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RE: Story Drift - Cracked or Uncracked Section Properties

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Depends on what you are after.  If you are doing some sort of analysis where
the relative stiffness is important, then it will not matter.  If you after
an actual deflection/drift, then it will DEFINITELY matter.  In the
situation that you describe, I would agree that the 1.43 value should be
included.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com] 
Sent: Friday, June 06, 2008 9:29 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Story Drift - Cracked or Uncracked Section Properties


Yes, the 10.11.1 section values is what I was referring to (lets just assume
we are using these instead of a more detailed analysis method for what I am
about to type). I had someone argue with me before that you should use the
10.11.1 values even for a serviceability drift check for wind. His argument
was that once the members reach this extent of cracking (guideline 10.11.1
factors) under design lateral loads, the building still needs to be
serviceable from this point onward (he disregards the 1.43 increase in the
commentary). But my thought was that a design wind or seismic event happens
once in the lifetime of the structure and some repair is expected after this
event. Therefore, applying the 10.11.1 values is not justified for a typical
occupant discomfort evaluation and the 1.43 increase is justified. What do
you think Scott?

WH

On Fri, Jun 6, 2008 at 8:26 PM, Scott Maxwell <smaxwell(--nospam--at)umich.edu> wrote:
> 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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