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Old 04-23-2012, 03:37 PM   #1
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two-story deck design question


Have any of you ever built a second level to a deck? I've built a few single-level decks and know all the rules for footings and attachments, beam loads and joist spans. But now I'm thinking of adding a second level as an observation deck. The existing posts and footings are adequate where I would add the second level, but I'm wondering about the attachment of the new posts to the existing deck.

The posts for the second level would be right over the 6x6 posts for the first level and the double 2x8 beams that ride on them. But I've got 2x8 joists running on top of those beams. Assuming the new posts fall exactly over existing joists, is that enough cross-section of wood to carry the load?

I guess what I am looking for is a standard for compressive strength of treated southern pine. I know I'll also have to attach the posts to the floor with brackets to take up lateral loads.

If anyone else have done this sort of thing (under the watchful eyes of their county inspectors of course), I'd be interested to know how you did it.

Thanks in advance.

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Old 04-23-2012, 03:59 PM   #2
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two-story deck design question


A 6x6 load bearing post resting on a 2x joist doesn't sound at all safe to me. You could add blocking on either side of the joist, behind the rim joist, but the whole thing seems very haphazard to me, and I would be surprised if that would pass inspection. Keep in mind that failure of this structure is almost certain to result in serious injury, if not death, to anyone on the first deck (anyone on the top deck would also not be having a good day).

I would expect new footings and posts that extend to the second story deck.

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Old 04-23-2012, 04:21 PM   #3
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two-story deck design question


The structural concerns for a second story deck are more complex than for a single story deck. Compressive load of the posts is probably not going to control. Here are some structural issues that you would need to design for:

1. Buckling of the posts. Buckling potential needs to be carefully evaluated at the connection between the top post and the lower post.

2. Sway of the structure. Normal sway bracing you probably installed for the first level is not likely to be adequate to handle second level needs.

3. If you live in a seismic zone, you need to check the connections of the lower posts to the footers, and design for lateral loading of the structure from earthquake events.

The problem is you are outside of normal code for a deck, so it is unlikely your code official is going to know how to evaluate your plans for safety and adequacy. If I were the local code official, I would probably insist on an engineering stamp on the design, just to cover my ass, since this type of structure is not a common occurrence.
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Old 04-24-2012, 12:05 PM   #4
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two-story deck design question


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Originally Posted by Daniel Holzman View Post
The problem is you are outside of normal code for a deck, so it is unlikely your code official is going to know how to evaluate your plans for safety and adequacy. If I were the local code official, I would probably insist on an engineering stamp on the design, just to cover my ass, since this type of structure is not a common occurrence.
Thanks for the engineering tips. I expected the county might ask for a professional engineering assessment. But they said to send in some plans first and we'd go from there. Sometime I think I'm more worried about the safety of the structure than they are.

I've got the footings and bracing pretty much over-engineered. While not zero, seismic concerns are small. Wind is a bigger concern. I'm thinking I may need bolted gusset plates to connect the lower and upper posts.
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