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splicing 2x12's to get longer member

splicing 2x12's to get longer member

(OP)
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

RE: splicing 2x12's to get longer member

Use TJI's instead.

Mike McCann, PE, SE (WA)


RE: splicing 2x12's to get longer member

(OP)
Not a good choice for several reasons.

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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.p...

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

(OP)
thank you hokie66

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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...

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

(OP)
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.

RE: splicing 2x12's to get longer member

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.

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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

RE: splicing 2x12's to get longer member

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.

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