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RESOURCES FOR SIEMENS DIGITAL INDUSTRIES SOFTWARE

 

 

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From:This email address is being protected from spambots. You need JavaScript enabled to view it.
Sent:Wednesday, July 11, 2018 10:55 AM
To:'Chris Hinnant'
Subject:RE: Cbush elements

 

Chris,

I defined the element orientation on the element card.

01

I used a node (at 0, -500, 0) rather than a vector because it worked well for all CBUSH elements.

02

I built an FBD to look at my reaction forces. The resultant force is in line with the cable:

03

Now it’s time to go nonlinear. I’ve attached an example model that uses a function for CBUSH stiffness. My example is intended to represent a bolt that loads up and breaks, but you can set up the curve however you’d like (i.e., tension only).

Cheers,

Adrian Jensen, P.E.

Sr. Staff Mechanical Engineer

This email address is being protected from spambots. You need JavaScript enabled to view it.

www.PredictiveEngineering.com

 
Land Line: 503.206.5571 ext 120

FAX: 866.215.1220

From:Chris Hinnant [This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent:Tuesday, July 10, 2018 11:52 AM
To:This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject:RE: Cbush elements

 

Adrian – Thanks.  Using a CBUSH is OK, but brings me back to my original question….I can’t seem to get axial only stiffness on the CBUSH.

Below is my original issue and model attached:

I am trying to create CBUSH elements with an axial-only stiffness.  However, instead I’m getting stiffness in the global X direction instead of stiffness along the local axial direction of the CBUSH.  I tried some of the orientation vector settings on the element, but that didn’t seem to achieve my goal.

 

In the attached model, the applied force at the top of the tower is in the Z direction and the tower deflects uncontrolled.  If you change the force into the Fx direction, the the CBUSH elements pick up load and help restrain the X displacement.  But, what I was expecting is the CBUSH would pick up load no matter which direction the load was applied because the CBUSH has an axial stiffness (like a beam).

Thanks,

Chris Hinnant, P.E.

K&H Fabricators, Inc.

PO Box 695 / 170 Loop Road

Smithville, TX 78957

This email address is being protected from spambots. You need JavaScript enabled to view it.

Office: 512-237-5020 x101

Cell: 512-906-8000

Fax: 512-237-2795

From:This email address is being protected from spambots. You need JavaScript enabled to view it.[This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent:Tuesday, July 10, 2018 12:55 PM
To:Chris Hinnant
Subject:RE: Cbush elements

 

Chris,

Nope. Gap elements are simple springs that can use different tension and compression stiffness values.

I think your best bet is converting the effective modulus chart into a force vs. displacement curve and using a CBUSH.

Adrian Jensen, P.E.

Sr. Staff Mechanical Engineer

This email address is being protected from spambots. You need JavaScript enabled to view it.

www.PredictiveEngineering.com

 
Land Line: 503.206.5571 ext 120

FAX: 866.215.1220

From:Chris Hinnant [This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent:Monday, July 09, 2018 8:41 AM
To:This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject:RE: Cbush elements

 

Thanks Adrian.  I’m downloading the model files now.

In this case, since the gap element is used, is the solution going to take into account the effects of cable sag?  My guess is “no”. 

If the answer is no, I still need to use the equivalent modulus method to account for cable sag and the effect on the modulus.  But, I’m not really sure how to achieve that since the modulus varies as a function of the load.  Ideally I could provide the elastic modulus as a function of the load, but I don’t see where that input is allowed in Femap.

Unlike transmission towers, typically for our flare stacks the cable pretension loads are only about 8% or 10% of the breaking strength of the cable to allow for thermal expansion of the stack due to hot or cold gasses.  When wind is applied to the stack, some of the cables will unload (i.e. those downwind).  As the cables unload, they will experience nonlinear loading due to the sag effects.

As the cable tension changes, the effective modulus of elasticity changes as predicted by Ernst.  This effect for my case at hand is shown below.  As you can see, if the tension increases, there isn’t much change in the modulus and it would be relatively safe to assume a constant effective modulus.  But if the tension decreases, the effective modulus will drop rapidly as the cable begins to become slacked.

I can convert the effective modulus chart into a force vs. displacement chart as shown in the 2ndchart below.  In that case, I could specify force vs. displacement function for the CBUSH as in the video I sent you before.  This was another reason I was looking at the CBUSH element.

Is there a way to incorporate this into the GAP element?

04

05

Thanks,

Chris Hinnant, P.E.

K&H Fabricators, Inc.

PO Box 695 / 170 Loop Road

Smithville, TX 78957

This email address is being protected from spambots. You need JavaScript enabled to view it.

Office: 512-237-5020 x101

Cell: 512-906-8000

Fax: 512-237-2795

From:This email address is being protected from spambots. You need JavaScript enabled to view it.[This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent:Monday, July 09, 2018 10:11 AM
To:Chris Hinnant
Subject:RE: Cbush elements

 

Hey Chris,

Hope all is well. Check out the resources on the Applied CAx website and you will find aseminar on SOL 106. There’s an example of a radio tower with guywires modeled using gap elements.

In the future, please send tech support emails toThis email address is being protected from spambots. You need JavaScript enabled to view it.and they will reach the whole team.

              

Cheers,

Adrian Jensen, P.E.


From:Chris Hinnant [This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent:Monday, July 09, 2018 7:57 AM
To:This email address is being protected from spambots. You need JavaScript enabled to view it.
Subject:Cbush elements

 

Adrian –

 

I’m trying to model some cable supports (guywires) for a stack.  The CBUSH elements seemed like a reasonable place to start.  I came across the video below, which seems pretty straight forward.

 

https://www.youtube.com/watch?v=kiIL3tzYc8s

 

But, I’m not getting results like I’d expect.  I am trying to create CBUSH elements with an axial-only stiffness.  However, instead I’m getting stiffness in the global X direction instead of stiffness along the local axial direction of the CBUSH.  I tried some of the orientation vector settings on the element, but that didn’t seem to achieve my goal.

 

In the attached model, the applied force at the top of the tower is in the Z direction and the tower deflects uncontrolled.  If you change the force into the Fx direction, the the CBUSH elements pick up load and help restrain the X displacement.  But, what I was expecting is the CBUSH would pick up load no matter which direction the load was applied because the CBUSH has an axial stiffness (like a beam).

 

Ultimately where I’m heading with this is that I need a tension-only element.  From there, I was planning to apply an adjusted modulus to account for the cable sag (i.e. Ernst’s method as explained below).

 

But, if you have other recommendations for modeling catenary cables, I’d appreciate the guidance. 

 

 

 

06

 

 

 

Thanks,

 

Chris Hinnant, P.E.

 

K&H Fabricators, Inc.

PO Box 695 / 170 Loop Road

Smithville, TX 78957

This email address is being protected from spambots. You need JavaScript enabled to view it.

Office: 512-237-5020 x101

Cell: 512-906-8000

Fax: 512-237-2795