Anchoring baseplates into slab on grade? 4

[mfstructural]

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?

 

[clarke1973]

Most of the responses here provide the advice you need but I would also add one point regarding the slab. Even if it is a slab on grade you cannot be sure that the ground / sub-base directly beneath your columns is adequately compacted. when applying point loads to a slab on grade I usually allow for a soft spot of roughly 1.0 to 1.5 sq m, i.e check that the slab can span a distance of 1 - 1.5m with the point load. un unreinforced slab will not have capacity to do this. also, the proximity of your column load to any joints in the slab should be assessed.

 

[dik]

clarke... that's one of the nice things about cutting a hole... it gives you a chance to see what you're bearing on...

Dik

 

[faromike]

So I've gotten updated drawings finally. Showing a 100' x 40' mezzanine with columns spaced at about 20'. Plus they want joists with slab on deck (probably 4" slab). So that puts me at 40 psf for slab + 10 psf for partitions + 50 psf LL (office). 100 psf x 400 ft sq. for center column gives you 40 kips (without moment). This is way too high and you would need decent size footings 3'x3' to get the bearing to 4.4 ksf. I'm not even sure what the bearing capacity is and I doubt they'll want to drill, so we'll have to estimate based on rack loading. I'm thinking you'd have to go with a moment frame in each direction for lateral.