Concrete Moment Frame with 2 way slab

[Astructengr]

I have a 6-story automated parking garage (around 50' tall, floor to floor heights are very small). There is no good way to have shear walls in the east-west direction without eliminating spots. Due to floor height restrictions and parking restrictions, the building has to be a moment frame building with a 2 way flat plate structure as the LFRS. The building is odd and I do not believe there are many automated garages in the US. Attached pdf has 2 sheets.

Foundation (4500psf net allowable)
1. There are columns along the perimeter that have to go deep,9-10ft, below the first floor slab to avoid neighbors foundations, vaults, pits, etc. What should the unbraced length of these columns be? Could I rely on either the soil or casting the SOG against them without an isolation joint to provide lateral stability? Any other clever ways around this?
2. I need to determine K factors for a sway frame using the alignment charts. I have read that the Psi(Y)of a footing should be 10 if it is pinned or not rigidly connected to the footing. What do other people assume at concrete column footings? What width of slab should I assume at elevated levels? I have read people will use the column strip width but I cannot find anything in ACI on what to use for gross properties in 2-way slab systems. RAM structural will not even calculate it unless it is an actual beam so it has to be done manually.
3. In order to control deflections/drift, some columns will need fixed bases, which I try to avoid. Can I tie some footings together with grade beams and design the grade beam for the column moment? Or should I just completely combine some footings and create a mat?
4. At the elevated levels, the slab is dropped 9" for the transfer car for the entire width of the building. Is there any way quantify the diaphragms are adequately tied together or any detailing recommendations that you can make?
5. There will be an exterior stair from the ground to the roof that will need to be attached at each floor level. I have never really given much thought to stairs as they are always delegated, but since the building will not be overly stiff is there any detailing issues here? Does the delegated designer just need to be aware of designing the columns for P-delta effects?

 

[Lomarandil]

1a. Embeddment in soil is enough to restrain the column against buckling, but not necessarily to (immediately) provide lateral support to the frame structure. You would want to perform analysis with the lateral "pile" capacity factored in (through nomographs, Lpile, soil springs etc.)

1b. Yes, tying the slab on grade to the column is one option for lateral support. It can raise other issues in your slab onngrade design.

1c. In the context of a parking garage (which often has precast beams on seats), be doubly sure you are confident in any assumptions here that affect the drift of your structure and subsequent bearing seat length.

2a. If the soils are stiff, you may be able to rationally justify partial rotational resistance (and a lower Psi value) in the spread footing base. The PCA manual has a method to assess this.

2b. Been a while, but the PCA notes on ACI318 (different from the manual) may have this in one of their worked examples and can be obtained free on their website.

3. Yes, both of these are options, depending on your local economic factors.

4. Sorry, I won't address this item today.

5. This depends on the connection between the building and stairs. If they are detailed with a slip joint (with adequate range specified) no problems. If the connection is rigid, it can attract lateral load from the building and have some notable negative effects, especially for flexible moment frames.



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just call me Lo.