## Fastener Pattern Force Hand Calc vs FEM

## Fastener Pattern Force Hand Calc vs FEM

(OP)

Hi All,

I am trying to compare using FEA results to calculate the bolt forces with the hand calculation from Shigley example... but I can't match it... I extract the forces and also moment... but I am not sure if the FEA results are correct although summation of forces are matching to applied force...

Anyone can tell what I miss out in the FEA?

I am trying to compare using FEA results to calculate the bolt forces with the hand calculation from Shigley example... but I can't match it... I extract the forces and also moment... but I am not sure if the FEA results are correct although summation of forces are matching to applied force...

Anyone can tell what I miss out in the FEA?

## RE: Fastener Pattern Force Hand Calc vs FEM

A Major problem you have is that you don't have force equilibrium ... 16 down applied, 20.5 up reacted ???

another day in paradise, or is paradise one day closer ?

## RE: Fastener Pattern Force Hand Calc vs FEM

## RE: Fastener Pattern Force Hand Calc vs FEM

But why don't you have a total reaction of 16 kN ?

another day in paradise, or is paradise one day closer ?

## RE: Fastener Pattern Force Hand Calc vs FEM

The bolts near to applied load take up most of the forces...

I re-run the ansys with free the moment at constraints, the reaction forces remain the same... comparing to hand calculation or joint fitting calculation, we transfer the force to the centroid of fitting and distribute forces evenly as well as induce moment...but FEA gives the reaction forces very much different...

## RE: Fastener Pattern Force Hand Calc vs FEM

Brian

www.espcomposites.com

## RE: Fastener Pattern Force Hand Calc vs FEM

## RE: Fastener Pattern Force Hand Calc vs FEM

Regardless, use a cbush between the mesh and the spc'd node to represent the fastener/joint shear stiffness, and set the stiffness a few orders of magnitude below that of the plate. If everything else is modelled correctly you should see a reasonable correlation between the hand calc and fem.

Ansys is horrid in my opinion, though it has its place.. Try using nastran if you can, and get used to manipulating the deck by hand: that way you have certainty over the solver inputs, unlike in a GUI preprocessor.

## RE: Fastener Pattern Force Hand Calc vs FEM

I shifted the applied force at the centroid of bolts with moment force manually, I can get the reaction forces pretty close to hand calculation...

Does it mean that it does not transfer the moment to the centroid of Bolts in FEA? I understand that usually the bolts near to the applied force will take up more loads...

## RE: Fastener Pattern Force Hand Calc vs FEM

The hand calc assumes the plate to be rigid, and the bolts are assumed flexible. Whereas in your FEM the plate has a finite stiffness, and from the way you've modelled it, it sounds like the bolts are rigid (mesh->rbe->grounded node).

Statically indeterminate problems are all about stiffness - add some flexibility to the fastener (use a bush between the the and ground).

## RE: Fastener Pattern Force Hand Calc vs FEM

Thanks!

## RE: Fastener Pattern Force Hand Calc vs FEM

## RE: Fastener Pattern Force Hand Calc vs FEM

Now I re-run the FEA using RBE3... I can get the same results as hand calculation...

## RE: Fastener Pattern Force Hand Calc vs FEM

Brian

www.espcomposites.com

## RE: Fastener Pattern Force Hand Calc vs FEM

## RE: Fastener Pattern Force Hand Calc vs FEM

## RE: Fastener Pattern Force Hand Calc vs FEM

2) do not model that bracket with solid elements.

3) connect the bracket nodes at fasteners to ground or adjacent parts with spring or bar or fastener elements with appropriate fastener stiffnesses.

4) use the spring/etc element results to get fastener loads

## RE: Fastener Pattern Force Hand Calc vs FEM

consider modelling the problem in the same way as you are solving it. constraint 6 dof at the centroid of the fastener pattern, then read the fastener forces at the fastener nodes.

another day in paradise, or is paradise one day closer ?

## RE: Fastener Pattern Force Hand Calc vs FEM

Brian

www.espcomposites.com

## RE: Fastener Pattern Force Hand Calc vs FEM

when you run the model with the load at the centroid, as per the hand calc, you get equivalent answers.

when you run the FEM with the load applied as per the real world, you get different answers.

the hand calc assumes an infinitely rigid beam, for all intents and purposes cantilevered at the centroid of the bolt group. But your beam is not rigid ... it'd be "interesting to see what happens with a beam 100 (1000?)x stiffer, 100 (1000?)x softer. My guess is you'll see the vertical reaction approach a SS beam, and the lateral reactions would be a localised couple.

If you assume a SS beam, then the RH bolts react 16*500/150 = 53.3kN, 26.7kN per ... slightly more conservative than the bolt group.

another day in paradise, or is paradise one day closer ?