Need help with Deck Beam Design

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Old 06-18-15, 09:48 AM
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Need help with Deck Beam Design

I'm in the process of designing a new deck above a walkout basement. My design calls for 4, 6x6 posts spaced 8'2" apart (center to center). My joists are 12' (2x10s) and my beam will be made from three 2x12s. I'm debating how to splice the 2x12s together. Many resources say that splices must be made over the posts. Other resources say having the beam continuous over the post and making the beam splices between posts is better. I tend to believe the latter and have come up with the following hybrid design (see link to picture for a visual representation).

Beam Design_zpsrpn9mrmv.jpg Photo by Chris_Rasko | Photobucket

The entire width of the deck is 28', which includes 1'9" cantilevers at each end.

Beam 1: 16' across posts 1&2 spliced to a 12' across posts 3&4
Beam 2: 9'11" across post 1 and half of post 2, spliced to a 8'2" across half of posts 2&3, spliced to a 9'11" across half of post 3 and all of post 4
Beam 3: 12' across posts 1&2 spliced to a 16' across posts 3&4

This way the outsides of the beam are continuous across all posts and there are no beams supported by only 1 post.
Alternatively, the center beam could be made from 6',16',6' pieces, but the 6' pieces are only supported by 1 post.

I thought about using 20' 2x12s to span 3 posts, but the lumber yard said 20' 2x12s tend to twist and warp and recommended against using them.

Just looking for some clarity if I'm going about this correctly. I was planning to use Simpson BCS2-3/6Z post caps to fasten beams to posts, and they specify to use continuous beams.

I was also hoping to use strong-drive wood screws from from Simpson, rather than nails to assemble my beam. Any concerns? Should I also use glue when assembling the beam?

Thanks,
Chris
 
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Old 06-18-15, 09:58 AM
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Personally, I would rather have beams meet on a post as long as the post is on top, of the proper footing. I know that some people believe that the splice should be between the footings. To me, the strongest point is directly above a footing.
 
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Old 06-18-15, 04:17 PM
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I like your design, you split the splices between directly on top of the posts and in between them. I know some people like to put the splices directly on top of the posts but I like them staggered instead of all in one place. As an alternative could you put them all on top of posts but stagger them between the various posts? Rather than using screws however I would probably use bolts directly through the beams. I probably use 1/2" SS bolts in a staggered pattern.
 
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Old 06-18-15, 05:38 PM
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http://www.awc.org/publications/DCA/DCA6/DCA6-12.pdf

Section 8 figure 4. Beam joints shall only bear on posts. You may get a local variance on that, as it isn't written in blood. I, too, don't like joints bearing adjacent to each other. #10 x 3" screws or Ledger Locks in 4" should work just fine.
 
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Old 06-21-15, 10:49 AM
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The bending moment curve for a 3-span (equally loaded), continuous beam shows maximum bending stresses occurring at the two interior columns. Contrary to what DCA 6 says, putting splices at the inflection points, where bending moments transition from positive to negative and are theoretically zero, would result in better beam performance.

Thousands of bridge designers can't be wrong--the next time you're out driving, look up at where splices are located on continuous steel girder bridges, over a typical freeway--you'll never see them over a pier, but rather offset by about 20% of the total span length to take advantage of lower stresses at the inflection points.
 
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Old 06-21-15, 03:47 PM
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According to a structural engineer I have worked with on repairing rotten joists under a failed bathroom. To make any kind of splice, you need a minimum 3 feet overlap on either side of the splice with an offset nail pattern alternating top to bottom of 4" apart.

While bridgeman is probably correct, it is different dealing with structural I-beams that are bolted to each other with the resulting sum being stronger than the individual parts. Compared to a butt joint wood beam and a saddle joint. I would opt to make your breaks directly over the posts. Or, add a post, shorten the span so there is less waste of material. Span 6 foot so that a 12" beam can be used and alternated as to where the breaks are over the posts (every other).
 
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Old 06-21-15, 05:55 PM
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Thousands of bridge designers can't be wrong
Thousands of Indians who walk over hot coals can't be wrong either but I'm not doing it.

All jokes aside, engineers are designing both decks & bridges so there must be a difference between the designs. Maybe it's because one is curved & the other is straight. I don't know.
 
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