Ratio of Reinforcement is higher then max Ratio of Reinforcement in concrete beam - ACI code 3

[XinLok]

Dear all,
Maybe this question look a bit silly for some Engineers, but I would like to understand If I got that 𝛒 > 𝛒 max in a concrete Beam, do I have to switch to double reinforcement?

Regards

 

[BridgeSmith]

You'd have to include properly confined compression reinforcement in the beam and calculations, so that the beam is not overreinforced and subject to possible brittle failure.

Unless you're tightly constrained on space, it's usually more economical to use a larger beam than meeting all the requirements to consider compression steel. It takes alot of steel to reach an overreinforced condition; you'll have to watch the minimum bar spacing requirements fairly closely.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[XinLok]

@BridgeSmith Thanks a lot for your reply, So it can treated as a Double reinforced concrete?

 

[jayrod12]

It can be provided you use the appropriate confinement for the compression steel where it is required to bring your reinforcement ratio back down below the allowable amount.

You can't account for compression steel unless you adequately confine the bars being utilized as compression steel.

 

[XinLok]

@jayrod12 Thanks a lot for your reply.
I would like to take example:
if 𝛒 = 0.02 → As = 0.02 x 40 in x 18 in = 14.4 in²
if 𝛒 max = 0.015, then 𝛒 > 𝛒 max

So, ACI code will not allow to use 𝛒 = 0.02, right?

So, to solve this issue, I have 2 solutions:
- Increase the Size to Beam
- use 𝛒 max for Tension reinforcement, and get the required reinforcement for Compression.

Is that right?
But AS I understand, in order to treat a beam as double reinforced concrete beam, I should have the bending moment greater then Balanced Moment, correct?

 

[jayrod12]

You were correct right up to this

But AS I understand, in order to treat a beam as double reinforced concrete beam, I should have the bending moment greater then Balanced Moment, correct?

In order to treat the beam as a double reinforced concrete beam, you need to ensure that the compression steel is tied as per the column reinforcing requirements.

 

[XinLok]

@jayrod12 Dear Sir, can I be little annoying by asking if you have any paper showing an example so I can follow?
Thanks and Regards

 

[XinLok]

I saw somewhere that 𝛒 max = 4 / (b x d) and in other place 𝛒 max = 0.85 x β1 x fc′ / fy x εu / (εu + εt)ice

Which one to follow according to ACI code?

 

[Lomarandil]

Not only is it more economical to increase beam size, deflection considerations will typically also point toward a larger beam size. Make sure your (cracked) deflections are OK before you dive into the details to cram more steel in.

----
just call me Lo.

 

[r13]

The linked article will help your understanding. Link

 

[XinLok]

@retired13 I was expecting your useful reply.
I read this tutorial before, you just hit where I am getting confused:

But As in tension is:

So, for double reinforcement, we should have 𝛒 > 𝛒 max cause we have: As = As1 + As2 && As1 = 𝛒 max x b x d.

 

[r13]

XinLok,

The sole purpose of count on, or add, the compression steel is applicable for a beam with excessive moment demand and with constraints in altering cross sectional dimensions, and material properties, fc' and fy. More often than not, it will result in an over-reinforced condition.

Under the situation that requires a reinforcement ratio in exceeding the maximum allowed by code, the options are,
1. increase cross sectional area,
2. rearrange floor/support layout to reduce the demand,
3. consider axial force in beam-column situation for the load case that causing excessive steel demand,
4. use higher strength reinforcement.

The graph below might help you to get better pictures on the effect of compression steel.

 

[r13]

You might ask what is the effect of compression steel in a typical design/detail, which provides but ignores the compression steel. The answer is "no effect", as tension steel will yield prior to any effect from the compression steel. However, the beam might not have failed as the tension steel can continue to elongate (with the increase in deflection) until reaching ultimate state. Therefore in normal design, the compression steel can be considered an insurance.

 

[XinLok]

@retired13 Really well explained.
Big thanks for you and for the other.
God bless.

 

[r13]

You are welcome.

Just remember one thing, if you have slab cast with the beam, you can utilize the slab on each side of the beam, and design the tension steel using the larger effective width (b_EFF) for compression. Thus, you may reduce the tension steel to an extent.