Deck Connections
Deck Connections
(OP)
I'm looking at decks a little more in-depth than I have in the past and just thinking about framing options. Do most people use knee braces or do you use x-bracing or do you assume the decking itself acts like a cantilevered diaphragm? It seams that if you have the tension ties to the house to create a diaphragm that is 2-3:1 then you wouldn't need knee braces and could assume a cantilevered diaphragm via the decking connection to the deck joists.
Any thoughts?
Any thoughts?






RE: Deck Connections
RE: Deck Connections
The following site has some good guidance. According to the info on the first page it's based off Publication DCA6 by the American Wood Council.
http://www.fairfaxcounty.gov/dpwes/publications/de...
RE: Deck Connections
RE: Deck Connections
RE: Deck Connections
RE: Deck Connections
You can also flagpole your columns out of big footings as well is the deck is not too tall.
RE: Deck Connections
Decking placed diagonally is excellent for making the deck more rigid as opposed to perpendicular to the joists (no hidden fastening systems!) but the decks tested (great videos BTW) were pretty resilient to damage even if the deflections were out of this world, like in feet not inches. Composite decking vs. wood seems OK. PVC?
Sorry I don't have a good reference doc or link or anything but start googling those names mentioned and "deck" & "lateral". Also Simpson Strong-Tie has jumped on the bandwagon and has a few good references.
Personally I want to use diagonals braces and not little ones 2' down from the top either. Unfortunately as mentioned, nobody likes them and the bigger they are the worse it is. There have been times where I have insisted on full diagonals however and the connectors are not generally just a couple bolts for these. Depending on the client diagonal braces are a good sell in some form or another. Usually I am resorting to Tie-back method with Diagonal decking if I can get it. If the diagonal decking is a no-go then I put in more braces and/or tiebacks. I have tried something like what Excel mentioned but the size and # of diagonals placed on the underside gets to be interesting. I can never seem to get on board with the whole embedded post thing. Usually the wood is not treated to the correct rating for continuous ground contact due to purchasing ignorance. I sometimes make the treatment spec really noticeable so it gets purchased correctly but can't say if it is ever followed.
What is appropriate? Seems like a lot of inspectors are pretty clueless still and most decks get built by DIY'ers so appropriate is definitely a market thing right now from the owner/builder perspective. From an engineering perspective there is a building body of knowledge and evidence that it must be dealt with. Regardless of what you hear in the news, deck failures and injuries related are a serious problem. Most deck failures go unreported by the press unless they are spectacular. A lot of failures are either really incompetent installs or severe decay.
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MAP
RE: Deck Connections
That document refers to the IRC and has it's own conclusions as well. It says that the bracing requirements are required unless it is actually engineered. Where does one draw the line? How much difference in lateral stability is diagonal bracing going to create compared to perpendicular bracing when considering a deck that extends no more than 6 to 8ft from the house and is more than 20ft long? The proposed knee braces and connection seem kind of puny when considering the size of some decks.
I'm sure we will eventually have a better understanding in a few more years after more research. Seams like there are still a lot of unknowns and a lot of people designing in many different fashions. It somewhat appears that this is an issue that is client/contractor driven by aesthetics as well.
RE: Deck Connections
Basically - you design the the deck as a horizontal shear wall (I also like the idea of combination cantilevered posts mentioned by Excel). The NDS or SDPWS gives values for allowable shear for horizontal and diagonal decking. Then you must have holddowns on each end that tie back into the structure as discussed by Excel above; due to the fact that the hangers are not rated for withdrawal loading.
The tests conducted by Virgina/Wash U were interesting in that diagonal or "V" diaphragm bracing below the deck did not help much as it was very stiff and created a 'brittle' failure. In regards to the horizontal diaphragm - The joist hangers contributed quite a bit of resistance to withdrawal/holddown forces when screws were used. Also the holddowns did not form a T/C couple rather each hold down was subject to tension due to a prying force created by the geometry of the holddown/joist connection. Ultimately neither the hold-downs nor the hangers failed rather large splitting of the joists occurred due to stress perpendicular to the grain where the deck was fastened to the joists (small T/C couples formed).
EIT
www.HowToEngineer.com
RE: Deck Connections
In Chicago, I find the multi story decks are always tied into the main building structure and we rely on that for the later system. We do get some stability from having interior columns and stair stringers.
For short single story structures that are in the 3 to 6 feet off the ground category, I've used kickers pretty successfully. Architects don't mind in this case, since this is just a crawl space anyhow.
It also depends if the structure is clad or not. If it is an open structure there simply isn't too much area for wind to collect on.
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Deck Connections
Thank you for the information! I am going to see if I can get a copy of the seminar to watch.
What you spoke about with tension ties aligns with my thought that if you place tension ties to create a diaphragm length to width ratio between 2:1 and 3:1 then you have in effect created a "cantilevered diaphragm" or as you said a horizontal shear wall. It seams that in this instance, knee braces or kickers might not do much if the system is already tied into the structure for decks that are 6-8ft in width off the structure. The tension ties should limit the deflection of the diaphragm as well.
I added the following detail about a year ago to my work so that even if a contractor used nails for connecting the joists, I still had everything tied together at the exterior beam (although it does place the porch beam in cross grain bending). Now, that is just for the detail shown. It's different for joists floating over the beam.
RE: Deck Connections
In short:
1) decks without bracing of some kind will be quite flexible.
2) decks with high deformation are susceptible to second order failure modes so too much flexibility = bad (not to mention uncomfortable)
3) the tiedowns allow the deck to remain attached under high loads and distortion and the tension and compression side are not what you think under high deflections.
4) the lateral loading on deck due to occupancy is not well understood or codified so the tiedowns forces required are ???
5) decks are a frequent source of:
a) poor installation methodology
b) highly susceptible to corrosion of the fastening elements
c) highly susceptible to rotting due to poor moisture control techniques
*** all which make me want a redundant system
______________
MAP
RE: Deck Connections
Dhoward - it is a good seminar if ever in your area.
EIT
www.HowToEngineer.com
RE: Deck Connections
I took my Timber Design class from Dr. Bender and it was the best class I took while in school. It's what really got me interested in wood structures. He is a great teacher and has a boat load of knowledge. Haven't had the opportunity to take any course work from Dr. Woeste though, but I recall Dr. Bender talking about him.
focuseng: thank you for the summary, things are making more and more sense to me and I'm adding lots of good ideas to my knowledge base. Thanks.