Regarding Single Shear Lug Joint

[kilbchoi]

Hellow!

I have a question about single shear lug joint strength.
If assuming I get the nominal axial strength of a lug, Fa, then
one of my references, German design hand book, says that

Fa* = Ka x Fa x (Sbr / Stu)

where, Fa* = final lug strength of sigle shear lug joint
Ka = factor dependent on diameter, lubrication condition,
and moment arm distance(unique one for single shear
joint)
magnitude range = normally 0.3 to 0.6
Sbr = bearing stress of the lug at Fa
Stu = tenstile strength of the lug

In this case, for pretty much general thickness of lug, Fa* is
too much less than Fa.

I am afraid the above formula is too conservative.

Anybody who has a better idea or more realistic formula ?

Thanks,

 

I would suggest using a resource like Bruhn. Lugs are subject to multiple modes of failure such as net tension, shear-bearing, shear tear out, hoop tension, bolt bending, bushing yield ... etc. A single check will not be sufficient to properly evaluate the lug.

Bruhn (Analysis and Design of Flight Vehicle Structures) contains a fairly comprehensive set of checks for single and double shear lugs subject to axial and/or oblique loading.

Good luck

 

[kilbchoi]

Hi Visitior 2002,

Thank you for your reply.

In the formula which I introduced, Fa is the selected minimum
load from a set of strength based on net tension, shear bearing, ...,
you mentioned.

My exact question is for futher reduction of lug strength of
single shear joints based on double shear ones since the single
shear is more stressed due to prying effect for balancing moment
than the double shear. I know single shear lug joint is not good
practice desgin. But, my designer insists that design due to certain
reasons.

Thanks,

 

kilbchoi,

The reduction in allowable static (and fatigue) load for the condition you are describing can be quite severe. What leads you to believe the method in your design book is overly conservative? Have you performed a beam-socket analysis to see what sort of stress peaking you expect? Do you have any associated test data handy to support or invalidate the method in your design book?

As an aside, if the joint you are describing is intended for use in primary structure or in a critical control system, in my opinion it would be prudent to convince your designer that a change is necessary.

Best of luck -