Which one is preferable? Multiple layers of reinforcements or one layer of reinforcement?

Question

For RC beams, which one is preferable to be designed for construction? Is it multiple layer of reinforcements or one single layer? and why?

Eg. If the beam required 3T20, is it preferable one layer of 3T20 or multiple layers: 2T16+3t16?

Thanks

Answer 1

On average, it's best to limit the number of layers of rebar. There are a few reasons for this.

Firstly, the more layers you have, the more complex the construction of the beam becomes. The first layer of rebar is easy to place: it simply rests on the stirrups and is held in place with some tie wire. All other layers, however, require the placement of plastic supports and more tie wire. So, in terms of ease of construction, the fewer layers you have, the better (with an especially big difference between one and two layers).

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Another constructibility difference is in pouring the concrete. The more layers of rebar you have, the harder it becomes to properly vibrate the concrete. Theoretically, so long as the horizontal spacing is sufficient, the number of layers shouldn't be much of a problem, but the more layers you have, the greater the odds that the vibrator will accidentally touch the rebar (which it shouldn't).

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Also, the more layers you have, the more you risk segregating your concrete's aggregates. This is because the rebar will behave like a "Plinko board". This allows smaller aggregates to reach the bottom of the beam faster than the coarse one, as can be seen in this video (though to a lesser extent, since the rebar will hopefully be vertically aligned). Obviously, this effect is relatively small unless you have many, many layers.

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And then there's the difference in strength efficiency between many or few layers. The more layers you have, the less efficient your reinforcement becomes. That's because the beam's strength is proportional to its effective depth, which is the distance from the steel's center of gravity to the opposite fiber.

If you have a single layer of rebar, then the effective depth $d$ is equal to:

$$d = h - c - \phi_s -\frac{1}{2}\phi_l$$

where $h$ is the beam's height, $c$, the concrete cover, $\phi_s$, the stirrup's diameter, and $\phi_s$, the longitudinal steel's diameter. If you have more layers, then the effective depth becomes:

$$d = h - c - \phi_s - \dfrac{n}{2}\phi_l - \dfrac{n-1}{2}d_l $$

where $n$ is the number of layers and $d_l$ is the vertical distance between faces of rebar.

So, the fewer layers you have, the more efficient your reinforcement becomes (requiring less area to get the same strength).

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Having more layers, however, does offer some advantages. It implies in using more bars of smaller diameter. This usually means your adopted steel area is closer to the necessary steel area (making your layout more efficient). For instance, if you calculate that you need 7.8 cm² of steel, you can choose between one layer of $4\phi16$ (8.04 cm²) or two layers of $10\phi10$ (7.85 cm²) (5 bars per layer). The smaller step increment in area when working with smaller diameters can be an advantage. In the specific case you mentioned, though ($3\phi20$ or $5\phi16$), this doesn't apply, since $3\phi20$ has a smaller total area (9.42 cm²) than $5\phi16$ (10.05 cm²).

Also, while your design may be more efficient with more bars of smaller diameter, these efficiency measures are really just financial measures: "how much money are you wasting by using more steel than is needed?" One question which must be raised is whether the labor costs of placing so many more smaller bars (which must each be placed individually) will overrun any benefit from using less material. That's a question I can't answer since it is entirely dependent on the local market conditions (and I wouldn't know how to calculate it even if I had the information).

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Another advantage of using more bars of smaller diameter is crack control. Using more bars of smaller diameter increases the surface area available to transfer force between the steel and the concrete, therefore reducing the tensile stresses in those regions and therefore reducing the cracking. For example, $28\phi10$ (21.98 cm²) and $11\phi16$ (22.11 cm²) have basically the same cross-sectional area (error of 0.5%). However, $28\phi10$ has a total perimeter of 87.96 cm, while $11\phi16$ has only 55.29 cm, almost 40% less.

So, if crack control is essential to your design, using more bars of smaller diameter may be interesting.

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It should be noted that the two advantages I just gave for using smaller diameter bars have nothing to do with the number of layers, however. So, even if one wants to make use of these advantages, the bars should be placed so as to minimize the number of layers.