Help from a welding engineer please

Date 11-19-2002 15:21

Okay here is the situation.

We are welding 8" sch. 80 304 Stainless pipe to a 1" cap plate and 1-1/4" base plate (all also 304 Stainless) Our engineer has designed the piece to have a 1" fillet weld around the top and bottom. He says he needs to have a 1" weld to handle the moment which could be put on the fabrication. However he is a little bothered by the old adage that you do not make your weld any larger that the thickness of the thinnest base metal.

Could you please e-mail me an explanation of this and help me explain it to him a little better as far as the rules for weld sizing etc. I appreciate any and all help you can give me.

thanks for your time,

please use this e-mail address

dsanders@fall-arrest.com

Don Sanders

By pipewelder_1999 (****)

Date 11-20-2002 03:47

I AM NOT AN Engineer of ANY Kind (But wish I was)

A single fillet weld will have an effective throat of .707 of the leg size. In a statically loaded joint with no account for any bending moments or the difference in strength of weld metal/base metal, this joint would not achieve "full strength". If a simple "tee" joint were to be fillet welded BOTH sides with a fillet weld with legs equal to the thickness of the thinner member, then you would have an effective throat greater than the base metal.

Items that are subject to fatigue/dynamic loading don't behave the same from what I have learned and the rules for sizing/contouring welds change.

Have a nice day

Gerald Austin

By jwright650 (*****)

Date 11-20-2002 13:34

Another alternative to the 1" fillet would be a CJP groove weld. This way tension or compression normal to the effective area would be the same as the wall of the pipe provided matching filler metal is used. Keep in mind there is more prep time preparing the joint to be welded vs a multipass fillet.
A CJP groove weld will handle any moment the schedule 80 wall will handle. If it is subjected to shear you are still limited to what the wall of the pipe can handle. See Table J2.5 Allowable Stress on Welds in ASD 9th Edition or AWS D1.1:2002 Table 2.3 Allowable Stresses.
Best of Luck,
John Wright

PS. I'm not an engineer either, but I cheated and talked to our resident P.E.

By Niekie3 (***)

Date 11-20-2002 20:32

The basic principle here is simmilar to the old story of a chain breaking on the weakest link. It does not matter if you have some links stronger than other links, the chain can only carry the load of the weakest link.

If you place a 1'' weld on a 1/2" material, then the weld will be the stronger link in the chain. The total weldment will however not be able to carry any more than the 1/2" wall thickness pipe can carry. It therefore does not make economic sense to place a large weld on a thin material. In addition, large welds result in distortion of the materials that can lead to other problems.

If your engineer calculated that he needs a 1" weld to carry the loads, then he has under designed the pipe, or made a mistake somewhere else.

Hope this rather untechnical answer is sufficient.

Regards
Niekie Jooste

By billvanderhoof (****)

Date 11-21-2002 05:35

In a world of lawsuits though you don't want to be the smithy who forged that weak link so maybe you put a little extra material in yours. Of course if everyone does that things can get out of hand.
Bill (not an engineer either)