Residential Deck Engineering 3

[medeek]

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.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

This thread is my first exposure to DCA6. I'd actually be quite comfortable cantilevering your deck off of the main building if details like those shown below were installed. Are they ever? I've never seen anything of the sort in the regions that I've practiced. I'd think it would be tough to even get the floor joists and deck joists aligned.

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.

 

[Archie264]

>>>...explain the prohibition on bolting through the posts...<<<

Medeek, I'm referring to Figure 9 in AWC's document. See link: AWC DCA

 

[JingaluTheMagician]

Hello Guys !!
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.

 

[manstrom]

Regarding bolting the beam through the posts. My guess is that it is prohibited to keep from relying on just bolts in shear. A better detail would be to notch the posts and then bolt it. That's how I always do it.

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

 

[medeek]

After informing the client of my "rollcoaster" truss work beneath the deck they now have decided to connect to the building with a ledger board and holdowns. Sometimes I feel like I'm on a rollercoaster...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[ChiefInspectorJeff]

KootK: strong diaphragm : weak column. Moment connection in wood: hard

 

[BUGGAR]

If your kids can detect any sway in the deck, they will likely see just how much they can make the thing sway. This is the way kids learn about impulsive loads and structure frequency. Isn't that kind of play that got us interested in engineering?

 

[medeek]

Based on the three papers listed above that deal specifically with lateral loads on decks I came up with the following spreadsheet:

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

http://www.medeek.com

 

[BUGGAR]

I don't have ASCE in front of me. How did you get the gust factor of 0.85? Is that for a rigid structure?

 

[medeek]

Yes, the white paper went through the calculations to get the gust factor but then after all of the math stated a .85 gust factor would probably be appropriate per ASCE 7-10 Sec. 26.9.1 if we assume a rigid structure.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

A copy of the spreadsheet is here for those who are interested in comparing the numbers to the white papers given above:

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

http://www.medeek.com

 

[medeek]

A new twist/development in my hot tub deck saga. The deck ledger board does not line up well with the rim joist of the existing structure so we've determined that in order to support the gravity loads we will need three more inboard posts and a beam (ie. non-ledger deck). However, as can be seen above I have a 3,000 lbs lateral load from the wind trying to shear the deck from the house. Due to the deck aspect ratio the holdown forces are not ridiculous so I think it will be possible to utilize the new provisions of the IRC 2015 and use the 750 lbs holdowns DTT1Z (min. qty. 4) instead of two DTT2Z holdowns.

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-DECKLAT15.pdf

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

http://www.medeek.com

 

[BUGGAR]

As a licensed engineer, etc., I recommend that you consider gust effect factors for flexible structures.

R. Garner, S.E.

 

[medeek]

Based on ASCE 7-10 Sec. 12.8.2.1 the approx. fundamental period is .112 Sec. which translates to a fundamental frequency of 8.9 Hz.

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

http://www.medeek.com

 

[LorenAWC]

On a related note, does anyone know why the AWC DCA6 in Figure 10 prohibits the use of knee bracing on center posts?

This...

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?

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.

 

[Archie264]

LorenAWC,

Thank you very much.