Field weld vs. shop weld

[braves25]

Hello all.....when determining the capacity of a field weld vs. a shop weld, is there any additional safety factor to be applied to the field weld? I have always been told to provide an additional 2.0 FS to any field welds; however, I cannot find any documentation of this. Would a field weld require additional inspection to have the same capacity of a shop weld (all other things equal)?

Thanks and have a great day.

 

[CANPRO]

There is no code requirement that I know of, but if you feel it is necessary to increase a factor of safety for any given circumstance that is your call as the designer. However, if you show say a 1/4” fillet or a CJP on your drawings, it is the contractors responsibility to provide the capacity you require/specify...the difference in a field vs shop weld is in the cost. It is more costly to provide the same capacity weld in the field vs the shop. Inspection and testing of the weld in the field is more expensive as well.

There might be situations where field welding isn’t allowed at all. I’ve rejected repair procedures for work before because I didn’t believe the field weld could be reliably completed.

 

[WARose]

I've never really put a safety factor on the strength of the weld. However I have (at times in the past) taken into account how hard some things may be to reach for a field weld. A good example is when I have added stiffeners to beams in the field. If I think those corners will be hard to reach, I may discount the segment near that corner.

 

[Lomarandil]

In theory, field welds and shop welds should both be specified to some standard (in the US, typically AWS D1.1 or D1.4). When followed, this specification will develop the same functional capacity and quality of welds whether performed in the shop or the field. (Granted, shop welds may be "prettier" or may have additional QA/QC policies implemented by the shop themselves). But there is no requirement to penalize a field weld for being a field weld.

Of course, practical experience shows us that some field welds are produced to lower qualities than would be accepted by the specification, and some projects/contractors do not fulfill the inspection/QA requirements specified for them. As such, many engineers (myself included) may increase the weld size to account for a difficult-to-perform field weld, or a field weld performed by a contractor who has a poor history for quality.

I typically don't increase any weld capacity by a factor of 2 -- but my contractors have been by and large pretty good outfits.

----
The name is a long story -- just call me Lo.

 

[BridgeSmith]

We typically try to avoid field welds for locations where fatigue is an issue, due to the difficulty of assessing and testing the quality of the weld in the field. We apply no reductions on strength.

 

[TehMightyEngineer]

It also really depends on the access to the weld. If they're forced to weld overhead in a tight location then even the best welder will struggle. If they're putting a 3/16" fillet weld on a easily accessed horizontal joint there should be almost no difference in quality.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[MotorCity]

Also note that for field welds, wind can be an issue as well as temperature of the base material. I think the perceived difference in quality between a field and shop weld really comes down to the position and comfort of the welder making the weld. If the welder is working outside on a sunny day making a weld standing on flat ground, you will probably get the same quality as a shop weld. If the welder is hanging upside down clinging to the steel while making the weld in a driving wind, I'd have my doubts.

 

[cliff234]

Forty years ago, when I started out in the profession my first employer (where I worked for two years) had a standard practice of reducing the strength of field welds. Ever since, I have not encountered this practice. I use the same strength, regardless of whether they are shop welds or field welds. That said, it is important for engineers to consider challenges in making field welds and considering those challenges when developing details. If a field weld is impossible to make, then adding a safety factor of 2 is not going to help. In fact, if you double the size of an already difficult weld, making it twice as big only makes it twice as difficult (and expensive). If constructability considerations might result in a sloppy weld, then as an engineer who is ultimately responsible for the safety of the structure, my solution would be to reconfigure the detail, not make the weld bigger. In fact, one of the guiding principles of good weld design (that I have heard many times in many welding seminars) is to NOT arbitrarily make welds bigger than needed for strength. Consideration of constructability, weldability, fatigue, etc. during design is the key to getting the strength you need – not making the weld bigger. Likewise, requiring trained and certified welders is equally important.

In my opinion all engineers should have to opportunity to do some welding in order to understand the challenges of making good welds. (After 40 years of designing welds, I have yet to actually weld – but it’s on my bucket list!)

On a related note, I took my 11-year-old grandson to a science fair in Philadelphia a couple of weeks ago. To my surprise Lincoln Electric had an exhibit there where kids could try their hand at virtual welding with some incredibly sophisticated welding simulators! Attached is a photo of him making a virtual fillet weld. “Welders” are graded by the computer on a number of different things (speed, angle, etc.). Thank you Lincoln Electric!

 

[TehMightyEngineer]

Great thoughts Cliff. Another item I'd add is watch some YouTube videos on welding and fabricators. Seeing how they tackle problems or how they explain the correct procedures, combined with some nice camera work so you can actually see the weld puddle, really helps understand the welding process.

Thanks also for reminding me that I still have a steel plate in my backyard I was going to turn into a metal workbench as an attempt to learn a little welding.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[braves25]

Thank y'all for your responses. Most helpful! Have a great day.

 

[Settingsun]

A factor of two seems high. The Australian standard has two categories of weld: General Purpose, & Structural Purpose. The design capacity of a GP weld is taken as 0.75*SP weld. All major structural welds are meant to be SP category, but even a fairly ordinary quality of structural weld should meet GP requirements. Fairly common IME to specify SP field welds but design for GP strength so you don't have to worry about the final quality achieved in the field.

 

[Lomarandil]

Interesting Steve... what additional requirements are made of SP welds? More inspection, I presume?

----
The name is a long story -- just call me Lo.

 

[Settingsun]

More inspection, fewer defects, and I think pre-qualification of the weld procedure (or more stringent pre-qualification). I'm a bit sketchy on pre-qualification: it's a nice-to-know for design engineers, not essential knowledge.

 

[Settingsun]

I should also say that my approach is to design as GP and specify GP. If the contractor is confident of achieving SP quality, you'll get an RFI requesting the change to save money. Welding is expensive and no-one will thank you for over-specifying if they find out.

I offered the GP vs SP ratio (1÷0.75=1.33) for comparison with the FOS=2 in the original post.

 

[Ron]

A weld is a weld is a weld....well mostly!

Shop welds are easier for the welder. They can be done in convenient positions, the conditions are more controlled and can be done using any of several welding processes (GMAW, FCAW, SMAW).

Field welds are more of a challenge for the welder. You are generally limited to two processes (FCAW and SMAW) since shielding gas can easily be compromised by even light winds in the field. The position is "as you find it" in the field so vertical and overhead welding are often necessary, both of which require greater skill on the part of the welder.

As Lo noted, the welds are typically done to some code standard and if done to those standards and inspected, whether field or shop, there should be no additional factoring issue.

 

[Agent666]

One thing to note with the Australian/NZ SP/GP weld scenario discussed above, is that GP (General Purpose) welds are not allowed in any seismic systems or any systems subject to fatigue provisions in New Zealand. SP (Structural purpose) welds are always required in these scenarios. Australia and New Zealand share joint welding standards.

I wanted to clear these following points up as its quite important:-
My understanding is the difference between GP and SP welds is purely just the level of inspection required, that's all that differentiates the two. It's just recognising with a lower level of inspection there is the possibility of higher risk of defects not being detected. The level of acceptable defects is the same for both categories.

The fact is because they are the same, the welds share the same welding procedure, and therefore with a competent welder should essentially achieve the same weld quality. Specifying a GP weld should not result in a lower quality weld (or acceptance of a lower quality weld) as seems to be a common misconception with designers. It is the same weld deposited using the same procedure with a lower level of Non Destructive Testing (NDT) highlighting the higher risk associated with accepting a lower total percentage for the testing, essentially manifested in the lower strength offsetting this increased risk.

I just design welds for the design load, no oversizing, except where I want to be able to develop the full strength of the parts being joined such as in seismic design where members may be yielding. Its false economy in my mind to oversize any welds in the design. See the following example:-

Take a 8mm SP weld, an equivalent GP weld based on strength would be a 12mm weld (using 490MPa electrodes). Remembering they are the exact same weld in terms of placement, the 12mm weld probably requires anywhere from 4-8 passes to build up the required size, the 8mm weld 1-2 passes usually. Yes you have additional testing but it generally works out in your favour cost wise if you consider the larger GP weld takes up to 4 times the number of weld passes (and time) to achieve. May be different elsewhere in the world regarding labour and consumable costs. You have to remember going up in weld size exponentially increases the number of passes, and hence cost, its not a linear thing at all.

Hopefully gives original poster some food for thought!

 

[Agent666]

I'd also add, if you know that a certain weld needs to be done in the field, then at least put some thought into how to make it easy for the welder and those testing the weld (i.e. thinking about where you might be locating field splices for example to enable welding from above, or the ability to flip/turn/re-orientate the piece before erection).

 

[Settingsun]

AS 1554.1 does have different limits on imperfections between GP & SP. See Table 6.2.2. I understand SP is similar to AWS D1.1.

I have a 1998 document (maybe the costs have changed since) from the Welding Technology Institute of Australia and the Australian Institute of Steel Construction (joint publication) - Technical Note 11: Commentary on AS/NZS 1554 - that says to choose GP where appropriate (eg seismic/fatigue needs SP as Agent666 said):
"The design engineer should always endeavour to specify GP weld category where appropriate in order to take advantage of the less stringent qualification requirements, minimal testing and lower production costs associated with this weld category."

 

[XR250]

You are generally limited to two processes (FCAW and SMAW) since shielding gas can easily be compromised by even light winds in the field.

Ron,
I though flux core was limited to the 110V homeowner buzz boxes? I have never seen anything but stick welding in the field.

 

[Agent666]

steveh49, thanks for that, obviously I'm wrong on the level/degree of acceptable imperfection differences!

In NZ our steel standard NZS3404 (appendix D if interested) has suggested similar levels of testing percentages for GP and SP categories (informative though), with final percentages agreed between principle and engineer (this bit is normative, so must be complied with). For example suggested level requires upgrading the GP 0-2% magnetic penetrant testing from AS/NZS1554.1 for GP welds to be identical to SP welds (i.e. 0-10%), and a higher 0-15% testing requirement for radiography/ultrasonic testing for SP welds (up from 10% in the welding standard). As far as I can tell AS4100 does not have the same requirements, it simply directs the designer to AS/NZS1554 for the requirement.

Visual examinations are required to min 25% for GP and 100% for SP welds in NZ, vs the lower 5-25% (GP) and 10-50% (SP) in the welding standard. In a NZ context the actual degree of testing ultimately required between the two categories can reach levels that are quite similar. In NZ at least, I believe the majority of welding is specified to SP standard by default (simply because of the seismic requirements requiring it), so I guess its just something everyone lives with here. Hence my comparison regarding longer welding times using larger GP welds of equivalent strength (using essentially the same welding specification procedure) actually not stacking up in my experience in combination with similar NDT testing percentages. Suppose it ultimately depends on volume though, how many welds can you can actually classify as GP for example to offset time/testing/cost in the various processes.