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Deck Foundation Sizing Survey

Glenn

Glenn

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Joined
Nov 1, 2012
Messages
889
Location
Denver
I am working on a deck code proposal for the 2015 IRC with a group of folks from NADRA, NAHB, CLMA, VBCOA, AWC, ICC BCAC, CCICC and others. There are folks that wish to require the minimum size pier/footing for decks to be about 17 inch diameter.

While this does calculate out based on the default 1500 psf soil compressive strength, I believe this size may be quite larger than what works just fine in many regions...even with 1500 psf soil.

Please provide your perspective and experience in this short online survey.

https://mountainllc.wufoo.com/forms/deck-foundation-survey/

Please respond here if you have questions, comments or suggestions regarding the topic, survey or questions asked. You can also PM me. If you are nervous about following the link, let me know and I will find some time to re-write it in this thread.

Thanks, and please keep this thread as a positive and constructive source for working together on this topic.
 
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Done....

FWIW, we have always accepted 8" piers in this jurisdiction, no methodology that I am aware of, just what has always been allowed. Have never seen a situation where that size has proved to be inadequate.
 
Done.

8-10" with sonotube is typical here. 3500-4000 psi clay, 42" frost depth

mj
 
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8" minimum here if using a 4 x 4 post. Going to 17" would not be well received by many including myself.
 
always used 10 or 12 inch in vermont, with a good bell footing at the bottom , well below frost line ( at least 4"6") here, we rarely get frost. i dont think a 17 inch pier would get over well, (especaiily here), but even in the coldest places. 10 or 12 inches seems seem plenty, reinforced properly and with a bell at the bottom, and getting below frost that really counts. i've seen folks build decks on the ground on those prefab concrtete footings and be fine ? .
 
I like the NC prescriptive code; 16x16x8, use of 4 solid 4x8x16 cmu's laid perpendicular permitted. (unless it has changed) Here we pretty much go with the DCA6 manual.
 
Glenn (and others),

Will the 2015 IRC requirements be for (downward) vertical & lateral loading only, or is

there any language regarding wind restraints, ..to prevent the deck posts & framing

from separating from the in-ground piers, or is this already addressed somewhere

else? Short question, but is wind restraint being included in the design

"requirements" of decks & associated framing? Thanks!

.
 
Same as Sifu, two 80# bags/hole. 12" pier if they insist on a hinge at grade.
 
The Wood Deck Construction guide has a chart based on spans on page 9 showing footer sizes and thickness.

http://www.awc.org/publications/dca/dca6/dca6-09.pdf

You can also calculate it:

Tributary area: ½ distance from post to post or beam and/or house

Multiply area by 40lb live load & 10lb dead load=50lb

Total load divided by soil bearing capacity (1,500psf)

Square root = size of footing in square feet

Square feet times 1.12 give you the diameter if using round footing (nothing scientific about the 1.12 factor. It just works and saves you steps.)
 
kyhowey said:
The Wood Deck Construction guide has a chart based on spans on page 9 showing footer sizes and thickness. http://www.awc.org/publications/dca/dca6/dca6-09.pdf

You can also calculate it:

Tributary area: ½ distance from post to post or beam and/or house

Multiply area by 40lb live load & 10lb dead load=50lb

Total load divided by soil bearing capacity (1,500psf)

Square root = size of footing in square feet

Square feet times 1.12 give you the diameter if using round footing (nothing scientific about the 1.12 factor. It just works and saves you steps.)
Click to expand...
Sorry. I forgot to include in my first post that the AWC is also part of the group I am working with on this. I have edited that in now. The DCA 6 is a big starting point for much of our work.

The problem is that calculating the bearing area is yielding some sizes that are well, well, well in excess of what appears to be working just fine across the country for decades. This is the problem and why I am asking for input regarding how you size them in your region and how they have been performing.

It appears that regions with no significant frost depth, are pouring large bearing areas in their shallow foundations, likely because it's not that big a deal. Also, when your not going very deep, it probably just "feels" weird to not put much concrete in the hole. So it's likely still a two bag, wide footer.

On the other hand, regions with deep frost depth (3+ feet) are digging narrow and straight down. Still a two bag hole, but not crazy wide. If only calculating bearing area, these foundations would appear to be grossly insufficient. However...they seem to be working just fine all over the place. These regions will not take well to having to pour wide footers at the bottoms of their 3 foot holes. Homeowners also do not want large amounts of exposed concrete at the surface, though there are other ways around that.

Please share your experiences and deck foundation methods...before one ends up in the 2015 IRC. Why are 10-inch deck piers working just fine all over the place? Why should they now be so much bigger?

ENGINEERS! Could it be possible that even at only 3 or 4 feet deep that there is some skin friction working in favor? How would that compare in an un-cased pier (straight against the dirt) vs. one that poured in a forming tube?

Thanks so much to all who have responded. There is a large proposal of new deck code underway that will be submitted Jan. 3rd. Whether a deck foundation sizing table is included has a lot to do with the group agreeing...which in my mind has a lot to do with how the country responds. This survey is going out to many other places.

I encourage you to help by copying the survey link and sending it to your ICC chapter, contacts or any other group that is interested in responding and helping to shape the future 2015 IRC.

Thank you.
 
globe trekker said:
Glenn (and others),Will the 2015 IRC requirements be for (downward) vertical & lateral loading only, or is

there any language regarding wind restraints, ..to prevent the deck posts & framing

from separating from the in-ground piers, or is this already addressed somewhere

else? Short question, but is wind restraint being included in the design

"requirements" of decks & associated framing? Thanks!

.
Click to expand...
We discussed uplift on decks. No one was able to produce any experiences where decks have experienced significant uplift from wind events. It's usually topographical effects, as I am aware of some in the rocky mountains, but due to wind patterns in valley's coming up a mountainside to an overhanging deck. Pretty rare. Wind does not affect decks like it does roofs because there is no pressure difference from below the deck to above. Actually a couple in the group spoke of homes with roofs removed from wind and deck left sitting just fine.

If you have any experiences or opinions about how wind affects decks and produces uplift, please share with me. PM if you wish.
 
fatboy said:
Done....FWIW, we have always accepted 8" piers in this jurisdiction, no methodology that I am aware of, just what has always been allowed. Have never seen a situation where that size has proved to be inadequate.
Click to expand...
You must be on great soil/rock, otherwise, I do not agree whatsoever.
 
18" bigfoot with an 8" sono tube is pretty common here. One contractor just did 4 decks on existing homes this year and used helical piers for the foundation. Said they where faster more accurate less disruptive to the existing yard and in the long run less exspensive.
 
Glenn,the building department at Cowlitz County has a good handout for homeowner/builders to do deck design. It requires a minimum deck footing of 12”x12”x8” for all decks. Decks over four feet in height will require an 18”x18”x8” footing.

Bottom line, the footings should be sized to support the calculated design loads. Using a soil bearing capacity of 1500# psf, a 17" diameter footing has a capacity of supporting over 2250#. It is not uncommon for middle footings to have loads greater than 2000#.
 
one thing we have seen, although not prescriptive is the use of engineering to include coefficient of friction in analysis for depths exceeding frost depth. This allowed one developer to use all 8 inch dia footings by making them 56 inches deep versus the 42 inch frost depth required. If I remember it was a 12 x 9.5 deck.
 
jar546 said:
You must be on great soil/rock, otherwise, I do not agree whatsoever.
Click to expand...
Just good old sandy, little to no clay farm ground. Like I said, been the same requirement for the 30 plus years I've been in/around the trade, have not seen any issues with it.
 
Papio, your deck handout is one of the best I've seen, as Garth Algar would say "Excell..........ant!

pc1
 
fatboy said:
Just good old sandy, little to no clay farm ground. Like I said, been the same requirement for the 30 plus years I've been in/around the trade, have not seen any issues with it.
Click to expand...
Over the years as the code changes, I see some things appear and others go away. Now and then I get the feeling that some of the changes are done not for any reason other than that the people that make changes need to eat too. They need to do something rather than nothing or they might not be needed in the future. A lot of stuff that ain't broke gets fixed.
 
Wow, 8" diameter piers - haven't seen those since BOCA. No concerns over cross sectional area of the post, centering of the post, etc?

Using a 4 by post = 10" minimum diameter

Using a 6 by post = we have allowed 12" diameter but strive for 14" minimum diameter
 
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