T beam minimum Reinforcing

[nickky]

Hi Guys,

Currently I am designing a one way slab which has beams (T-beams).

We know that when the neutral axis is in the flange the beam acts like a regular (bf x d) rectangular beam (bf is the effective flange width). In this case if the calculated reinforcement ratio is less than RHOmin then we must use RHOmin to calculate the minimum rebar.

I remember from the school time (and also saw in "Winter" concrete design) where it says the bw (web width) must be used for min reinforcement calculation but here is the problem:

If we use the bw then the available flexural strength of the beam will be way lower than the demand. How this could be explained? Why we must use the bw for Asmin?

Thanks

 

[frv]

As,min is just that. A minimum. If you need more, use more. I fail to see the big issue.

If I had to make an educated guess, I'd say that the As, min based on the web width is just for the beam; you'd still have to satisfy the minimum reinforcement for the slab that's acting as a T beam.

 

[BAretired]

frv is quite right. You can use as much reinforcement as you can fit into the web.

BA

 

[JAE]

Well, there are limits to As as well. It's called As(max).

Don't want the concrete to fail first - rather we want the steel to yield first, correct?

 

[Lion06]

It's quite often the case the As,min provides less capacity than the demand required of the beam. As,min is there to prevent sudden failures where the cracking moment has greater capacity than the reinforced section. There is nothing that requires you to use As,min. If you need more steel, use more steel.

JAE - The only thing I would say differently is that there is no As,max anymore. The phi factor changes based on tension-controlled, compression controlled, or transition zone conditions, but there isn't an upper limit on the reinforcement. I know from a practical standpoint the old As,max was 0.75 rho,balanced. I don't think that always falls in the tension controlled condition, though.

 

[frv]

StrEIT..

Did the 08 code modify section 10.3.5?

I don't have 08 in front of me, but 05 places a minimum strain requirement of .004 on non-prestressed flexural members. This implies that at most you can be in the transition region, not compression-controlled region.

 

[Lion06]

No, that's still true. I just forgot about it.

 

[JAE]

Yes, the 05 and 08 versions of 318 limit reinforcement in a different manner (i.e. by limiting/defining strains) but the "max" effect is still there.

 

[nickky]

Thank you everyone for the responses. Although I agree with what all of you are telling but still my question is there. Which width (i.e bf or bw ) must be used?

On the other hand, this is how I solved my problem to be on safe side but yet not to specify extra amount of rebar. If you check ACI-05 clause 10.5.3 it gives the designer the permission not to satisfy the rule of 200/fy if he can provide an As which is 1/3 greater than that required by the analysis. In my case the 1.333xrho(analysis)=.0015 which is much much better than 200/Fy=.0033 (half of it!)

So I used .0015xBf(flange)xd which provides enough flextural strength against the demand yet the amount of rebar is reasonable and it is per CODE. What do you think?

Thanks again everyone.

 

[BAretired]

According to the Canadian Code, CSA A23.3 1994 and also ACI 318-63, the minimum reinforcement in a flanged member is based on the web width, not the flange width.

BA

 

[rapt]

The logic is based on the cracking moment as StructuralEIT explained.

Simplified logic of minimum reinforcement based on percentages of steel are therefore misleading, especially for non-rectangular sections, sections with different concrete strengths or sections with high covers, or with reinforcement at different layers or of different types (eg PT).

The locical way to calculate this is to base it on the cracking moment for the face that is in tension. Normal rule is that the Ultimate Capacity must be > 1.2 Mcr (AS3600, ACI318 PT and BS8110). That is what As,min is based on also.

Then you do not have to worry about what width to use. Calculate the Section moodulus for the shape you have (T in this case) at teh tension face and then calculate a minimum ultimate capacity for that face from the tensile strength of the concrete.

This general rule covers all combinations of the above mentioned variables!

PS Or use RAPT which does it all for you based on this logic!

 

[rowingengineer]

"PS Or use RAPT which does it all for you based on this logic!"

While RAPT is a great program, I would suggest you get to understand the requirements of the codes, such as min Ast/1.2 Muo, before adopting a black box approach. I did all my calcs by hand for the first 3 years before I moved across to RAPT. I am not Suggesting that RAPT was suggesting that you adopted a black box approach this but I sure it would be tempting right now.

However in saying that RAPT does have the best manual for learning a program/concrete. Most others, even if they are more complex, generally give methods of drawing in there help manuals not modelling.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[rapt]

RE,

I did not realise you were one of the converted!!

I agree entirely. In too many cases, I see engineers expecting a design program to apply the the rules of a design code or design methodology for them without knowledge of the basics in the first place. A recipe for a disaster. We always suggest that this knowledge is required to prospective users and trainees but a lot do not listen. if you cannot do it by hand, you should not be using software!

Unfortunately, sometimes codes often provide super simplified solutions (eg Ast,min = .13% in BS8110) and do not explain the limitations or the more refined solution (limitation to that one is basically 20MPa concrete with minimum cover and no restraint, none of which are ever applicable these days so everyone designing in countries using BS8110 and derivatives is grossly under providing for minimum reinforcemennt in slabs (except the RAPT users), and the same problems exists for beams).

More and more often we are getting requests for software that produces a final engineering design drawing from an architectural drawing, without engineering interference in the process. I am still holding out!!

 

[Tomfh]

I use a super simplified check most of the time. Ast,min = 0.25%

 

[rapt]

TomFh,

That would be ok for most slabs. Does not work for many Tbeams, especially with higher concrete strengths (30MPa or more) and especially for top reinforcement (flange in tension)!

 

[hokie66]

rapt,

Are you aware of any actual slab failures attributed to this problem? The ACI and Australian provisions are different, but still wouldn't provide enough reinforcement to meet the theoretical requirement with higher strength concrete. Solution: don't use high strength concrete in slabs. Raises the question: if a slab is specified to be 32 MPa, but is actually 60 MPa, is that reason for rejection? I think not.

 

[rowingengineer]

RAPT,
At the risk of taking over this thread, I need to express an opinion or two.
1st. I am half converted, I use a general 3D program to do lateral analysis combined with RAPT when the structure needs it.
2nd. hold out, hold out, hold out. I am almost at the point of shooting someone who puts the next design in front of me that doesn’t understand how the computer came up with the drawings. How is the computer meant to know that you need reo over you column for punching shear, it has no idea.
3rd. I would support the development of a 3D modeller that you can derive strips from and do lateral analysis (mind you the help manual would need to be good).


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[asixth]

Calculate the cracking moment (Mcr) of the T-section (fcf*Z...where fcf is the tensile capcity of the concrete and Z is the elastic section modulus. The elastic section modulus will differ for positive and negative moments because the cross-section is not symetrical). Multiply the cracking moment by 1.2 and this will the miminum design moment for critical regions. Critical regions are generally referred to regions where the face is in tension. Generally speaking, reinforcing 2-3% of the cross-section in the tension face will satisfy this requirement.

I think a 3D renderer of the individual frames would be helpful for visualization of the entire structure. An example where that would be helpful is cross-panel deflections. A 3D modeller for lateral analysis would also be helpful to account for torsional moment that arises from unsymetrical stiffness. Quite ofter these moments can become quite significant.

And personally, I never use a computer program without verifyng the design by hand calcs.

 

[rowingengineer]

“Raises the question: if a slab is specified to be 32 MPa, but is actually 60 MPa, is that reason for rejection?”

I think the slab would go close to qualifying as unreinforced, if all you had to provide is minimum reo.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[hokie66]

asixth,
Did you mean 0.2 to 0.3%?

RE,
Huh?