Original author: Zhang Xiangrong
Whether the steel bar is calculated according to the length of the skin, or calculated according to the length of the centerline, has been debated for a long time, until now there is no standard answer, some areas have clear regulations on the quota, some areas do not, many budgeters are confused, in the end according to what length to calculate?
Today's content, I still can't give you a standard answer, but I can give you a judgment of the standard, whether it is the length of the skin, or the length of the centerline, you choose according to your position.
— １—
What is the difference between the length of the skin of the reinforcement and the length of the centerline?
If a rebar is straight, the length of the outer skin and the length of the centerline are equal, as shown in Figure 1.
Figure 1
Therefore, straight section rebar does not have the outer skin length and the centerline length, only the bending steel bar or the curved hook rebar, there is the outer skin length and the centerline length.
(1) Several bending methods of steel reinforcement in construction
There are three main forms of bending of steel reinforcement in construction, as shown in Figure 2.
Figure 2
This is the main form of bending of steel reinforcement in construction, of course, there are other bending methods, but very small, we mainly talk about these three.
Let's look at the difference between their outer skin length and centerline length one by one.
(2) The difference between the length of the 90-degree angle outer skin and the length of the centerline
I drew a schematic diagram of the length of the skin at a 90-degree angle and the length of the centerline, as shown in Figure 3.
Figure 3
In this way, the length of the outer skin and the length of the center line are not equal, which is large, that is small, we will calculate it later.
(3) The difference between the length of the outer skin of the 135-degree curved hook and the length of the centerline
This is a schematic diagram of the length of the 135-degree angle outer skin and the length of the centerline, as shown in Figure 4.
Figure 4
At a 135-degree angle, the length of the centerline and the length of the outer skin are not equal, and it is obvious that the length of the centerline is greater than the length of the outer skin, which can be seen by the naked eye.
(4) The difference between the length of the outer skin of the 180-degree curved hook and the length of the centerline
This is a schematic diagram of the length of the 180-degree angle outer skin and the length of the centerline, as shown in Figure 5.
Figure 5
At a 180-degree angle, it is clear that the length of the centerline and the length of the outer skin are not equal, and the length of the centerline is greater than the length of the outer skin.
— 2—
What is the difference between the length of the rebar skin and the length of the centerline?
(1) What is the difference between the outer skin length and the centerline length of the 90-degree angle reinforcement?
Let's take the schematic, as shown in Figure 6.
Figure 6
Outer skin length = R + d, here is the inner skin of the bending radius, not the center line, d is the diameter of the reinforcement,
Centerline length = diameter ×π× 90/360
Diameter = (R+d/2) ×2
Centerline length = (R+d/2)×2×π×90/360
Centerline length = (R+d/2)×π/2
Difference = centerline length - outer skin length = (R +d/2) ×π/2-R-d
As long as the bending radius R can be determined, this difference can be calculated.
1) How is the bending radius R determined?
Turning to page 57 of Atlas 16G101-1, there are clear rules on the bending diameter, as shown in Figure 7.
Figure 7
The atlas tells us what a curved diameter is, as shown in Figure 8.
Figure 8
Regarding the bending diameter, the atlas does not give a hard rule, but only uses the vague word "should not".
2) How does the software determine the bending diameter?
When the software is designed, it must give an accurate bending diameter, otherwise the software program cannot be written.
When compiling the software, Guanglianda gave some hard and hard rules for the bending diameter, as shown in Figure 9.
Figure 9
The bending diameter D is determined, the bending radius R can be determined, and below we can calculate the difference between the length of the 90-degree angle outer skin and the length of the centerline.
3) The difference between the outer skin length and the centerline length of the 90-degree angle first-level reinforcement
Let's take the schematic diagram of the calculation, as shown in Figure 10.
Figure 10
When the bending diameter of the first stage steel is D=2.5d, the radius R=1.25d, let's calculate the difference.
Difference = centerline length - outer skin length
=(R+d/2)×π/2-R-d
=(1.25d+d/2)×π/2-1.25d-d=0.499d≈0.5d
Here, the centerline length > the length of the outer skin, and the difference = 0.5d
4) The difference between the outer skin length and the centerline length of the secondary and tertiary steel bars at a 90 degree angle
When the bending diameter of the second and third grade steels is D=4d, the radius R=2d, let's calculate the difference.
=(2d+d/2)×π/2-2d-d=0.927d≈0.93d
Here, the centerline length > the length of the outer skin, and the difference = 0.93d
5) The difference between the length of the outer skin of the 90-degree angle four-stage reinforcement and the length of the centerline
When the bending diameter of the four-stage steel is D=6d, the radius R=3d, let's calculate the difference.
=(3d+d/2)×π/2-3d-d=1.498d≈1.5d
Here, the centerline length > the length of the outer skin, and the difference = 1.5d
(2) What is the difference between the outer skin length and the centerline length of the 135-degree angle rebar?
Let's take the schematic diagram and derive the formula, as shown in Figure 11.
Figure 11
=fe-(R+d)
= Diameter×π×135/360-(R+d)
差值=(R+d/2)×2π×135/360-(R+d)
1) The difference between the length of the outer skin of the 135-degree angle first-level reinforcement and the length of the centerline
Difference
=(R+d/2)×2π×135/360-(R+d)
=(1.25d+d/2)×2π×135/360-(1.25d+d)
=1.873d≈1.9d
Here, the centerline length > the outer skin length, and the difference = 1.9d
2) The difference between the outer skin length and the centerline length of the 135-degree angle of the second and third-level steel bars
=(2d+d/2)×2π×135/360-(2d+d)
=2.89d
Here, the centerline length > the length of the outer skin, and the difference = 2.89d
3) The difference between the outer skin length and the centerline length of the four-stage rebar at a 135 degree angle
=(3d+d/2)×2π×135/360-(3d+d)
=4.25d
Here, the centerline length > the length of the outer skin, and the difference = 4.25d
4) The bending radius affects the summary of the bending adjustment value
Just like the highway, the sheep intestine trail turns a small bend, the first-class highway turns a big bend, the highway turns a larger bend, the driver must see 100 meters ahead, the angle is the same, the radius is not the same. This is shown in Figure 12.
Figure 12
Note that the bend adjustment value is not the bend arc length, but the difference between the arc length and the dimension length. This must be remembered. This is shown in Figure 13.
Figure 13
Bend adjustment value = centerline length - outer skin length.
(3) What is the difference between the outer skin length and the centerline length of the 180-degree angle reinforcement?
Let's take the schematic diagram of the calculation of the 180-degree angle, as shown in Figure 14.
Figure 14
Let's derive the formula.
Outer skin length = R + d
Centerline length = diameter ×π/2
Centerline length = (R+d/2) ×2×π/2
=(R+d/2)×π
Difference = centerline length - outer skin length = (R +d/2) ×π -R-d
1) The difference between the outer skin length and the centerline length of the 180-degree angle first-level reinforcement
=(R+d/2)×π-R-d
=(1.25d+d/2)×π-1.25d-d
=3.248d≈3.25d
Here, the centerline length > the outer skin length, and the difference = 3.25d
2) The difference between the outer skin length and the centerline length of the 180-degree angle two- and three-step rebar
=(2d+d/2)×π-2d-d
=4.854d≈4.85d
Here, the centerline length > the length of the outer skin, and the difference = 4.85d
3) The difference between the outer skin length and the centerline length of the four-stage rebar at a 180-degree angle
=(3d+d/2)×π-3d-d
=3.996d≈7d
Here, the centerline length > the length of the outer skin, and the difference = 7d
(4) Comparison of the difference between the hand calculation and the software calculation
Most of the differences calculated by hand and software are the same, only 180 angles of two- and three-grade steel are a little bit worse, manual calculation is 4.85d, software calculation is 4.86d, I estimate is rounded error, this error can be ignored, everyone can press the value of the software.
Figure 15
— 3—
Precautions when using
In practice, do you choose the length of the outer skin or the length of the centerline?
From the above calculations, we can know that they are all centerline length > skin length, some people have said, I definitely choose the centerline length.
That's not necessarily true, I'll give you an example and you'll see. For example, the difference calculation of the stirrup, as shown in Figure 16.
Figure 16
For the stirrup, the outer skin length is greater than the centerline length.
Therefore, the same project, whether to take advantage of the skin or according to the centerline, is really not certain.
If you use software to calculate, summarize in two ways, you know that it is more advantageous.
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