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#76354 - 04/01/22 10:29 PM Moments on Linear Restraints
Saeedeng3 Offline
Member

Registered: 11/20/15
Posts: 2
Loc: Pakistan
Hello Everyone,
I want to ask some expert, why don't we have moments when we see restraint summary for linear restraints, some one may say we don't have moments on liner restraints, unless we block rotation we cannot have moments, but in some configuration (as sketch attached here) of piping with guide or axial stop we should have moments on restraints.
In sketch, when pipe expands due to temperature change, at guide we should have moment about Y axis. Help me get out of this confusion.

[img]https://drive.google.com/file/d/1RhBZkw0q70vRQNESpZ2WaoFlz8Rts-wh/view?usp=sharing[/img]

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#76355 - 04/02/22 12:48 AM Re: Moments on Linear Restraints [Re: Saeedeng3]
danb Offline
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Registered: 04/22/05
Posts: 1426
Loc: ...
You have moment in the element node, but not on the restraint.
Look at "Global Element Forces"
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Dan

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#76356 - 04/02/22 01:02 AM Re: Moments on Linear Restraints [Re: Saeedeng3]
Saeedeng3 Offline
Member

Registered: 11/20/15
Posts: 2
Loc: Pakistan
Dear danb,
It shows moments for piping element nodes, but not on restraints, as we can see in restraint summary, Mx, My and Mz are zero. It means we have moments on piping nodes but not those on restraints. What is the reason for that?

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#76357 - 04/02/22 01:48 AM Re: Moments on Linear Restraints [Re: Saeedeng3]
danb Offline
Member

Registered: 04/22/05
Posts: 1426
Loc: ...
Because the element node is not tied to the restraint for the moments. So whatever moment is, it is not transferred outside here. it is transferred to the next element. This is a simplistic representation. In fact is more complex.
Because the moments and forces are not disappearing in the air, you need to find somewhere else this moment. In your case part of it will shows up in the opposite forces in the two first guides. part will be absorbed by the flexibility of the elements.
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Dan

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#76358 - 04/02/22 03:39 PM Re: Moments on Linear Restraints [Re: Saeedeng3]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 6164
Loc: Houston, Texas, USA
Translational restraints (like those shown in your image) do not restraint rotation, therefore there will be no moment on these restraints. For a moment resisting restraint, you need to restrain RX, RY, and/or RZ.

If you select "Anchor" as your restraint, all six DOFs are restrained and the restraint report will show three forces and three moments.
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Regards,
Richard Ay - Consultant

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#76372 - 04/06/22 12:49 PM Re: Moments on Linear Restraints [Re: Saeedeng3]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 1000
Loc: Louisiana, US
If support number 1 is the anchor, and 2 through 5 are progressively farther away...

Typically, we would model 1 as an anchor, 2 through 5 as +Y, and 3 through 5 as possessing a guide.

Different users have different practices, but my world would say 2 through 5 have a friction value on the +Y and 3 through 5 will have a gap of 0.125 inches.

I would not expect CAESAR to report moments on any supports except for the anchor.

As for real-world effects...
1. The anchor definitely possesses moments in the real world.
2. The guides - particularly at 3 - might possess moments in the real world. If the pipe from the anchor to the first bend grows 0.125" or more, I'll need to see rotation of almost 5 degreees in the Y axis at the first guide before both guides are contacted simultaneously, which is the only case I would consider non-negligible moment to apply. I'll note that if there is no displacement from the anchor to the first bend, you'd only need to see around 2.5 degrees of rotation in the Y axis to contact both guides at once.

So why no moments on the +Y supports? If the pipe is sitting on a beam, as soon as the pipe pitches forwards or backwards, the pipe is acting as though it's pinned on either the near side or the far side of the beam. For there to be moment - as seen by the pipe - it needs to be more than a pinned condition. But I will note that if you have a hold-down, clamp, etc, you could definitely see a moment and model accordingly.

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