From: William Haynes [mailto:gtg740p@gmail.com]
Sent: Thursday, September 04, 2008 1:15 PM
To: seaint@seaint.org
Subject: Re: Minimum Slab ReinforcingIf there is tension at the top and bottom, then you need to provide 0.0018*Ag at EACH face.Will H.On Thu, Sep 4, 2008 at 3:06 PM, David Topete <d.topete73@gmail.com> wrote:
I've always checked the 0.0018*Ag for top and bottom min steel. i.e., if i have a 24" thick footing (mat or grade beam), I'll check As,min top = 0.0018 * 12" * 24"/2 = 0.259 sq.in./ft. Use #4 @ 9"o.c., or something like that...--
On Thu, Sep 4, 2008 at 11:28 AM, Josh Plummer <joshp@risatech.com> wrote:
In ACI 318, section 10.5.4, the code states that the minimum flexural reinforcement for slabs and footings should be the same as given in 7.12 (which is the temperature / shrinkage requirements).
I've interpreted this to me that regardless of what the area of steel required by analysis is, you can never let your total slab reinforcing (As_top + As_bottom) be less than the minimum amount required for temperature and shrinkage.
Now, I've got another engineer who is arguing that the Asmin = 0.0018 * Agross requirement applies for BOTH the top steel reinforcing and the bottom steel reinforcing. Since temperature and shrinkage controls the design of most relatively thick mat foundations, this interpretation would effectively double the amount of steel required in these mats.
Any opinions? Has anyone heard of someone interpreting this section of code in this manner? Is it common?
Sincerely,
Josh Plummer, SE
David Topete, SE
Thursday, September 4, 2008
RE: Minimum Slab Reinforcing
The code does not refer to "each face", just to "area of shrinkage and temperature reinforcement" based on gross area of concrete. The "common" practice is to provide half of this area in each face as a minimum (except for thin slabs with one mat of steel). But the code doesn't say it has to be done this way - if the combined effect of the top and bottom steel areas meets the code minimum for S&T steel, then the code has been met. (You still have to provide tension steel in any face that has design tension.)
Bill Sherman
CH2M HILL / DEN
720-286-2792