Column Effective Length with Flat Slab Floor

[haynewp]

I have an existing 3 story building I am analyzing the columns on. The LFRS of the building appears to only be using the flat slab and the columns.

For checking these unbraced columns under only the gravity axial and moments from the 2 way slab, how do you get the effective length for the columns? There are no beams at the top and bottom of the columns, only the flat slab. So do you use the inertia of the column strip part of the slab times the 0.25 cracked inertia factor and then enter the alignment chart?

What are the equivalent beams to use at the top and bottom of the columns when figuring the effective length of the columns?

 

[asixth]

I would do as you suggested, calculate the I value of the column strip and multiply by 0.25 as per Cl 10.11.1 of ACI318-05.

 

[theonlynamenottaken]

Based on Pecknold's research I also use the torsional rigidity for the column strips perpendicular from your plane of evaluation. You'd be surprised how much rotational stiffness they add, especially at exterior columns. Flat plate buildings where the slab comprises the "girders" of the lateral load resisting system are typically value engineered until they hardly pass code/deflection checks - so this may make the difference between pass and fail...