Inter-rivet Buckling Fixity Co-eff for Rivet Tails

[graemew]

I have an application where a strap on the reverse side of a spar cap needs checking for inter-rivet buckling. This is the rivet tail side of the joint.
I do not feel comfortable using C=4 for such a configuration (even when using MS20470AD rivets).
My gut feel tells me that I ought to be looking at C=2.5 to 3 approximately. A C=3 is used for brazier and round head rivets in several publications available to me.
Has anyone seen an end-fixity coefficient for sheet under rivet tails? It surely can't be so uncommon that it has never been considered.
Any thoughts?

 

[Stezza]

The only reference I was able to find was from the Boeing structures part II notes. In the buckling chapter on page 15 the fixity coeficient (c) for a rivet upset head is quoted as 2.0 for a strap. Not a great reference but I hope it helps on a ballpark number.

Cheers
Mat

 

[Sparweb]

Inertia4U:

Bruhn, C7.12. C=4

You can reason that if in a row of rivets the sheet will always buckle the same way (ie. away from the spar cap), then under each rivet head, the slope of the curvature is horizontal again. If the strap between two rivets is drawn as a free body, it doesn't matter if the resisting moment under each rivet tail comes from the rivet tail or from the strap beyond the FBD.

I hope that was clear - a picture would save me a thousand words.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[graemew]

Thanks Mat,
The C=2 for an upset head smells of "Boeing conservatism" to me but it is worth knowing about.
Steve,
I was thinking along those lines myself where the underlying structure is very significantly stiffer (read thicker).
I think that the inter-rivet end-fixity factors were determined mostly for thin sheet and thin underlying structure where the rivet head contributes significantly to the restraint over the full effective width of the sheet.
Regards
Graeme