Placing a backing bar in an inaccesssible location 1

[canwesteng]

We have an issue where we welding in an area with little access. I have come up with the attached detail, but I believe it requires pre tacking a backing bar to the tee being welded to the wide flange, or pre tack it to the wide flange. Is this acceptable?

 

[Guest102023]

It's probably doable, but to be perfectly honest the entire stacked-up configuration looks like a dog's breakfast made from found materials.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[dhengr]

Canwesteng:
Who says that those three welds need to be full pen. groove welds, with back up bars? You have to provide sufficient weld to take care of the shear flow at each location, and not much more, unless you have something unusual going on. On the top two .5" pls., make a groove (3/8ths grooves) so that you leave an 1/8" land on the back side. Fill the groove and add a reinforcing fillet if needed. But, now you should recheck the weld btwn. the ‘C’ and the WF top flg. to be sure that they are o.k. for their added shear flow. Do the same thing with the stem on the WT. Maybe make the grooves only 1/4" (thus 1/4" lands) and slightly larger reinforcing fillets. Good fit-up, and a good root weld into the land, is really no different than welding into a back-up bar, except the back up bar means a larger groove. Groove welds are generally more expensive than fillets for a given weld cap’y.

 

[canwesteng]

Without a doubt the full pen is more expensive, I'd be using two fillets on each side if I could. My worry with a partial pen is that there is some eccentricity in the weld causing stress raisers of some kind. Now maybe no backing bar is needed for full pen, is that what you're proposing?

 

[wannabeSE]

Tacking backing in place is common. Blogett has an article on backing at

http://weldingdesign.com/archive/blodgetts-basics-good-weld-design-details.

Too bad the figures are missing from this online edition.

 

[racookpe1978]

Rule of thumb.
Absolutely for inaccurate for anything but illustrating a point.

Changing the WF (or angle iron or plate) to a more expensive but stronger shape = $1.00 per foot.
Finding, cutting, installing, and then welding a backing bar to an angle iron or plate or WF = $100.00 per foot.

 

[canwesteng]

No doubt that the field labour is colossally more expensive than the material. In this case though, shrinking the material helps substantially because it reduces the thickness of the CJP. And the difference in labour between a 3/4" CJP and 1/2" CJP is about a factor of two.

 

[KootK]

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[canwesteng]

Let's forget everything above the C12 for now, I've designed around that. If I'm welding the T to the bottom of the W21, and it's a fillet on one side only, the shear flow is eccentric to the centroid of the beam in its weak axis, and this will induce a) some weak axis bending in the beam (no issue) or b) some bending in the weld (boom, brittle failure). Am I imagining this, or is there some substance to my worry?

 

[KootK]

Am I imagining this, or is there some substance to my worry?

Theoretically, I don't think that there is much substance really. Thoughts:

1) The eccentricity will be under an inch. I'd be inclined not to worry about it for the most part.

2) The eccentricity should create a tendency for the wide flange to bow left and the WT to bow right. Since the two members are connected, however, each will exert a countering lateral force on the other rectifying things. In the absence of the stabilizer plates that I recommended, that force would need to go through the WF/WT weld. That's my primary reasoning for the stabilizer plates.

3) Despite there not really being a theoretical basis for it, I can't shake the feeling that the WT might experience some tendency to roll which would put the weld in bending (your sage worry). I feel that the stabilizer plate does a fine job of ensuring against that as well.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

If you dont put in the stiffener in KootK sketch, I would not use a single side fillet weld. Lateral deformation of the Wtx will cause a crack in the backside of the weld and reduce capacity.

 

[WKTaylor]

Anyone thought about accessibility for good quality visual inspection and NDI?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[CANPRO]

if this is a dynamically loaded structure, I believe that you must remove the backing bar once the weld is complete. Looks to me like this is a crane rail - so you might want to take a look at the backing bar & dynamic load requirements

 

[canwesteng]

I know backing bar needs to go for seismically loaded structures, but this is a maintenance crane subject to relatively few cycles.