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Author Topic: Eastern White Pine Span  (Read 315 times)

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Offline jdtuttle

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Eastern White Pine Span
« on: June 07, 2017, 01:22:23 pm »
I'm building a Community Park Pavilion and need help to confirm the size of the beams are appropriate to carry the loads. The Ground snow load is 40. I'm using 9' 6x6 post 11' OC, with 6x6 diagonal braces 30" out on beam. The beam is 6x8. The roof system will be engineered truss 4' OC with 2x4 purlins 2' OC & metal sheathing. I'm not an engineer & just want to be certain of the loads.
Thanks, Jim
Have a great day

Offline ChugiakTinkerer

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Re: Eastern White Pine Span
« Reply #1 on: June 07, 2017, 03:30:19 pm »
Hi Jim, that sounds like a fun project.  I'm no engineer either, just someone who thinks he knows enough about wood structures to be dangerous.  :)

Actually with Don P's patient explanations I have learned enough that I can work my way through your situation.  As I understand your description the 6x8 beams will be supporting the trusses.  Assuming full snow load and 10 lbs/sf dead load, the total load for the roof will be 50 psf.  Your posts are 11' on center, so each beam will have a tributary area that is 11' wide and as deep as half of the structure width.  I'll assume a width of 20' but you'll want to adjust this calculation based on the actual width.  So each tributary area is 11' x 10' = 110 sf.  Total load on a beam is then 5500 lbs.

Using the toolbox available on the left below the sponsor links, I'll use the Uniformly Loaded Simple Beam calculator.  Inputting values for load and length, we have 5500 lbs and a span of 132".  Eastern white pine is in the 2015 NDS Supplement.  A beam of EWP #2 has engineering properties of Fb = 575, Fv = 135, and E =1.1 x 10^6.  Plugging that into the calculator, the max load the beam can support is about 2200 lbs.

Using those same numbers and making the beam deeper, a 6x10 can support up to 3450 lbs and a 6x12 can support up to 5000 lbs.

This is using the engineering values for #2 graded beams.  Better quality beams are stronger and can support greater loads.  Were I in your shoes I would look at going to a 6x12 beam or using a much stronger wood such as Douglas fir.  Otherwise it's probably a good idea to bring in an engineer to help determine the best path forward.
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Offline ChugiakTinkerer

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Re: Eastern White Pine Span
« Reply #2 on: June 07, 2017, 03:47:11 pm »
Where I said above I know enough to be dangerous, here's an example...

Thinking some more on this, I think a built-up beam would be much better.  If you use three 2x8 nailed together it will be much stronger.  Here's how you can do the calculation:

1) Use the engineering values for Eastern White Pine as above, which come from Table 4A in the NDS Supplement. 

2) Apply a size factor of 1.2 to the bending strength, which you get from having a narrow deep board.  The resulting Fb = 690 psi.

3) Apply a repetitive factor of 1.15, which you get when using three or more joists or beams close together.  Resulting Fb = 793.5

4) Use that Fb in the load calculator to get a max load for a 6x8 beam (three 2x8 together) of 3,000 lbs.

If you consider using three 2x10 together in a built-up beam, then the size factor for a 2x10 is 1.1.  The resulting Fb = 727.35 psi.  The max load for that beam with an 11' span is 4400 lbs.

Finally, for a 6x12 beam built up from three 2x12, the size factor is 1.0 and the max load is ~5,750 lbs.  Again, this is based on #2 grade.  If you can get better grade timbers you will be better off.
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Offline jimdad07

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Re: Eastern White Pine Span
« Reply #3 on: June 07, 2017, 09:54:00 pm »
Good job Tinkerer.
Hudson HFE 30 Homesteader bandmill w/28' of track
Couple tractors, a bunch of chainsaws and not enough time to use them.