Minimum Steel Ratio In Mat

[mhdhamood]

Dears;
The minimum steel ratio =0.0018 is for one face or for the 2 faces of the section (top & bottom)??
As far as I know that 0.0018 is for the face exposed to tension (not the 2 faces). Since the Mat suffers tension at top and bottom of the section at the same section so we provide 0.0018 for 2 faces .
Am I right???
Regards

[EngrUzair]

Minimum reinforcement in a mat footing, is to be provided as per Section 7.12.2 of ACI 318-08, as specified in ACI Section 15.10.4.

 

The amount of reinforcement to be provided (e.g., 0.0018 b h for Grade 60 reinforcing steel), is for each face (top or bottom).  This is because of the fact the mat reinforcement is provided to resist tensile stresses (rather than temperature or shrinkage) on the relevant (top or bottom) face of the mat, similar to minimum reinforcement provided in RC beams.

 

Regards.

[mhdhamood]

Thank you dear engineer

[Waqas Haider]

Dear Uzair,

You mean in slabs or isolated or combined footings, this 0.0018bh is for GROSS AREA i.e. 0.009bh per face because they need minimum reinforcement for temperature control but mat and beams are exception?

[EngrUzair]

Dear Waqas,

 

No. It is not so.

 

This As,min = .0018bh is actually minimum flexural (tensile) reinforcement for each face of mat (NOT the temperature reinforcement), although its amount is is same for the both.  In case of flexure, you have to provide the full amount (=0.0018bh) at each of the relevant location/face.

 

For example, if the uniform thickness of mat is 15 inches and you are using Grade 60 rebars,

 

                       As, min = 0.0018x12x15 = 0.324 sq. in./ft width of mat per face of mat.

 

This amount of reinforcement (0.324 sq. in./ft) has to provided in full at top face of mat at mid-span, and same amount has also to be provided at bottom face of mat near the supports / edges of the mat. This is however, subject to the condition that required flexural reinforcement at these locations is less than 'As,min', otherwise you will have to provide the reinforcement amount required by flexural calculations.

 

Regards.

 

[Waqas Haider]

You are talking about mats/rafts. I am talking about footing slab and floor slab. But I m very much confused now. Let me put my confusions here.

 

Quote

 

According to ACI 7.12.2.1, it says

 

Deformed reinforcement conforming to 3.5.3

used for shrinkage and temperature reinforcement

shall be provided in accordance with the following:

 

7.12.2.1 — Area of shrinkage and temperature

reinforcement shall provide at least the following ratios

of reinforcement area to gross concrete area, but not

less than 0.0014:

 

(a) Slabs where Grade 40 or 50

deformed bars are used .................................0.0020

 

(b )  Slabs where Grade 60

deformed bars or welded wire

reinforcement are used...................................0.0018

 

© Slabs where reinforcement

with yield stress exceeding 60,000 psi

measured at a yield strain of

0.35 percent is used ......................0.0018 × 60,000/fy

 

Now here it is saying provide this minimum temperature reinforcement to the Gross area.

 

and According to it commentary, it says

 

R7.12.2 — The amounts specified for deformed bars and

welded wire reinforcement are empirical but have been used

satisfactorily for many years. The area of reinforcement

given by 7.12.2.1 may be distributed near the top or bottom

of the slab, or may be allocated between the two faces of the

slab as deemed appropriate for specific conditions. Splices

and end anchorages of shrinkage and temperature reinforcement

are to be designed for the full specified yield strength

in accordance with 12.1, 12.15, 12.18, and 12.19.

 

Here it also says this 0.002bh or 0.0018bh is for gross area or either we can distribute between two faces or we can provide at center. 

 

But According to 7.12.1.2, it says

 

7.12.1.2 — Where shrinkage and temperature

movements are significantly restrained, the requirements

of 8.2.4 and 9.2.3 shall be considered.

 

And in its commentary it says

 

R7.12.1.2 — The area of shrinkage and temperature

reinforcement required by 7.12.2.1 has been satisfactory where

shrinkage and temperature movements are permitted to

occur. Where structural walls or columns provide significant

restraint to shrinkage and temperature movements, the

restrain of volume changes causes tension in slabs, as well as

displacements, shear forces, and flexural moments in columns

or walls. In these cases, it may be necessary to increase the

amount of slab reinforcement required by 7.12.2.1 due to the

shrinkage and thermal effects in both principal directions (see

References 7.7 and 7.16).

 

 

 

I m confused when our slabs offer this movement and when does not. Because in slabs we cast slabs monolithically with beams and slabs will be offered restraints at ends of slab. 

 

Till now what i have been doing is i provide half of minimum reinforcement at top and half of reinforcement at bottom in footing slabs but if i m providing only one layer of steel I provide full 0.002bh or 0.0018bh steel. 

[EngrUzair]

Dear Waqas,

 

Don't be confused. It is very simple.

 

All you have referred to in your post, is about and applicable to minimum area of reinforcement, required to resist 'shrinkage and temperature movements' in slabs and mats.

 

On the other hand, I have discussed the distribution of minimum reinforcement, required to resist 'shrinkage and temperature movements' only, reinforcement distribution shall be as mentioned in ACI section 7.12.2 (referred by you).

 

However, when the slab or mat has to resist 'primarily, distribution of reinforcement shall be as per flexural requirements. In this case, total amount of 'As,min' has to be provided at each face of slab or mat subjected to '

[Waqas Haider]

Thanks brother. Previous confusion is clear. What i m confused now is how to decide when i m to provide temp/shrinkage reinforcement and when to provide for flexural minimum. Our slabs/ mats always going to experience flexural stresses because no strata is ever hard enough. Slab/mat will bend in any case. So what would be the case when  temp/shrinkage reinforcement only and when to provide for flexural minimum? 

Also if u can refer me some study material regarding flexural minimum reinforcement I will be very thankful.