Building W-Shapes from Plate

[uwmadd]

I have a situation to which I have lack experience and kindly request some discussion.

We have a structure that uses approximately 10,000 feet of 175mm deep I beam (H175x175x7.5 – A Chinese shape). The Chinese fabricator is asking to build up all of the sections from plate material (apparently a sourcing problem for hot-rolled shapes). The welding of the flanges to the webs will be done with continuous fillet welds, however, a penetration weld is NOT to be performed through the web (fabricator is worried about warping). I have my doubts regarding the quality control and NDE that will be performed on the welds. I have been told that building beam profiles is common in this region. As the structure was designed using hot rolled shapes, I have some obvious concern over whether built-up beams from plate will be sufficient or not. Without significant experience with this sort of situation, I thought I would post.

The structure could be re-designed using Plate Girder techniques, but am hoping to minimize re-work. I have concern over the level of residual stress introduced into the structural shapes due to their small profiles. Additionally, manufacturing the exact geometry of the hot rolled profiles would be very difficult when considering the radius between the web and the flanges.

Surely, this subject has come up before but my searching on the forum has not yielded much success. Thank you in advance for any discussion you may be able to offer.

 

[HgTX]

You say, "The structure could be re-designed using Plate Girder techniques, but am hoping to minimize re-work." I'm confused. If what they're proposing is not to replace the rolled sections with built-up plate girders, then what are they proposing? You don't have to redesign, just take their proposed design and analyze it.

If the fabricator is asking for a substitution, you are in a position to dicate NDE requirements, which will probably come down to what percentage of the length of the weld you want them to MT.

This is a very small section. How thick is the web? What welding process are they going to be using? Are they going to be welding both sides of the web simultaneously? With a thin web, they may very well wind up with full penetration regardless unless they're using a very low-heat process.

Hg

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[mrMikee]

This happened to me once on a project. What I disliked the most was how the flanges were butt welded together at random locations (to use up smaller pieces I guess) where I had no idea what the splice detail was. I think are correct to be concerned.

Regards,
-Mike

 

[UcfSE]

You can't just substitute a built-up shape for a hot-rolled shape. You have some differences in design requirements that you'll have to check. The other concern is exactly what you stated: they cannot manufacturer the exact dimensions of a rolled shape from plate. You will have some changes in geometry to take into account.

You're other option is to require them to hire their own engineer for the work they want to change, that you will then review and accept and reject.

 

[apetr26542]

Mike has a point about the splices, you need to specify that no splices are to be permitted unless you first approve them, make them do shop drawings of all the beams, you would get shops anyway, Right, quality control is important.

 

[uwmadd]

Thanks for the responses so far. I am happy to see similar concerns.

I realize I can dictate the NDE requirements, but that does not constitute there will be 100% joint efficiency between the web and flange unless perhaps I spec 100%RT... however, that would be ridiculous on this much steel. The members are stress to capacity under maximum loading conditions.

I realize that I can take their proposed design and analyze it, but that would constitute re-work that we are trying to avoid.

Not sure about the welding process (none yet proposed) but the steel is thin (web will be 6mm). The heat process will be high I am sure due to the amount of welding that will be required. They may get penetration due to high heat, but that could also result in higher residual stress. If all of the members are built up, there will be over 25,000 lineal feet. That would be over 50,000 ft of welding!!!!

It has dawned on me to have the fabricator prove their proposal to me (via engineer) for our review and of course is an option.

Welcome any additional comments and look forward to reading them.

Apparently, this small member building technique is more common in countries where material is the driving costs, not fabrication. I am trying to get a feel for whether or not this option will be worth the risk/work required on our end.

 

[apetr26542]

Has this fabricator done jobs like this in the past, can they provide you a list of successful jobs? Once they propose their weld procedure, (likely a stitch weld), would they do a load test to failure on the beam.

Is this similar to a bar joist, where you specify a size and joist manufacturer certifies it?, maybe you should just specify your Izz and Sxx and let them and their engineer design the shapes and have their engineer seal the drawings and calcs.

 

[HgTX]

There is no way to allow this without analyzing the proposed fillet welds to see if they can carry the required shear between web and flange. You could save yourself a little bit of work by requiring them to submit calculations and checking those, but it has to be done one way or another. They also need to demonstrate that they have the required cross-sectional area, moment of intertia, torsional properties, or whatever else you feel is relevant.

You need to watch out for the full penetration, actually. If what they wind up with is something longer from one side of the web to another than it is deep, you can get a crack that separates the web from the flange. See p. 9 of

http://www.jflf.org/pdfs/wixiv2.pdf

Why does amount of linear feet mean a high-heat process?? If anything, they'll be trying to go as fast as they can.

Hg

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[uwmadd]

I like the idea of specifying the critical profile properties (such as Izz, Sxx... etc) and having the fabricator's engineer design shapes that meet the criteria.

Regarding HgTX comment regarding a high-heat process. I am making an assumption that due to the lineal feet of welding that they will try to lay as much weld down as they can in one pass, and therefore intorducing more heat into the weld than desirable.

Appreciate the comments.

 

[IFRs]

I'd also specify dimensional tolerances - ie flatness, camber, etc. Depending on their process and QC, they may not wind up with straight beams within the tolerance for rolled shaeps.

 

[HgTX]

You're within your rights to require them to submit a welding procedure. If it looks hinky to you, you might get away with making them run a qualification test.

But with such a small section (it's 175 mm, not 175 cm, right?), your fillet weld size will probably be governed by the minimum size required for heat purposes. What code are you operating under? For AISC and AWS D1.1, the minimum weld size is 6 mm for plate thickness 20 mm and under, and 8 mm for thicker plate. An 8 mm weld is typically done in one pass regardless, so I don't see them doing anything untoward to "lay as much weld down as they can in one pass".

Given that this is probably inherently a one-pass weld, I still think your worry may rather be insufficient heat because they're trying to run too fast.

Hg

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[JStephen]

I'd be concerned about welding distortion. If welds are not continuous on both sides, proper painting is a problem.

 

[HgTX]

To piggyback onto JStephen's & IFR's points--there are also standards for flange tilt & cup that they'll need to meet. Check out both AWS D1.1 (section 5.23.8) and the dimensional tolerances of ASTM A 6.

And you'll want approved procedures in place for correcting unacceptable distortion.

Hg

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[uwmadd]

Thank you everyone for your comments.

I have never run across a situation where built up members for this member profile could even be considered an economical option.

The fabricator feels confident that it can be done successfully, but is cautious to take on liability.

Regarding the welding, since it is Chinese fab, I suspect all welding will be done by hand not with an automatic machine. I am not sure, but find it difficult to believe that welding flux core by hand would result in more than 6mm weld leg in a single pass while still getting proper fusion. I suppose if time is not an issue and your welders are skilled it is possible. The beams are 175mm deep, and the plate thickness will range from 6 to 9 mm.

Weld distortion, residual stress, tilt and cuping all are issues I wholly agree with.

Fabricator indicates usual practice is 10% MT or PT but is willing to increase the % to satisfy concerns. They also indicate that actual geometries of built-up beams will be within plus/minus 2mm.
Mind you, geometries and post-weld tolerance of steel are two different things, at least to me.

Bottom line seems to be that there is good reason to be concerned. I think everyone agrees that post-welding distortion could be an issue. I think the consensus is that a welded fabricated beam is NOT the same as a hot rolled shape and unless further analysis is done, therefore, there cannot be a direct substitution. Further investigation is merited.

I appreciate your comments.

 

[HgTX]

For the fabricators I deal with, 8 mm tends to be the cutoff for single-pass welding, even for handheld FCAW, and you're probably looking at 6 mm leg size. (Why do you say a larger weld would give less likelihood of proper fusion?)

I'm in an industry where built-up plate girders are standard, so it's hard to feel too excited about it, but I can see how thinner sections can be more vulnerable. (Also if built-up sections are not standard in your industry, you're not as well set up to handle them in terms of inspection etc.)

I agree that it seems highly unlikely that this is an economical substitution, but if it's a question of availability as they say, then that trumps economics.

Funny--we had a big steel shortage here in the US for a while, blamed largely on Chinese consumption. So now there's an availability issue in China??

Hg

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[uwmadd]

Thanks Hg,

Interesting comment regarding the 8mm weld size. My concern regarding the fusion stems from circumstances where the welder is more concerned with laying a large bead and getting quite a bit of undercut or lack of fusion. However, this is probably more likely due to poor welding techiques than anything else. I have seen it happen quite often, however, the welding is typically not for structural steel.

Our industry typically does not deal in built up plate girders, therefore my post. I am fishing for some experience in that area, especially when related to smaller member profiles. Availability does trump economics... well sort of... economics drives our sourcing approach. We can always try sourcing "beefier" steel if it is more readily available.

LOL on the steel availability comment. To my understanding, China is still a root cause for the steel spike. The reason why there is an availability issue on THIS particular project is due to a less common material grade that the members are to be made of. Please don't ask. That subject is for a complete different day... maybe when I get some energy up to discuss it Reality is that the material should be sourcable as we have done so successfully in the RECENT past, however, now it appears that the tune has changed some since then... or we need to try and source the material with more vigor!

 

[apetr26542]

uwmadd, I would give them the section properties and materials you designed for, or, since you provided WF sizes. Stipulate that the built up shapes must be of like material, with minimum I and S of that section. If there are problems fabricating, it is not your problem. I would write a letter to that effect, cc owner, architect etc.

 

[HgTX]

Sounds like you should investigate pricing the larger members, unless that builds in a heap of redesign cost. I can't imagine how building up these little members wouldn't be a significant price increase.

[tangent]We had a design come through that required building something like 21" plate girders. I didn't find out about it till after it was in the fab shop. I tracked down the designer and it came down to some thing like well, within the maximum depth they could use for clearance purposes, no rolled section worked, and they asked ONE fabricator whether cover plates or building a brand-new section would be more economical, and that ONE fabricator said building the section. Meanwhile the fabricator who did get the job had to reconfigure their equipment in order to make something so small. At least the kid in the design office who I had help me track down the designer learned the valuable lesson that it's often a good idea to include two alternatives rather than guess at what a fabricator might find more economical.[/tangent]

Hg

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[JStephen]

I can't imagine they'd weld that by hand- would guess some type of fixture they can just feed stuff through.

We have had applications where we welded two channels together in a T-shape, and wound up with a gradual twist in the whole thing.

 

[EddyC]

Is there some fabrication code that these built-up sections need to conform to? Why is the liability on the engineer for what is a fabrication issue? We engineers in the USA don't have to worry about the quality of a beam. That is the supplier's responsibility.