splicing 2x12's to get longer member 2

[keyPitsimplE]

I am considering splicing (2) 2x12's together in order to get a longer rafter, but also get the bending strength of a double rafter. 24' is the typical stocked length in our area and I have a roof that has a 25' span. The dead and snow loads require 2x12 DF2 @ 12" o.c. I would like to splice (2) 24' pieces together with 1' offset at each end and set them 24" o.c. I believe with the right nailing (about (4) 10d @ 12" o.c.) and some glue for safety, this should function well as a double 2x12. The max shear load is plenty for a single 2x12 and this allows me to use single 2x12 hangers at the ridge. Does anyone have any major objections or am i missing something? We save a lot of money and lead time for specialty lumber.

Thanks

 

[hokie66]

Just reading this, and am giving you a star for your patience, keyPitsimplE. Nothing wrong with your solution.

 

[keyPitsimplE]

thank you hokie66

 

[woodman88]

Two 20' lapped for a 25' span should wouk with 10d common nails (@ 12"oc with a cluster at each end) with three full depth blocking along the lapped section. IMHO

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[CELinOttawa]

The only trouble I see here is the incidental eccentricity. It may seem very small, but a great deal of buildings were brought down by a failure which propagated from an eccentric bracing detail where the eccentricity was below a half-inch.

I would make damn sure that the members are blocked out at the supports (as I'm sure you would do), but I would also provide blocking as if this 25' length was spanned by a 2x12 member rather than a 2-2x12. I hope you see what I mean...

 

[hokie66]

CEL,
Good advice, but I think what you are referring to were compression braces, not bending members.

 

[keyPitsimplE]

We are going to block it at the bearings and mid span. I thought about this awhile and the double offset rafter will actually be quite symmetrical. Each end will bear evenly such that it should not twist at all, even without blocking.

 

[CELinOttawa]

Quite right Hokie, but my point is that every engineer who looked at the detail believed the eccentricity to be dismissively small. That only changed once they started coming down under ridiculously low snow/wind/dead loads on the roofs.

I would prevent rotation in the system to not allow any "surprises" in this very long lap joint or the bearings.

 

[keyPitsimplE]

Indeed, CEL. Do you have a link to reports or info on the failures you are talking about?

 

[CELinOttawa]

Here you go:

http://www.dbh.govt.nz/practice-advisory-12

 

[keyPitsimplE]

Thank you! Several of the technical advisories on that website are interesting and informative.

 

[CELinOttawa]

NZ is one of the few jurisdictions which take Structural Engineering proper serious.

 

[KootK]

I'm going to weasel in here and be the first to say that the OP's solution isn't just acceptable, it's bloody clever. KootK approved! I've done something similar with metal plate connected wood trusses that had to be shipped in two pieces.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.