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Field welding to Exst member under load

Field welding to Exst member under load

Field welding to Exst member under load

(OP)
I have an existing 36WF300 member (A7 steel) supporting a regenerator vessel and was asked to look into the existing shear stress at the copes. The web plate flexural stress is approx 30% over allowable due to the loss of flange steel at the cope location and a reinf bar has been proposed at the cope location to strengthen the section (see attached).
 
However the vessel load cannot be removed and shoring is not an option for the duration of the welding of the new reinf bar. Having referenced AISC first quarter 1988 Field Welding to Existing Steel Structures it indicates an AWS recommendation of limiting calculated stresses to 3ksi for unshored welding and that this is a conservative approach. Even if product is removed from the existing vessel I will not be able to get down to 3 ksi.
 
Does anyone have more information on allowable stresses in existing members during welding?
 
Any other ideas or insight on how to weld to steel that is at or near maximum stress or any other ideas on how to reinf this cope while under stress?
 
Any advice is appreciated.
 
Thanks
Roger
 

RE: Field welding to Exst member under load

im not sure how the bars shown are going to add any shear capacity?  

RE: Field welding to Exst member under load

We usually don't consider flange are in the calculation of shear capacity of wideflanges....web area only...albeit project through the flanges....or in other words total depth x web thickness

RE: Field welding to Exst member under load

(OP)
Toad

You are correct, the web is carrying the shear (80% of allowable) but the web in the current condition has to carry the bending ( R * Cope length) due to the lack of flanges at the cope location.

The proposed bars are for flexural strength in positive bending.

   

RE: Field welding to Exst member under load

Sorry...read the post a little to quickly...
my apologies.
I don't think the bars added will be engaged or take any force if the load is not removed or some sort of jacking is not done.  

RE: Field welding to Exst member under load

Any possibility of encasing the beam in concrete and adding rebar to lower the shear stresses?  

Or wrapping the area with a composite material?

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Field welding to Exst member under load

...Still, you need to relieve load prior to doing the reinforcing or the new materials added will be useless.  

RE: Field welding to Exst member under load

Apparently, though, some load reduction can be done without  shoring by product removal from the tank.  But would this be enough?  

What percentage of the total load seen at the joint is the product?  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Field welding to Exst member under load

That same thing crossed my mind Mike, but I don't know what a "regenerator vessel" is.

Was the W36 coped to fit into a column web?  

RE: Field welding to Exst member under load

Whatever it is, using a W36, it's got to be heavy...

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Field welding to Exst member under load

The vessel had to be craned in there when originally placed.  

Any chance of re-hooking a crane (or two) to it for deloading during welding?  You don't have to move the vessel, only support it.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Field welding to Exst member under load

hmmm...if this thing is like any of the power plant vessels i have worked with, there is no way you're getting anywhere near it with a crane

RE: Field welding to Exst member under load

I don't know how many greenbacks you have in this job, but my first thought is; let's test a coupon from a low stress area of the web. The material is likely to be much better than minimum strength.

I agree with Toad, as shown, there is no effect. The only successful idea I can dredge from memory is to weld a longer bar to the left side of the notch first, then chill the middle, alongside the notch with dry ice and quickly nail the right end down. As the center warms, it tries to expand.

I haven't actually seen this done in this conformation, it was used on some joists. I would place my hope on testing a coupon.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Field welding to Exst member under load

The beam is 36.7" deep.  You have coped the web 3" top and 3.5" bottom, so the web is approximately 30" deep for a length of 13".

It is not safe to weld reinforcing plates as you have shown because the web is under too much stress.

I suggest the following:  

Place 1/2" x 29" x 18" plates one side of web (see attached).  Fillet weld b-d first.  The stress is very low in this region.  Then weld a-c.  You might want to use intermittent welding on this line and fill in later.  This will add flexural strength to the web.

Let the welds cool and do the same on the opposite side of the web.  Finally, on each side, weld a-b and c-d.

This uses more material, but it is a safer procedure.  I agree with ToadJones that this or any procedure will not actually affect the overstress in the web, but it will improve the ultimate capacity of the web in flexure.

BA

RE: Field welding to Exst member under load

(OP)
Thanks for the all replies, A few clarifications.

The product is approx 20-25% of the end reaction (240 kips max). I dont think this reduction will get me there.

The beam is coped and frames into another W36.
 
A crane was used to set the vessel back around the time my parents were born. Getting a crane in to lift it for these modifications I do not believe will be an option.

The beam was encased in concrete for fireproofing. I considered encasing it with concrete (or fiber material as suggested) but still not sure how to transfer the reaction from an encased beam to the suppoorting W36 beam.

I understand your statement about the new steel not being effective in resisting flexural stresses if it added while the beam is under stress. The proposed bar was also intended to prevent a shear failure crack to develop at the bottom cope location and run into the web. I believe this bar will do that even without a removal of loading. The existing cope is also going to be re-done and grinded smooth to help prevent crack propagation.

I like the idea of a test coupon to verify the material strength. I think that along with a protective coating to prevent any corrosion may be the best options.

 

RE: Field welding to Exst member under load

(OP)
BA,

I like your approach / idea. When weld along a-c is performed (even if skip welded) wont I be in the same position of loosing base material strength due to melting at the connection location where shear is greatest?

I am not following how adding the weld along b-d first will strengthen the beam end for the time weld a-c is performed.

I am misunderstanding your approach?

  

RE: Field welding to Exst member under load

BA-
I can see where your approach may help mitigate future failure, but adding anything while the beam is already beyond capacity is a bit like wiping after pulling your pants up, no?  

RE: Field welding to Exst member under load

Fillet weld b-d does not add strength to the existing web, but weld a-c starts to add strength immediately.  Both of these welds are performed in flexurally low stressed areas.  The moment at a-c is only 4/13 = 30% of the moment at the end of the cope.

BA

RE: Field welding to Exst member under load

ToadJones,

Nice comparison, TJ.  I posted at the same time as you, so you have not seen my response.  The first two welds would be performed in flexurally low stressed zones, although the shear stress is fairly high.   

BA

RE: Field welding to Exst member under load

I guess the web would actually have to crack in order to mobilize your reinforcing plates then, right?  

RE: Field welding to Exst member under load

The web would have to yield flexurally in order to mobilize the reinforcement plates.  If it didn't yield, the web would continue to carry load just as is it has done for many years.  But the safety factor against collapse would be improved drastically.

Why do you think the web would have to crack?  

BA

RE: Field welding to Exst member under load

Change "drastically" to "dramatically".

BA

RE: Field welding to Exst member under load

(OP)
TJ,

I see what your getting at. My thinking is that the current web is overstressed (by 30%) which includes a reduction in thickness for corrosion allowance which has not been verified via testing, so the existing condition may be slightly overstressed but within the factor of safety of the orig design. The current support has been in service for 60 years plus and we are not increasing the loads on it, only reporting on the current state and recommending possable modifications.  

I think that if the bars are added, the current condition is not changed, but if an increase in load does happen (large wind gust, etc..) and the web becomes overstressed to a point of failure the reinf would become active and act composite with the web when a larger deflection compared to when the rebar was added was experienced.

I am still mulling all this over and looking into BA's suggestion.

RE: Field welding to Exst member under load

A regenerator is often used in making gasoline.  They typically weigh in at 500,000 to 1,000,000 lbs!!  Yes - there are cranes that can lift them.  And when they do - eveyone holds their breaths and crosses their fingers!! QUITE A SIGHT.  We have shored a few of them while under going repairs/retrofits.

If this was set so long ago - why are you now worried about the beam capacity??  Has something changed??

If it isn't broken don't fix it!!!  That said, you may have discovered a long exisitng and worrisome problem that does need attention.

Shoring posts do exist where you could probably get most of the load supported while making repairs.  Also, there are hydraulic jacks that can be fine tuned using weight cells that can tell you exaxctly how much load you are lifting.

All of this is rather expensive - but it can be done.

RE: Field welding to Exst member under load

TL,

Could you add a heavy W8 stub bracket welded to the top (or bottom) of the weak beam.  Size it to transfer the required load to the web of supporting W36 beam, may have to be 24" long or more.

 

RE: Field welding to Exst member under load


One of my first drawing jobs when I got on the board, was the riveted plate girders of the Catcracker structure for Fawley Refinery, oddly, they gave it to me because it was simpler to draw than the circular platforms and ladders on the towers.

For anyone not familiar with these, the two vessels were connected by 56" dia. pipes. The stems on the valves were over 20" long.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Field welding to Exst member under load

(OP)
Mike,

Nothing has happened other than reports of other similiar vessels of the same time period having issues/ cracking in the webs with large copes that may not been accounted for properly. You are correct about the estimated weights, we have estimated (on the high side hopefully) of about 1,500,000 lbs to 1,800,000 lbs for calc purposes so hopefully the actual stresses are lower then we determined. Build it bigger stuff to say the least.

I am starting to realize the existing condition may be "livable" providing we can make modifications (adding the bottom bars for additional flexural stresses at web yielding and preventing crack propagation)and as BA indicated make the safety factor against collapse improved dramatically.

The question I still have is can the welding take place to add the bottom cope bars in the as is stressed condition safely?

Will the high tempertures during welding reduce the existing web capacities to a point below the applied loads. I have looked at Blodeggett's (sp) welding text (great reference) and it indicates that the temp effects on strength are short lived, over a short time period and do not extend very far. The comments may have been made on fillet welds and not PP welds, I need to revisit the text tomorrow.

I still have more mulling over to do and to consider if the double plate BA was thinking about would be a better approach for welding in the as is condition.

Thanks for all the input.

 

RE: Field welding to Exst member under load

If I remember correctly, they have to remove and replace the refractory lining periodically, that should provide a temporary reduction in load. Again, from memory, on the big vessels, it takes several weeks to strip out the worn brick and lay the new lining.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Field welding to Exst member under load

BA-
yield would be a better word....
Having said that, if the thing is already overstress, at might be already yielded.
I am NOT trying to argue here, believe me.
But, if you were standing straight up and down and I tied a rubber band to the back of your neck and the belt loop of your trousers, it wouldn't stretch until you bent over to pick up the morning paper.
We have done a lot of industrial demo jobs using pancake jacks and shoring towers in situations similar to this. As mentioned above, the jacking system can pretty accurately gauge the load being lifted.  

You may also consider welding diagonal stiffeners on the web as a stirrup.
 

RE: Field welding to Exst member under load

ToadJones,

I much prefer to shore the load during welding, but this is not an option in the present case.  If the coped web is 30% overstressed, it has not reached yield strength, but it is close to it.

If welding is to be performed while the beam is under load, it is preferable to (a) reduce the load as much as possible and (b) locate welds in areas of least stress.  

The risk of failure during welding must be carefully assessed.

BA

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