Column Stiffness Modifier - Slab Deflections

[Drapes]

What stiffness modifiers, if any, are recommended for concrete columns when designing a slab for deflections (i.e. under service loads)? Using a non-linear cracked section analysis program like SAFE, obviously the slab stiffness is inherently accounted for, so I have no issues in capturing this. But I am unclear on what to use for the column stiffness.

I understand that under ultimate gravity loads, a stiffness modifier of b/w 0.7Ig and 1.0Ig is used for columns depending on the axial load (low rise vs high rise structure), although from a punching shear point of view, the general consensus is to take 1.0Ig.

Is it reasonable to take a column stiffness value of 1.0Ig under serviceability conditions as well? Or is that being too unconservative?

 

[JAE]

Here's an approach:

1. Assume Ig for your columns
2. Run a service-level analysis using alternating live loads
3. Get the associated column axial and bending moments for each combination
4. Check for cracking using: f = P/A + My/Ig

If the column cracks (compared to fr = 7.5 x Sqrt(f'c), or perhaps assume a more conservative value of 5.5 x Sqrt(f'c):
1. Use the 0.7Ig or
2. Calculate an Ie directly (more work without much gain I think).

Or you could just assume 0.7Ig and go with it - getting perhaps a conservatively higher value of slab deflections than the more rigorous analysis would get you.


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[slickdeals]

I would use caution with this approach when it comes to designing for punching shear in the slab. I would recommend using 1.0 Ig (lowest I would personally go is 0.7 Ig) and ensure that you don't have punching shear issues.

 

[JAE]

Agree
I was speaking only for the slab design checks for deflection and strength. For the punching shear you want to stay on the conservative side which means 1.0Ig for the column.

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[Drapes]

Thanks for the feedback JAE and slickdeals