jar546
Forum Coordinator
Chat GPT had some interesting answers once I fed it some very detailed information about a hypothetical deck. You folks should try it.
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Lateral support for free-standing decks
- Thread starter Mr. Inspector
- Start date
jar546
Forum Coordinator
How accurate is this?
Using the formula F = 0.00256 x Kz x Kzt x Kd x V^2 x A, and the given information that the deck is 8 feet above grade, in a 115 mph wind zone, has a 10 psf dead load and a 40 psf live load, and is 12' x 12' in size, we can calculate the wind force on the deck as follows:
First, we need to determine the exposure coefficient, Kz. For a deck that is 8 feet above grade, in a 115 mph wind zone, and with an exposure category of B (typical suburban terrain with scattered obstructions), the value of Kz from ASCE 7 is 1.15.
Next, we need to determine the topographic factor, Kzt. For a square deck with no abrupt changes in elevation, located in an area with surrounding terrain that is lower in height, the value of Kzt from ASCE 7 is 1.0.
The wind directionality factor, Kd, is 0.85 for wind loads acting normal to the deck surface.
The wind speed, V, is 115 mph.
The area of the deck, A, is 12 feet by 12 feet, or 144 square feet.
Finally, we can substitute these values into the formula:
F = 0.00256 x Kz x Kzt x Kd x V^2 x A
F = 0.00256 x 1.15 x 1.0 x 0.85 x (115)^2 x 144
F = 4828 pounds, or approximately 4.8 kips (kips stands for thousand pounds)
Therefore, the wind force on the deck is approximately 4.8 kips. This calculation provides an estimate of the wind load, and additional factors such as dynamic effects and building response may need to be considered for more accurate designs. Additionally, the design should be verified to comply with local building codes and regulations, and it is recommended to consult with a licensed engineer to perform a detailed analysis and provide a design that meets local building codes and regulations.
Using the formula F = 0.00256 x Kz x Kzt x Kd x V^2 x A, and the given information that the deck is 8 feet above grade, in a 115 mph wind zone, has a 10 psf dead load and a 40 psf live load, and is 12' x 12' in size, we can calculate the wind force on the deck as follows:
First, we need to determine the exposure coefficient, Kz. For a deck that is 8 feet above grade, in a 115 mph wind zone, and with an exposure category of B (typical suburban terrain with scattered obstructions), the value of Kz from ASCE 7 is 1.15.
Next, we need to determine the topographic factor, Kzt. For a square deck with no abrupt changes in elevation, located in an area with surrounding terrain that is lower in height, the value of Kzt from ASCE 7 is 1.0.
The wind directionality factor, Kd, is 0.85 for wind loads acting normal to the deck surface.
The wind speed, V, is 115 mph.
The area of the deck, A, is 12 feet by 12 feet, or 144 square feet.
Finally, we can substitute these values into the formula:
F = 0.00256 x Kz x Kzt x Kd x V^2 x A
F = 0.00256 x 1.15 x 1.0 x 0.85 x (115)^2 x 144
F = 4828 pounds, or approximately 4.8 kips (kips stands for thousand pounds)
Therefore, the wind force on the deck is approximately 4.8 kips. This calculation provides an estimate of the wind load, and additional factors such as dynamic effects and building response may need to be considered for more accurate designs. Additionally, the design should be verified to comply with local building codes and regulations, and it is recommended to consult with a licensed engineer to perform a detailed analysis and provide a design that meets local building codes and regulations.
steveray
SAWHORSE
5000# seems a bit much to me....for wind...
I have spent over a decade researching and dealing with the subject of deck lateral loads. This is not an off-hand statement or casual guess:
It's because no one knows the magnitude of lateral load being resisted by the braces. Knee braces transfer the lateral loads on the deck into the post mid span. This creates additional buckling force on the post, but no one knows how much. Lateral live loads are not standardized, not even in the ASCE 7. The shape of a deck changes the loading. A center posts with two knee braces takes twice as much force on the post than a corner brace with only one. Since no one knows how much load that is, we only know 2 is more than 1. AWC was not comfortable with twice the unknown load and that is why it is prohibited in their document. This is the same reason for the universal post height maximum of 14 feet in the DCA and the IRC. We are making a lot of guesses about decks and a lot of the DCA and IRC is "this should work" kind of validation.
Waiting a decade to get help from others for deck lateral load codes and getting ZERO help, I wrote my own proposal, on behalf of myself, on my time, for the 2024 IRC. RB 188-21 All I got was a line of opposition testimony and a confused committee. I fought for it, alone, through both hearings. I begged for help. I got none. (One opposition professional actually did offer me a floor modification suggestion to address his concerns. I thank that person for helping)
I'm so beyond exhausted with this subject.
bill1952
SAWHORSE
I thought people movement was the primary factor in designing for lateral loads on a floor or deck. Not sure where I'm recalling but somewhere between portable platforms - like head tables, boxing rings, and portable stages - and bleachers and grandstands - I recall 20% of the live load was the lateral load design. So 1000 sf platform with a 50 psf live load would have 10 pound lateral load psf or 10,000 overall. Was not a problem for the companies I know if manufacturing these kind of portable projects.
wwhitney
Registered User
Thank you for the extensive response.
The above is true if the knee braces can act in both tension and compression. So a detail that would allow them to act only in compression, but not in tension, would avoid this issue. E.g. cut your knee brace in half transversely, drill coaxial holes in each cut end, and install a smooth steel dowel of sufficient length.
Cheers, Wayne
Sifu
SAWHORSE
- Joined
- Sep 3, 2011
- Messages
- 2,809
I get the frustration. I have been frustrated with this for years, as I was for many deck related issues. Thanks in large part to Glenn, many of the issues have been solidly codified. A prescriptive method would be nice, however I can attest that even when I could explicitly cite the NC code, there was still a whole lot of push-back. That is the rub with prescriptive provisions, the code can never anticipate or direct everything. I have come around a little to the performance aspect for lateral bracing, but only after many years of building and inspecting decks. But, as Glenn's frustration implies, being able to build or inspect a single deck based on experience and knowledge, is a far cry from putting it in a code, especially in a one size fits all code.
FYI, Glenn's proposal was better than what we have IMHO. I am sure much is lost in a 1 paragraph disapproval, but my reading of the proposal would have decreased reliance on engineering, not increased it as the disapproval observes. I had it on my AS list...but then I found out I couldn't vote.
One other thought. I have seen many engineered decks without any lateral bracing. Many fewer with lateral bracing. I have never asked for calculations, but I wonder what would happen if I did.
FYI, Glenn's proposal was better than what we have IMHO. I am sure much is lost in a 1 paragraph disapproval, but my reading of the proposal would have decreased reliance on engineering, not increased it as the disapproval observes. I had it on my AS list...but then I found out I couldn't vote.
One other thought. I have seen many engineered decks without any lateral bracing. Many fewer with lateral bracing. I have never asked for calculations, but I wonder what would happen if I did.
Mr. Inspector
SAWHORSE
I started this thread because of legal problems I had because a plan review I approved for a free standing deck did not show lateral bracing. Usually I require some kind of bracing at the inspection or the contractor realizes it need it but I did not do the inspections, a new green unexperienced inspector did the inspections. I was told the deck was ready to fall over because of the missing bracing.
I hate to do it but for now on I will require sealed plans for free standing decks, at least for the bracing.
I hate to do it but for now on I will require sealed plans for free standing decks, at least for the bracing.
Robert
Registered User
I'm not an engineer, but the code used to place columns into 3 categories....short, medium and long, based on the L/D ratio (length over column dimension). The longer the column, the greater the moment at the base. So the bracing was able to transfer the diaghram stress into the column at a lower height, (thus reducing the effective length of the column) which could mean using a smaller column.
Genduct
Registered User
Seems that the definition of a Structure is: that it ( whatever it is) can stand-up and remain standing, covers the situation.
So if the Designer / builder chooses to "Plant" those columns in concrete or use angle braces at the column - beam / joist connection to make it a "Rigid" Moment Connection, i.e. Laterally Supported is a design choice
I think it is self evident, although I can see where we all know that Common Sense is Not Common
Genduct
Registered User
Robert, I seem to think that the L/r is the slenderness ratio and it is used to figure out the bearing capacity of the column. I believe the lateral bracing is a separate matter
Genduct
Registered User
I believe it transfers even more than half the load to the center column.
I think the prescriptive approach already ignores the fact that the center column takes half of the tributary load
That footing should probably / technically be 2X the other footings to be proportional. Not sure anyone cares about differential settlement of a deck
Mr. Inspector
SAWHORSE
The tributary area of a deck center footing using table R507.3.1 will always be more then the end footings which would require the center piers to be lager then the end piers.
Genduct
Registered User
Thanks Rich, I'll look it up
I didn't remember seeing it, and here I thought they had ignored it.
Also am pretty sure about the braces at center post would move even more load to the center column
mark handler
SAWHORSE
Mr. Inspector
SAWHORSE
All the answers that i am getting is giving me more reason to have engineered seal drawings for free standing decks.
Genduct
Registered User
I posted this earlier:
I believe it transfers even more than half the load to the center column.
I think the prescriptive approach already ignores the fact that the center column takes half of the tributary load
That footing should probably / technically be 2X the other footings to be proportional. Not sure anyone cares about differential settlement of a deck
someone posted that the center column of that did neeed a bigger footing
Sifu
SAWHORSE
- Joined
- Sep 3, 2011
- Messages
- 2,809
It does take more load, and if knee braces are used, it transfers even more from the corners to the center but how much is a guess because it would be based on the deflection of the beam. Which, I imagine, is why AWC doesn't even try.
chief-inspector
Registered User
The wind resistant provisions apply so the post would have to be either braced, or moment connection at the top or bottom.
Genduct
Registered User
To add to the comp[lexity, let's not forget about UPLIFT