Known displacements of Structural members

I have been given the displacements of the structure (due to storm criteria resolved in SACS). I need to impose these displacements on my pipe via the pipe supports connected to the structure. All supports act in both the Y and GUIDE directions. I think the best way is with the CNODE, but I struggle with exactly how to do this. Once I have the displacments modeled, do I need to treat them as occasional loads in the load case editor?
Any guidance would be appreciated.

You have for example a restraint at node no. 100. You specify Restraint: node 100, cnode 101, type +y, mu 0.3 and node 100, cnode 101, type guide (or X or Z or whatever you need), then in Displacements: node 101 and you insert the displacements (let say x=4", y=2", z=8", rx=0, ry=0, rz=0).

Also read manual regarding restraints with cnode.

For second question, no, displacements are not ocassional.

Regards,

Danb,

Thank you for your quick reply. I don't need to define where node 101 is with respect to node 100 (direction and distance)?
Wouldn't displacements due to wind/wave (storm criteria as mentioned above) be occasional?

No, you do not define any direction and distance. 100 is connected to 101, but 100 belong to pipe and 101 belong to structure. You only define the relationship between 100 and 101.

Even storms are occasional events, displacements of any kind are not.

Regards,

What if I have two pipes supported on the same support? When I try to use the same CNODE for both pipes, so I only have to define the displacement once, it shows the two pipes on top of one another graphically. Do I need to turn off the "Connect Geometry Through Cnodes" in the configuration editor?
These displacements are due to wind and wave which are occasional events. By inputting them as displacements, am I modeling them incorrectly?

etacarinae76 -

You are correct, you need to change that Configuration Directive "Connect Geometry Through Cnodes" to false. This will prevent the plotting routines from considering CNODEs.

You are correct again, your displacements are induced by Occasional loads (storm waves), hence they should be classified as Occasional. (If the displacements were caused by thermal or settlement events, they would be classified as Expansion.)

danb -

Look at it from this view point, instead of applying the displacemets, say you modeled the structure and applied the storm waves to the structure. That would be an occasional event. So if you take the support displacements due to that event and used those as input (instead of modeling the structure), you still have an occasional event.

Richard,

Quite strange for me, but I was thinking that is clearly stated in B31.3.
Of course maybe I'm wrong.

319.2.1 Displacement Strains
(c) Externally Imposed Displacements. Externally
caused movement of restraints will impose displacements
on the piping in addition to those related to thermal
effects. Movements may result from tidal changes
(dock piping), wind sway (e.g., piping supported from a
tall slender tower), or temperature changes in connected
equipment.

(d) Total Displacement Strains. Thermal displacements,
reaction displacements, and externally imposed
displacements all have equivalent effects on the piping
system, and shall be considered together in determining
the total displacement strains (proportional deformation)
in various parts of the piping system.

But technically, if you model the structure, how external displacements can be handled as displacements strains? Interesting.

Regards,

I think some of our terms get in the way here.

A displacement (or strain) may be caused by an occasional event (note I did not call it a "load" as loads could be understood as applied forces and moments). But should it be treated as an occasional stress?
Look at the evaluation method - sustained and occasional stresses are checked to assure the system does not collapse, expansion stresses are evaluated to assure the system will not develop a through the wall crack caused by fatigue.

Before I start rambling let's give an example. Let's say we have piping attached to a vessel. Wind load moves the vessel. If I model the vessel and piping and apply the wind load, we would assume that all response would be force-based and the wind stress in teh piping would be occasional. Now, say you didn't want to model the vessel but you know how much it moves under wind loads. So you enter (wind) displacements at your nozzle connections. To get the same occasional stress results as the complete analysis, you must call these displacements occasional rather than (the default) expansion. This is where Rich's occasional designation collides with Danb's quote of the Code (but Danb concludes with the same point - "Interesting.")

And what if that structure providing the "imposed displacement" was a bit weak and actually relied on the piping for added strength? (I think there are cases where the pipe holds up structure rather than the reverse.) Any yield in the piping may form a second hinge that allows the structure and piping to collapse. Here that displacement is not stress limited and should be categorized as occasional.

Here another angle - B31E provides two evaluations for stress -

1) the expected sustained plus occasional stress and
2) a check on the "resultant force (tension plus shear) due to seismic anchor motion".

That second check focuses on a major concern in seismic analysis - it's the support defelction, not the g-load that brings down piping. But what if I have pipe "anchors" in a building and I include the building model in my seismic analysis. Is there "seismic anchor motion"? How is it identified? Or I guess more practically, how could c2 figure out that these induced relative displacements require separate evaluation? I think a conscientious engineer would build a second model of the piping, without the structure, and look at those displacements alone.

I think we can find the right answers but it takes some thinking.

I'd really like to have John Luf opinion on this. Thank you for your answer, it is a polite one.

Best regards,

I found these two:

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=644#Post644


http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=3747#Post3747

Best regards,

In my opinion, you may consider some guidance in Interpretation: 20-45
Subject: ASME B31.3-2004 Edition, Combining Displacements
Date Issued: October 18, 2005
File: 831-05-1213
Question (1): In accordance with ASME B31.3-2004 Edition, are strains caused by cyclic support displacements during seismic events considered to be externally imposed displacements in accordance with para. 319.2.1(c)?
Reply (1): Yes.

Question (2): In accordance with ASME B31.3-2004 Edition, shall externally imposed displacement strains, excluding single cycle events, be considered in conjunction with thermal displacement strains?
Reply (2): Yes; see para. 319.2.1(d).

I would consider some key words as "cyclic support displacements" and "single cycle events" as they make the difference.

Best regards.

One more thing, to answer to Dave. Also para. (b) consider the flexibilities of the supports in the following way. Restraint Flexibility. If restraints are not considered rigid, their flexibility may be considered in determining displacement stress range and reactions.

So this makes the things even harder. The problem remain. "But technically, if you model the structure, how external displacements can be handled as displacements strains? Interesting"

But this is why Engineering is beautiful. Because of the challenges.

Can you please consider this a "sticky" topic until some satisfactory answers will be founded? Of course if you consider this interestig enough.

Best regards,

A little bit confused.

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=10051#Post10051

Regards,

If I have a small bore line connected to a big line, how do you consider the displacements of the big line? If you model the big line, you will consider the wind displacements as occasional, if you do not model the big line, you you will consider the wind displacements as secondary stress as per code requirement?

Where is the truth?

My humble opinion is that the code was appropiate for the times when we had simple tools. Now we are more complex, appendix p was introduced as an answer to the computer era, but confusion is the result. Is the code not appropiate to the modern time? Or we, with the computer oriented mind are not appropiate for the code?

I figure if you're concerned about collapse, it's occasional. If fatigue, it's expansion; but then you have to consider accumulated damage.
In either case you should confirm that your subsystem does not alter those main line deflections.

Thank you Dave

Best regards,