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Welding between a thick plate & a thin plate

Welding between a thick plate & a thin plate

Welding between a thick plate & a thin plate


Which type of weld should be used to weld a 1 3/4" plate to a 1/4" plate? As far as I know the weld size should be smaller than the thinner plate, but in this case the other plate is too thick. Will complete penetration work? Or will it affect the 1/4" plate.

Your help is greatly appreciated.

RE: Welding between a thick plate & a thin plate

Not enough information to help you. The choice of a weld joint detail is based on loading and service conditions.
What is the application?

RE: Welding between a thick plate & a thin plate

This is a spreader bar for lifting a 8 ton motor. The spreader bar is W6x15 and nearly 3'-0" long. A lifting lug of 1 3/4" thick plate needs to be welded to the top of the top flange. The top flange thickness is about 1/4". The motor is hooked up at the ends. Please see the attachment.

RE: Welding between a thick plate & a thin plate

Looks to me like an external fillet weld would work just fine with this spreader beam.

RE: Welding between a thick plate & a thin plate

So the fillet weld size should be less than the 1/4" ,am I right?  

RE: Welding between a thick plate & a thin plate

No. Size the fillet weld for load applied from lifting + safety factor through the structural member.

RE: Welding between a thick plate & a thin plate

Thank you very much for your help.

RE: Welding between a thick plate & a thin plate

Do I understand your condition?  A 1 3/4" PL (lug) is welded to the top or bottom surface of the beam flange, at the centerline parallel the beam web?  

If this is the case AWS minimum fillet weld size for material thickness is not an issue.  A 5/16 fillet can be applied on both sides of the lug.  However the wide flange must also be adequate for the loading.  Also two 5/16 fillet welds will not match the tensile strength of the 1 3/4" lug.   

RE: Welding between a thick plate & a thin plate


RE: Welding between a thick plate & a thin plate


In addition to uncleyd's references ASME has released a Below the Hook design guideline (BTH-1) that does a good (or at least better than before) job outlining what is required for lifting beams and other similar devices.  I recommend reading through it if you haven't already as unlike some other specifications it's relatively clear.

Also if you are in the United States OSHA has requirements that cover riggind devices such as lifting beams.  Besides the 5:1 safety factor one of the more important ones is that these are required to be marked with a safe working load and proof tested prior to use to 125% of the SWL.  I mention this because lately I've been running into these one off designs and this step is missed.  OSHA section number 1926.251(a)(4).

RE: Welding between a thick plate & a thin plate

Thank you very much for the help, I appreciate everybody's attention.

RE: Welding between a thick plate & a thin plate

You may also find most offshore lifting specs will require full penetration welds. Your client will need to specify the required lifting standard, as your design and testing can vary significantly based on specs.

RE: Welding between a thick plate & a thin plate

Fillet full penetration  (both sides) 5/16''. Welding process GTAW ER70S-6 2.4mm

RE: Welding between a thick plate & a thin plate


You have a very nice CAD drawing of a symmetrical spreader beam, very short, very light, and non-compact too.  I sure would like to see the design calcs. for that spreader beam, including some consideration of stability and buckling.  Then, let's see your rationalization of the load path and stresses, at each level, from the top lifting lug (at 16kips), through the light flg., to the light web, through another light flg., to the two bottom load lugs (at 8kips ea.).  It's actually a fairly complicated little structure, which causes very complicated load paths, and direction changes, through a bunch of potential hard spots and stress raisers.  These are really very complex, tri-axial, stress conditions at a point.  The old std. beam design approach we learned in college does not always lend itself well to this type of equip., but isn't to be ignored either.  The beam with its light flanges and web and the web stiffener welds which you didn't ask about present some real potential problems too.  And, you are wondering about the weld size btwn. the lifting lug and the 1/4" top flange, which seems to me to be the least of the potential problems?

You are most likely to get a poor HAZ, creators, undercuts, etc. at the start and stop of the weld of the lug to the top flange, these are all stress raisers, four of them at the lug corners.  That's right where you have an identical welding condition and stress raiser on the underside of the flange at the web stiffeners.  These are awful, hard spots, right where the stresses are transmitted through the flange, in the through thickness orientation.  Otherwise, it is assumed that the lug welds transmit their load through the bm. flanges acting as short cantilevers off the web and its radius to the flange.  But, this bending load transfer, in the flange, won't happen until you get some considerable yielding, or maybe worse, at the four hard spots, at the corners of the lug.

Pin bending and bearing stress (Hertz stresses) and the real max. stresses in the pin plates are important to look at too.  You will quick discover, that for a lot of equipment like this, it really doesn't pay to try to use structural shapes, right out of the book, they just don't meet our needs very nicely.  Built-up sections, where we can pick our web and flange size often fit the bill better.

Some engineers and specs. are inclined to weld everything with full penetration welds, at great cost, and potential detriment to the structure, when there is nothing about the loading or stresses which require this. Imagine what a pretzel that light beam would look like after welding those 1.75" lugs with full penetration welds. To your original question; two 5" fillet welds would be fine, except for the issues above.  Thus, (load factor)(16k)/(2)(5") = shear flow (kips/inch)/ (allowable weld stress) = (design throat of weld)/.707= filet weld size.  That being said, it is important to pay attention to OSHA, ASME and the like, but they are not design guides, they are requirements, to be met.  Do offshore lifting specs. really require full penetration welds, which might be detrimental to the structure, or seal welding of all joints?

Why not consider using a 3/4" plate, or some such, 38.25" long and 16"± high, flame cut it to a shape that the loads and stresses would tend to take, btwn. the three load pin points.  A boomerang shape comes to mind, and that way it would come back to you when they're done with it.  At the three load points (pin holes) apply 1/2" stl. pl. doughnuts, (6 total), fillet weld all around, with a 6" O.D.; the 1&5/16" I.D., for your 1.25" bolts, is drilled or line bored after everything is welded up.  I wouldn't change hole sizes on the links either, that just makes the machinist change tools.  You sometimes need some web stiffening flange plates on an arrangement like this.  These are just some ideas for you to think about, you do your own design.


RE: Welding between a thick plate & a thin plate

So, considering that excellent summary, align the lugs with the lifting forces (at an angle to vertical but with the lug's main axis on the main beam not transverse to the beam's web) so the force go directly through the lugs down to the web of the lifting beam?


RE: Welding between a thick plate & a thin plate


A plate is only a few pounds fer foot, and only a few dollars few foot length.

Get a thicker plate for the lugs?   It would be cheaper than the time spent analyzing in advance of a (potential) accident and penalties.    

RE: Welding between a thick plate & a thin plate

DNV at least requires full penetration, particularly for things like ISO frames. If the lug and beam are sized correctly then you shouldn't get stress hardening. Either way the gussets should prevent any distortion through the web. Don't worry about the stress concentration in the weld, in reality (unless there is a flaw in the weld) it will only affect fatigue.

@ racookpe1978
He already has his lugs aligned correctly. Some spreaders require that the lugs be traverse, however in these cases it is very important to try and balance the forces, and factor in any eccentric bending in the strut/beam for buckling. Most lifting specifications also cover all this anyway.

Check out the Woodside lifting spec for a good general onshore and offshore lifting spec. You should be able to find it online for free.

RE: Welding between a thick plate & a thin plate


Look back a the thread on 1" insulation board.

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