Dear members,

ASME published ASME B31E " Standard for the Seismic design and Retrofit of above ground piping system"

This standard provides rules to qualify existing above ground piping for seismic adequacy.

My basic question is related to higher stress allowable in B31E

Question-1: Why B31E has two condition for seismic evaluation where as B31.3 has only one condition? condition are describe as below.

1) B31E requires two equation to satisfy
(i) PD/4t + 0.75i (Msustained + Mseismic) / Z <= min [ 2.4 Sh, 1.5Sy, 60 Ksi]
(ii) Fsam / A <= Sy

where as B31.3 requires to check sustained stress + Occasional stress <= 1.33 Sh.

Question-2:
Does ASME B31E has higher allowable (2.4 Sh) compared B31.3 ( 1.33 Sh) only because of consideration of seismic anchor motion ( SAM) in B31E ? as B31.3 is silent on seismic anchor motion.

Question:-3 How to calculate Fsam ?
Fsam= Resultant force (tension plus shear) due to seismic anchor motion
assume D is SAM

(i)W+P+T+U+D ====> eq.1

(ii)L1= W+P+T+U+D
L2=W+P+T+U
L3= L1-L2 ====> eu.2

(iii)
L1= W+P+T+U+D
L2=W+P+T
L3= L1-L2 ====> eq.3


which is most appropriate equation to calculate Fsam out of three eq.1, eq.2, or eq.3 ?

consider all this with seismic with static approach.

Kindly share your view to clear my understanding..
thanks!

Hardik
hardik.pilojpara@ril.com

Yes, B31E offers an additional stress check on the strain associated with seismic anchor motion and also changes the allowed stress for the SUS+OCC combination. In my opinion, the B31 code books have not adopted the B31E checks because of these two issues. That's not to imply that the individual books do not accept the physics promoted by B31E. Let's take a look at these two concerns.

Seismic anchor motion - in many instances, the strain caused by independent support motion is more destructive than the inertial loads. The B31 codes throw strain into the flexibility (expansion stress range) calculation. B31E itemizes the seismic strain for separate check here. It's a bit of a strange calculation - the resultant force (both axial and shear) due to this seismic strain is divided by the pipe cross section. That stress must less than yield - not 2/3 yield (as in Sh).

Higher SUS+OCC limit - this gets back to the allowable stress design (ASD) used by B31 and the load resistance factor design (LRFD) used by structural codes. ASCE 7 is often cited as a source for these seismic inertial loads. As a structural code, ASCE 7 uses the LRFD approach. ASCE 7 states that if ASCE loads used for ASD, the ASCE loads can be reduced by 30%, i.e., ASD=0.7(LRFD). But that is only for stress evaluation. Those ASCE loads should be used without reduction if you are after structural response of the piping system - pipe position and restraint load. This implies that there should be two analyses (again, this is my opinion) - one for stress - with the lower seismic load and a second analysis for system response - with the full load.

What I see in the higher allowable SUS+OCC stress is a way to eat your cake and have it too. Use the high load for a proper structural response (no 0.7) and then boost the allowable stress so those loads can also be used for stress evaluation.
Now, your other question - how to isolate those seismic anchor motions (SAM) in a static analysis. First, I must assume that supports are grouped by the deflections of their supporting structures. (If all supports moved in unison, there would be no resulting strain.) If the system is linear, I would run the SAM displacements as their own load case - one for each support group independent of the others. (Any group could be completely out of phase with the others - one structure is moving left while the next one is moving right.) Then these displacement sets should be summed (absolutely?). If your piping model is not linear, I figure an "operating plus SAM" minus "operating" would be the proper approach. But you should also consider "operating minus SAM" minus "operating".

One other B31E note: if using the ASCE 7 seismic g load for piping, B31E states that Rp shall not exceed 3.5. ASCE 7 lists an Rp=12 for typical B31 welded piping. Rp appears in the denominator. If you still use 12, your load is over 3 times too low.

Dear Dave,
Thank you very much such detailed and quick response. Really appreciate.

i request you to throw more light on my few more ignorant question, correct me please...

My few more questions..
In my file model is Non-Linear.
My load cases are ( P= Pressure, T=Operating temp. D=SAM )
L1= W+P+T (OPE)
L2= W+P (SUS)
L3= W+P+T+U (OPE) --- WITHOUT SAM
L4= W+P+T-U (OPE) --- WITHOUT SAM
L5= W+P+T+U+D (OPE) --- WITH SAM
L6= W+P+T+U-D (OPE) --- WITH SAM
L7= L3-L1 (OCC) ---- PURE SEISMIC
L8= L4-L1 (OCC) -----PURE SEISMIC
L9= L5-L3 (OCC) -----PURE SAM
L10=L6-L4 (OCC) -----PURE SAM

TO CHECK FOR CONDITIONS OF B31E WILL FOLLOW FOLLOWING STEPS..

STEP-1:
(1) PD/4t + 0.75i (Msustained + Mseismic) / Z <= min [2.4 Sh, 1.5Sy, 60 Ksi]
To calculate Msustained will use load case L2= W+P (SUS)
And finally calculate
Msustained = √ (MX2+My2+Mz2) -- (i)
To calculate Msiesmic will use load case L7= L3-L1 (OCC) – PURE SIESMIC LOAD
Mseismic = √( MX2+My2+Mz2 ) -- (ii)
With eq. (i) and (ii) will calculate
PD/4t + 0.75i (Msustained + Mseismic) / Z <= min [ 2.4 Sh, 1.5Sy, 60 Ksi]
Similar for both + and – cases.

STEP-2: CHECK CONDITION 2 OF B31E.

CONDITION (2) Fsam / A <= Sy

To calculate Fsam, I will use L9 = L5-L3 (PURE SAM)

Will list Faxial and Fshear from L9 case.

Fsam = √ (Faxial) ^ 2 + (Fshear) ^2 --- (iiI)
And
Finally calculate Fsam / A <= Sy using (iii) ..


My main doubt is about calculation of Fsam, should i use load case

L9 == Pure SAM or load case L3= W+P+T+U (OPE)


It would be great if you comment on load case..

thanks a lot again for your detailed answer..

if any one has more details available kindly send detail to my personal mail
(hardik.pilojpara@gmail.com) also..

Thanks Dave!!

Regards.
Hardik.

hardik.pilojpara@gmail.com / hardik.pilojpara@ril.com

That looks pretty good.
CAESAR II can do the SUS+OCC combination for you but the calculation will also include an axial stress due to direct axial load (F/A) term. If you send your output to Excel, you can easily grab the terms you wish and calculate your B31E stresses in Excel.
I question your SAM loads. If all supports see the same seismic displacement, then there will be no strain from SAM. You get strain by having different support groups with their own maximum displacement and their own "timing" (these groups hit there peaks at different times).
So I think you need more than a single displacement set.
Since these support groups are independent of one another, each one should be evaluated alone and then combined with the other(s). This would accommodate the situation where support group 1 is wagging to the left while support group 2 is wagging to the right. In a dynamic analysis, these support group strains would be summed absolutely. B31E does not state how to treat these strain summations.
CAESAR II can sum stress absolutely. Using your load case combinations, isolate response to each support group (+D1, -D1, +D2, -D2, etc.). You might then collect the maximum of +D1 & -D1 and the maximum of +D2 & -D2 (as opposed to evaluating all the +D's, then -D's). Then sum the loads from these maxima (1, 2, etc.) for your B31E SAM force calculation. From here you go back to Excel to develop the resultant force and use this to calculate F/A.

I would add that M_Seismic is the elastically calculated resultant moment amplitude due to seismic load, including inertia and relative anchor motion.
In other words, must be the bending moment that includes the SAM effects.

Second, note that after years 2000, ASCE 7 adjusted the seismic parameters corresponding to a return period as less than 2%-50 years (2475 years return). A factor of 2/3 was considered to scale-down the stress results. I don't think you can apply directly B31E philosophy working with other standards considering 475 years return seismic events (as many other standards consider).