Anchoring baseplates into slab on grade?
Anchoring baseplates into slab on grade?
(OP)
Hey all,
I have a situation where we are adding a mezzanine to an existing warehouse building. The warehouse is about 30' high and the mezzanine will be about 10' off the slab. I am working with another structural engineer and architect on this project.
The mezzanine is planned to be 100' x 40', with 3 columns per frame. There will be 8 frames spaced at about 12'.
My issue is that they want to anchor the column base plates directly to the slab on grade which is 6" thick. I had suggested providing footings but they claim anchoring to the slab is sufficient. They are also proposing to have the columns designed as flag poles and this would be the lateral system...no bracing or moment frames. The building is in northern Michigan so not a high seismic area but I'm not sure I like the idea of anchoring directly to an existing slab.
In the past I've always provided footings. I suppose if you show the anchors are sufficient with a sufficient amount of clearance between the bottom of slab and end of anchor hole and also that the slab can resist the moment (assuming no reinforcement) then this would be feasible. The mezzanine will be used as an office so it's 50 psf LL.
We are planning on spanning 2x12's or wood trusses between the frames so the DL will be low. I'm not sure if this will be feasible; the architect is looking into fire protection requirements.
Any thoughts or has anyone ever anchored to an existing slab on grade?
I have a situation where we are adding a mezzanine to an existing warehouse building. The warehouse is about 30' high and the mezzanine will be about 10' off the slab. I am working with another structural engineer and architect on this project.
The mezzanine is planned to be 100' x 40', with 3 columns per frame. There will be 8 frames spaced at about 12'.
My issue is that they want to anchor the column base plates directly to the slab on grade which is 6" thick. I had suggested providing footings but they claim anchoring to the slab is sufficient. They are also proposing to have the columns designed as flag poles and this would be the lateral system...no bracing or moment frames. The building is in northern Michigan so not a high seismic area but I'm not sure I like the idea of anchoring directly to an existing slab.
In the past I've always provided footings. I suppose if you show the anchors are sufficient with a sufficient amount of clearance between the bottom of slab and end of anchor hole and also that the slab can resist the moment (assuming no reinforcement) then this would be feasible. The mezzanine will be used as an office so it's 50 psf LL.
We are planning on spanning 2x12's or wood trusses between the frames so the DL will be low. I'm not sure if this will be feasible; the architect is looking into fire protection requirements.
Any thoughts or has anyone ever anchored to an existing slab on grade?






RE: Anchoring baseplates into slab on grade?
Given the lack of information, I would tend to argue for a small footing.
RE: Anchoring baseplates into slab on grade?
1. The slab-on-grade as "6 inches" may not be exactly 6 inches thick. Many times slabs vary in thickness due to the variation in subgrade.
2. You say that the slab is unreinforced. If so, you would need to check punching shear and have a thick and wide enough base plate.
3. With an unreinforced slab, and a cantilevered column, how are you resisting the column base moment? The slab has no reinforcement and this has very little (if any) moment capacity.
4. Mezzanines inside in low seismic areas still need to resist 0.01 times the dead weight of the mezzanine laterally.
5. Also - you technically could argue that any lower and upper walls on the mezzanine could experience some air/pressure differential due to exterior wind forces (i.e. the lateral 5 psf required in the IBC for example). So your lateral could be significant.
6. ACI recommends a minimum of 6" thickness for footings. If your slab is just at or below 6" you would be in violation of that provision.
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RE: Anchoring baseplates into slab on grade?
I suggest using individual footings.
RE: Anchoring baseplates into slab on grade?
I'm not licensed in the state the building is being erected in so I'm just on board to prepare the calc. I'm only a PE, SE in Illinois. regardless of sealing it or not, I can't agree to do something like this if the numbers or my judgment is against it.
RE: Anchoring baseplates into slab on grade?
I have done this before, but only for a wood stud bearing wall. Any isolated column or column at the end of a beam I have opted to saw cut the slab and install a new spread footing - no exceptions unless you are dealing with a minimum 8" slab with two matts of reinforcing, not an unreinforced 6" slab.
You mentioned cantilevering the column off the slab for the lateral. You may be able to do this in theory for steel, but not for wood columns. Personally, I would use wood shear walls or steel moment frames, but the Architect must be a better structural engineer than me.
Mike McCann
MMC Engineering
RE: Anchoring baseplates into slab on grade?
RE: Anchoring baseplates into slab on grade?
RE: Anchoring baseplates into slab on grade?
Run your calcs, provide to the EOR, and if he chooses to ignore calcs...talk to the owner.
RE: Anchoring baseplates into slab on grade?
Hilti has adhesive anchors that are quite strong and don't have to go very deep. 1/2" diameter anchor embedded 3.5 inches into normal weight concrete has an allowable tension capacity of 3kip and allowable shear of 3.7k based on bond strength as an example.
I'd also try and keep the leveling grout to a minimum.
Is the mezzanine going to be near any exterior walls/columns? I'm guessing this is a prefab metal building? You could just tie to them for lateral support.
What about slab reinforcing. I have to believe it has something. I'd determine a safe allowable load for the slab and space the columns accordingly. Tie the mezzanine floor to some exterior walls for lateral bracing so you won't have to worry about high overturning base plate loads and/or switch to steel/aluminum and use moment frames.
Why does fire protection make it not feasible and why is there fire protection even needed? It's not a separate structure, it's inside an existing one. The same people that use the warehouse will be using the office space, no?
RE: Anchoring baseplates into slab on grade?
RE: Anchoring baseplates into slab on grade?
Still, much better to just pin the bases and use moment frames. Putting moment into a minimally reinforced slab-on-grade seems like a terribly inefficient design to me. Also, the large baseplates required to transfer such a moment may present a tripping hazard.
Maine EIT, Civil/Structural.
RE: Anchoring baseplates into slab on grade?
Initially he didn't want to tie off to the existing building, but yes it's a prefab warehouse structure. I'm assuming there is some sort of reinforcing in the slab just not sure yet. I'm supposed to get more info tomorrow.
I don't know much about fire protection. I'm still waiting on information. I don't think it should matter because like you said, you're inside an existing building. It's not a new independent structure.
I agree with Teh; no reason to introduce large moments into the slab. So I'm going to go a different route and go with the moment frames.
Thanks all
RE: Anchoring baseplates into slab on grade?
RE: Anchoring baseplates into slab on grade?
Dik
RE: Anchoring baseplates into slab on grade?
One of the items I struggle with in a case like this is the location of the control joints or cracks in the slab. You need to make sure whatever assumption you use to check your design works with the joints and cracks in the vicinity of your posts. Sometimes this is not known and gives me great pause.
That said, there are many, many heavily loaded racking systems which are set on warehouse slabs throughout the country to no ill effect. The architect is right to push you a bit on this one.
RE: Anchoring baseplates into slab on grade?
Mike McCann
MMC Engineering
RE: Anchoring baseplates into slab on grade?
Maine EIT, Civil/Structural.
RE: Anchoring baseplates into slab on grade?
Mike McCann
MMC Engineering
RE: Anchoring baseplates into slab on grade?
I think the vast majority are probably braced system (x-braces) or some kind of semi-moment frame system - or tied into the adjacent structure.
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RE: Anchoring baseplates into slab on grade?
RE: Anchoring baseplates into slab on grade?
Dik
RE: Anchoring baseplates into slab on grade?