Unit Weight of Steel: Definition and Calculation
Steel is a ductile material commonly found on construction sites, and engineers need to determine the unit weight of steel in their designs. So what is steel? Steel is iron with the addition of carbon and other metallic elements in small amounts. These additional materials change the properties of steel. Steel is magnetic just like cobalt, nickel, and iron.
What is The Unit Weight of Steel?
The weight of steel is calculated as follows
Weight = Density x Volume
The unit weight of steel density is also defined as the ratio of its mass to volume. Specifically, it is 7850 kg/m3 or 78.5 kN/m3, or 8050 kg/m3 or 80.5 kN/m3.
The density of steel ranges from 8050 kg/m3 to 7850 kg/m3, depending on the type and grade of manufacture.
However, the unit weight of steel in design standard EN 1991-1-1 Table A.4 is commonly used as 78.5 kN/m3 with a bulk density of 7850 kg/m3. It is also used by civil engineers in other countries as a density for steel.
The most common diameters of steel bars in the construction industry are 40mm, 32mm, 25mm, 20mm, 16mm, 12mm, 10mm, 8mm, and 6mm.
Weight of Steel Bars
Bar size diameter (mm) | Perimeter (mm) | Weight of Bar | |
Kg/ft | Kg/m | ||
6 | 18.85 | 0.728 | 0.222 |
8 | 25.1 | 1.296 | 0.395 |
10 | 31.4 | 2.031 | 0.619 |
12 | 37.7 | 2.913 | 0.888 |
16 | 50.2 | 5.180 | 1.579 |
20 | 62.8 | 8.100 | 2.469 |
25 | 78.5 | 4.554 | 1.388 |
32 | 100.5 | 20.712 | 6.313 |
40 | 125.6 | 32.362 | 9.864 |
How to Calculate the Unit Weight of Steel: Steel Weight Formula
The density of steel = 7850 kg/㎥
Weight of steel per meter = density x volume
Where, volume = length x area
Where, length = 1000 mm
And, area = π/4 x d2
Therefore, the weight of steel per meter = 1000 x π/4 x d2 × 7850 kg/m3
= 0.785 × 1000 × d2 × 7850 kg / (1000 × 1000 × 1000) (meter = 1000 mm)
= 785 × 0.00000785 x d2
= d2/162 kg/m
Now let’s understand calculating the unit weight of steel better by taking two examples of 8 mm and 16 mm steel bars.
Steel Weight Per Meter of 8 mm Bars
Number of steel bars = 5
Diameter of the steel bar = 8 mm
Length of the bar = 1 m
Density of steel = 7850 kg/m3
Therefore,
Area of circular bar = π/4 x d2 = π/4 x 8^2 = 50.27 mm2
We know,
Volume = area x length = 50.27 x 1000 = 50270 mm3
Therefore, the steel weight per meter of a 8 mm bar = 50270/(10^9) x 7850 = 0.395 kg/m
Note: To convert mm3 to m3, multiply it by 1 x 10^9 (1,000,000,000)
Steel Weight Per Meter of 16 mm Bars
Number of steel bars = 5
Diameter of the steel bar = 16 mm
Length of the bar = 1 m
Density of steel = 7850 kg/m3
Therefore,
Area of circular bar = π/4 x d2 = π/4 x 16^2 = 201.06 mm2
We know,
Volume = area x length = 201.06 x 1000 = 201061 mm3
Therefore,
The steel weight per meter of a 16 mm steel bar = 201061 /(10^9) x 7850 = 1.578 kg/m
Note: To convert mm3 to m3, multiply it by 1 x 109 (1,000,000,000)
A Few Words About The Uses of Steel Bars
Steel bars are a commonly found in reinforced concrete, that takes the tensile stresses in concrete because concrete is not good in handling tension loads, unlike compressive loads. Reinforced concrete and steel bars are used in the construction of slabs, columns, dams, beams, and other such structures.
The density of concrete with no steel bars is 2400 kg/m3. But when we calculate the unit weight of steel in concrete, we increase the density of concrete to 2500 kg/m3 to take into account the steel bar weight.
A Final Word: What is The Yield Stress of Steel bars?
Yield stress or YS is the stress level at which steel bars begin to permanently deform. At this point, the bars cannot retune to their original size or shape even if the applied stress is released. Usually, YS is 500 N/mm2. To determine the yield strength of steel bars, you must refer to manufacturers and national technical standards. The yield strength of concrete steel bars ranges from 400 to 600 MPa (N/mm2).
