Seismic Anchor Motions vs Inertial Loads

Hi,
I wonde rif anyone can answer the following:
How does Caesar treat Seismic Anchor Motion (SAM) cyclic loads?
Are these included with the inertial loads and combined with the Sustained Loads for the SUS+ OCC Code check or are they treated as the ASME B31.3 Code intent whereby the SAM cyclic loads are treat as a displacement stress range and therefore combined with other displacement stress ranges such as the thermal stress range?

Regards

DSB1954

The Seismic Anchor Movement defines the absolute displacement of the restraints included in a spectrum shock case for independent support motion analysis (ISM). This displacement is applied to all restrained nodes in the node group, and is used to calculate the pseudo-static load components representing the relative displacement of the individual restraint sets.

This option is only available for Dynamic Analysis, when Earthquake (spectrum) is selected as the Analysis Type.

So in answer to your specific question, SAM is considered as part of the (dynamic) inertial loading.

I've seen seismic anchor motion handled both ways - as strain included in the expansion stress range and as loads contributing to occasional stress.

B31E (the general B31 document on seismic evaluation of piping systems) kind of straddles the fence. Bending moments used to evaluate (sustained plus occasional) stress include "relative anchor motion". But there is a second stress check - the "resultant force (tension plus shear) due
to seismic anchor motion" divided by pipe cross sectional area must be less than or equal to pipe material yield strength.

Richard,
From your reply I gather that Caesar does not follow the B31 Code intent as per B31.3 Interpretation 2-17 and just lump the inertial and SAM loadings together. Why does Caesar not follow the Code Interpretation 2-17 ?

I read Interpretation 2-17 differently.
This is what I see:

Interpretation 2-17
Subject: Seismic Loads
Date: June 8, 1983

Question: How shall stresses in a piping system due to restraint deflection from seismic motions, and those due to seismic inertial loadings, be applied in accordance with ANSI/ASME B31.3?

Reply: It is the intent of 302.3.6 that the inertial loads from earthquake be included in the calculation of stresses due to occasional loads. The effects of displacement of restraints due to earthquake may be treated as an externally imposed displacement in accordance with 319,2,1(c).

It says seismic displacements MAY be included as part of the expansion stress range.

As I said earlier, (the more recent document) B31E includes the effect of those displacements in the sustained plus occasional stress evaluation.

Remember, too, that interpretations do not necessarily extend to Code updates. There have been many changes since 1983.

One final point, the Code committees now avoid addressing "intent" questions. Published interpretations must draw on what is written rather than propose why it says what it says.

Thanks Dave for the response.

However how do you respond to the fact that the well renowned "Pipe Stress Engineering" book by Peng ( who I believe is a B31 committee member) indicates that the SAM displacements should be added to the displacement stress range ? Are you saying Mr Peng is wrong? I have also seen this approach stated in other books. Are you saying these other esteemed Authors are wrong?

So the approach in Caesar is correct and the "interpretation" by the Code (2-17) and these Code committe people is incorrect ?

Even the CEN Code EN 13480 is incorrect also ?

It is amazing that all of these different Authors / interpreters have got it wrong !!!

Regards

DSB1954

Obviously there is no consensus here.
I would consult the Engineering Design Specification. If that does not define the approach, then work with the client to settle on the method before starting work.
Engineering is judgement.
And you can do it any way you want in CAESAR II.

Hi, I’m dealing with Seismic Anchor Movements and B31E and i need some help.
I’m running my model under Dynamic Analysis module, Earthquake (spectrum).
For seismic evaluation, B31E has two condictions:
1) PD/4t+0.75i(Msustained+Mseismic)/Z<= min(2.4Sh,1.5Sy, 60ksi)
2) Fsam/A<=Sy
I understand condition 1 can be evaluated using a Load Case combiniation, S1(W+P1(SUS)) combined with D1 in the Static/Dynamic Combinations tab.
How can i evaluate condition 2? Should i evaluate only SAM effects? in this case how can i get this effects without inertial effects?

Thanks,

JPB