Re: Software

From: <Christopher>
Date: Thu Jan 05 2006 - 13:33:00 EST

On Jan 5, 2006, at 5:44 AM, Steve McKenzie wrote:

> Ug22 Sec8 and 302.3.5 B31.3 rquire the design has to meet (within the)
> the
> code allowables including all loading (not excluding upset/transient )
> loadings.

Not quite true. UG-22 provides that other loadings than pressure need to be addressed if they exist. It doesn't say anything about allowables. The Code allowables apply where Codes rules are set out for particular details. U-2(g) is the blanket section for details not covered by Code rules and it states that the details provided shall be 'as safe as those provided by the rules of this Division.' That doesn't say anything about allowables either, just about relative safety.

The hooker with that language is that it puts a helluva lot of responsibility onto a designer to establish true margins of safety, to understand just what the Code actually provides and to match that. Just applying code allowable stresses blindly could be overly conservative or unconservative. For example if you design a detail against fatigue using the Code allowable as your endurance limit, you're way over-designed; conversely if your detail is subject to buckling the Code allowable may very well be too high for stability. If you have a crack and you try to evaluate it using the Code allowable, you're flat wrong. That means you must use judgment, because you can't just pretend that the Code will keep you safe in situations which it doesn't address.

> Is it necessary to exercise judgement or do you guys always work to
> code?

It's always necessary to exercise judgment, but the first element of judgment is not to ignore experience or the mistakes of others. The Codes are a distillation of almost 100 years of field experience and lab work supervised by experienced working engineers. Only a fool ignores that kind of experience.

Remember that Code allowables are matched to the accuracy of the design methodology. When you use a Code you really need to get to the intent behind the rules. Not meaning to slander any particular individual, but this list gets a helluva lot of questions from people who clearly don't understand the Code they're trying to interpret or worse, from people trying to cherry-pick provisions like 302.3.5. If you don't understand intent, your judgment's no better than any other hip-shot. If you're going to use a non-linear material like plastic and consider transient loading, you really need to understand where the Code allowables come from and whether there's enough of a margin against creep or momentary high stress or temperature. If your conditions are out of the scope of the intent of the Code, you have to use judgment, but it has to be based on first principles and study.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=ZyBR8P2YPIkKhPG4_ws-H62aHKGw1FbzBcaFAglbFVVUGFTfcovFQ6XT32H4s_kXshmdyr3FU9rKWFc">chrisw@skypoint.com</a> | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)
<a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a> Received on Thu Jan 05 13:33:00 2006