Advice on Stress Simulations of Timber Structures (Screws and Staples)

[GavoGarmo]

Hi There,

I'm wondering if anyone could offer some guidance on conducting stress analysis on timber frames. 

We manufacture very light timber structures, and more often than not, failures occur due to the strength of fixings rather than the sectional stiffness of any of the members.

Our fixings are primarily staples into softwood, and I was wondering what would be the best approach to approximate this in Inventor's Stress Analysis environment. (I'm assuming actually modelling the staple would be OTT and not very accurate)

We don't need anything pin point accurate, but it would be good to get it close enough to limit the number of real world prototypes.

Thanks in advance

Gav

[raviburla]

Hi,

Inventor Stress Analysis does not support orthotropic materials. So I am assuming that you are using isotropic materials with appropriate properties. 

In order to design fixings, we will need to understand the forces acting on them. The fixings would be typically represented by contacts in the FEA analysis, so this would mean that if we know the contact forces, we will have enough information to design the fixings. However, contact forces are not directly provided in Stress Analysis environment, but Contact Pressures are provided, so looking at the contact pressures, we could estimate the contact forces.

There is another work-around to estimate the contact forces - Lets say that we want to estimate the contact forces in a contact between two parts. We can replace the bonded contact with spring contact with known stiffness Kn (normal stiffness) and Kt (tangential). Solve the problem with appropriate loading and probe the displacement values at a representative point on each side of contact (one point on each part, perhaps the centroid of the contact region). Knowing the displacements, we can estimate the forces are Fn = Kn(u1n-u2n) - this gives the normal forces that are taken up by the fixing. We could do similar for the tangential components. Please note that this gives an estimate and an approximation of the actual contact forces.

Please let us know if this helps you in solving your problem. Also, let us know if you have questions/comments.

Thanks,

Ravi Burla (Autodesk)

[GavoGarmo]

Hi Ravi,

That was a big help. Thank you!

In my case the the conversion of the contacts to 'Spring' gave the type of results I'd expect to see approximateed in real life.

As almost all of our real world connections are of this type, is it possible to set the Automatic contact creator to set contacts to 'Spring' and have the values that we have gathered from emperical study?

I'd expect this to be an API/iLogic task. But as we have hundreds of connections in a single frame, automation of this type would be extremely useful.

Thanks again.

Gav 

[raviburla · Accepted solution]

Hi Gav,

I am glad that this workaround was useful for you. 

With respect to "defaulting to spring contact" - Please see the attached images - we can edit Simulation properties ( RMB Simulation -> Edit Simulation properties) and set the default contact type to what ever makes most sense for that particular analysis. In addition, this option can be set globally (for all simulations that will be created ) by Stress Analysis Settings in the Ribbon. 

Please let me know if you have more questions/comments.

Thanks,

Ravi Burla (Autodesk)

[GavoGarmo]

That's perfect! 

Thank you very much fro your help.

Gav