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#73163 - 04/15/19 04:14 AM Loads on restraints for spring can with friction
Alessiaccio Offline
Member

Registered: 10/15/08
Posts: 63
Loc: Italy
Hi esteemed experts,
I modeled a spring can with friction according to Caesar II 2018 Application guide.

I noticed that horizontal loads on restraint X and Z @ node 10 / cnode 15 are different front horizontal loads on +Y restraint with friction placed directly on node 15.

Please, can You explain me why there is that difference?
Thanks.
Best regards

Alessio
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#73169 - 04/15/19 07:16 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 5933
Loc: Houston, Texas, USA
Can you attach the job so we can look at it?
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Richard Ay
Hexagon PPM (CAS)

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#73172 - 04/15/19 09:05 AM Re: Loads on restraints for spring can with friction [Re: Richard Ay]
Alessiaccio Offline
Member

Registered: 10/15/08
Posts: 63
Loc: Italy
Thanks for Your reply, Mr. Ay.

Model_1 @ node 430 has spring can modeled with all rigid elements described in Applications Guide; instead Model_2 @ node 430 has the +Y restraint with friction placed directly on Cnode.

Horizontal loads are different for:

a) Node 432 and 433 as regards Model_1;

b) Node 433 and 434 as regards Model_2.

Thanks.
Best regards

Alessio


Attachments
Model_1.zip (33 downloads)
Model_2.zip (28 downloads)



Edited by Alessiaccio (04/15/19 09:06 AM)
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#73181 - 04/18/19 11:17 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
Your Model_1 includes the height of the spring can while Model_2 does not.

These systems respond in a similar fashion as expected (and as suggested by the note in the Application Guide).

It looks to me that the loads transferred to the pipe at Node 430 are identical between the two models. See below. (I am not using the same version of CAESAR II as you.)


Attachments
1.png

2.png


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Dave Diehl

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#73195 - 04/25/19 04:40 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Alessiaccio Offline
Member

Registered: 10/15/08
Posts: 63
Loc: Italy
Dear Mr. Diehl,
I didn't notice a difference between the results of the two models.

Instead, I noticed a difference in horizontal loads of node 432 and 433 in the Model_1; i.e. it seems that horizontal loads on X and Z restraints are different from horizontal loads on +Y restaint along X and Z direction respectively.

Perhaps I modeled X and Z restraints as a Guide because the rigid element is vertical but this doesn't change the results (at this time I have not the model here).

I noticed the same difference in horizontal loads of node 433 and 434 in the Model_2.

Can you explain me, please, why is there this difference?

"+Y restraint" is the upper circular plate of the spring can on which the pipe saddle is placed; "X and Z restraints" are the can ("the frame" that contains spring and allows to upper circular plate to move in vertical direction only).

So, I supposed that each horizontal loads applied by the pipe saddle on +Y restraints should be carried by the X and Z restraints; therefore horizontal loads on X and Z restraints should be the same on +Y restaint along X and Z direction respectively.

Thanks.
Berst regards

Alessio
_________________________
La potenza e' nulla senza controllo.

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#73212 - 04/30/19 08:20 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
In Model_1, I see the difference you noticed - the horizontal restraint loads at 432 do not equal the reported internal loads on the single pipe running into 432. A static analysis should have the system in equilibrium.
I believe this apparent imbalance may be caused by boundary conditions provided in this model - 432 has RX & RZ to hold moments while the other end (434) is free to rotate.
It does look odd though.
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#73214 - 04/30/19 11:28 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Faizal K Offline
Member

Registered: 07/21/08
Posts: 138
Loc: USA/Malaysia
When I opened Model_1, I get the following results from restraint summary for case 2.

@432
-20857 -70238 -2997 -1705 0 11865

@433
-23730 -70238 -3409 0 0 0

The resultant FX and FZ are as follows:
@432 = 21071
@433 = 23974

So the Resultant at 432 seems to match FY * 0.3 friction coefficient. But the element forces for 434-432 match the magnitudes of the loads at 433.

I started playing with the Friction Normal Force Variation in the config editor. I lowered the value until 0.001 and I noticed that the FZ and FZ of both nodes start to converge to the common values. Shown below are the Friction Normal Force Variation setting versus the percent difference between the FX of nodes 432 and 433 (also applicable to FZ).

0.15 - 13.77%
0.05 - 0.59%
0.001 - 0.01%

So with 0.001, I get the following values of FX, FY and FZ
@432
-20843.266 -70256 -3129.442

@433
-20840.896 -70256 -3129.087


I'm not sure of the exact mathematical explanation for it. Help file says the Frictional Normal Force Variation is "the amount of variation in the normal force that is permitted before an adjustment is made in the sliding friction force." So we're tightening the tolerance, forcing the solver to iterate enough to meet the tolerance?

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#73215 - 04/30/19 11:50 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Dave Diehl Offline

Member

Registered: 12/14/99
Posts: 2376
Loc: Houston, TX, USA
Good work.
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Dave Diehl

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#73222 - 05/02/19 12:14 AM Re: Loads on restraints for spring can with friction [Re: Alessiaccio]
Alessiaccio Offline
Member

Registered: 10/15/08
Posts: 63
Loc: Italy
Thanks!

Alessio
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