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Old 10-08-2016, 10:44 AM   #1
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Multi-ply deck beam - flat on top or flat on bottom


Quick question - I'm assembling a multi-ply deck beam (in this case a 20 ft long 2x10 3 ply beam), and I think it is more likely to win the lottery than find 3 20ft long PT boards that are perfectly matched in height over their entire 20 ft length. These are #1 boards from the same pallet but still every board wants to be unique in one way or another. Even with the closest matches, I can make the top flat and have ply gaps at the post caps, or I can make the bottom flat so all plies sit evenly on their posts but have uneven tops.

The deck requires two of these as drop beams, which the joists will lay across. So the two beams should be flat to each other as far as the joists are concerned.

Thanks!
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Old 10-08-2016, 10:48 AM   #2
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Re: Multi-ply deck beam - flat on top or flat on bottom


Just install them all crown side up.
Not a big deal if one does not make full contact, it's going to act as one piece once installed and the boards are going to shrink anyway.
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Old 10-08-2016, 11:21 AM   #3
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Re: Multi-ply deck beam - flat on top or flat on bottom


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Originally Posted by joecaption View Post
Just install them all crown side up.
Not a big deal if one does not make full contact, it's going to act as one piece once installed and the boards are going to shrink anyway.
Thanks for the quick response. So you install them flat at the top and then use the post caps to get the beam square? What I mean is, when the bottoms aren't flat, the beam may rest on one or two of the plies, and the beam is coerced into plumb using the nailing holes on the post cap. Isn't it a concern for a substantial portion of the load to be carried through five nails on one side of the post cap? I assume the gaps under the plies that don't reach the post cap need to be shimmed?

If I do have 2 of the 3 plies that are reasonably close in size, can I hide the "shorter" ply in the middle of the 3 so all sit flat on the post, but joists rest on only two of the three plies? 16D nails spaced evenly along the beam would ensure load is shared among all the plies. I guess I'm wondering whether there is an advantage to ensuring joists make contact with the 3 plies, or an advantage to ensuring the 3 plies make contact with the post.

Appreciate your feedback.

Cheers
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Old 10-08-2016, 12:02 PM   #4
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Re: Multi-ply deck beam - flat on top or flat on bottom


The purpose of multiple plies is to increased the moment of inertia of the beam, which makes it stronger in bending. Typically bending controls in a beam application, so the fact that one or even two of the plies does not physically touch the top of the post makes no difference, since shear at the post does not typically control.

Just make sure you use the correct nailing pattern to ensure that the plies all work together as one beam. Also, make sure all splices occur over a post. If the splice does not occur over a post, you have a very different situation, and you need to do special calculations to make sure the splice zone is strong enough in bending.
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Old 10-08-2016, 03:24 PM   #5
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Re: Multi-ply deck beam - flat on top or flat on bottom


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Originally Posted by Daniel Holzman View Post
The purpose of multiple plies is to increased the moment of inertia of the beam, which makes it stronger in bending. Typically bending controls in a beam application, so the fact that one or even two of the plies does not physically touch the top of the post makes no difference, since shear at the post does not typically control.

Just make sure you use the correct nailing pattern to ensure that the plies all work together as one beam. Also, make sure all splices occur over a post. If the splice does not occur over a post, you have a very different situation, and you need to do special calculations to make sure the splice zone is strong enough in bending.
Thank you for providing this explanation. It sounds like as long as the beam plies are nailed in a pattern like you describe, there is no need to shim the gaps at each post. If only one ply is contacting the post, nailing lets it act with the strength of three plies.

Is a nailing pattern of 3 16D nails vertically, 12" OC, from each side, sufficient to make the beam act as a single unit?

Thanks
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Old 10-08-2016, 03:42 PM   #6
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Re: Multi-ply deck beam - flat on top or flat on bottom


Stagger those nails.

That is do not put nails from both sides next to each other, nail one side as stated, on the other side halve the distance between the nails, and start there with your OC spacing.


ED
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Old 10-08-2016, 03:59 PM   #7
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Re: Multi-ply deck beam - flat on top or flat on bottom


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Stagger those nails.

That is do not put nails from both sides next to each other, nail one side as stated, on the other side halve the distance between the nails, and start there with your OC spacing.


ED
Will do - thanks for the help!

Cheers
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Old 10-08-2016, 06:03 PM   #8
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Re: Multi-ply deck beam - flat on top or flat on bottom


Staggering nails is a good idea, I always do it that way. Also, when I build a three ply beam, I start by nailing one ply to the second ply, typically using 10d 3 inch long galvanized nails (for exterior use), then I nail the third ply to the two ply beam from the opposite side. Make sure the nails do not hit each other. I typically use 6 inch spacing between nails, each nail about 2 inches from the edge, staggered, for a 2x10 beam. You don't want your nails too close to the edge, this can cause splitting. It is possible to do a complicated analysis to compute the required spacing for nails, but realistically 6 inches OC works fine, nails are cheap, and your building inspector is not likely to want to see five pages of shear and bending computations for a typical built up beam.

Most important, make sure your splices are above a post. You can also nail the first two plies together, set the beam, and nail the third ply on, this saves weight an makes it easier to install the beam.
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Old 10-08-2016, 08:31 PM   #9
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Re: Multi-ply deck beam - flat on top or flat on bottom


Crowns up, stagger nails, splices over posts......in most cases. In this case, the OP is buying 20' long beams for a 20' span so no splices. Sthat said, many years back I was helping a friend and his plan called for a 40' triple two bye twelve with three intermediate posts not equally spaced. So I draw up the layout for him, planned splices over the posts, gave nailing pattern.....and go to see how he did a few days later after he put the beam in place.....only to find that someone had turned the layout the wrong way before assembling the thing....meaning splices not over posts. VERY big pain in the butt to take out and spin around a 40' triple two bye twelve!!
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Old 10-09-2016, 01:58 AM   #10
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Re: Multi-ply deck beam - flat on top or flat on bottom


Well, not to confuse the issue, and the previous replies should be sufficient to answer your question, but here are my thoughts.
Flush on top, crown up, and plane the bottoms at the beam bearing points so they are all the same as measured from the flush top of the beam. That may make the beam an eighth or so shallower than the full dimension, but it will still work fine.
The top of the beam, where the joists sit, is your reference face for the decking and the post heights will all top out at the same elevation if you do it this way.
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Old 10-09-2016, 06:47 AM   #11
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Re: Multi-ply deck beam - flat on top or flat on bottom


Don't know if your pt is going to be relatively straight to assemble all 3 crown up. You may have to start with crown up and one down. Nail the edges first then use the toenail to bend it but the finish should be crown up. You don't want the crown to be too much. As it dries, the crown could stay permanent. No more than 3/16 or so.
Must use double dipped nails and buy these from lumberyard. I think homedepot nails may be little thin on zinc.
I would bolt them anyway. Nails corrode and 1/2 bolts with big washers every 16" oc would feel better for me. Search for bolt schedule.
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Old 10-09-2016, 10:58 AM   #12
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Re: Multi-ply deck beam - flat on top or flat on bottom


Quote:
Originally Posted by ront02769 View Post
plan called for a 40' triple two bye twelve with three intermediate posts not equally spaced. So I draw up the layout for him, planned splices over the posts, gave nailing pattern.....and go to see how he did a few days later after he put the beam in place.....only to find that someone had turned the layout the wrong way before assembling the thing....meaning splices not over posts. VERY big pain in the butt to take out and spin around a 40' triple two bye twelve!!
Why a 40' one piece beam when you have three intermediate post in the run? I don't get it.
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