# doubling floor joists



## ratherbefishing (Jan 13, 2011)

I don't know offhand of an online calculator. Oh look; here's one: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp I googled "joist span calculator".
I also remember reading a while back (maybe here) about nailing a 2x4 to the edge of a joist, to form a sort of I-beam. Seems like it had a surprisingly large effect.


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## titanoman (Nov 27, 2011)

I don't think doubling up a 16' 2x8 will all of a sudden span 30'.
Stacking, not either. Not even a 12 micro will span that far.
I would think the doubled up's would span further because you can attach one to the other more thoroughly.


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## titanoman (Nov 27, 2011)

ratherbefishing said:


> I don't know offhand of an online calculator. Oh look; here's one: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp I googled "joist span calculator".
> I also remember reading a while back (maybe here) about nailing a 2x4 to the edge of a joist, to form a sort of I-beam. Seems like it had a surprisingly large effect.


That's why they call that a strongback.
And attaching a 2x4 to a joist makes the 2x4 stronger, not the other way around.


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## TarheelTerp (Jan 6, 2009)

mae-ling said:


> anyone know of an online span calculator or chart for doubling up floor joists.
> 1. I vaguely remember from my college days that 2 of anything side by side are twice as stong as one. Is that right?
> So can it span twice the distance? *NOPE*...


I think any real strength only comes when you slip a piece of steel plate in between the two joists (or at least some plywood)


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## mae-ling (Dec 9, 2011)

ratherbefishing said:


> I don't know offhand of an online calculator. Oh look; here's one: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp I googled "joist span calculator".
> I also remember reading a while back (maybe here) about nailing a 2x4 to the edge of a joist, to form a sort of I-beam. Seems like it had a surprisingly large effect.


I use that one, it only does 2by material not doubled up ones.


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## mae-ling (Dec 9, 2011)

When I mentioned stacking it is going from say 2x4 to 2x8, not just setting one on the other. Should have been clearer.


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## mae-ling (Dec 9, 2011)

TarHell - it is amazing what even a thin piece of metal does between 2 pieces of lumber.

Even stuff so thin you can screw or nail right through it.
One time we needed to move a post in a basement remodel. did not want to go wider or deeper with the beam. We were thinking we might use steel. Archy/engineer whoever spec'd a metal plate in-between the plys of the beam. It was less than 1/8 insh I think 1/16th. A while ago.


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## woodworkbykirk (Sep 25, 2011)

doubling up wont double the span but it will increase the deflection rating and the ability to take more load on it. as for stacking. ive never heard of this making something stronger unless its finger jointed and bonded with epoxy 

unless its ok'd by an engineer i would mess around with it personally


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## mae-ling (Dec 9, 2011)

just read online someone say it adds 1/3 the original distance. But had nothing else to back it up.


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## titanoman (Nov 27, 2011)

mae-ling said:


> When I mentioned stacking it is going from say 2x4 to 2x8, not just setting one on the other. Should have been clearer.


Okay. I was wondering, like "I've never seen that done before..."


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## Daniel Holzman (Mar 10, 2009)

The strength of a beam loaded in bending (which is what you get when you impose a uniform load on it) depends on three key factors:

1. The allowable stress (typically abbreviated with Greek letter sigma) in psi
2. The moment of inertia I of the beam. For a prismatic beam (rectangular beam is prismatic) the moment of inertia = bd^3/12, where b is the width, d is the depth, and ^3 means cubed
3. The distance y from the centroid of the beam to the outside edge 

The critical equation is Stress = My/I, where M is the maximum bending moment applied, y is the distance from the centroid of the beam to the edge, and I is the moment of inertia.

So let's see what this means in practice. If you double a 2x4 by adding a second 2x4 (sistering), I doubles, y remains the same, allowable stress remains the same, and the strength doubles. This is somewhat harder to achieve in practice than it sounds, because the 2x4's need to be adequately fastened together to act as a unit.

If you double the depth of a beam, say by attaching a second 2x4 to the bottom of the first 2x4, thereby changing the depth from 1-3/4 inches to 3-1/2 inches, you increase the moment of inertia by a factor of 8, since d doubles, and I goes as the cube of the depth. The distance y doubles, since the beam is twice as deep. Assuming you use the same type of lumber top and bottom, the allowable stress does not change, so it turns out that the maximum moment the beam can handle (the strength) goes up by four times. By the way, doubling the depth of a beam is difficult, because you need adequate fasteners to insure that the allowable horizontal shear between the two beams (the top one and the bottom one) is not exceeded, and computation of horizontal shear is tricky. Failure to adequately fasten the top and bottom beams together results in a drastic reduction in strength, and unfortunate behavior of the composite system.

How far you can span is more complex, because the moment typically increases as the square of the span. And computation of deflection (stiffness) is much more complicated, because deflection formulas depend on the modulus of elasticity of the material.


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## Gary in WA (Mar 11, 2009)

I use this one, I've posted it 6-7 times on here before, last one less than a month ago.....send check or money order to:http://www.awc.org/pdf/wsdd/C2B.pdf

Gary


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