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Need assistance correcting insufficient deck footings

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Silver Spring

Structural
Sep 23, 2020
2
Homeowner here. Here's the story:
I hired a guy to make drawings and size out the materials for my deck, which I then submitted to the city for my permit. The permit was approved so we dug our footing holes, had the footing inspection, poured concrete, and started framing.
I called for a framing inspection once we had the basic shell up (beams but not joists, so this was not a required inspection; I just wanted to make sure we were on the right track before we went too far). When the inspector came, he informed me that I was not in compliance because although the city had approved the permit, they had made corrections on the plans that I was not aware of. He told me where to look for the corrections and he left. The corrections said that my footings and my beams were required to be bigger than what was on my submitted plans. By that point, my footings were already buried and the city has no idea they are insufficient in addition to the beams he had pointed out to me. We've since beefed up the beams and we're ready to really get rolling on joists (just a couple are in) but it's really niggling in my mind that I probably have a footing issue, so before we continue, I need some consultation on the footings.

Here's the build detail:
We used 12" sonotubes, which are 48" deep, and we filled them 12" deep with 5000psi concrete. The posts sit on the concrete and are buried the rest of the way with dirt/gravel. We're doing 12" OC joists because we're using Trex decking installed at an angle, so there's a lot more weight here than there would be with a cedar deck at 16"OC. So I'm kinda freaking out about the insufficient footings. I'm not so worried about the 12"ers that were supposed to be 14" (apparently my city's minimum, not necessarily code) but the city said two of them were supposed to be 17" and I'm really concerned about those ones. My city does not allow diamond piers, but since they don't know about the insufficiency, I can't see how they would argue if I voluntarily added a couple of those.

Answers:
1) Permitting was all done online so I had no idea they had made and uploaded corrections....their website kept sending me in circles and I barely got the permit printed.
2) Apparently the footing inspector was only looking at depth, not diameter. He approved it and told me I could pour.
3) Yes I've seen the load charts online but it's difficult to know how they apply to a partially round deck.
4) The original drafter guy is in the wind and I can't talk to the city about it because they won't tell you how to fix things. When he told me my beams were underspanned, he wouldn't even just tell me to change them from 2x10 to 2x12. They will not offer any recommendations whatsoever because "that would be designing your deck for you". So if I tell them about the footings, it's a whole shit show and they'd probably make me disassemble and dig up the whole works.
5) Gravel. I'm told this neighborhood was literally a gravel pit once upon a time, so it's all gravelly rocky soil. No sand or peat, barely any actual soil.
6) I'm in MN, so 42" was required but the sonotubes are 48 and we pretty much went level with them.

IMAGE ATTACHED with questionable footings in red.
Inkeddeck12_LI_jxmu7q.jpg
 
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Is there frost heave that affects footings? Can you add footings as shown? Can you have a geotech engineer review the footings?

Dik
 
I think you need to hire another structural engineer to evaluate your situation. This forum is not the place to resolve your issues.
 
What region are you located in OP? Will help know if your foundations are adequately deep.

If the ground is mostly gravel, that works in your favor. Is that what you found when you dug the sonotube holes?

Adding a few more footings & posts is probably your best bet as Dik noted. Probably would help with some of your framing as well, which looks a little suspect to me in a few locations.

With no changes, it's possible that you may get some excessive settlement at the central footings that are supporting most of the load.
 
strucbells... we typically have 6' frost penetration... if he has drained granular, it's best and good confined granular may have fairly high footing values... geotekkie may help.

Dik
 
Wow! I usually work in areas with 30"-36" frost depth that I think of as cold/snowy. I've never personally worked on a project exceeding 48" (assumed that was plenty deep!). Will edit my comment accordingly.
 
First off as Retired indicated you need to hire a local engineer.

The posts sit on the concrete and are buried the rest of the way with dirt/gravel.
Because you did this it will be pretty trivial to shore what you have built trim back the posts and install new proper size foundations with isolating post bases. Buried wood like you did is generally not a great idea for decks because any rot that begins to take hold is hidden below the soil leading to a possible sudden failure of the deck.



My Personal Open Source Structural Applications:

Open Source Structural GitHub Group:
 

In some juridictions you may not require an engineer. The local code may have prescribed solutions. He should check to see the the proposed remedy is adequate.

Dik
 
dik,

The engineer is not for the local authority, which has already issued the permit, rightfully or wrongfully. I got headache after reading the thread, so decided refer to another engineer to sort this mess out, so OP won't fall into another trap/mistake again. But maybe someone here can handle it. My mistake.
 
Dik,
I'm in MN so 42" is the requirement and we're at about 48 simply because that was the length of the sonotubes. I'm not worried about heave, but am worried about it sinking over time.

(and as to other responses, we did beef up the beams already from what is on the plan so no worries there)
 
Then, add more supports, as suggested by dik, to spread the load out is a good approach.
 
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