Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Concrete Code Interpretation

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Jim,

Your interpretation of the 10.5 requirement seems to be a little backward.
The requirements is that you MUST use the greater of 3*sqrt(f'c)*b*d/fy or
200*b*d/fy for As (section 10.5.1) UNLESS the actually As provided is 33%
greater than that required by analysis (section 10.5.3).  While this is
very similar to what you describe, it is not the same.  At no point are
you permited to "reduce" the moment capacity, rather you are increasing
the amount of steel by 33% ove what is required by the moment capacity
calculations.

Thus, if the amount of steel in your existing member is 33% more than what
you are detemining as required by your flexural calculations (Mu/phi),
then you don't have to meet the requirements of section 10.5.1 (the 200/fy
stuff).

Now, since your member seems to be structural slab or footing, then it
would appear that section 10.5.4 would apply.  Thus, you would not need to
apply the requirements of section 10.5.1 (the 200/fy stuff).  This becomes
more understandable when you read the commentary for section 10.5.1.  The
intent of the section 10.5.1 is to prevent sudden failures that occur when
the section is under reinforced.  This obviously becomes less critical
when the member is also supported by soil such as a structural slab or
footing, which even if a "sudden" failure of the section were to occur,
the soil beneath would precent a catastrophic failure and limit it to some
cracking and sinking of the member at the cracks which could damage the
machine.

BTW, I tend to still use the requirements of 10.5.1 for footings, but then
that is my own call.

HTH,

Scott
Ypsilanti, MI


On Wed, 25 Sep 2002, Lutz, James wrote:

> In ACI-318, section 10.5, if flexural steel ratio provided is less than
> 200/fy, you can only use 75% of the computed moment capacity. However, in
> paragraph 10.5.4, a lower minimum steel ratio is provided for slabs and
> footings per section 7.12. My question is this, can you ignore the 200/fy
> requirement and still use the full computed moment capacity for a slab on
> grade as long as you meet the minimums in 7.12? I am reviewing an existing,
> lightly-reinforced thick concrete pad for possible mounting of some heavy
> equipment. It computes if I can avoid taking the discount for the low steel
> ratio.
>
> Jim Lutz, P.E., S.E.
> Earth Tech, Inc.
>
> 10800 NE 8th Street - 7th Floor
> Bellevue, WA 98004
> (425) 455-9494
> (425) 453-9470 FAX
>
>
>
> ******* ****** ******* ******** ******* ******* ******* ***
> *   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
> *
> *   This email was sent to you via Structural Engineers
> *   Association of Southern California (SEAOSC) server. To
> *   subscribe (no fee) or UnSubscribe, please go to:
> *
> *   http://www.seaint.org/sealist1.asp
> *
> *   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
> *   send to the list is public domain and may be re-posted
> *   without your permission. Make sure you visit our web
> *   site at: http://www.seaint.org
> ******* ****** ****** ****** ******* ****** ****** ********
>



******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********