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

 

[msquared48]

Use TJI's instead.

Mike McCann, PE, SE (WA)

 

[keyPitsimplE]

Not a good choice for several reasons.

 

[keyPitsimplE]

I know. My first inclination was that it was a bad idea, but the more I thought it seemed doable.

 

[dcarr82775]

It should work. I would use blocking to resist rotation where (1)play ends, and I would use screws instead of nails. That being said, I don't like the idea. Never heard of long lead times getting a simple TJI.

 

[EngineeringEric]

I would think the ductility of a nail is better than the brittleness of a screw. Again, i'd say don't do it but it is do able

 

[jayrod12]

It is possible however again, for the cost of the double 2x12's, nails, glue and labour. you can buy a TJI that is full length.

 

[jayrod12]

You should abide by your screen name and keep it simple.

 

[dcarr82775]

I like screws with glue. The glue is really doing the work and the screws provide a better clamping than nails.

 

[keyPitsimplE]

Thanks to those willing to consider it. As I was hoping, no one has any good reason not to do it except our natural aversion to a spliced member (probably due to having seen it done poorly so many times in existing buildings). TJI do not work because I have to match 11.25" depth elsewhere, cannot notch or rip the flanges, sloped bearing, have to fill webs and sister on tails, etc. I have priced all options and it is the cheapest and quickest way for us to go FOR THIS PROJECT. The labor to splice them washes out. If it were as simple as doing TJI I would not have posted. My biggest question was if anyone saw a reason it would not function structurally the same as a full length (2)2x12.

 

[jayrod12]

You could buy 11 1/4" deep lvl or lsl members and space them out further. I'm not saying it doesn't work. I'm saying it requires way too much effort and attention from the contractor.

 

[EngineeringEric]

no LVLs either?

your 24' foot long 2x12 may be finger spliced to begin with. splices are not always bad but can be. make sure you use exterior grade glue and fasteners that cannot rust out. the problem is those nails and glue are far more critical than anything else.

 

[dcarr82775]

Natural aversion to lap splicing wood members, but for me it is the certainty that the framer will not do what I specify on the drawings (stupid overkill engineer, I don't need that many nails) unless I watch them like a hawk.

 

[keyPitsimplE]

The Framer (a good one whom I know) is the one asking for options. 2x12's are solid DF2. LVL and LSL have to be ripped down to 11.25" or special order and are more $ per LF.

 

[SlideRuleEra]

The U.S. Department of Agriculture, Forest Products Labs has looked into bending in bonded multiple-member wooden beams. Their conclusion is that for common grades of lumber bonded 2-ply beams perform better than two separate individual members. See "Bending Strength of Vertically Glued Laminated Beams with One to Five Plies" at this link:

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp333.pdf

IMHO, if the two members are properly bonded over the 23 foot overlap (which should be pretty easy to do considering the length) then the splice in not a problem.

Likewise, since the one foot length of single 2 x 12 at each end is adequate for shear this is not a problem either. In principle, this is not much different than coping of a steel beam at the connections... and that's done all the time.

I agree with dcarr82775 that the glue is most important; don't cut skimp on that.

Additionally, the span to depth ratio of the beam is very high, 26.7 (i.e. 300" / 11.25"). It is asking a lot of a simply supported beam of any material to perform well with that slenderness. Be sure to recheck to make sure deflection is ok.

Finally, I suggest that you pick the "best" end of each individual member to at the end of the beam. You want the best wood there to carry the shear load.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[keyPitsimplE]

Immensely helpful, SlideRuleEra! Thank you very much! And my thanks is not because you are supporting my hunch, it is that you took my problem at face value and dove into it with me to help me solve it. I really appreciate that.

To everyone else, please know that I am certainly appreciative of your time trying to get me to think out of the box. I was just growing a little impatient since I have deeply considered all of the options, spent time on the phone with the supplier & framer, and just needed to discuss the technical merits of my actual question.

Keeping it simple was the 2 options I had put on the plan. We were beyond that.

And for the record, TJI's suck for sloped roofs that don't have full depth tails.

Thank you all for being a great resource for one-man engineering operations to bounce ideas off of.

 

[racookpe1978]

If so, drill and bolt through with galv or SStl bolts, if using treated wood/outdoor exposure, over the glued joints.

Don't trust a nail or nail gun, bult clamp the two together with 5/16 or 3/8 bolts and washers, both sides.

Offset your joints with the through spans:

For a 24 foot-long joist, use 6 ft + 12 ft + 6 ft spans on one side of the joint. 12 ft + 12 ft on the other mating 2x12 joist.

 

[keyPitsimplE]

This is indoors and covered. Using 2 full 24' DF2 rafters, spliced together and offset such that we have one member that is 25' long, with only single 2x12 protruding at each end, allowing LUS210 hangers. That is a 23' lap splice. Glue and nail gun no problem. Done.

 

[racookpe1978]

No, move your joint a bit further from the end to develop a stronger moment arm across the actual joint the joint against the end rotation moment and shear: 21+ 4 foot will be better than 24 + 1 foot.

 

[keyPitsimplE]

Thank you racookpe1978, but I don't think you understand what I'm proposing. There is no "actual" joint. Just a 25' double member with 23' overlap. That is a 23' moment arm, as I think you are seeing it.