COLUMN K VALUE

[johndeng]

The one story parking garage is attached to a building.
Parking slab is precast plank, and supported by big steel girders and masonry wall both ends. The other direction are only small tie beams and no masonry wall.
Wall direction (e-w) can be treated braced, the other direction (n-s) may be not (don't know the building part). Please see plan sketch. All connections are simple connections.

What would be the column k value? Should I just treat the columns as gravity column only (that's what I assumed original design)?
Column is 3 feet down below grade, if I encase the column (extend the footing up to grade), will that cut off the unbraced length? or I have to cut column and add piers below?

Thanks

 

[KootK]

For EW buckling, you can use K=1.

For NS buckling, you'll need to figure out what the lateral system is before you can design your columns. If the columns are the lateral system K could be as high as two for a base assumed to be rigid and as high as infinity for a base assumed to be flexible.

You can get some modest milage from encasing the column. Some would call the slab on grade a braced point, if you have one.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johndeng]

Thanks KootK.

Since the building is on one side, even it's braced, the NS direction of the garage column is still not braced as I think.
Not sure the soil condition, but I know it's always tough to call the base rigid. If I encase the column, maybe I can count it as rigid? or anything I can do to make it more rigid (such as add studs on column to combine with the column encasement)?

 

[KootK]

Is this a new or existing structure?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johndeng]

it's a existing structure, and we are going to add a roof on the garage.

 

[KootK]

If I had to guess, I would say that your garage looks like a three sided building from a lateral perspective, with that existing wall on the west side acting as the sole shear resisting element in the north/south direction. Do any of the construction details support that hypothesis? If so, you would need to design your roof in a way that it's lateral system pays homage to the existing three sided system below. On the positive side, your column K values would be unity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johndeng]

Yes, your guess might be reasonable.
With the building on north side and the walls on east and west, the garage does look like a three sided shear wall building, with a rigid diaphragm, the columns will not take the lateral, gravity only.
I don't have any details/drawings, but since it's plank slab, slab always doweled to the shear wall three sides. masonry wall are just brick wall, west side 8", east side 12", no reinforcement in walls.

New roof would be relatively light, I am thinking only column moment connections top & bottom, no walls and bracing. This way, column can only be welded to cap plate of column below.
If roof column can be pin connection on bottom, moment connection on top, I would just bolt column thru the slab.