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ashen 07-30-2008 08:35 PM

Structural integrity of fir 6x6 beams
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Hello all.

I'll get right to the details here.
Ive just built a post and beam porch out of nominal rough hewn douglas fir.
Ive used 2x8 rafters resting on 6x6 beams as well as a 6x6 ridge beam.
The roof has been decked with 3/4 hem-fir shiplap with a layer of 3/4 inch standard sheathing to account for roofing nail penetration (so you dont see the ugly nails from below)

I cant seem to rest easy about the fact that I may be reaching the weight limits of the 6x6 beams. Everything seems very solid but I just cant get the thought out of my head that I maybe miscalculated. (I'll give ten bucks to the first person that suggests 'you should've used 6x8 or 6x10')

Each 6x6 beam only spans 12 feet, of which 2 of those feet are supported by a knee brace. So technically the beams only span 10ft.

Perhaps someone out there could rest me at ease that what Ive built isnt going to topple down under the combined weight of itself and the asphalt shingles that will be applied to it.

Pic Included.


buletbob 07-30-2008 09:38 PM

Best to consult with a structural engineer. nice looking project. How deep did you make those bird's mouth's I would be concerned about the rafter tails cracking.

ashen 07-30-2008 09:55 PM

Theres still at least 4 inches of meat left at the thinnest part. But I think between the 1 1/2" of decking above the tails spreading the load and the fact that we dont ever get much of a snow load here everything is ok. (I hope)

Termite 07-30-2008 11:44 PM

That looks rock solid. I wouldn't have any concerns about the 6x6, given the fact that it is just a roof structure. The rafters are overcut at the birdsmouths, but the only problem it could cause is some cracking/twisting at the heels. You should secure the heel of the cut to the beam pretty well.

The ridge is supported to the 6x6 that runs perpindicular to it. Are the ends of that 6x6 bearing on the posts or are they butted to the parallel beams supporting the rafter heels? There's some fair load there, and there needs to be some bearing at that point. Perhaps the angle brackets could be let into the post 1" and into the perpindicular 6x6 1", which would actually let the bracket do some structural work.

My only other concern would be the connection between the 6x6 posts and the horizontal structure. There can be a considerable amount of uplift. If they're only nailed or screwed, I'd consider having some 1/4" steel cut, drilled, and painted, and use it to make bolted connections between the two.

ashen 07-31-2008 12:16 AM

Rock solid. Right on.

It was too late for me to lower the beams supporting the rafter heels, and I didnt want to let the beam in any for concerns of weakening it. I had to swallow the fact that the birdsmouths were deep, but I'll see if I can sneak in some more fastners to prevent any sort of movement.

The beam that supports the ridge has been notched to the posts with a half lap joint and some lag bolts accompanied by some 3" deck screws. The tiny little king post has been let into both the ridge beam and the support beam beneath it and has been secured with 5" screws.

As for the posts being connected to the beams (read: horizontal structure), the connection was made with an 8"long 3/8" lag screw ( or bolt, depending on where you come from). That means that the lag is penetrating into the post at least 6 inches (due to the half lap joint at connection point over the posts)

Thanks for the 'uplifting' comment. This is the first independent project Ive taken on since learning how to swing a hammer. Im finding it very easy to second guess myself about my abilities and knowledge.

Thanks for your comments

Termite 07-31-2008 12:40 AM

From your description of the connections, it sounds like it was done right. Most first time carpenters aren't doing work that looks that good...Trust me, I see a lot of it! :yes:

Yoyizit 07-31-2008 02:34 PM

Stressed beams will sag in the middle or bend at the end. Stretch a line and measure the deflections.

With the measured deflection plus knowing the wood type [which gives you the modulus of elasticity] plus the "lumber engineering" book [you borrowed]
will tell you exactly how much stress the beam is under.

The beam is then its own strain gage.
Using a spreadsheet will keep all the calculations in some kind of order.

Termite 07-31-2008 03:38 PM

And then you'll have a spreadsheet full of analytical mumbo jumbo...And no solution to any potential problem. :huh:

It is too late to engineer it now, and to do so seems futile. I don't think there's going to be any sort of structural issue with this structure based on the picture and the descriptions posted, and the insignificant loads.

ashen 07-31-2008 07:31 PM

Thanks termite. Now that its all together and all the knee braces are up, and Ive hung the deck ledgers and all the other little things I feel a bit more confident with it. Thanks again for the encouragement and advice.

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