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Robot Structural Analysis

New Member
Posts: 2
Registered: ‎02-06-2013

Orthotropic Stiffness Coefficients Panel Instability

1133 Views, 3 Replies
02-06-2013 06:58 AM

Hi all,

Modelling a 3D building I'm trying to artifically reduce the stiffness of the shell elements that model the floor slabs to ensure minimal framing action and that the stability loads are transferred to the core. To do this when defining panel properties i'm selecting an orthotropic model and then in the geometrical parameters modifying the coefficients. In some cases I am editing the f (membrane) and others the k (flexural) stiffnesses. Having entered this data when i then click on display button to check the stiffness matrices the values are correct for the factors. However I'm unsure they're working correctly and when i try and check them in a test model i have instability problems which gives me inappropriate results.

I have 5 wall panels all the same support conditions and the same thickness but with modified material or panel properties. The first one is 250thk homogenous panel with the material as default concrete, the second is a homogenous panel with a different material i've created with half the E-value (Youngs Modulus) of the concrete, the third has a "reduction of the moment of inertia" of 0.5 * Ig, the fourth is an orthotropic panel with the coefficients of flexural (k) stiffness all set as 0.5, and the fifth is an orthotropic panel with the coefficietns of membrane (f) stiffness all set as 0.5. I am applying test loads both in-plane (nodal force at the top) and out-of-plane (uniform pressure) but when i run the model i have instability type 1 problems for most of the mesh nodes in the last two panels which then give me ridiculous deflections as i'd be expecting the deflections to be double if i'm halving the flexural stiffness?

To create all the models i initially created one panel and then copied 5 times to form the basis of each wall panel before i then changed the material properties only. Why am i having instability problems with the final two wall panels only where the only thing i have modified are the stiffness properties? The model is a 3d type and the support conditions are pinned at the base and fixed in the X-direction (out-of-plane to the wall) at the top only.

Jpeg below (and attached) of problem with screenshots. Thanks for any help.

Attached is a pdf to show the main information. All panels are

Product Support
Posts: 5,498
Registered: ‎04-26-2010

Re: Orthotropic Stiffness Coefficients Panel Instability

02-06-2013 07:11 AM in reply to: ajweng

PLease send us this test rtd file

Rafal Gaweda
Product Support
Posts: 5,498
Registered: ‎04-26-2010

Re: Orthotropic Stiffness Coefficients Panel Instability

02-07-2013 06:45 AM in reply to: ajweng

However I'm unsure they're working correctly and when i try and check them in a test model i have instability problems which gives me inappropriate results.

Remark 1

Orthotropy direction can not be perpendicular to panel plane.

Use Automatic or Dir Z

Rafal Gaweda
Product Support
Posts: 5,498
Registered: ‎04-26-2010

Re: Orthotropic Stiffness Coefficients Panel Instability

02-07-2013 06:51 AM in reply to: Rafal.Gaweda

Remark 2: after correcting model as mentioned in  remark 1 the results seem to be OK.

Rafal Gaweda