Residential Deck Engineering
Residential Deck Engineering
(OP)
I have found plenty of design guides for decks specifically aimed at contractors and builders but not a whole lot for engineers. Given the recent deck failures I've seen in the news I am specifically wondering about lateral loads due to occupants moving about on a deck and how to best address that.
I have a free standing deck with a large hot tub. The deck is rather high (9' from grade) so as to bring it in level with another two story deck positioned on the residence. The deck supports a large 3,000 lbs hot tub (water + tub) on six 6' diameter poles. The owner purposely designed the deck to be independent from the house so in the case of a seismic event it could break away and not take down the house if it were to fail catastrophically. Apparently this was a similar approach she had from a previous residence in where a structural engineer had suggested this method given the high seismicity of the area.
The posts on the concrete bases are essentially pin jointed and with a couple of 5/8" bolts into the beams above, not offering any significant lateral or moment resistance. I will design some knee bracing that will provide the required lateral strength, in both directions.
I am not too worried about the typical lateral wind and seismic loads or even the vertical loads. What I am wondering about is how best to quantify a bunch of rowdy teenagers running and jumping about on this deck, I am thinking about 10-12 people could theoretically be on this deck surrounding the hot tub at any given time. Does anyone have any past experience with a a similar situation or references to papers on this subject.
I have a free standing deck with a large hot tub. The deck is rather high (9' from grade) so as to bring it in level with another two story deck positioned on the residence. The deck supports a large 3,000 lbs hot tub (water + tub) on six 6' diameter poles. The owner purposely designed the deck to be independent from the house so in the case of a seismic event it could break away and not take down the house if it were to fail catastrophically. Apparently this was a similar approach she had from a previous residence in where a structural engineer had suggested this method given the high seismicity of the area.
The posts on the concrete bases are essentially pin jointed and with a couple of 5/8" bolts into the beams above, not offering any significant lateral or moment resistance. I will design some knee bracing that will provide the required lateral strength, in both directions.
I am not too worried about the typical lateral wind and seismic loads or even the vertical loads. What I am wondering about is how best to quantify a bunch of rowdy teenagers running and jumping about on this deck, I am thinking about 10-12 people could theoretically be on this deck surrounding the hot tub at any given time. Does anyone have any past experience with a a similar situation or references to papers on this subject.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com






RE: Residential Deck Engineering
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
RE: Residential Deck Engineering
http://www.deckmagazine.com/Images/MeasuringLatera...
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
http://www.deckmagazine.com/images/windloadsanddec...
http://www.deckmagazine.com/Images/SeismicLoadDete...
However, I am noticing that all of these decks are attached to the primary structure and DCA6 states that decks should be attached to the primary structure. What is the thinking on decks that are free standing?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
RE: Residential Deck Engineering
1) When decks have diaphragms with some stiffness, I suspect that they mostly cantilever laterally from the house proper and the knee braces at the front don't do much.
2) When deck diaphragms are soft, the knee braces at the front become much more important. I believe that deck failures occur most often in this scenario due to P-delta effects. These stability issues are exacerbated by tall posts, effectively pinned bases, and the flexibility inherent in the knee brace connections.
3) I don't see a deck delivering enough load to a house proper in a seismic event to materially affect whether or not the house collapses. For the reasons listed above, however, I think that a typical deck is much more likely to collapse if not attached to the house proper as the deck then lacks that one stiff shear resisting support provided by the house. Is the concept to accept that the deck will collapse in a seismic event and design it to a lower standard that would be objectionable for the design of the house proper?
I'm fine with designing decks as free standing so long as consideration is given to stability and connection flexibility. I feel that it is inherently riskier than attaching to the house however.
My wife built us a little bench for the end of our bed. See the photo below. The diaphragm was much stiffer than a free standing deck diaphragm and the moment connections at the top of the legs was probably about as stiff, relatively speaking. Yesterday I sat on it to adjust one of my shoelaces. The bench P-Delta'd over slow enough for me to perceive it but too fast for me to react and prevent my other foot from getting squished.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering
RE: Residential Deck Engineering
Here's something that will throw a wrench into your works - None of the Simpson or USP bases are rated for any shear load except the ones that are cast into the footing (the ones the contractors hate)
So, technically, none of our lateral bracing works anyway. (I generally ignore the lack of shear testing on the bases as they seem to work fine)
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
RE: Residential Deck Engineering
+1 to that. I consider it atypical of Simpson's otherwise excellent product information and customer service. If the bases really had no capacity then it would be rare that they could be used. While we model statically determinate structures with roller connections we don't usually use them to model 3-D space frames; pinned connections are more the norm. In any case, if they they truly had no lateral capacity then we certainly couldn't justify using them on a free-standing deck, at least, not theoretically.
RE: Residential Deck Engineering
I don't know why AWC prohibits use knee bracing on the center posts. The only guess I can come up with is that since the center posts carry twice the vertical load of the corner posts perhaps they want to make sure no undue bending moment is applied to them as well.
A related question I have is if anyone knows why they prohibit through-bolting lumber to the sides of posts? Are they trying to keep the beam plies adjacent to each other for better lateral bracing?
RE: Residential Deck Engineering
The steel rod bracing does sound attractive but I have no idea how to implement that solution or details showing its construction.
However, before I get that far I am still working on my spreadsheet which examines the lateral loads due to wind, seismic and occupancy.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
At the very least, you (someone) would/might cut or drill half the post way, right at a max. load and moment point on the post, in an effort to attach two knee braces, without a second thought. Some builders claim to be good enough with a chain saw, that they work fine for them for this kind of joinery. If you could use some hardware which did less damage to the post and you designed for all of the various loads and moments, I’ll bet there isn’t any code prohibition against knee braces on the center post. Remember, that “AWC DCA6 in Figure 10 “ is written for builders, DIY’ers. and homeowners who aren’t qualified engineers and usually don’t know how to follow the IRC, or understand it, let alone understanding or following the IBC. If the deck builder follows DCA6 to the letter, they will usually stay out of serious trouble, and probably won’t need an engineer’s stamp when they go into the AHJ for a permit.
As for various post bases by Simpson or USP, they just don’t know how some duffus will install them, so they are much safer saying nothing. The bolt or bolts are usually cantilevered up a few inches out of an unknown blob of concrete, called a pier or footing, with unknown embedment, it is just too difficult to pin down a lateral cap’y. I suspect that in an engineering discussion both of the manufacturers would admit that there is some lateral cap’y. in many of those bases, when installed properly. But, then they would immediately launch into a discussion on their liability when they start putting numbers in a catalog.
Free standing decks are dangerous because they don’t have the considerable lateral stability of the house to help support them laterally. When properly attached to the house, and properly tied back into the floor diaphragm in the house, decks of a reasonable proportion work pretty well. When not properly attached they can peel/unzip, right off the bldg. and maybe pull the rim joist/board off with them. The free standing deck, and particularly one with 3000-4000lbs. of sloshing water up at 8-10' above the foundation, and in a seismic region, needs to provide all its own lateral stability within its legs and foundations. Most homeowners don’t like to see knee braces or x-bracing on their decks, they like the cleaner lines, and think nothing of lateral stability until the deck folds over and is on the ground.
RE: Residential Deck Engineering
Does anyone have any bracing ideas or details that would work well with 6' round peeler poles. I'm pretty familiar with knee braces using conventional methods with square/rectangualr timbers, lag bolts and/or simpson brackets (KBS1Z). Perhaps there are better methods to apply bracing to round poles that I am yet unaware of.
I'm looking at the Simpson CJT right now (CJT3 w/ 4x8 knee brace), see sloped view:
http://www.strongtie.com/ftp/catalogs/c-c-2015/c-c...
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
Full on lumber x-bracing or steel rod bracing is the direction I'm taking now.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Residential Deck Engineering
Medeek, I'm referring to Figure 9 in AWC's document. See link: AWC DCA
RE: Residential Deck Engineering
I am from Canada and I am really enjoying this discussion. This is my first post/ comment on this website ..
I am more involved with the steel structure and not as much into the wooden construction.
I have these two cents:
1) Use of round bar as a bracing member is prohibited or at least discouraged in the steel construction
2) Would it not be a good idea to stick with a wood brace(say 2X4). If not then why ? and also what could be the limiting slenderness ratio (KL/r) for a wooden brace. For the steel I can tell that this limit is 300 as per the Canadian codes.
Thanks in advance.
RE: Residential Deck Engineering
As far as lateral bracing, I would prefer to attach back to the house laterally with hold downs like KootK posted. Simpson has newer stuff that attaches to the outside only to provide some restraint. Knee braces will do something, but won't be as strong as a true joint to joint brace.
Don't neglect lateral bracing of the diaphragm itself. The entire run of deck boards could rack if they aren't properly connected. I'd recommend a diagonal attached to the bottom of the joists. Or you could always run the deck boards at a 45 degree angle.
I built a deck off my house last year and I got intimate with the DCA 6 as a PE. Most of it is pretty good, some is just to keep idiots from doing something dumb. The deck is free standing (not bearing vertically on my house), but laterally, I tied back to the house with some HDU connectors and threaded rod back to the house. No knee braces, but I did add diagonals to the bottom of the joists to stiffen up the diaphragm.
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
RE: Residential Deck Engineering
RE: Residential Deck Engineering
In this case the wind load governs over the other two (seismic and occupants) due to the large (mostly solid) profile created by the double deck, the high basic wind speed and the exposure D (right on the beach). Otherwise the occupancy lateral load probably would control. Also note this particular deck has double the dead load due to two levels of decking and an additional 3000 lbs for a large hot tub as well as an increase in 10 psf per the owner's request.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
RE: Residential Deck Engineering
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
http://design.medeek.com/resources/deck/
The seismic analysis done by Bender's paper is slightly different that the methodology I used. His was more complicated due to the deck connecting to a 2nd story level. My calculator assumes a first story deck and a simple calculation. My approach may be incorrect.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
Take a look at Simpson's new tech. bulletin that attempts to comply with the new IRC 2015 provisions, I found it quite helpful:
http://www.strongtie.com/ftp/bulletins/T-C-DECKLAT...
Some really nice details and options for making the attachments to the deck and an excellent option when the more traditional DTT2Z is not an option.
Due to concerns of the client with large parties loading up this deck and as an added safety margin I will probably call out six DTT1Z holdowns.
@Manstrom
With your deck project did you rely on the HDU holdowns for lateral restraint (shear) as well as tension. I guess what I am getting at is there will be no easy way to get bolts/screws into the house from the deck and I have no lateral values for the DTT1Z holdowns (only tension) from Simpson. Will the six holdowns be enough, my thinking is probably not.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
R. Garner, S.E.
RE: Residential Deck Engineering
According to ASCE 7-10 Sec. 26.2 if the fundamental frequency is greater than or equal to 1 Hz the structure can be assumed rigid and the gust effect factor can be assigned to 0.85.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Residential Deck Engineering
This...
No, we just want to rely on bearing instead of the connector for force transfer. After the wet service factor is applied, bearing just holds a lot more.
In DCA6, the knee bracing is just for added stiffness. It isn't the primary loadpath.
RE: Residential Deck Engineering
Thank you very much.