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Double 2 x 12 span

Uncle Bob

Uncle Bob

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Joined
Oct 17, 2009
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1,409
Location
Texas
Beam/Girder spans. I have been talking to local framers in this new area and they seem to be of the opinion that if you sandwich 1/2 " osb between the 2 X 12s, you extend the span limit to 12 feet. Without engineer's specifications I can't accept this. Anyone heard of extending 2x12 lumber spans by using this method?

Thanks,
 
This method is not prescriptive.

If there is a joint in the span then the increase is negligible at best

PE's that I spoke with say it can add from 0 to maybe 4% depending on how and where, maybe more.

If they are under spanned prescriptively but point to this as an excuse, I give 2 options: 1) Meet the span tables or 2) Get a structural engineer to stamp it.
 
People will also use foam blue board for the sandwich to get a little R value but the charts only recognize the framing members. Good catch.
 
This reminds me of another pet peeve. Calling it a beam or girder prescriptively when there is a space between the members such as a deck that calls for a double 2x10 beam and they have one 2x10 on each side of a 2x6.
 
Agree with the others, LVL or engineer design.

In the old days they use to sandwich a piece of steel between the 2x's, don't hardly see that anymore?

pc1
 
Right on Jar, split "girders" bothers me too. Another thing to watch for is the practice of using thin strips of plywood at the top and bottom of a header, giving it the appearance of a solid piece. I think an argument could be made that one could properly fasten a solid piece of pywood between two members and have them act as a single member however I think you can still only count on the values of the members. And if they aren't fastened together properly you may be reducing the loads to less than the tables indicate.

The NC code acually has (or had as of 08) prescriptive flitch beam load tables for the steel sandwich.
 
And, by the way, I used steel flitch plates all the time when I was building.
 
And I'm sure they helped, last time I checked, 2 X 12's would not even make a 9' garage door span with a 24' bearing total roof span. But, even with the flitch plate, would still need engineering today. BTW, never seen a fliitch plate used around here, had just heard/read about them before.
 
doing a review without information

How can any comments be made without any idea of the load. I see too many headers where they are not bearing any load.
 
elowpop said:
How can any comments be made without any idea of the load. I see too many headers where they are not bearing any load.
Click to expand...
Excellent point. I guess we are assuming there is a load, otherwise, what's the point of the conversation.
 
Inspector Gift said:
Does anyone else use StrucCalc? ...Uncle Bob? ...Anyone???2-2x12 DF #2 floor beam or roof beam spanning 12 feet with a 4 foot tributary load -- passes.
Click to expand...
Hey Terre, :)

Nope, I don't do engineering; don't want to put those good people out of work. How do you know what the "tributary load" is? Why does the code table (2009 IRC, Table R502.5(1) restrict 2-2x12 douglas fur any # to not more than a span of 9' 9" where the building width is 20 feet; and only allows 7' 6" where the building width is 36 feet?

With your Struc-Calc; do we just tear the code tables out of our books and say a software program voids the code?

The code states that compliance with the code is required accept where spans exceed the limits of the code and then engineering is required. Terre, do you require code compliance or not?

Uncle Bob
 
Great topic, and good questions, Uncle Bob... As you know, the span tables in the IRC are not all inclusive. Given a set of plans, we will check beam sizing with StucCalc if there is a concern. And I will allow alternate methods and materials. G
 
Which is according to accepted engineering practices, no differently than if I have the salesman check the same girder as an LVL or if I refer to the NHBA's flitchplate design tables; Flitch Plate and Steel I-Beams.

BTW in a flitchplate girder the steel is carrying the load, the wood prevents buckling. A flitchplate for a given load and span will I believe, always be heavier and thus more expensive than an I beam. When LVL's came out they largely replaced flitchplate beams.

In a <8' header you could add the section of the plies with grain oriented along the beam's axis to that of the solid sawn wood. If you're splitting that hair you're cutting it too fine. Once there are joints in the ply it's getting pretty deep.

As an aside and I'm curious. I worked with one seasoned old carpenter who would unroll flashing inside a girder between plies and have us nail it up tight. He claimed it made the girder stiffer. I think it was too thin not to ripple its way out of that problem but am not certain, the bottom strap would be reinforced.
 
DRP,

Thanks for the flitch plate info, put it on my favorites for reference.

Like I posted before, don't see the flitch set-ups. LVL's rule the day with engineering and install directions.

pc1
 
I just worked one typical example and picked up about 4.5" of increased span with 1/2" ply assuming that it is spliced at a point of low moment and well connected. I also assumed the ply was of a higher strength species and grade than the solid sawn lumber... when you make a composite like that the design values of the weakest member are used. In other words if the lumber was SYP and the osb was aspen it wouldn't make sense to try to increase the allowable span using aspen strength numbers, it would be better to ignore its' presence in there.

I was delighted to see lvl's, drilling a bunch of bolt holes in 1/2" steel plate and then getting it up and aligned to bore and bolt the wood was often the best part of a day, and scary with that guillotine at times. LVL's are lighter, lift em up and shoot, and then whine about how hard they are to shoot :)

Another is to play with species and grade, for one the code tables are defaulting to the weakest species or group they call out in the heading. I'm playing with some beech at the moment
 
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