×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Symmetry with Cadre Pro

Symmetry with Cadre Pro

Symmetry with Cadre Pro

(OP)
Have a question concerning symmetry using plate elements with Cadre Pro.  I assembled a quarter section of a circular cylinder under a uniform pressure. The cylinder has a radius of 30ft and a height of 30ft.  The horizontal length of the elements are at 3 degree increments and the vertical are at 1 ft increments. At the 0 degree boundary points I restrained the x translation and the z-rotation and at the 90 degree boundary points I restrained the y translation and the z-rotation.  The top and bottom boundary nodes were left free to model a free sliding base and top.  The elements were the plate bending elements.  The uniform pressure was set normal to the plate elements; the load vectors showed radial.  When I ran the solution the error message indicated that the model was unstable indicating rigid body motion.  Shouldn't the restraints I applied be adequate?  Any additional restraints at the 0 and 90 degree boundarys indroduce forces and deflections that are not consistant with a circular cylinder under uniform load.  Any suggestions would be appreciated.  

RE: Symmetry with Cadre Pro

Hi, mlm54.

I can't see where you have restrained your model against linear motion parallel to the axis of your cylinder - ie you have no Z translation restraint, and the cylinder is free to disappear to infinity along its centreline.

RE: Symmetry with Cadre Pro

(OP)
austim:

I had restrained the model in the z-direction but still got the unstable model error.  In order to get the model to calculate I set additional rotational restraints.  At 90 degrees I restrained rotation of the cylinder about the y-axis, this was in addition to restraint of translation in the x direction and rotational restraint about the z-axis.  At 90 degrees I figured the cylinder should be able to move freely in the y direction as well as rotate about the x-axis.  At 0 degrees I restrained rotation of the cylinder about the x-axis and this was in addition to translational restraint in the y direction and rotational restraint about the z-axis.  At 0 degrees I figured the cylinder should be able to move freely in the x direction as well as rotate about the y-axis.  The results from the calcuation didn't match those you would expect for a free end cylinder under constant internal pressure load.  I extended the cylinder model to 180 degrees and had more reasonable results.  Any suggestions on getting the 90 degree model to work?  I try and use symmetry as much as possible to speed up the solving process.  

RE: Symmetry with Cadre Pro

Hi, mlm54.

I really have no worthwhile experience with Cadre (I was just looking in to see what was going on), so I am probably not your best source of good advice.  However, if I am all that you have, it looks as if you may be stuck with me.  Let's see what we can do .

Firstly, let's clear up some confusion in my mind.  

I do not really understand why you would want to model a cylinder under uniform internal pressure, unless it is purely for the purpose of gaining confidence in the use  of Cadre.  (Which would be a perfectly adequate reason, in my opiinion).  Otherwise, why use FEA for something that can be analysed in a couple of lines of hand calculation?

In your two posts you referred to two different arrangements of translational restraint.

In post 1 you told us "At the 0 degree boundary points I restrained the x translation and the z-rotation and at the 90 degree boundary points I restrained the y translation and the z-rotation".  

In post 2 you said "At 90 degrees I restrained rotation of the cylinder about the y-axis, this was in addition to restraint of translation in the x direction and rotational restraint about the z-axis.... At 0 degrees I restrained rotation of the cylinder about the x-axis and this was in addition to translational restraint in the y direction and rotational restraint about the z-axis."

Thus first time you said that your x restraints were at 0 degrees, 2nd time they were at 90 degrees.  Likewise the y-restraints were transposed.  Have you in fact changed your model?

The restraint arrangement that you describe in you 2nd posting is what I would have expected to work, WITHOUT your y-rot retraints at 90 degrees and x-rot at 0 degrees.  (I am assuming, by the way, that your x axis is at 0 degrees, your y axis at 90 degrees, and your z axis 'out of the paper' ?)

Having said that, an experience I had 9 years or so ago, with very different software (DOS based) and a very different structure, may be relevant.

I was designing a piled roadway slab (to carry a main road over an abandonded municipal garbage tip).

I modeled a typical length of slab with a grid of near-rectangular plate bending elements. (The road was on a curve, so elements could not be exactly rectangular, and no symmetry was avilable to me).  My axes were: x  along road centreline at end 1;  y horizontal perpendicular to x; z vertical.

I applied z-trans restraints at all piles, x-trans and y-trans at road centreline end1, y-trans at road centreline end 2.

Thus I had restrained rigid body translation in both X and Y, plus vertical deflections at every pile, and due to the y restraints at both ends, had also restrained rigid body rotation about a vertical axis.  (Rigid body rotations about the x and y axes were restrained by the piles).

I confidently started the analysis, and promptly got an error message not unlike yours.  My structure was unstable.  There was some reference to the z-rotation degree of freedom of the last node in the model.

I re-examined all of my logic,and remained satisfied that I had not omitted anything obvious (this may be sounding familiar ?).  Then, for want of anything better to do, I added a z-rotation restraint to the last node, and it analysed without any further difficulty.  Subsequent investigation showed that it did not matter where I put the z-rotation restraint, just so long as there was at least one somewhe in the model.

If the plate bending element in Cadre is similar to the element used by my software, then it may be that you have struck the same problem, and the solution by  'austim's dummy restraint' may also work for you.  

I would suggest that you delete all of the x-rot and y-rot restraints that you added, and just try a single x-rot restraint at (radius,0,0) and see what happens.  If you still have a problem, try adding a y-rot restraint at (0,radius,0).  If that doesn't work, see what Cadre tech support have to say.  (If it DOES work, then let us all know, and we can raise our glasses together )

Good luck.

RE: Symmetry with Cadre Pro

Hello mlm54,

I modeled the quarter cylinder as you described it and used your restraint conditions (X,Zrot at zero degress and Y,Zrot at 90 degrees).  The model solves fine and seems to give reasonable results.  How about we trade models by Email and see where the difference lies.

James (CADRE Analytic)  email: cadre_analytic@compuserve.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources