Adding cover plates - weld under tension

[crossframe]

We are adding cover plates to an existing continuous steel girder bridge as part of a strengthening rehabilitation, and are trying to give the contractor as much flexibility as possible by allowing both field welded (with bolted end connections) and field bolting options. Some of the flange areas will be in tension at the time the cover plates are added.

Are there any guidelines for field welding on a member under tension?

I've looked in Blodgett, AWS D1.5, and every other reference I have, but no joy. I've found plenty of information for members that WILL BE under tension in service, but nothing useful for welding under tension. Shoring is not a practical option (over water), but we can somewhat modulate the tension and compression zones to some degree through adjusting the areas and sequence of the deck removal. Most of the weight will be off the areas to be cover plated, and the stress at that time will be less than 0.1fy.

Thanks.

 

[MikeHalloran]

The interesting part of the problem is that you have to consider all the load case(s) where the load is carried by the member exclusive of the HAZ, which is molten during the welding process, and not capable of carrying any load.


This seems to be on point:

http://www.modernsteel.com/steelinterchange_details.php?id=412

Mike Halloran
Pembroke Pines, FL, USA

 

[crossframe]

Thanks, Mike. Yes, I read through that link and also the "Field Welding of Existing Steel Structures" by David T. Ricker, AISC Engineering Journal, 1st Quarter 1988, but I found nothing definitive as far as recommended stress limits, etc.

The stress on the member is expected to be low compared to its capacity, so I don't anticipate issues with a reduction of member size due to molten areas of steel.

 

[Bobber1]

If the bridge is not experiencing live load during the repairs, the stress in the beam is likely to be low relative to the allowable stress of the beam with full dead and live load.

 

[rceconsulting]

I would hesitate to perform field welding of any kind if at all possible. Bolting is a much better way to go in my opinion.

 

[crossframe]

Owner (DOT) wants to offer both welded and bolted options, and their engineers are confident welding is a viable option but have not yet been able to provide any guidance on the tension aspect. The cover plates are at the piers, so we can eliminate the tension by specifying which portions of the deck may be on the beams when the cover plates are installed, but this gets into prescribing the contractor's work sequence. This is a difficult rehabilitation, and we (the DOT and me) want to offer the contractor as much flexibility as possible in sequencing the work.

 

[VoyageofDiscovery]

How can the DOT engineers be confident with field welding! What is the QC on field welds on bridges! Your cover plates or details will finally be compromised by this and fatigue stresses for certain details may well be around 0.1fy.

 

[Qshake]

I have to agree with VOD. But welding in certain parts of the country where welding is common place is likely ok. My experience is that those locations are limited to where oil and pipe line operations are. There construction workers weld long hours and produce quality. Other places welding is really not done enough in the field to expect quality.

That said and the note you made about the deck being removed in places, is it possible then to set up a strong back beam with a hoist system? THe hoist would be connected to the girder and the strongback. This will pull the beam up and remove tension. This way the tension or most of it can be removed and the loads moved back to other parts of the beam under question. Now, if the tension is near the pier.....well, good luck.



Regards,
Qshake

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

 

[HgTX]

If you're not in Texas already, contact someone at the Texas DOT Bridge Division, Field Operations Section. Texas has an active field welding program, both new construction and rehab/repair, and they can probably offer your DOT client some advice.

Hg


Eng-Tips policies: faq731-376

 

[crossframe]

Thanks for the additional replies.

We've eliminated need to weld in areas under tension, so this is pretty much an academic question at this point.

Field welding is another issue and the DOT feels they can adequately control the quality.

 

[ajose]

Your post is an interesting question and i would like to add some words about it.

When you weld on stressed member of a structure you are dividing in phases the construction. First stage, the stressed member and the reinforcement doing nothing. Second stage, when additional loads come in there is a stress variation on member and its reinforcement. To make it more complicated there is other factor and is the stress relieve due temperature.

So if a structure is being reinforced with service loads on it, there is going to be stages, and every stage have to be verified.

 

[crossframe]

Understood. The amount of tension would've been relatively small - from member self weight, not from full service loads - but tension none the less. And, the effect of the loading phases mentioned above have been considered.

It's an academic question now, but it lead me to wonder about the general statement of "no welding on the tension flange" that is prevalent in bridge construction.

 

[VoyageofDiscovery]

Risk