Rehab options for steel bridge girder!

[bgrove]

I have a situation were a bridge has two beams badly rusted at the bottom flange and the bottom half of web. The only "rehab" options that seem cost effective is either to bolt on angles or weld plates to build up the girder. Generically what is everyone's opinion on bolting vs welding? My understanding is that welding in tension areas is not good. This brings me to my next question does all live load and deadload need to be relieved before this work is undertaken? There doesn't seem to be much guidance for this type of work in AASHTO, can someone point me in the right direction to find standards on this type of work?? Thanks for the help!

 

[VoyageofDiscovery]

Hi bgrove,

Yes, for rehab, welding in tension areas is not preferable as it would be a field weld and enclosing the area during rehab may be costly. That being said as long as fatigue loading is accounted for it can be done.

With respect to building up the girders (I assume they are the main girders), they should be jacked up and shored for dead and live load because if it isn't, they will have locked in stresses prior to installation of new components. This is not a good thing.

Can you jack up and shore the entire bridge for the work? If you can't, at least try to jack up and shore the corroded girders without damaging the deck.

With respect to design standards for this type of work, I do not know of any, however the final design should comply with AASHTO and new connections should be able to transfer loads to attain overall girder capacities.

Regards

VOD

 

[bgrove]

Thanks for the reply! You have confirmed most of my feelings on the matter but one thing is still a mystery to me. How do you typically go about jacking a bridge like this? Would you go to the midspan and set jacks under diaphragms or the bottom flange of beams? Do you jack the bridge up the distance equal to the original dead load deflection including beam self weight? In this case I think we would need to jack the entire bridge since the deck information is unknown and the deck is exihibiting some spalling. I don't think I would trust localized jacking of the deck. Do you think I'm fast approaching a point on this job that it is more cost effective to put on an entire new deck and replace the rusted beams?

 

[VoyageofDiscovery]

Jacking costs would be in the shoring falsework as well as the complexity of this falsework.

Yes, jack it to the original camber with an allowance for live load.

For the corroded girders, it is preferable to have jacking points cantilevered from the shoring at the underside of the top flange so as to clear the bottom flange, however you may be able to work around the jacking points if they are to the bottom flange.

If the shoring is complex, yes, you are fast approaching that point where it would be more cost effective to replace the girders and redeck.

Regards

VOD

 

[SlideRuleEra]

If the economics even hint that replacement may be cost effective, please give it serious consideration. On this type work one set of problems (corroded steel, spalled concrete) often just leads to another (maybe bad connections, frozen expansion joints, etc).

 

[bgrove]

Thanks to both of you. This web site is great. Sometimes even if you think you know something it's always good to get some feedback from other engineers. In this particular case I will develope cost estimates for different alternatives but I think I will make the recomendation that any slight cost savings will not be worth the unforseen risk involved in rehab.

 

[Qshake]

There is little information regarding rehabbing bridges and a lot of other structures. What is out there is mainly in papers and firms that are experienced in rehabilitation.

Regarding your situation, I agree with all of the post made to date. The advice you have will place you in the right direction.

For my $0.02, please consider the deck as a whole and consult with the owner before deciding that the last option is to remove the entire deck. Often times, you can remove part of the deck over the girder, cut the girder and lift the deteriorated section from a crane positioned on top. A new section can be spliced in and the deck rebar and concrete replaced. This has been done in the past several times. Obviously not all conditions will favor this removal.

Cover plates and welded plates are good ideas and for the reasons noted above I would favor the bolted solution. Jacking is complicated but you'd be surprised at how resourceful good contractors can be. Jacking the entire structure at once is questionable since it requires several jacks to work together which means more apparatus to fumble with.

Another consideration could be external post-tensioning.

Lastly, here are some resources that I've found useful,

Illinois DOT Structural Services manual. A manual dedicated to documenting what retrofits have worked in the past. And it includes a section on replacing decks.

Bridge Strengthening and Rehabilitation by Xanthakos.

Good Luck.

 

[bgrove]

Thanks again for all responses. Now that the rehab of the beams looks more and more improbable I know the client is going to want an estimated design life of the structure in it's current condition. One rehab option is to simply blast and paint the beams but this is very costly and by the time you crunch the numbers it almost seems like you should just replace the beams and deck. I guess my bottom line question is this: Is there any good way to estimate future rusting if no rehab is undertaken? Is there a generic (section loss/year) growth rate that I can tell the client for rust loss? Does anyone know of good resources on this subject? Any thoughts would be appreciated.

 

[curvbridger]

There has been much good advice here. Let me add:

You can estimate an in-service rate of corrosion by dividing section loss to date by the age of the structure (or the age since paint loss if available from inspection reports). This rate then can be used to estimate time to loss of serviceability based on reduced ratings, etc.

If your deck and beams are both deteriorated, I would suggest deck removal, then jacking (at quarter points) then steel repairs/strengthening, then placement of a new deck.

If the bridge was built earlier than, say 1965, it is a candidate for a major rehab including new bearings, expansion joints, etc.

Peace,

Curvbridger

 

[Qshake]

One of the references that I mentioned gives section loss in steel members for years of service. Please refer to the Xanthakos text.

 

[Ho2]

Hi bgrove:
Unless your deck condition survey indicated that the deck needs a lot of repair, I don't see replacing the entire deck being cheaper than just repair two girders. I assume the girder ends are rusted due to leaking expansion joints. I designed some similar repairs. What I did was temporarily supported the girders in front of the abutment or pier, cut out the rusted portion of the web and flange and field weld back a new section. You may also want to consider to eliminate the expansion joint by introducing semi-integral abutments.