rect reinforced concrete beam with one main bar only

huangyhg

rect reinforced concrete beam with one main bar only
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
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.
besides being able to tie the rebar cage together, it is also important to re  
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
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?
as marcbse points out, another reason for a greater quantity of smaller bars is for crack control.
you should be able to get (2) #3 to (2) #5 into a 6" section.  this is assuming #3 stirrups and 1.5" clear.
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!
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.
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.
single bar in center of 6" x 6" fence post.