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RE: Story Drift - Cracked or Uncracked Section Properties

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If you mean the approximations for Icr that are given in section 10.11.1,
then strictly speaking, those are purely use INSTEAD of doing a full
second-order analysis to determine the effects of "moment magnification"
(i.e. second order effects).  They are meant to represent a cracked section
at close to failure and have some even MORE conservative "fudge" in them.

In reality, many engineers commonly use these values, but they can result in
an overestimation of the second order deflections...which is probably where
the 1.43 factor came into play by someone.  By default, the values in ACI
318 section 10.11.1 are .875 of those in the reference document that is the
basis for that section.

Point is that this is a commonly accepted way...but it is not strickly
speaking what those values are for in the code.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com] 
Sent: Friday, June 06, 2008 7:42 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Story Drift - Cracked or Uncracked Section Properties


For the question on when to use cracked vs non-cracked:

If I remember correctly, you can multiply the cracked section factors given
in ACI by 1.43 when checking service level drift. I don't believe you are
able to assume the moment frame sections are completely uncracked in any
case.

WH

On Fri, Jun 6, 2008 at 5:06 PM, Scott Maxwell <smaxwell(--nospam--at)umich.edu> wrote:
> 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.
>
> Regards,
>
> Scott
> Adrian, MI
>
> -----Original Message-----
> From: Paul Blomberg [mailto:paul.blomberg(--nospam--at)gmail.com]
> 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?
>
> Paul.
>

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