Building W-Shapes from Plate
Building W-Shapes from Plate
(OP)
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.
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.






RE: Building W-Shapes from Plate
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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RE: Building W-Shapes from Plate
Regards,
-Mike
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
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
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
RE: Building W-Shapes from Plate
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
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate
RE: Building W-Shapes from Plate
And you'll want approved procedures in place for correcting unacceptable distortion.
Hg
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
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
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate
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!
RE: Building W-Shapes from Plate
RE: Building W-Shapes from Plate
[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
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate
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.
RE: Building W-Shapes from Plate
RE: Building W-Shapes from Plate
I personally would opt for a built-up box beam for section stability.
RE: Building W-Shapes from Plate
We engineers in the USA in the QA field have to worry about making sure the suppliers are actually meeting their responsibility. (We do a better job of this with the fabricators than with the mills, who seem to be answerable to no one at all...)
Hg
Eng-Tips policies: FAQ731-376
RE: Building W-Shapes from Plate