Dears,

As per the latest edition of ASME B31.3, Sustained SIF Multiplier - 0.75 or 0.0001, which one is right? What was the position as per 2004 Ed of B 31.3 ?

regards,

Sam

PS: Today, many of us are worshipping the Goddess of Learning, Mother Saraswati, heralding the coming of Spring! Pray that success comes to M/S Coade & all the forum members' efforts in learning & sharing knowledge!

Sam,

Sustained SIF multiplier 0.0001 does not seem feasible. Is it a typo?

Regards
Siv

Surely not! B31.3 Code Interpretation 6-03 dated December 14, 1987 permitted Caesar-II users to ignore the stress intensification for sustained and occasional loads. To comply with this interpretation, we can enter 0.0 in B31.3 Sustained SIF multiplier; isn't it ?

So, the query is :

As per the latest edition of ASME B31.3, Sustained SIF Multiplier - 0.75 or 0.0, which one is right? What was the position as per 2004 Ed of B 31.3 ?

regards,

sam

sam, i think this is where the engineeer is supposed to appreciate why a Sif of 0 is allowed for occational loadings.

I believe, most engineers stick to .75 to allow for potential collapse in the sustained case, however, in a tight spot i have used a sif <<.75, but only in emergency relief systems where fatigue is not an issue (single events) and only after much deliberation. (ie,chasing those red numbers)

Dear Superpiper,

Thanks for reply.

But, fatigue is not an issue in sustained case,too. Then, why will we use SIF?

I am interested to know what B31.3:2006 & B31.3:2004 specifically ask for, if any ?

regards,

sam

Take a look at B31 Case 178. (Available from teh B31.3 web page.) Here (0.75*i) is the recommended moment multiplier for longitudinal stress due to sustained loads.

Don't confuse CAESAR II switches with Code equations. That 0.0001 is a CAESAR II switch to eliminate i. Since (multiplier*i) must be greater than or equal to 1, (0.0001*i) is 1.0.

As I understand it, while i reflects fatigue "weakness", (0.75*i) produces a similar measure with respect to collapse.

So for fatigue analysis we use (i)>1, but for static analysis we use (0.75i)>1 ?

How is it posible for the SIF to reduce to 25% in the sustain and occasional loading? and even be ignored? Can someone clarify?

Regards!

But, what will happen after 1st May 2008, when B31 Case 178 expires ?

regards

sam

http://cstools.asme.org/csconnect/pdf/CommitteeFiles/15243.PDF

At the last B31.3 Section Committee meeting (September 2007 in Seattle) a motion was passed to remove the Expiration Date from B31 Case 178. The process will now take this ballot to the ASME Standards Committee. Should they accept the change, the expiration date will be removed and the Case will live on. Expectation would have the B31 Case incorporated into the base Code by the 2010 Edition.

Why is there a difference between moment multipliers between expansion and sustained stres calculations?
Because piping components do not fail with the same load magnitude when failure is determined by collapse or by fatigue.

Sir Dave,

I just found Sir John luf article in this topic. maybe later I will raise clarification.
Thanks for the effort of replying.

Regards!

Sir Dave,

I am grateful to you for this clarification on sustained SIF in ASME B 31.3.

regards,

sam

Sir Dave,

After reading Sir Luf article in newsletter and those Technical report given by Sir Breen. I now follow up my question..
The SSI(Sustain stress index) base on B31.1 is .75i as confusing post by B31.3 Interpretation #1-34 and #6.03.
I understand that the definition of SIF(i) is iS=245000N^-0.2(for carbon steel), so I conclude that the last post I make is due to ignorance of this definition. Further, I would like to clarify the question of Mr. Sam on why should SIF be use in sustain if the SIF is derived from fatigue test. But in my thoughts, lifting of supports during temperature loading as per sustain is concern, the SIF is then declared.(am i correct?)

and...

How is it posible for the commitee to obtain this .75i rather than any value for SSI and when can we say that it is .75 or .001?

This shall fulfil my little thoughts of this subject.

Thanks in advance!

Regards!

B31.3 code case 178 took all doubt out of the matter.... 0.75 SIF was the SSI no if ands or buts.

Sir John,

How about, how?
Caesar2 multiplier allow user to use .75 or 0.0001<-- when and how?

Regards!

Dear,

What I have understood from Luf-Sir's comment is that B31.3 Code case 178 is a rule of law at present - to be followed while conforming to B 31.3 regarding sustained & occasional stress, whether you like it or not. It's OK.

regards,

sam

Hi sam,

Well its the code... As he said no but's and if's. And I will end up my questioning with this subject and used the said .75i which a practice writen in the code. I think it's not yet the right time for me to find the reason of why .75i.

Regards!

I dont get it. Why are we going to use 0.0001 SIF? It says in the paper that is the greater of 0.75i or 1.0.

Hi McLovin,

Yes.. Its .75 or 1.0, as my practice I use that .0001*SIF to determined the effect of SIF on sustain. There it is much easy to defined what nature of failure will occur in the system. But I "never" use this for complaince even SIF are mainly fatigue issue..

Regards!

McLovin,

You said it right "0.75i" or "1.0". don't confuse CAESAR II switches with the Code. The Code says (or implies) stress=(index)*Moment. That index can be "i" or "0.75*i" but never less than 1.0.

CAESAR II has this: index=(multiplier)*i with the stipulation that the index minimum is 1.0. So, in CAESAR II if:
multiplier=1, index=i but not less than 1.0
multiplier=0.75, index=0.75*i but not less than 1.0
multiplier=0.001, index=0.001*i but not less than 1.0, so index=1.0

There is a "Code Interpretation" that says you can ignore the SIF for the Sustained case. The way you accomplish this in CAESAR II is to set the mulitplier to a small number - say 0.0001. When CAESAR II determines "multiplier*SIF", the minimum value allowed is 1.0, so you have effectively eliminated the SIF.

This B31 case 178 expires last May 1 this year which say's that which ever is greater.. so when Io is large enough say SIFo=10, then I have to consider SIFo=7.5 for calculation of my Sl. Using SSI=.0001 will make that SIFo=10 to SIFo=1. "which no evidence or proof i found in my reading".

Maybe Sir Richard you will guide as on what year or no# of interpretation it is stated.

Regards!

B31.3 Code Interpretation 1-34 dated February 23, 1981 File: 1470-1 states that for sustained and occasional loads an SIF of 0.75i, but not less than 1.0 may be used. This setup directive allows the user to enter their coefficient. The default is 1.0. To comply with this interpretation the user would enter 0.75.

B31.3 Code Interpretation 6-03 dated December 14, 1987 permitted users to ignore the stress intensification for sustained and occasional loads. To comply with this interpretation, the user would enter 0.0001. (.0001*SIF will yield a value less than 1.0 - CAESAR II will override this and use 1.0, effectively ignoring the SIF.)

Sir Richard,

Many thanks..

Regards!

Just wondering, if Code Case 178 is the law (has it been officially reaffirmed? If not, is it a case of ignore at your peril?), why is there no reference to supersede interpretations 6-03, and 1-34?

If only interpretation 6-03 is followed, then Code Case 178 could be violated.

Is my understanding correct?

If you go to the B31.3 web page now and view the Code Cases, the opening page now reads "All B31 Code Cases in effect as of Septeber 21,2007 will remain available for use unless anless annulled by teh B31 Standards Committee."

So CC178 will not expire. Hopefully, the method will be moved to the base Code in the 2010 Edition.

Regarding Interpretations...

Interpretations are fixed to specific Editions of the Code. To quote page I-1 of B31.3 - 2006: "Subsequent revisions to the Code may have superseded the reply." So, old Interpretations may be invalidated through Code update.

When code user opts for "B31.3 Code Interpretation 6-03 dated December 14, 1987", the CAESAR II switch 0.0001 is selected and an SIF of 1.0 is selected by CAESAR II, the stresses are reduced.

Any other precaution, consideration that we should take before opting for this code interpretation?

In my case I am having a system with high temperature (800 degree Celsius).
Please help!

Dear Sam,
It is conservative to use SIF multiplier of 1.0, Because you can get high value of code stress(Actual) when compared to "0.75","0.0001".

stressengineer,
You have not read all the posts.
It MUST NOT be less than 1, 0.0001 is only a CAESAR II switch it is NOT a value for SIF.

Dear Shahid Rafiq,
I have read all the posts and i have understood that ignoring SIF and reducing SUSTAINED STRESS is not conservative and the same i have replied in my previous post.