Garage Mezzanine (Concrete question)

[gsmith22]

Wow thanks for all the replies, folks. Definitely gave me a lot to think about. Super helpful forum!

I did wind up talking to my concrete guy and he says there's really no way to do it legally while having it rest in any way upon the slab itself. No amount of additional concrete will really conform legally (For reasons mentioned earlier. Frost heave, potential cracking, etc. )
I'd need to use footings of some sort that go 48" down.

He recommended that I could just use 16" sonotubes 48" down to get the job done. Considering it would add almost nothing to my total building cost, I'm likely going to go that route.

I'll definitely be using some of the advice from this discussion when I go to build the actual 2nd floor or Mezzanine...But at least whatever I decide, there will be a legal and solid base to build it on.

sonotube doesn't spread the load to the ground (its concentrated on a slightly bigger footprint than say a 6x6 column). footings spread the load. sonotube is fine as a pier (a vertical load path) to keep a wood or steel column above grade and concrete below grade. But you want a footing below the sonotube to make sure bearing pressure on the soil is kept within limits specificed by building code. 3x3 is my minimum as I said back in post 2. actual size based on your loads You want to do this once, not tear up your floor in 10 years because you put in a 2x2 footing, plans changed, and need something bigger

 

[drxlcarfreak]

I just stumbled across this thread. I was going to share the concrete thickness calculator I used to figure out the depths needed for my mezzanine. Now im worried that the depths I chose in the interior of the slab aren’t going to work because of the heaving. They just poured the perimeter footer for the stem wall today, so I need to quickly figure out how best to get this figured out. I am having trouble finding a detail that works for an actual deep footing. Is there one out there, or is this frost protected stuff only for thin footings?

 

[billconner]

If you're heating building, that's probably going to keep frost heave from an interior column, so just a footing big enough to support mezzanine concentrated load. If not heating, like me, I'd use a footer form and sonotube, try to finish sonotube at final floor elevation, and keep slab not restrained around sonatube.

 

[drxlcarfreak]

I do plan to heat the building, but would want a contingency plan in case of an extended power outage or other failure.

I am actually kind of confused about the whole isolating thing. The drawings for the steel structure call out a monolithic pour tying the slab to the foundation. Wouldn’t that make the entire slab not floating? Or should I expect major cracking along the control joints with the heaving?

 

[OCD]

Music to my ears (eyes) LLWillysfan. I'm really hoping that is the case, and I appreciate the fact that your signature seems to have a lot to do with Concrete. More to come once I consult with my guy!

LLWillysfan is the King of Concrete, I would say he knows what he is talking about!!

 

[billconner]

Different opinions on the slab being free or tied to foundation. I like separate so it can move without edge being restrained if prolonged freezing is likely.

 

[drxlcarfreak]

The Frost protected stuff isn't nearly as stringent as I thought it would be. For like 80% of the country, the horizontal portion isnt needed at all and just a 1.5" perimeter wrap is enough. That said I am having trouble figuring out a solution for where I currently am. My footers are poured and the team is starting on the CMU stem wall today. The problem I am having is how to take care of the exposed insulation cheaply. They do own parging the CMU, but I imagine this would be a considerable extra cost. The other issue is the 4" thermal gap because the steel structure people require the slab to be 4" wider than the structure on each end. If it was just that issue and not needing to pay more to protect the insulation, I think I would just make due.



The only other option I see is to move that insulation to the inside wall and run it to the top of the slab. Since this would make the slab floating and the structure walls bridge the gap, wouldn't that require me to basically run a footer directly inside the footer? I feel like I am missing something pretty basic here. I just want to have my ducks in a row before I talk to the concrete guys.

 

[tstaude]

This is an interesting topic, a two post lift just needs 4-6 inches of concrete depending on what lift and capacity.
I can't imagine a mezzanine in a garage requires much more to support a similar load.

A guy that I know had a slab poured with a thickened area for his lift, it cracked all around the thick area so that may not be a good idea in comparison to a real footing or just going to a 6" slab.

 

[drxlcarfreak]

Hmm, that is the first I've heard of that. I wonder if it was due to the frost heave issue, or lack of wire mesh, or what that could be, but I hadn't heard of that before!

I actually went through and calculated the loads out for the mezzanine for me assuming 100PSF, found a calculator for concrete depths, and thought I was good... until reading this thread. The interior walls were second worst since they had the largest tributary area at 1,500lb/linear foot, which for a 2x6 wall is a 3,200PSF load onto the concrete. The middle column was the worst since I calculated a 12,000lb point load on the base plate. All of them were under 9 inches thick assuming 24" wide, so rounding up to a 10" thickened slab about 24" wide seemed like a good place to be. Now, I am trying to avoid cutting another 36" deep trench down the middle with this insulation wrap, but having a hard time wrapping my head around the best way to do it.







It's kind of crazy since the overall steel structure only requires a 12x12" footer at the perimeter of the entire building, but the structure itself isnt that heavy and I think the most of that is to support lateral loads from wind. The mezzanine will see a lot more loading than the structure ever would.

It's also probably good to note that like the OP (sorry I kind of highjacked your thread), it really isnt a mezzanine and its more of an entire freestanding second floor that I designed more like a freestanding deck or more like a 2nd floor of a house since I am using 2x6 walls at the perimeter to spread the loads out as much as possible. A true mezzanine wouldn't have nearly as large of a load since the overall footprint is much smaller!

 

[billconner]

The Frost protected stuff isn't nearly as stringent as I thought it would be. For like 80% of the country, the horizontal portion isnt needed at all and just a 1.5" perimeter wrap is enough. That said I am having trouble figuring out a solution for where I currently am. My footers are poured and the team is starting on the CMU stem wall today. The problem I am having is how to take care of the exposed insulation cheaply. They do own parging the CMU, but I imagine this would be a considerable extra cost. The other issue is the 4" thermal gap because the steel structure people require the slab to be 4" wider than the structure on each end. If it was just that issue and not needing to pay more to protect the insulation, I think I would just make due.



The only other option I see is to move that insulation to the inside wall and run it to the top of the slab. Since this would make the slab floating and the structure walls bridge the gap, wouldn't that require me to basically run a footer directly inside the footer? I feel like I am missing something pretty basic here. I just want to have my ducks in a row before I talk to the concrete guys.

the last diagram is the way I prefer, all foam on inside. I question if the wall can be so not centered over footing and stem wall.

Yes, heated building - foam down or out - and depending on climate - not that far.

Energy codes here would not permit your first diagram - edge of slab not insulated. Pout footing, build stemwall, build walls and roof, then I stal foam and pour slab, at least that's my approach. Less likely to crack if not restrained by foundation.

 

[drxlcarfreak]

That seemed to be the best of both worlds for a normal construction, but with the requirements from the steel structure manufacturer it really is a bear and I can't find a detail that seems to work.

Unfortunately, I dont see a way to... originally the footer was going to be 12" wide full depth until the concrete vendor realized that the fall over the footprint of the pad was too high and we had to swap to a CMU stemwall deal. The steel vendor requires the concrete pad to be at least 8" wider than the structure itself, 4" on each side, I am guessing so that there wont be a possibility with blowout of the concrete anchors?

And even more unfortunately, I have to have the entire slab poured and photos taken before the structure people will put me on their installation schedule.

I have even another little piece of constraints... I am planning to interleave the 2x6 structural wall inside the steel ladder structure so I dont lose even more footprint of my buidling (I am pretty miffed that they went to the ladder structure without telling me and losing 2' interior clearance, when I originally asked to do a 32' wide structure and they said it would be a large expense and I wouldn't even gain useable space since they would have to switch to a ladder structure, but that's another story).

Assuming I can put the 2x6 stud wall in there, I really only get about 3-1/4-3-1/2" of bearing right on the edge of the slab. I'd guess that's okay since there are details of exterior walls cantilevering on the edge of the structure, like the below image I found online. Granted, none of this has to meet energy codes, I am trying to add the insulation to save myself money in heating the space, and trying to do the perimeter insulation to mitigate any frost heave issues. Any input would be greatly appreciated.




Example of cantilevered structure