering the beams, then moving to the other end of the beams to repeat the cycle.  Then a second shift of 3 or 4, welding the bearings into position, and removing the weldments, moving jacks, etc. for the next jacking shift.  This puts you down below 1000 man hours and less than $45,000 for labor, bottle jacks are only a few hundred dollars, traffic control is a couple of thousand, and the only other expense is another $25,000 or so for the bearings.  So for $100,000 and ten days, you get the bearings replaced. And, trust me, this will be cheaper than renting synchronized jacking systems, that routinely need to be bled every shift, have at least a 5% jack failure, the two days set up, the day test run, the three days spent trying to get the planned one days work complete, the claim for the wearing surface contributing extra weight not shown on the plans, and all the rest, that is going to put you over $100,000 and ten days.  Let me know when you are going to sell the project...