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

 

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!

 

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

 

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).

 

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.

 

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.

 

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.

 

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

There will be a small cantilever over the beam

 

Thanks everyone for your help.

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

 

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.

 

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.

 

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

 

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