Attachment to pad, or pad around attachment

[cycguy]

A client has supplied a design of a 15"H x 12"W temporary support lug welded to a 30"H x 30"W x 1"thk pad to be welded to 3/8" thick support skirt. There is a 14" moment arm on the load. Most times we would weld the lug to the shell, then cut the pad to fit around the lug, and weld the pad to the lug, and of course weld around the outside of the pad. We feel this is a better attachment to eliminate the kind of suction cup effect of welding the lug on top of the pad. Does anyone know of any guidelines for such an application. When one way should be used in leiu of the other by Moss or Bednar. Thanks

 

[vesselguy]

Did I read you right? You said, and I quote; "Most times we would weld the lug to the shell, then cut the pad to fit around the lug, and weld the pad to the lug" If you weld the lug to the shell instead of on the reinforcement pad (where it belongs), what is the point of adding the reinforcement pad? If you actually do this, please STOP now.

Weld the external attachment directly onto a solid (read: do not cut holes to fit around the attachment) repad. the whole point of using a "REINFORCEMENT" pad is to reinforce the shell thickness so that the combined thicknesses can resist the local bending moment generated by the external attachment (read: lugs, clips, etc.).

If you actually done what you said you've been doing, please inform you previous Clients of this and fix the problem before a failure. I do hope I am wrong when reading and interpreting your message.

 

[cycguy]

The attachment of the lug to shell is still reinforced by the pad which is welded at the junction of the lug to shell. It's common practice. See Moss's, Pressure Vessel Design Manual,3rd edit, page 431, Type 1, showing a pad around the trunnion. In the side elevation you can see the pipe protuding thru the pad. The pad is not a circle but a ring or doughnut shape. The same as a pad at a nozzle. Maybe my wording made it confusing, sorry. The lug is like a fabricated box beam section welded to the shell. The pad has the box-beam shape cut out of the center to fit tightly around the lug. The pad is welded around the outside perimeter, and welded around the perimeter to the lug/shell. I hope this describes it better.

 

[chaulklate]

If my opinion means anything, I agree with cycguy's method of attaching the lug and repad, it's the same way I was taught.

 

[code1]

Previously when I was designing vessels, I'd do a localised stress calculation using WRC 107 as well to check on the attachment, in addition to sizing (1) the fillet welds and (2) the lifting lug

By using the welding detail described by cycguy's method, I'd check the local stress at the edge of the lug using a shell thickness of the shell, ts + thickness of pad, tp.

The local stress at the edge of the pad would be checked by using the shell thickness only.

 

[cycguy]

Thanks for your responses. Yes a WRC-107 analysis has been done for the edge of lug and edge of pad, with the thicknesses described by code1. The way I see it if the lug has a large load on it, say producing stresses to 3/4 the allowable stresses then I think the lug should be welded to the shell and the pad welded around it. If the loads only produce stresses that are 1/4 of the allowable stress then the lug may be welded on the pad. A way of visualizing my concern is, say a lug is welded on a pad and has a high (P) load, per WRC orientation radially outward enough to distort the pad away from the shell. Then the shell's thickness does not really contribute to the WRC stresses at the edge of lug. If the lug is welded to the shell with the pad around it, then the pad and shell thickness act together in reinforcing and in the WRC analysis the pad thickness and shell thickness is valid for the analysis at the edge of lug. I recall seeing a note in Bednar's that the pad width to thickness ratio should not exceed 12, and if it does then plug welds may need to be used. But I have not seen anything else about when to weld the lug to the shell first then pad around, or weld the lug on top of the pad. So I guess this is the real question. Has anyone seen published guidelines to when to use lugs on a pad or lugs on the shell then paded around?

 

[doct9960]

Has anyone seen published guidelines to when to use lugs on a pad or lugs on the shell then paded around?

So far, I have not seen any published guidelines.

IMHO, the most common and practical way is the "lugs on pad" because it is easy to fabricate, weld, inspect and NDT.

 

[arto]

With the "donut" method the Shell, Lug & reinforcing pad are all tied together.

With the lug only to the pad, the load goes to the shell through the pad OD dimension, but you also have to check the lug-to-pad for tearout, etc. But if you had a stainless shell & a steel lug, the pad could act as a "poison pad" too