Wood Deck 2

[spats]

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?

 

[Archie264]

If you've properly attached to the house you might have satisfied lateral bracing requirements, specific situation depending, of course. Here's a good resource, fyi.

http://www.awc.org/publications/DCA/DCA6/DCA6-09.pdf

 

[SteelPE]

I'm glad I don't have to design your deck. If you are looking to get some "diaphragm" capacity out of the decking, good luck. You may want them to apply the decking diagonally to the direction of the framing, this will help but I have found getting code allowables is difficult.

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

 

[msquared48]

Just apply diagonal 2X4's flat to the underside of the joists and nail them off at each joist. That should stiffen it up sufficiently. At any joints in the 2X4's, apply metal straps.

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

 

[msquared48]

You could also use diagonal metal blocking with continuous Simpson CMST straps over the bottom of the blocking, or the top if the deck is not yet constructed.

Mike McCann
MMC Engineering

 

[smb4050]

See American Wood Council publication DCA 6. I believe it addresses all of your questions. Be careful with the height of the column versus acceptable lateral movement. On a large deck with tall columns, we had to install knee braces and attach to the building to gkeep the movement down to an acceptable limit.

http://www.awc.org/codes/dcaindex.php

 

[MikeHalloran]

Telephone poles, sunk deep?

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

 

[mijowe]

I have seen the flat bracing on the underside of the joists, and embedded poles.

 

[msquared48]

If it is done properly, Mike, there should be no problem with attaching to the house. Mine has lasted for 40 years and is still going strong in a very wet and seismic environment.

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

 

[dhengr]

Spats:
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.

 

[spats]

Thank you all for the responses. I've decided to embed the columns in concrete like a pole structure.

dheng: I'm curious what your "give us a break" is about... not really following that.

 

[LorenAWC]

Lateral load on decks is tricky since there is no codified loading provisions. Decking has little diaphragm capacity per SDPWS, but since there is nothing to compare it to, one doesn't know if it is overloaded. I know of no way to laterally engineer a deck without loads. That is why prescriptive requirements are common and are being recommended. However, noting the number of deck failures in the news, we should all give pause.

Spats, how deep are you going to embed the posts? What size of posts/collars are you going to use?

 

[a2mfk]

I think dhengr means you did not give us hardly any info about your problem, ie, the geometry of the deck. It is routinely his comment on the OP when they are vague posts, but don't let me put words in his mouth. I do agree with a lot of the posts on this message board not containing adequate details about the problem at hand. And most should post a sketch.

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=1797

 

[dhengr]

Spats:
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.

 

[spats]

I agree that some posts are too vague. However, my question was about general ideas and concepts. I wasn't asking anyone to engineer my particular situation. Certainly, if there was anything unusual, I would have included that information, so as not to waste anyone's time on a wild goose chase.

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.

 

[a2mfk]

Well, if I may be a bit snippy, you have typed a lot of words on this post and never gave us one sentence about the geometry of the deck How will I sleep tonight without knowing the actual deck dimensions!!

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.

 

[Engineerataltitude]

Take a look at this code compliant prescriptive deck design from the Mono County CA website:

http://www.monocounty.ca.gov/sites/...894/40_psf_high_deck_plan_package_3-26-13.pdf

It may help answer some of the practical side of your questions about how to brace a deck to improve diaphragm stiffness.

 

[LorenAWC]

a2mfk brings up a good point about the tension/compression couples to the existing house. This is how it is prescriptively done in the IRC with 1500# hold downs. The house diaphragm's ability to resist that force isn't mentioned.

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?

 

[mijowe]

do we know if there is going to be dancing on this deck?

 

[a2mfk]

mijow- I sense you are joking, but if curious, read my other thread on the deck study. It is not a live load or even vibration issue that would be caused by dancing that we are talking about. It is a lateral load that would be caused by several people running in unison from one side of the deck to the other and stopping in unison.

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.