Shrinkage and Temperature - This requirement is satisfied by ensuring that the total reinforcement in the 2 faces is covered by 10..7.4 (7.12). This requirement is normally applied where there is no significant flexural action and if in cases where the slab would not fail if this reinforcement were to fail, in other words if a load path still exists without this reinforcement,. eg transverse reinforcement in a one way slab. ACI318 does suggest that this requirement also applies in raft slabs but I cannot understand why as there is significant flexural stress. Just because it is on the ground does not mean that failure of the reinforcement is not critical so, personally, I would ignore this as a code slip up.
Strength/Brittle failure - This requirement is satisfied by 10.5.1 etc and requires a minimum area of reinforcement at each face of the concrete in tension. The reason given in the commentary is that the cracked strength must be greater than the uncracked strength, and there is good reason for this as it explains. The other forgotten reason is that we never check the strain level in reinforcement at ultimate strength. The smaller the area of reinforcement , the higher the strain in the reinforcement at the tension face The minimum reinforcement rule is also meant to limit this strain in the reinforcement to avoid snapping of the reinforcement which is a brittle and possibly catastrophic failure. So, where there is significant flexure and the load path for the slab requires strength at a cross-section, minimum reinforcement is required at an section where there is tension at a concrete face. This does not necessarily require reinforcement at both faces everywhere as someone suggested, only at locations where there is flexural tension.
So, in conclusion, if you are relying on reinforcement to provide flexural strength and to support the structure, the minimum is required on any face that requires tension reinforcement.
At 07:01 AM 5/09/2008, you wrote:
As Bill noted, to my knowledge, the intent of the provision is to provide the TOTAL amount of shrinkage and temperature steel in the cross section that you must provide. The code then leaves it up to you to decide how to distribute that steel with in the cross section, with the only limitations being on bar spacing in the "out of plane" direction...although you could also use those spacing limitations in the other direction for layers of bars if you have a slab of sufficient thickness (i.e. greater than 18 inches).
- -----Original Message-----
- From: William.Sherman@CH2M.com [ mailto:William.Sherman@CH2M.com]
- Sent: Thursday, September 04, 2008 3:36 PM
- To: firstname.lastname@example.org
- Subject: 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
- From: William Haynes [ mailto:email@example.com]
- Sent: Thursday, September 04, 2008 1:15 PM
- To: firstname.lastname@example.org
- Subject: Re: Minimum Slab Reinforcing
- If 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 <email@example.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 <firstname.lastname@example.org> 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?
- Josh Plummer, SE
- David Topete, SE
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