Field welding to Exst member under load

[TheLinker]

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

 

[ToadJones]

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

 

[ToadJones]

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

 

[TheLinker]

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.

 

[ToadJones]

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.

 

[msquared48]

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

 

[ToadJones]

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

 

[msquared48]

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

 

[ToadJones]

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?

 

[msquared48]

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

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

 

[msquared48]

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

 

[ToadJones]

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

 

[paddingtongreen]

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.

 

[BAretired]

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

 

[BAretired]

Sorry, I forgot to upload the sketch.

BA

 

[TheLinker]

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.

 

[TheLinker]

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?

 

[ToadJones]

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?

 

[BAretired]

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

 

[BAretired]

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