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Lateral support for free-standing decks

Mr. Inspector

Mr. Inspector

SAWHORSE
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Nov 28, 2009
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Location
Poconos/eastern PA
I was looking for a requirement for lateral support for a free-standing decks. I can only find it required when only attached to an exterior wall in 2018 IRC sections R507.8 and only to a band joist in R507.9.
Is there a section that requires free-standing decks to have lateral supports?
 
Rick18071 said:
I was looking for a requirement for lateral support for a free-standing decks. I can only find it required when only attached to an exterior wall in 2018 IRC sections R507.8 and only to a band joist in R507.9.
Is there a section that requires free-standing decks to have lateral supports?
Click to expand...
The requirement comes from R311.5, but it has some baggage with it.

You are pointing out something that has been a little convoluted and largely ignored. In my opinion, the code, as written does not really contain prescriptive provisions specifically for free-standing decks. The sections you are looking at call out connection at the exterior wall or "band joist". Band joist isn't defined but could be understood to be either end. These sections of code do strongly imply it is the joist at the house they are referring to, but it isn't super clear, and they provide two example methods for connecting at the house band/rim which can only work at that location. So the code only provides two prescriptive examples, it does not require they be used. So now we have to figure out how to accomplish the performance requirement to transfer the loads to the ground or structure. For a free standing deck those loads need to be resisted on 4 sides, and in two planes, a little more than a ledger connected deck. Here is where it gets a little messier IMHO. R311.5 says that exterior landings, decks, balconies, stairs etc. "shall be positively anchored to the primary structure to resist both vertical and lateral forces (this is your requirement) or shall be designed to be self supporting". It does not say "engineered" but it has been interpreted in many other circumstances that "engineered" is what "designed" means. In order to clarify this I spoke with an AWC engineer (several years ago), and they confirmed the intent here was to have free-standing decks engineered. The main reasoning is the multiple planes and locations for the lateral load resistance.

So now you have to ask yourself: Are you going to require an engineer for every free-standing deck? Technically, I think you would be within the limits of the code. I also think the code may come up short here and some latitude and judgement can be used. IMHO, most of the prescriptive codes can be used for a free-standing deck except 507.8/9, but there are many ways to adequately brace a deck, free-standing or not, that are far superior to the examples given in the code and they don't require an engineer. The deck codes are constantly undergoing refinement and the lateral load connection issue has been identified as a sore subject. Hang out and see if Glenn picks up on this. You can also check out his book, I just checked it and he has a whole chapter on lateral bracing. Not sure if he addresses the free-standing thing...guess I need to read up.

I bet he has some videos on it as well....Glenn, you out there?

Deck Construction based on the 2021 International Residential Code
Glenn Mathewson
 
"Lateral loads shall be transferred to the ground or structure capable of transmitting them to the ground."

There you go. It is required.

In 2015, a freestanding wood deck collapsed at a restaurant in Georgia, injuring 32 people. The deck was about 10 feet above the ground and was not attached to any structure.

Now, where is the prescriptive lateral restraint requirement in the I-Codes for free-standing decks?
 
This is a residential thread, so not sure how a restaurant relates...Jeff, there is no prescriptive way to build stairs in the IRC, but we all approve them and they don't generally kill people by collapsing...

R301.1 Application. Buildings and structures, and parts
thereof, shall be constructed to safely support all loads,
including dead loads, live loads, roof loads, flood loads,
snow loads, wind loads and seismic loads as prescribed by
this code. The construction of buildings and structures in
accordance with the provisions of this code shall result in a
system that provides a complete load path that meets the
requirements for the transfer of loads from their point of
origin through the load-resisting elements to the foundation.
Buildings and structures constructed as prescribed by this
code are deemed to comply with the requirements of this
section.
 
This deck is far from a wall, pool or building
OK, For a free standing deck Lateral loads shall be transferred to the ground. But where does it say does it say Lateral loads need to be dealt with at all for free standing decks in the first place?
Can someone show me an example of transferring lateral loads to the ground?
 
There is no specific guidance for free-standing decks because they are not within the scope of the IRC. As pointed out, the provisions for decks in the IRC are for decks that are positively anchored to a structure (311.5). All of the provisions in 507.9 are for decks anchored to a structure. Per 507.1 if the deck is "using materials and conditions not prescribed" then go to section 301. Of course, with some experience and rational analysis, many of the same methods prescribed in 507 could be used to construct a free-standing deck, but lateral resistance is not one of them (for that matter, if you don't want to use the two methods they allow you to use, you can't actually get prescriptive guidance for ANY deck lateral resistance in the IRC).

If you are encountering a free-standing deck that you can't approve, don't worry about picking out the specific provisions, just require a design per 311.5..
 
If you have a long narrow deck attached to a house (on a narrow side - like a dock) it seems possible you'll need some means of lateral support at the end away from house so it doesn't collapse sideways. Also doesn't seem that difficult with a few properly placed and fastened diagonals.
 
Detached decks are in the IRC...sorta:

10. Decks not exceeding 200 square feet (18.58 m2) in area, that are not more than 30 inches (762 mm) above grade at any point, are not attached to a
dwelling and do not serve the exit door required by Section R311.4.
 
steveray said:
Detached decks are in the IRC...sorta:

10. Decks not exceeding 200 square feet (18.58 m2) in area, that are not more than 30 inches (762 mm) above grade at any point, are not attached to a
dwelling and do not serve the exit door required by Section R311.4.
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Good point, but that only says you don't need a permit. If you had a detached deck > 200sf², then you would need a permit...and a "design". Do not get me wrong, I am not advocating that every detached deck needs an engineered design. I think you can use most of the prescriptive code, using experience and rational analysis for the performance requirements, to "design" one. But the OP was looking for a code that says a detached deck must have lateral load resistance, and since I don't think the code covers these decks they didn't bother putting in any requirements specific to them.

FWIW, I see positives to a properly built free-standing deck. Namely, the ledger flashing and water intrusion that is the largest cause for deck collapse, and the fact that none of the connections are concealed and often ignored. The biggest issue with them is the lateral load resistance, but that is easily mitigated with common methods.
 
jar546 said:
The DCA6 manual from the AWC does, in fact, have prescriptive stair solutions. The DCA6 is not referenced in the IRC although I believe it should be.
Click to expand...
Would anyone know if the AWC have info on lateral load for free standing decks?

R507.1 Decks. Wood-framed decks shall be in accordance
with this section. For decks using materials and conditions
not prescribed in this section, refer to Section R301.

R301.1.1 Alternative provisions. As an alternative to the
requirements in Section R301.1, the following standards
are permitted subject to the limitations of this code and the
limitations therein. Where engineered design is used in
conjunction with these standards, the design shall comply
with the International Building Code.
1. AWC Wood Frame Construction Manual (WFCM).
2. AISI Standard for Cold-Formed Steel Framing—
Prescriptive Method for One- and Two-Family
Dwellings (AISI S230).
3. ICC Standard on the Design and Construction of
Log Structures (ICC 400).
 
Attaching NC residential code appx. M for decks. Not the most recent edition, just the most recent I have on my computer. You can see the most recent on their website, so you may want to check them to see if anything has changed. Obviously, this may not be usable in your AHJ, but it provides the best "prescriptive" provisions I have seen. These provisions have been through their department of engineering and I trust them and used them for years. You may be able to accept them as an alternate method.

I don't think the AWC has any prescriptive guidance, they don't cover free-standers in the AWC DCA 6 unless an update has occurred that I don't have.
 

Attachments

  • Pages from 2012_NC_Residential_Code.pdf
    816.1 KB · Views: 17
I just checked the 2018 NC residential code, it doesn't look like there is any significant difference.
 
I noticed that DCA6 Figure 10 includes the statement "diagonal bracing is prohibited on center posts," while the NC Appendix M Figure AM109.1 clearly shows them. Can anyone comment on the engineering reasons for or against diagonal bracing on center posts?

Cheers, Wayne
 
"Can anyone comment on the engineering reasons for or against diagonal bracing on center posts?"

Just spit-balling here but....the expanse of the deck is a diaphragm with a force resisting element at each end..... if a force resisting element is placed in the middle, does that reduce the overall size of the diaphragm and therefor reduce the capacity of the diaphragm.

That seems counterintuitive if a brace is placed at every post....but then you only asked for a comment.
 
I'm pretty sure I spoke with someone (either AWC or more likely the engineers at NCDOI) and asked the same question, it was a very long time ago. If I recall correctly, the reason given was that by placing knee braces on the center from both sides it will naturally pick up vertical load from the beam deflection, and they thought the vertical load doubled was too much since a center beam carries twice the load already. I don't really see that as a problem, and apparently neither did NC. On the contrary, a single knee brace picking up load from only one side on a corner location would seem more worrisome in that it would tend to push laterally on the post, but from only one side.
 
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