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# Suggestions on improving usage of CBUSH elements in Nastran

## Suggestions on improving usage of CBUSH elements in Nastran

(OP)
Hello,
I have a few questions regarding usage of CBUSH elements for modeling fasteners in Nastran.

If 1 represents the X direction along the fastener length, then 2,3 represents the other two shear directions.

I usually use Huth Formulation for representing fastener stiffness.

Axial stiffness is calculated using formula, K1 = AE/L, (L = length of the fastener).

For rotational stiffness, either I give 10^9 or I calculate stiffness based on the formula:

K5,6 = (1/e)*(Max (K2,K3)*(Lcbush^2/4))

where 1/e =100
Lcbush = length of CBUSH element

For K4, I specify a low value of 100 or so.

I usually find that axial stiffness is very high value compared to shear stiffness. If the structure is a simple lap joint or such where one knows the deformation pattern based on loading, appropriate SPCs can be provided. However, in complex assemblies, I usually input all stiffnesses cause:

1. Failing which I get a rigid body motion error
2. Don't know the individual deformation before hand and thus difficult to provide individual SPCs

Sometimes, the above approach can provide incorrect results. For ex, in some cases, I find that rivets reacting much of the load in tension/compression instead of the preferred shear mechanism. I am looking for guidelines on how to overcome situations like the above in complex assemblies and if possible, get some pointers on best practices for using CBUSH elements.

Thx

EDIT: If I set the axial stiffness value to a low number lets say 500 instead of the usual 7 digits (units lbs/in), will it help? Thought is, making shear stiffness higher will force the load to flow in that direction instead of axial. I guess I will try and see if I run in to rigid body motion issues. Still, I would appreciate suggestions from experienced personnel.

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