Rect Reinforced Concrete Beam with One Main Bar only
Rect Reinforced Concrete Beam with One Main Bar only
(OP)
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
Regards
Richard






RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
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
RE: Rect Reinforced Concrete Beam with One Main Bar only
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?
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
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!
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
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.
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
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!!
RE: Rect Reinforced Concrete Beam with One Main Bar only
RE: Rect Reinforced Concrete Beam with One Main Bar only
I use BS 8110 but searching for other codes or reference on the subject matter irrespective of whatever practices or countries. Thank you!
RE: Rect Reinforced Concrete Beam with One Main Bar only
From the width of your members, it would seem that you are working with a joist, but definiing the qualifications of the US codes would be easier by purchasing them from www.concrete.org
If your code has no similar reference, having the US code could help justify or explain your reasoning.
Best of luck,
Daniel Toon
RE: Rect Reinforced Concrete Beam with One Main Bar only
If this is a continuous beam then be careful of the slenderness - there should be a limit in the code.
csd
RE: Rect Reinforced Concrete Beam with One Main Bar only
Dik
RE: Rect Reinforced Concrete Beam with One Main Bar only
For a 6" wide beam a single bar is close enough to each of the side faces to ensure crack control. Refer BS8110; Section 3.12.11.2 .
RE: Rect Reinforced Concrete Beam with One Main Bar only
It is a continuous beam satisfying the code limit of the slenderness for beam.