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Re: Story Drift - Cracked or Uncracked Section Properties

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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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