bundled or layered top bars at support in beam

[ticas]

The design of top bars at support of a 300mm width by 500mm depth rc beam has 7 bars. Is it recommended to put all 7 bars in one layer (top most) and bundled together in two's or distributed in two layers? What do you usually do in your practice? I think distributing it in two layers can diminish the moment capacity but with the advantage of less congestion.

 

[msquared48]

If the bars are no greater than 1/2" in diameter and this is a concrete beam with stirrups, 7 on the bottom will work, but I would consider using pea gravel concrete.

Any larger bars, stack 'em and compute the loss. Probably one extra bar will suffice making it two layers of four stacked.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

You cannot fit 7 bars in a 300mm wide beam in one layer and still provide adequate clearance for placing concrete unless it is a tee beam in which case the bars may be spread out beyond the 300mm width. Two layers would be better.

BA

 

[ticas]

The bars are 20mm 7 pcs in 300mm width beam (not tee beam). If two layers are used, what is usually the separation between the two laters? I usually handle 2 to 3 bars only. Is the moment capacity percentage decrease of two layers significantly great in your experience?

 

[BAretired]

The moment capacity is proportional to the effective depth, d. When two layers are used instead of one, the decrease in effective depth is easily calculated assuming a clearance of 25mm between bars. You should also have a clearance of 25mm horizontally between bars.

Perhaps you should consider using a larger diameter bar. Four 25mm bars will fit nicely in a 300mm wide beam.



BA

 

[ticas]

Maybe the 20mm can just be bundled two together up-down or left right and put in top most position and this is better than having 25mm separation between the top and next lower layer. Have you done this bundling in practice and why not if you do not?

 

[BAretired]

I have never used bundled bars but it is permitted by code so it could be done.

BA

 

[ticas]

Why have you not used bundled bars and preferred putting them in layers? What do you think is the disadvantage of bundled bars?

 

[msquared48]

300 mm = 11.81" width available...

2 X 1.5" clearance = 3"

2 X .375" (#3) stirrups = .75"

7 X .5" (#4) diameter bars = 3.5"

Sum = 7.25"

Net available width = 4.56"

Divided among 6 spaces = .76" between each bar (1.5X the bar diameter but 1" minimum spacing is required between the bars - I forgot that...)

BA is correct here. Stack the bars...



Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

I have also never used bundled bars in a beam. Either use a wider beam, or bigger bars.

Mike, why convert? Sticking with mm is so much easier.

 

[msquared48]

To each his own Hokie..


Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Why have you not used bundled bars and preferred putting them in layers? What do you think is the disadvantage of bundled bars?

Bundled bars were not permitted when I started practicing engineering. Can't remember when bundling was introduced, somewhere around 1980 I think. Never did agree with the concept. Bond between steel and concrete, I believe, is seriously compromised by bundling. In any event, there is no need to do it so why bother?

I have always found when on site, that the concrete placers were not too happy even when I used the spacing of bars permitted by code. There is no point fighting them. Give them plenty of room to pour and vibrate the concrete. There seems to be no problem with one bar for every 4" (100mm) of beam width, so that is what I try to stick to. But in your case, I would go to 4-25M bars.

BA

 

[ticas]

Are you saying even in columns, bundled bars seriously compromised bonding between bars and concrete? or is it only in beams? I commonly bundled the corners of a column

 

[hokie66]

I seem to remember bundling bars in columns...once. But if a column needs that much reinforcement, I generally use higher strength concrete, or a larger column.

 

[ticas]

Why is bundled bars in columns not a problem of bonding between bars and concrete while in beams it is a problem?

 

[hokie66]

I didn't say that. But bars in columns are in compression, while the bars in the beams you mentioned are in tension. Bundled bars in columns are not spliced by lapping, but rather by end bearing or couplers.

 

[ticas]

So the problem only occurs in the spliced sections of bundled bars? 5 of the 7 bars are just extra top bars at support. It doesn't need to be spliced to anything. There is no splice in the beams because 10 meter bars are used. So here bundled bars in beams are not problem as far as connections between bars and concrete are concerned? Or do you still think bundled bars without splice is still not good, and why is that?

 

[BAretired]

In order to be effective, a bar must develop bond with the surrounding concrete. The five bars you mentioned may not be spliced to anything, but they must develop their tensile capacity in bond with concrete, otherwise they don't do much good. Our code in Canada, A23.3 permits bundled bars but there are special requirements for developing the bar strength in a bundle (which I am not going to outline here).

BA

 

[ticas]

of course a bar normally develope bond thru the rigged surfaces of the rebars and the concrete. We used 4000 psi concrete, a single bar with rigged surfaces can bond with it, as is normally and usually the case in general construction. What I can't understand is if the bars are bundled two together, let us say two 20mm. Why is the bond between it and concrete poorer? It is the same 4000 psi concrete and vibrated as concrete is poured to the form works.

 

[hokie66]

Just a question...since we are talking about bundling bars in a 300 wide beam. What is this beam supporting? A section detail of the beam would help, in ways you might not realize.