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.

 

[ticas]

Imagine a column and continuous beam horizontal on left and right side of the column. The middle of the left beam (and right beam) has secondary beam perpendicular to it (so the floor slabs would be only 4" in thickness). The top 7 bars at support at column came out after calculations of the moment requirements at the support. Would bundling the 7 bars in two would be good idea or can I just add another bar to make it 8 and stack it up and down with vertical distance of 25mm. But before I do this. I want to know how bundled bars become poorer in the concrete connection to the steel.

 

[hokie66]

Spread some of the bars into the slab...but a 4" slab is tight. Why are your concrete dimensions so skinny? As to your query about bond of steel to concrete, I thought it would have been obvious that with bundled bars, there is not as much contact area available, as the concrete can't get into the area between the bars.

 

[ticas]

100mm or 4" thick slab is thick already. The span is only short that is why it is not thick (because of the secondary beam). In the columns I designed. I used 3 bundles (20mm) at corners. How bad is it? Concrete is already poured into it,

 

[hokie66]

You should still be able to spread some bars into the slab. How large is the column? When you talk about bundling bars in both columns and beams, I hope they don't clash.

 

[BAretired]

You told us earlier it is not a tee beam. Now you are saying it is.

To keep it symmetrical, you could place 5-20M inside the beam and 1-20M in the slab on each side. Or you could place 3-20M inside the beam and 2-20M in the slab on each side. I prefer the second option because it provides better opportunity for placing and vibrating concrete around the bars.

BA

 

[ticas]

not it's not a tee beam, just normal beam with floor slab connected to it. The column is 0.5x0.5. No problem with the calculations. my main concern now is I use 3 bundled bars (of three 20mm) at each corners. This is not good, how bad?

 

[hokie66]

Your beam IS a tee beam. That is how you define a tee beam. It is a beam with a monolithically placed slab on each side. You can distribute some of the reinforcement into the slab, as BA has outlined. I am not going to say how bad bundling the beam bars would be, as I just wouldn't do it. Glad to see that your column is bigger than the beam, which allows for no clashing at the joint.

 

[ticas]

My beam is a t-beam since slabs are monolithically connected to it. But is it common to put the beam reinforcement outside in the sides (of the t-beam)? I never use this. There may be some performance effect or calculation inaccuracy using this method. Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this?

 

[BAretired]

But is it common to put the beam reinforcement outside in the sides (of the t-beam)? Yes. You can provide reinforcement within the flange width of the T-Beam which is 12 times slab thickness (i.e. within a 48" strip).
I never use this. There may be some performance effect or calculation inaccuracy using this method. Not true. It is good practice. Some engineers include the temperature reinforcement in the slab for the width of the T-beam flange as part of the top reinforcement for the beam.
Also remember the 7 bars have 2 as continuous 10 meter bars (left and right beams of column) and 5 pcs as extra continuous L/3 bars at supports (top bars). Ten meters is a long bar to handle. You could use bars from each beam and lap them at the column. That gives you four top bars. You need three more, say one at center and one in the slab on each side.
So you are saying the 5 pcs can be only 3 at the beam and 2 distributed in the sides? How does this affect the performance. How common do people do this? Not very common. It is more common to lap the bars as described above.

BA

 

[ticas]

BARetired, you said earlier "Our code in Canada, A23.3 permits bundled bars but there are special requirements for developing the bar strength in a bundle". Can you just give a clue what's the special requirements?

By the way, when you lap bars, it has same appearance as bundled bars! So you don't bundle bars yet you lap bars and they look the same.

 

[BAretired]

If you are using the ACI Standard, I would think that the requirements for bundled reinforcement are spelled out in detail. I suggest you read them.

In CSA A23.3, tension lap splices require 30% more development length than that required to develop either bar. So if you know your theoretical cutoff point, you must add 1.3 times the necessary development length to each of the lapped bars.

BA

 

[ticas]

BARetired wrote: "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.".

I'll choose this option of two layers. Only 4 bars can fit at the top of the 0.3m width due to the stirrups 10mm diameter and Four 20mm. Assume the depth of the beam is 0.5m. Putting the second layer 25mm below the first one is like having a beam depth combination (and moment capacity) of 0.5m and 0.475m, isn't it. Know any rule of thumb of knowing whether to adding one 20mm bar to the second layer is enough to compensate?

 

[TXStructural]

The longitudinal reinforcement will not be all the way into the corner of stirrups. They will ride the radius, moving them in or down/up, changing the available depth or width, or both.

I seldom see bundled bars. The main focus for development is to be sure of proper consolidation and good concrete mix design. Using a polycarboxylate to help the paste flow and keep aggregates in suspension with more fluid concrete. The interlock of deformations on one bar to those on the adjacent bar work with the paste and with the surrounding concrete to develop the reinforcement.

With any very large bar or bundle (i.e., large area of steel) the surrounding concrete is asked to do quite a lot. The local compressive and tensile forces in the concrete can be quite high, including splitting forces.