Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

Register Log in
  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Placing a backing bar in an inaccesssible location 1

Status
Not open for further replies.
canwesteng

canwesteng

Structural
Joined
May 12, 2014
Messages
1,752
Location
CA
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?
 
 http://files.engineering.com/getfile.aspx?folder=fe5f37e8-6948-4330-b932-1f80191af6f8&file=sketch.pdf
Sort by date Sort by votes
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?"
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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?
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
Capture01_skd9ok.jpg


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.
 
Upvote 0 Downvote
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?
 
Upvote 0 Downvote
canwesteng said:
Am I imagining this, or is there some substance to my worry?
Click to expand...

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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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]
 
Upvote 0 Downvote
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
 
Upvote 0 Downvote
I know backing bar needs to go for seismically loaded structures, but this is a maintenance crane subject to relatively few cycles.
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

W
  • Locked
  • Question Question
Bar Joist Strengthening - Value Engineering
Replies
1
Views
2K
dik
dik
P
  • Locked
  • Question Question
Welding a member on top of an existing weld
Replies
7
Views
5K
pinaki_stahlbau
P
Hansac
  • Locked
  • Question Question
Use of Nitrogen as Backing Gas
Replies
16
Views
2K
gtaw
gtaw
struclearner
  • Locked
  • Question Question
Spacer's Dimensions used in a Bolted Joint
Replies
16
Views
2K
mfgenggear
mfgenggear
L
  • Question Question
Tension/Bending Checks for C shape in weak axis
Replies
3
Views
10K
ErikBjurSE
E

Part and Inventory Search

Sponsor

Back
Top