Hi,

This is regarding the load cases to be considered when doing static analysis with seismic load:

Uniform g-load : 0.3g

CODE TO BE FOLLOWED : B31.3

Its says "Piping should be designed for eqarthquake induced horizontal forces"

so , U1 = 0.3,0,0
U2 = 0,0.3,0
U3 = 0,0,0

Now, the model has got non linear components like onsided restraints, double acting restraint with gap on both sides.

Need to do code compliance and also from the operating case need to pick the displacement reactions i.e. forces and moments (flanges connected to main equipment are displaced due to thermal expansion of the equipment in operation).

This F&M values then to be compared with the equipment design guidelines to see the anlysed valued are within these limits.

I request for your inputs that to satisfy both this requirments which load cases to be set.

I put following load case combinition :

L1 :W+D1+T1+P1 OPE
L2 :W+P1 SUS
L3 :W+D1+T1+P1+U1 OPE
L4 :W+D1+T1+P1-U1 OPE
L5 :W+D1+T1+P1+U2 OPE
L6 :W+D1+T1+P1-U2 OPE
L7 :L1-L2 EXP
L8 :L3-L1 OCC (algebric)
L9 :L4-L1 OCC (algebric)
L10:L5-L1 OCC (algebric)
L11:L6-L1 OCC (algebric)
L12:L2+L8 OCC (scalar)
L13:L2+L9 OCC (scalar)
L14:L2+L10 OCC (scalar)
L15:L2+L11 OCC (scalar)


L8 to L11 will take care of non-linearity and L12 to L15 giving stress case for code compliance.
I can use Max of L3,L4,L5,L6 for displacement reaction in operating condition.

But here I am not taking the combined effect of both components while doing code compliance. So,it does not seem ok to me !?!?!

Otherway

L1 :W+D1+T1+P1 OPE
L2 :W+P1 SUS
L3 :U1 OCC
L4 :U2 OCC
L5 :L3+L4 OCC (SRSS)
L6 :L1-L2 EXP (ALGEBRIC)
L7 :L1+L5 OCC (ABSOLUTE)
L8 :L2+L5 OCC (ABSOLUTE)

But here the non linearity is not taken care of instead a resultant of both horizontal components is used. Is this correct?

Equipment load qualification is always as per INDUSTRY OR PROJECT STANDARD. It is not part of CODE compliance.CODE compliance is for STRESSi.e,

SUS

EXP

SUS+OCC

Now for the SUS+OCCC cases the CODe requires that you do the addition of the individual cases, which in CAESAR terminology is SCALAR combination.

Now excitations in mutually perpendicular directions being statistically independent, your SRSS combination is ok.

Now regarding ABSOLUTE summation of L1& L5, you can do a scalar commbination also. Since bending stress is sign independent and the longitudinal pressure stress is tensile,so these two methods usually do not show different results unless you have a negative F/A stress in your system.

I hope you are doing L1+L5 for equipment nozzle checking.

For checking the loads in OPE+OCC condition, it is better that you use load case like (w+d+p+t+u)-(w+p+t+d) to get pure U as considering system non linearity the individual additions are not same as the total due to non applicability of the superposition principle.

To take into account both compoments of seismic excitation , i would prefer to go by a case like L8+L10 or L9+L11 ( taken from your first set)by SRSS ( say new load case is L16)as they are statistically independent and do an ABS summation of W+P+D+T and L16. This will be conservative.

Regards

Anindya Bhatacharya

Hi,

For b31.3 need not consider vertical.

I think For OCC code compliance add resultant of seismic components to the SUS load and not only single component. I think this is what your confusion is.

This is what I think will do the needful. Anindya, ur comments plz

L1->W+D1+T1+P1 OPE
L2->W+P1 SUS --------->CODE COMPLIANCE
L3->W+D1+T1+P1+U1 OPE
L4->W+D1+T1+P1-U1 OPE
L5->W+D1+T1+P1+U2 OPE
L6->W+D1+T1+P1-U2 OPE
L7->L1-L2 EXP -------->CODE COMPLIANCE
L8->L3-L1 OCC(ALGEBRIC)
L9->L4-L1 OCC(-"-)
L10->L5-L1 OCC
L11->L6-L1 OCC(-"-)
L12->L8+L10 OCC(SRSS)
L13->L9+L11 OCC(SRSS)
L14->L2+L12 OCC(SUS+OCC)-->CODE COMPLIANCE
L15->L2+L13 OCC(SUS+OCC)-->CODE COMPLIANCE
L16->L1+L12 OPE(OPE+OCC)
L17->L1+L13 OPE(OPE+OCC)
L18->L16,L17 OPE (MAX) ----> HV THE DISPLACEMENT REACTIONS FOR YOUR EQUIPMENT LOAD COMPLIANCE FROM THIS LOAD CASE.

Kalpesh,

I agree with you except for" B31.3 need not consider vertical".What do you mean by this?

Thre is no specific guideline in B31.3 as to how to perform a seismic analysis ( I have not seen 2004 edition , so I don't know if they have come out with an Appendix to provide guidelines for seismic analysis).It refers to standards like ASCE, UBC etc which does.
B 31.3 only tells you that the stresses in the SUS & OCC cases need to be added which CAESAR II calls SCALAR addition.

What I think could be done is to consider seismic excitation in global Y in both U1 & U2 definitions.Say U1 consists of excitation in X &Y and U2 excitation in Z & Y.

Regards

Hi,

Correct me if my interpretation is wrong but was just reffering to B31.3 paragraph 301.5.3 (Earthquake)which states that : “Piping shall be
designed for earthquake-induced horizontal forces. "

Anyways, are the load cases put forward by me seems to do justice to the cause?