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RE: Story Drift - Cracked or Uncracked Section Properties

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Cd is purely the "switch" from an elastic deformation to an assumed inelastic (I say assumed as it is an approprimation...not an exact analysis).  When you do you elastic analysis, you should be using cracked section properties.
Adrian, MI
-----Original Message-----
From: Paul Blomberg [mailto:paul.blomberg(--nospam--at)]
Sent: Friday, June 06, 2008 4:25 PM
To: seaint
Subject: Story Drift - Cracked or Uncracked Section Properties

We are checking some concrete moment frame calcs and debating internally about using the cracked or uncracked concrete section properties for the structural analysis.  There is some discussion that Cd (deflection amplification factor) incorporates the loss of stiffness as the concrete cracks while most believe that Cd increases the elastic deflections (using cracked section properties) to to account for inelastic behavior and that the elastic deflection is based on a cracked section.  The code is a mix of a north African national code with incorporation of ACI 318-05 and ASCE 7-05 just for fun.
So on this fine Friday, anyone want to chime in on this debate.  When do you use the uncracked section properties of concrete versus the cracked section properties?