should cracked section be used for wind drift calculation?

[rejoicer]

When checking the drift of high rise building with concrete shear wall under wind, should the cracked section (ACI 318-05 10.11.1) be used? Are these cracked section factors for strength design only or for both drift check and stregnth check?

Thanks

 

[RARWOOD]

When I check the drift of a multi-story concrete building braced by columns and beams, subjected to side sway I use cracked moment of inertias.

 

[WillisV]

Determine if the wall is cracked by looking at the stress in the wall under 10-yr service level wind loads compared to the tensile capacity of the concrete - if it is, then use cracked properties. If it isn't, then don't.

 

[JAE]

I would try to calculate the effective moment of inertia of the shearwall for a building that high. I would include in the Ie calculation the axial forces in the shear wall at each level (include P/A in your determination of your cracking moment Mcr).

Then I'd also be sure to include drift in the wall from both flexural deflection and shear deflection in the wall. We've had some threads here before that discuss the equations that go into including both these effects.

But using Icr instead of Ie would certainly be conservative - just don't use Ig.

 

[Whiffle]

There is a very good paper by Horvilleur, Patel, and Young titled "Modeling Assumptions for Lateral Analysis" that explains when cracked properties should be used, and they give their opinions on what values to use for serviceability, strength, and stability. It may help you out.

It's found in SP-240, "Performance-Based Design of Concrete Buildings for Wind Loads" by ACI.

 

[slickdeals]

I agree with JAE, it is very important to use the P/A in your calculations because the cores on tall buildings typically have very high axial loads and that helps to overcome the tension due to moment.

As WillisV states, you can look at the wall stresses from your analysis. If you are using ETABS, you can look at the stresses in the wall and set your stress limits in the viewer. Any stress above the specified limit will be shown in the color of your choice. This will help to determine the stresses.

I think the important thing that has not been addressed is the cracking in the slabs. If the slabs are using a linking element between the core walls and the perimeter columns, it is very important to account for cracking. I have seen 0.5 Ig being used for PT slabs and 0.35 Ig for RC slabs.

@Whiffle: Thanks for the paper information. I will try to get a copy of that.