DDAM AND Bolts

[craigbrockman3028]

All, When I create my model with (8) hold down bolts using the bolt wizard in release 2012.

After I have run my DDAM analysis, when I search the results, should I consider the results of the element forces and moments (axial force, local 2 and local 3) menu for the bolt stresses, or as my previous instruction to use the Results/Stress/Beam and Truss/ Axial (local 1), Bending stress (local 2, and bending stress (local 3) menu item.

I ask this because  the second menu item always shows values 2-3 times more than the first menu item.

Also, will the values of stress be different when using 8 spokes vs 12 spokes.

Thanks in advance.

[PipePakPat]

It is typical that the beam element stresses are different than the beam element forces and moments.  The forces and moments are used, along with the beam cross sectional data, to determine the beam element stresses.  The stress values are typically used to correlate with the material's allowable stress (usually the material yield stress while considering some safety factor).

For example, a bolt having an axial force of 100 lbf will have a higher axial stress if the cross sectional area is smaller than another bolt having the same axial force, but also having a larger cross sectional area.  From this correlation, the larger diameter bolt will tolerate larger axial forces than a smaller diameter bolt, if both bolts are created from the same material, having the same yield stress.

Usually, the beam stresses should be evaluated in the line element that corresponds to the longitudinal axis of the bolt.  Increasing the number of spokes will distribute the bolt loads more evenly across the brick element part for which the bolts were added.

Using the bolt wizard is a means of evaluating the overall bolt loads experienced in a large assembly having many bolted connections.  If one is interested in the detailed stresses on the 3D brick element part for which the bolts are applied, one might analyze the system in stages.

In the first stage, the entire assembly might be analyzed using the bolt wizard to consider the bolts.  After reviewing the results, one might determine the portion of the model having the most significant bolt stresses, then create a new model of only that portion of the larger assembly. 

The bolt wizard would be used to once again create the bolted connection and that section (or sub-model) would be loaded in a manner that replicates the previous larger assembly results in this smaller section.  Then the bolt wizard beam element part would be suppressed and a brick element 3D bolt assembly would be considered, such that surface contact is also considered.  A refined mesh would also be used so that the flange stresses in the model can be properly evaluated. 

Please keep in mind that this refined approach is designed to evaluate the stresses in the bolted object, not the bolts.  Typically bolts are sized based on the loads they experience.  It is not typical to analyze 3D bolts, since bolt manufacturing is already a standard.  Please let me know if you have further questions.