# Beam strength sizes



## sannitig (Jan 15, 2012)

Hey guys very quick question here.

I'm building the beams for my deck, what's stronger and can support more load: 3-2x6 or 2-2x8?

using pressure treated spf...I can only find info on southern pine and ponderosa.

Thanks!


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## hand drive (Apr 21, 2012)

it looks like those two different beam configurations are very close to the same strength while in comparison to one another.


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## sannitig (Jan 15, 2012)

hand drive said:


> it looks like those two different beam configurations are very close to the same strength while in comparison to one another.


Thanks for the quick answer. Is this a guess or more based on fact. No intention to offend and please forgive me if I did, but is there math behind your answer or possible decades of experience? Just want to make sure!


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## hand drive (Apr 21, 2012)

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf


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## jklingel (Dec 21, 2008)

San: I could not get that link to work, but the carrying capacity of a beam is related to the square of the height. Thus, a 2x8 will carry 4x what a 2x4 will, all else being equal (approx, and not an engineering equation completely, but the squared part is in there).


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## sannitig (Jan 15, 2012)

So then the 2x8 is stronger than the triple 2x6 beam.

The pdf says the triple 2x6 is stronger


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## hand drive (Apr 21, 2012)

sannitig said:


> So then the 2x8 is stronger than the triple 2x6 beam.
> 
> The pdf says the triple 2x6 is stronger



for spf the 3-2x6 beam is stronger than 2-2x8 and for southern yellow pine the 2-2x8 is stronger than the 3- 2x6.

in order for the triple 2x6 to be stronger it has to be nailed in the correct beam nailing pattern and if your floor joists for the deck are 2x8 then a triple 2x6 beam does not work to well.


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## sannitig (Jan 15, 2012)

I am going to use 2x8 for the joists. Why would the triple 2x6 not work as well?

That's what I was planning to user since it was stronger with less height. It was perfect...


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## hand drive (Apr 21, 2012)

sannitig said:


> I am going to use 2x8 for the joists. Why would the triple 2x6 not work as well?
> 
> That's what I was planning to user since it was stronger with less height. It was perfect...


oops, forgot to say. if the beam is flush with the joists ( flush beam ) then it would not work but if it is a dropped beam then no problem.


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## thetalkingmule (Apr 6, 2012)

The triple will just make it a pain to connect to the posts anyway, so go with the double 2x8 so you can notch the posts on both sides.


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## sannitig (Jan 15, 2012)

Ya sorry I forgot to mention the beam is going under the joists.

There will be a small cantilever over the beam


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## jklingel (Dec 21, 2008)

sannitig said:


> So then the 2x8 is stronger than the triple 2x6 beam.
> 
> The pdf says the triple 2x6 is stronger


 Again, what I said is not the entire engineering formula, just a general rule for a single beam. "The numbers" put them pretty close, so I would not be able to bet on either one. If it is a long span or you suspect heavy loads, see an engineer for "the rest of the story."


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## sannitig (Jan 15, 2012)

Thanks everyone for your help.

I may just say screw it all and go triple 2x8


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## Daniel Holzman (Mar 10, 2009)

The strength in bending of any beam is related to the moment of inertia of the beam, which for a rectangular beam is equal to 1/12bd^3, where b is the width and d is the depth, and ^3 means cubed. For a triple 2x6, where each 2x6 is true measure 1.5 inches wide x 5.5 inches deep, the moment of inertia is 62.4 in^4.

For two 2x8's, where each 2x8 is 1.5 inches wide and 7.25 inches high, the moment of inertia is 95.3 in^4. The doubled 2x8's therefore have about 50% greater moment of inertia than the tripled 2x6. The actual beam capacity is a bit more complicated to compute, since we are primarily interested in extreme fiber stress due to bending.

In bending, the doubled 2x8 could support 264 lbs/ft on a 10 foot long beam assuming an allowable stress of 1500 psi. For the same allowable stress on a 10 foot beam, the tripled 2x6 could support 228 lbs/ft. So the doubled 2x8 beam can support approximately 15 percent greater load per foot than the tripled 2x6 beam, assuming the same maximum allowable fiber stress.


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## sannitig (Jan 15, 2012)

Daniel Holzman said:


> The strength in bending of any beam is related to the moment of inertia of the beam, which for a rectangular beam is equal to 1/12bd^3, where b is the width and d is the depth, and ^3 means cubed. For a triple 2x6, where each 2x6 is true measure 1.5 inches wide x 5.5 inches deep, the moment of inertia is 62.4 in^4.
> 
> For two 2x8's, where each 2x8 is 1.5 inches wide and 7.25 inches high, the moment of inertia is 95.3 in^4. The doubled 2x8's therefore have about 50% greater moment of inertia than the tripled 2x6. The actual beam capacity is a bit more complicated to compute, since we are primarily interested in extreme fiber stress due to bending.
> 
> In bending, the doubled 2x8 could support 264 lbs/ft on a 10 foot long beam assuming an allowable stress of 1500 psi. For the same allowable stress on a 10 foot beam, the tripled 2x6 could support 228 lbs/ft. So the doubled 2x8 beam can support approximately 15 percent greater load per foot than the tripled 2x6 beam, assuming the same maximum allowable fiber stress.


 
On page 5 of the pdf, the values in the table suggest that a trippled 2x6 is stronger than a doubled 2x8. I came to this conclusion because the trippled 2x6 is able to span a greater distance than the double 2x8. Would this not mean it is able to carry a greater load?

Unless, the pdf is flat out wrong...that's always a possibility.


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## jklingel (Dec 21, 2008)

Dan: Thanks for the equation. I mis-remembered when I said it was a square phenomenon; it is a cube. Big difference.


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## sannitig (Jan 15, 2012)

Thanks all. I went with triple 2x6 in the front and added an extra post and triple 2x8 at the back where the spam would be 8.5 feet


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