Wood Deck
Wood Deck
(OP)
I'm designing an elevated wood deck addition at the second floor of an existing house. One side attaches to the house, and the outboard edge is supported on wood posts/columns. They don't want any vertical bracing or even knee braces at the outboard edge. The structure will be decked with 2x6 planks, screwed to the joists. Is there any way to get diaphragm action with the decking so I can avoid bracing at the outboard edge? If not, any suggestions for a way to brace it without moment frames?






RE: Wood Deck
http://www.awc.org/publications/DCA/DCA6/DCA6-09.p...
RE: Wood Deck
Then there is the code required lateral attachment of the deck to the structure, you can find that discussion here:
http://www.eng-tips.com/viewthread.cfm?qid=345477
RE: Wood Deck
Don't forget to add your ties to the house structure at the ends of the deck to develop the "diaphragm" action.
Mike McCann
MMC Engineering
RE: Wood Deck
Mike McCann
MMC Engineering
RE: Wood Deck
http://www.awc.org/codes/dcaindex.php
RE: Wood Deck
Don't take my word for that. I claim no relevant skill.
I just wanted to suggest that you search this site for discussions about attaching a deck to the house. The consensus seems to be that it's a really bad idea and a false economy.
Mike Halloran
Pembroke Pines, FL, USA
RE: Wood Deck
RE: Wood Deck
The fact is that too many contractors and owners who don't know how to do it, to include plans examiners and inspectors. Too often, they are done without a formal design, inspection, or even a permit.
Mike McCann
MMC Engineering
RE: Wood Deck
Come on, give us a break, of course column length and buckling must be reviewed per the NDS, but they can’t take much lateral load without bracing or knee braces, etc.. But, the most important thing is the deck proportions and you’ve neglected to mention them. Does this deck have a 10' ledger attached to the bldg. and cantilever out from the bldg. 30'? Or, is the ledger on the bldg. 20' long and the posts and beam a 8 or 10' out from the bldg. Obviously, there is a difference btwn. these two. Tie backs, into the bldg. floor diaphragm are important, so the deck can’t just be zipped off the bldg. by a lateral load; and of course, they are dependant upon the framing direction within the bldg. to get that load back into the bldg. floor diaphragm. The IRC/Deck Design Guides have picked a conservative, and arbitrary value for these strengths. I prefer to put these connectors in a deck joist or two from the deck edge, so that the deck diaphragm can develop on either side of the connector, as opposed to being fixed to the edge deck joist. Of course, this reduces the lever arm on these connectors, that isn’t usually the issue. I have also used light cables, turnbuckles, eye bolts, from corner to corner of the deck and back into the bldg. floor diaphragm. I think the advantage of this is that it essentially puts the entire deck into compression back into the bldg. The difference is that one is prescriptive and anyone can do it without much thought, while the other is based on some engineering judgement and real engineering design. In terms of diaphragm action, the 2x6's are not appreciably superior to 5/4x6 decking, but diagonal orientation of the decking does improve the diaphragm action, and maybe it can even be kinda artsy too.
RE: Wood Deck
dheng: I'm curious what your "give us a break" is about... not really following that.
RE: Wood Deck
Spats, how deep are you going to embed the posts? What size of posts/collars are you going to use?
RE: Wood Deck
I think out of all the options, I like your flagpole columns the best, and it would seem the most efficient use of material. Put some thought into water proofing or preventing water intrusion at the base to prevent wood rot, and let the owner know this is an ongoing maintenance issue and slapping some sealant on the posts every year would be a good idea. Diagonal horizontal straps work in theory but then you get these point loads that you have to resolve into the posts and/or existing structure.
This article from the most recent Structure Magazine addresses this oft-discussed topic that doesn't have a lot of good code/spec guidance...
http://www.structuremag.org/article.aspx?articleID...
RE: Wood Deck
From my previous post... “But, the most important thing is the deck proportions and you’ve neglected to mention them. Does this deck have a 10' ledger attached to the bldg. and cantilever out from the bldg. 30'? Or, is the ledger on the bldg. 20' long and the posts and beam a 8 or 10' out from the bldg. Obviously, there is a difference btwn. these two.” In the first case, the outer hand rail is 30' out from the bldg., posts and beam at about 28' and some other posts and beams in there too. I just picked these dimensions as extreme examples of the two conditions.
Gravity loads a pretty easy to rationalize in either case, but lateral loads and people loads out at the outer handrail are quite different. The reactions to lateral forces on the two different decks act quite differently, and that’s what you are designing around/for, that’s the real design problem. Posts embedded in conc. seem to have some problems of their own, in the way of rotting, etc. And, they are rarely deep enough to really act as real cantilevers out of the ground, and you don’t have any moment connection at the top. How are the piers installed, soil re-compacted around them, reinforced, etc., so they might act as a cantilever? A2mfk did get my point, and the article he posted is a very good one. Those authors have been doing some meaningful work in this area of structural design.
RE: Wood Deck
Mainly I wanted to know if anybody had ever successfully considered the diaphragm capacity of individual deck boards. If metal decks can successfully transfer shear without sidelap connections, certainly a wood deck can as well. It just takes a little free-body math to compute shear strength. Of course I would not try it on a wide deck (perpendicular to the house), particularly with a crazy aspect ratio, as the deflection would probably be too great. However, to me the concept has real potential for a fairly modest deck, or one that is significantly longer than it is wide, which is normally the case.
Thank you Archie264 and a2mfk for the good references. Msquared's strapping idea has merit, but the end anchorage concerns me, and may be problematic. I didn't want to go with the moment frame or embedded post concepts if I didn't have to. Moment frames are a pain in the *ss. There are issues with embedded posts, but there is treated lumber, and there are waterproof coatings. Pole structures are not uncommon. In answer to LorenAWC's question about embedment/encasement... I haven't gotten there yet, but it's a simple, common calculation.
RE: Wood Deck
To address one of your points, you can use the deck as a diaphragm, Loren references where to find the values in my other post on decks. But if you cantilever it off the house with no braces at the far end, then you have tension/compression couples to transfer into the existing house structure. That doesn't sound very practical if even possible.
I would go with your idea if you cannot get the owner to live with a couple of knee braces.
What lateral loads are you going to use? Did you read through that other post?
Why not many lateral load failures of decks that are likely rarely designed by a structural engineer? I think because the cantilevered diaphragm action is adequate, real life lateral loading is small and rare, posts are often embedded in the ground/concrete and are cantilevered flagpole-type columns, and carpenters know to throw in some knee-braces.
RE: Wood Deck
http://www.monocounty.ca.gov/sites/default/files/f...
It may help answer some of the practical side of your questions about how to brace a deck to improve diaphragm stiffness.
RE: Wood Deck
Regarding knee braces, has anyone run a calculation on how much force they would have to take? (I know it is hard since there is no codified force, but assume the 12 or 7 psf from the research.) Has anyone calculated the relative stiffness of the posts and their ability to resist the force?
RE: Wood Deck
RE: Wood Deck
The other load case they did where people swayed back and forth in unison until the deck was swaying up to 7 inches or something seemed just a bit silly and unpractical to me.
RE: Wood Deck
When people are decked out, there is always dancing...
Mike McCann
MMC Engineering