Bolt strength reduction in shear lap joint with packer 2

[dooron]

In a bolted lap joint subject to shear and including a non structural packer (greater then 15% of the bolt diameter) what is the method of analysis for the bolt stress?

 

[Lcubed]

Consider the packer to be attached to one or the other of the joined members. That member will share load with the packer as a function of stiffness. That is, if the sheet is 0.050" thick and the packer is 0.030" thick, and the material has the same modulus, 3/8 of the load in the sheet is transferred to the packer. You can attach the packer with discrete fasteners having the necessary capacity, or simply add sufficient fastener capacity to the joint to transfer 1.375 times the load which would be at the joint in the absence of the packer.

 

[dooron]

Thanks, the packers I am considering, are floating, i.e. not attached to either of the load carring plates.

 

[Lcubed]

I noticed previously that you specified initially that the packers are non-structural.

If you cannot extend the packer and add discrete fasteners to make it structural, use the second suggestion of my first post, and increase the total fastener strength by the amount of load which would be transferred in the case of a structural packer. That should cover the problem.

As an additional check, it would be wise to do a detail analysis of the fasteners, as there will be additional bending and prying, but adding capacity as described above should result in heavier and or stronger fasteners which can handle the extra eccentricity and resultant loads.

Hope this helps.

Regards,
Lcubed

 

[Lcubed]

I've assumed so far that your packers are of structural material and the holes in the packers are not oversize clearance holes. If the packer material is not capable of picking up load, you must check the bolts by detailed analysis, but you should also re-think such a design if at all possible.

I was a little short of time when I wrote my last post, so I may have omitted some detail. Since you mentioned that the thickness of the packer is >15% of the bolt diameter, it sounds as if you are using Bruhn as a reference. Is that the case?

 

[dooron]

Bruhn is one source and 15% is derived from aircraft manufacturers old experimental data (so I believe),
I a looking for a research paper, experimental results AISC NACA etc, where this subject is treated. To date I have not found an eloquent and comprehensive analysis of thick packers in bolted joints and it suprises me.

 

[ChrisSpedding]

Best advice is, try to change the design. If bolts are shear-carrying, the danger is that the shear load tips them instead of shearing, so after repeated load application there is local bearing failure in the outer leaves of the joint, and the under-head radius of the bolt is subject to prying loads. Eventually there will be a fatigue failure and the bolt heads will be detached. To avoid this the bolt diameter will have to be increased disproportionately, resulting in a heavy and bad-looking design.