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Concrete beam minimum steel 1

EngDM

Structural
Aug 10, 2021
598
Hey all,

I'm just wondering if CSA minimum steel for flexural members, clause 10.5.1.2 requires the reinforcing steel to be in the tension zone. The clause doesn't appear to indicate that it HAS to be tension steel (though obviously this is ideal); when using Jabacus today I noticed that their min steel check was based on the steel in the tension zone only.

I was under the impression that so long as the tension steel is designed for the loading, that the min steel was for the gross cross sectional area. Hopefully someone can confirm or point me somewhere that outlines it a bit more clearly.
 
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The spirit of it is to provide a reinforced flexural capacity well in excess of the plain concrete flexural capacity so that any flexural failure will be ductile enough to provide occupants with some degree of warning prior to brittle collapse. So any reinforcing scheme that accomplishes that goal will do. Consequently any reinforcement scheme that satisfies the 1.2 Mcr thing would be compliant.

With regard to 10.5.1.2, I believe that the [h] term is a proxy for the depth to flexural reinforcement and surely assumes a [d/h] ratio upwards of 0.8 (I don't know the exact number off hand).
 
None of those clauses indicate if the reinforcement needs to be in the tensile zone.
The spirit of it is to provide a reinforced flexural capacity well in excess of the plain concrete flexural capacity so that any flexural failure will be ductile enough to provide occupants with some degree of warning prior to brittle collapse. So any reinforcing scheme that accomplishes that goal will do. Consequently any reinforcement scheme that satisfies the 1.2 Mcr thing would be compliant.

With regard to 10.5.1.2, I believe that the [h] term is a proxy for the depth to flexural reinforcement and surely assumes a [d/h] ratio upwards of 0.8 (I don't know the exact number off hand).
Makes sense. So if I required 3-20M bottom bars for tension to satisfy my postive moment, but needed 1200 mm2 via min steel 10.5.1.2 I could proportion it 3-20M top 3-20M bottom.

This is how I have been treating minimum steel requirements, I just saw the jabacus requirement and my heart skipped a beat for a second there.
 
None of those clauses indicate if the reinforcement needs to be in the tensile zone.

Makes sense. So if I required 3-20M bottom bars for tension to satisfy my postive moment, but needed 1200 mm2 via min steel 10.5.1.2 I could proportion it 3-20M top 3-20M bottom.

This is how I have been treating minimum steel requirements, I just saw the jabacus requirement and my heart skipped a beat for a second there.
I don't believe your distribution of splitting the steel between faces is accurate, the full 1200 mm2 would need to be placed in the tension face to ensure the flexural capacity exceeds the cracking moment by the specified amount.
 
I don't believe your distribution of splitting the steel between faces is accurate, the full 1200 mm2 would need to be placed in the tension face to ensure the flexural capacity exceeds the cracking moment by the specified amount.
That's where my confusion arises. 10.5.1.2 doesn't clarify where the reinforcing has to be, and coworkers I have asked have all said that as long as the total reinforcing in the gross section exceeds min steel, and your tension steel is designed for your design Mf then you're good to go.

If the clauses were worded like minimum tensile reinforcement or something, it would make it a little more clear.
 
None of those clauses indicate if the reinforcement needs to be in the tensile zone.
It belongs in the flexural tensile reinforcing zone... It doesn't state the reinforcing shall be placed in the concrete, either...
 
That's where my confusion arises. 10.5.1.2 doesn't clarify where the reinforcing has to be, and coworkers I have asked have all said that as long as the total reinforcing in the gross section exceeds min steel, and your tension steel is designed for your design Mf then you're good to go.

If the clauses were worded like minimum tensile reinforcement or something, it would make it a little more clear.
I think your co-workers may be mixing up temperature and shrinkage steel requirements and minimum flexural steel requirements, minimum flexural steel as far as I am aware is required to be placed nearest the tension face as you are trying to ensure As Fy (d - a/2) exceeds 1.2 Mcr to achieve a more ductile failure.
 
I think your co-workers may be mixing up temperature and shrinkage steel requirements and minimum flexural steel requirements, minimum flexural steel as far as I am aware is required to be placed nearest the tension face as you are trying to ensure As Fy (d - a/2) exceeds 1.2 Mcr to achieve a more ductile failure.
I'm curious how this translate to the wall (that experiences flexure) clauses in chapter 14 where they are 0.002Ag and 0.0015Ag for vertical and horizontal respectively? Is it to be interpretted as 0.002Ag for each tensile face?

Also to clarify your earlier statement, As Fy (d - a/2) must exceed 1.2 Mcr, or ΦAs Fy (d - a/2). I'm assuming the latter.
 
yep forgot the phi

For walls even ACI is a bit murky the wall chapter dictates minimums in each direction but doesn't speak to the face , ACI 318-14 used to prescribe minimums to each face for walls thicker than 10in but that language was removed in ACI 318-19. All of the new examples in the example manual now have language describing the behavior of the wall to justify reinforcement placement and adjusted minimums, one-way vs two-way action. Think there is too much ambiguity in the code right now on what the minimum should truly be but for the below lets assume that a buried basement wall pin top and bottom should be treated as a one-way slab for minimum flexural steel then:
the inside tension face needs to satisfy minimum flexural steel requirements for me that is 0.0018Ag this steel goes on the inside face
then there is vertical temperature and shrinkage and good practice would be to split this to each face so the 0.0012Ag would become 0.0006Ag each face and the flexural minimum would satisfy this on the tension face.
Then horizontal t&s again split evenly for me this is 0.002Ag so 0.001Ag each face.
 
Those simplified minimum flexural reinforcement rules (E.g. 0.2%) assume that steel is in the flexural tension zone. It's just a short cut to ensure the strength after cracking exceeds the cracking strength, i.e. to ensure the MR>1.2Mcr clause that dik mentioned. If that steel is in the top of a simply supported beam for example, then obviously 0.2% might not be enough to ensure this occur.
 
Is the 0.2 in CSA equation 10.4 a typo, companion formula in ACI would appear to be [3 sqrt(F'c)/Fy] bw d

Edit: nevermind, metric vs freedom units :)
 
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There was an old thread about this (well specifically ACI 10.5.4 I think) where the question was aimed at the ACI provisions for "slabs and footings" where the older versions of the code directed the engineer to the Temp & Shrinkage section for slabs/footings. The question on that thread was - 1/2 on top and bottom or ALL on the tension side?

The final (and correct) answer was ALL on the tension side and this was confirmed by ACI through emails.

For the other minimum steel provisions for bending - ALL should go on the tension side.

Even the Canadian code says: At every section of a flexural member where tension reinforcement is required by analysis, minimum reinforcement shall be..."

For flexural analysis - placing minimum steel in a compression zone makes no sense related to what KootK states above as the reasoning behind the need for a minimum in the first place.
 
nevermind, metric vs freedom units
Funny... most of the world uses metric units... The old Harley motto, "Live Free or Die" should be changed to "Live Free and Die."
 
Digging up this thread again to see what people think about minimum steel in a slab on grade. Both faces go into tension depending on soil properties and subgrade modulus, so how do slabs with only top steel get around this? Is it because SOG are often deemed non-structural elements?
 
It's because a slab on grade is only as good as the sub-grade beneath it. The reinforcing is only for T&S.
 
It's because a slab on grade is only as good as the sub-grade beneath it. The reinforcing is only for T&S.
That slab would still go into positive/negative bending based on point loads say from racking though right? Yes the subgrade prep would help spread out the load/transfer more of the load immediately to the soil but it still flexes the slab.
 
The whole point is that the sub-grade is prepared so the slab doesn't. I know a retired super flat consultant who has designed 10s if not 100s of millions of square feet of super flat concrete slabs on grade used for high precision racking systems. His philosophy was always to treat the slab as paint on the sub-grade.
 
The slab on grade in those cases in designed as plain concrete (see PCI method for example). If the slab on grade is designed as reinforced, then you provide reinforcement on both faces. If designed as plain concrete, the T+S is more optional that anything (have seen some plain concrete slabs, and personally I don't do more than 0.002 of the top 6" for plain SOG). You put the T+S on the top there because that is where the cracks are seen
 

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