Rect Reinforced Concrete Beam with One Main Bar only

[dcnnng]

For a rectagular reinforced concrete beam, what are the reasons that normally we provide 2 main bars? Can we just use only one main bar with the same area and any danger? Thank you!
Regards
Richard

 

[Lion06]

It is unlikely that one bar will give you the required are of steel unless you are using a #14 or something ridiculous. If you do use something that large you really need to pay close attention to your develpoment length.
Additionally, you need (2) bars at the bottom and (2) at the top just to build the rebar cage which includes the stirrups. You wouldn't want the stirrups around a single bar.

 

[MarcbSE]

Besides being able to tie the rebar cage together, it is also important to remember that the concrete beam performs better against cracking with more smaller bars rather than fewer larger bars.

 

[dcnnng]

Hi StructuralEIT,
Physically and practically, there is no problem to provide with one bar of the same or larger area and no problem for providing adequate anchorgae or lap length. Of course if one main bar is used then we also provide one hanger bar for the stirrups. Similarly, rectangular stirrups are provided for the shear. I am concerned if adopting one main bar only, the corner of the section would crack and ohter problem would arise!! May I have other experts' advice please. Thank you

 

[dcnnng]

Hi MarcbSE,
Yes, I agree that the concrete beam performs better against cracking with more smaller bars rather than fewer larger bars.e.g. I expect that theer would be carck if we provide only one main bar for say 12"(300mm) width beam. In fact I am referring to more specific narrow beam with width of some 5"-6" (125-150mm), would there be any problem? Any literature that I can refer to?

 

[Lion06]

as MarcbSE points out, another reason for a greater quantity of smaller bars is for crack control.

 

[Lion06]

You should be able to get (2) #3 to (2) #5 into a 6" section. This is assuming #3 stirrups and 1.5" clear.

 

[dcnnng]

Hi StructuralEIT,
Yes, we have been pratising putting 2 smaller bars and stirrup for 5"-6" beams though very tight but still manageable with due care and proper vibration of concrete. But we end up with little concrete cover then we would face with durability problem. By using one single larger bar, we can have adequate cover but also cheaper cost in steel and labour. That is why I am seeking advice from experts on this issue. Thank you!

 

[MarcbSE]

5" to 6" - That is a narrow beam. I'm assuming this beam would be for an above grade location that would not require a larger cover distance.

I'm not aware of anything in the code that would prevent the use of a single bar for a beam this narrow, but I would still prefer to use (2) small bars if space allowed.

 

[UcfSE]

You can't get two bars in a 6-inch wide beam with two-leg stirrups unless the bars are tied to the stirrups above the corner bend.

dcnnng, what you are asking about sounds a lot like concrete joists, which may have a single longitudinal bar and single-leg stirrups. Take a look for joists and soffit beam systems.

 

[civilperson]

Single bar in center of 6" x 6" fence post.

 

[Lion06]

UcfSE-
I disagree. If you start with teh 6", assume #3 stirrups, and #5 main bars, and 1" clear between bars, you can get (2) #5 in there while still maintaining 1.5" clear.

[6" - 2(0.375") - 2(0.625") - 1"]/2 = 1.5" which satisfies the minimum for clear cover assuming the concrete is not exposed to weather.

 

[UcfSE]

EIT, you are assuming that the bars will butt up to the stirrups in the corners. The stirrup corners have a radius that means you can't push the bars into the corner of the stirrup loop without losing some depth. You can tie the bars to the side of the stirrup or account for the radius at the corner. Try adding your number again by locating the long bar at the horizontal tangent of the stirrup corner radius.

1.5+0.375+0.75+0.625+1+0.75+0.375+1.5=6.875 >6.

Another method is accounting for the bar being tangent at the center of the corner, as shown in the PCI Design Handbook. This allows less required width but changes the rebar depth slightly. Using those numbers, different from what I reference above, you get

1.5+0.875+0.625+1+0.875+1.5=6.375 >6.

 

[Lion06]

You raise an interesting point. I thought the stirrup could be bent with a tight enough radius that the #5 would fit snugly inside it. Maybe that isn't true. I am looking at tables from 2 different textbooks - 1 is telling me that I need bw of 6" for (2) #5, the other is telling me I need 7".
I am taking an advanced concrete class in grad school right now and when checking cover requirements the professor never consider the radius of the stirrup.
Does anyone else have some insight on this? It isn't a big deal for normal situations, but when you are pushing the limits of clear spacing and cover, this can become an issue.

 

[UcfSE]

See also the CRSI Manual.

 

[BurgoEng]

could you use fiber reinforced concrete instead of traditional rebar?

 

[Lion06]

BurgoEng-
For a beam?
We haven't come to any sort of consensus as to whether that is even acceptable for T&S steel only. I know I would certainly not feel comfortable using that for any sort of structural application in lieu of reinforcing steel.

 

[civilperson]

Professors of reinforced concrete never tied rebar with 6 bar diameters for bend diameter. CRSI is correct and the corner bars are usually not in line with the others in that face unless pulled over to the tangent point.

 

[Taro]

ACI 318 section 7.13 is a good reference. For joists conforming to section 8.11, one bar is acceptable. For beams, two bars is the minimum.

 

[dcnnng]

Dear All,
In this case , I am not considering using alternative such fibre reinforced concrete nor bent up bars. In ohter words, we shall end up with inadequate cover for using 2 bar approcah and not sure how it would perform by using one bar.

Taro has raised interesting points based on ACI 318 section 7.13, for joists conforming to section 8.11, one bar is acceptable and for beams, two bars is the minimum.

I don't have ACI318, can you all experts commend on Taro's reference please. If one bar approach is not acceptable, could you please also state the reasons and reference please.

Thank you all!!