Replies

1. 
   Edgar76b | May 28, 2003 02:49am | #1

   If I understand you the "beam span", refers to a post every five feet?

   Can't say for sure without seeing it. And I can't tell you the load carrying ability, off the top of my head. But It sounds plenty strong. As long as you have the footers right.

   The only disadvantage to solid 6 x 6" beams or any square beam is that you are more likely to find a weakness in the grain. And in this situation the only disadvantage. I see is deflection. But it won't be a problem, because you have the posts every 5'. Built up 6 x 6" are stronger because the grain opposes the peice next to it and helps create a cross bracing effect. Not unlike the layers in a sheet of plywood.

   If i remember correctly 3- 2x6's can carry about the same as a 2- 2 x 8 of equal length. But the 4 x 8  has a better deflective value than the 6 x 6"

   Ask piffin or BossHog. They probably have the load tables handy.

   Where there's A wheel there's a way, got any wheels?

1. 
   scottyb | May 28, 2003 03:12am | #2

   Your local Building Codes will be the main factor.

   Here is the accepted spans for girders(for decks)in one town I am working in.(Suburb of Ghicago)

   4X6---8'0" 40lbs.per sq. ft.live load

   4X8---11'0"

   4X10---14'0"

   4X12---17'0"

   These are solid 4X stock,but 2X stock nailed together or one on each side of a post bolted together through the post.

2. 
   Piffin | May 28, 2003 04:25am | #3

   I'm presuming you are not adding a roof over this.

   You also don't mention whether the deck is only nine feet out, total - or if it cantilever's over this beam. Total size of deck is needed to calculate load.

   But as a general rule, with these assumptions and decent lumber grain, it will work if there are no heavy snow load requirements.

   I would use the fir 4x8 or doubled up 2x8 PT

   .

   Excellence is its own reward!

1. 
   TimInSeattle | May 28, 2003 10:02pm | #4

   4x8's it is then! Actually 2 2x8's with 1/2 inch treated plywood spacers every foot per instructions in decking book mentioned below.

   Thanx for the tips. I forgot about the overhang, it is only a foot, but worth considering.

   After the post i found Schuttners book on decks and his numbers agree with scottyb's Chicago area code values.

3. 
   JohnSprung | May 28, 2003 10:38pm | #5

   Section modulus is the thing you want to compare, it's equal to the width times the square of the depth divided by six.  For the 4x8, figuring 3.5" x 7.5", S = 32.8 cubic inches, for the 6x6, figuring 5.5" x 5.5", S = 27.7 cubic inches, or about 85% as strong as the 4x8.  So, the 6x6 might be OK.  If you prefer it for appearance or some other reason, it would be worth doing the math.

   -- J.S.

1. 
   TimInSeattle | May 30, 2003 10:53pm | #7

   John, 

     I think you are describing the beam stress equation that goes as b*h^2 for stress on the outer element.  Your description sounds good, i just never heard it called the section modulus.  Going even farther, you could look at the beam deflection equation that goes as b*h^3, you will find the 4x8 to be almost 50% stiffer.   I haven't looked at shear since this beam is so long but it might be worth checking also.  In any event, the doubled 2x10's are installed and should work.   I upped the dimension to 10 inches to allow for the overhang mentioned in an earlier post and 'just to be sure'.  Schuttner's book also calls out increased beam sizes for hem-fir and i am not sure what wood this PT lumber comes from.

   Tim

1. 
   JohnSprung | May 31, 2003 01:52am | #8

   For rectangular cross sections,

   Section modulus S = (b*h^2)/6

    Moment of Inertia I = (b*h^3)/12

   Very roughly, S is for breaking, I is for bouncing.  First design so it won't break, then check out if you can live with the bounce. 

   For 700 pages and all the rest of the equations, see the book "Simplified Engineering for Architects and Builders" by Parker & Ambrose. 

   -- J.S.

4. 
   Edgar76b | May 30, 2003 01:45am | #6

   The only reason I ever consider using anything less than 2 x 8's, is if there is an elevation problem2x6's can solve the problem if you need those extra 2".

   Where there's A wheel there's a way, got any wheels?