Steel is a widely used material in construction due to its versatility. It primarily consists of metallic iron and non-metallic carbon, with trace amounts of elements like silicon, nickel, manganese, chromium, and copper. This combination forms an alloy, and even though steel is mostly composed of iron, these other elements significantly impact its properties.

The unit weight of steel, also known as weight density, is the ratio of the weight of steel to its volume. Typically expressed in kilograms per cubic meter (kg/m³), it can also be represented as 7.85 g/cm³, 78.5 KN/m³, or 489.84 lb/ft³. This value, 7850 kg/m³, is commonly used for calculations.

Steel is classified into different types based on its chemical composition:

- Mild Steel
- Medium Carbon Steel
- High Carbon Steel
- Low Alloy Steel
- High Alloy Steel

Among these, the first three are known as structural steel, and they are widely used in building structures.

In accordance with the Indian standard IS:800-1984 (Code of Practice for General Construction in Steel), the following types of structural steels are recognized:

- IS:226-1975 Structural Steel (Standard Quality)
- IS:1977-1975 Structural Steel (Ordinary Quality)
- IS:2062-1984 Weldable Structural Steel
- IS:961-1975 Structural Steel (High Tensile)
- IS:8500-1977 Weldable Structural Steel (Medium and High Strength Qualities)

Steel conforming to IS:226-1975 is suitable for a wide range of structures subject to static, dynamic, and cyclic loading. It is also suitable for welding up to 20 mm thickness. Here are some physical properties of mild steel:

- Density or Unit Weight of Steel: 7850 kg/m³
- Young’s Modulus of Elasticity (E): 2.04 x 10^5 MPa (or N/mm²)
- Modulus of Rigidity (G): 0.785 x 10^5 MPa (or N/mm²)
- Poisson’s Ratio (µ): 0.3 (inelastic range)
- Coefficient of Thermal Expansion or Contraction: 12 x 10^-6 per °C or 6.7 x 10^-6 per °F

Different diameters of steel bars are utilized in construction work, ranging from 8 mm to 40 mm. Here’s a breakdown of their typical applications:

- 8 mm: Stirrups for beams and columns, distribution bar for slabs and other structural members.
- Greater than 8 mm: Main bar for various structural elements like foundations, columns, and beams.

To calculate the unit weight of steel bars, we employ the formula: Weight = Density × Volume, where Volume is derived from Area × Length. For instance:

**Example:** Diameter of Bar = 10 mm, Length = 1 meter

- Area of Circular Bar = π/4 × d² = 3.14/4 × 10² = 78.5 mm²
- Volume = Area × Length = 78.5 × 1000 (1 meter = 1000 mm) = 78500 mm³

Unit Weight of 10 mm Steel Bar = (7850/1000 ×1000 ×1000) × 78500 = 0.616 kg/m (1 kg/m³ = 0.000000001 kg/ mm³)

Similarly, the unit weight of square and rectangular bars can be calculated.

Given that the density of steel is 7850 kg/m³, we derive the formula:

Weight of steel per meter = Volume × Density [ Volume = Area × Length ]

= π/4 x d² × 1000 × 7850 kg/m³ (length = 1000 mm)

= 0.785 × d² × 1000 × 7850 kg / (1000 × 1000 × 1000) [ 1 meter = 1000 mm ]

= 785 x d² × 0.00000785

= d²/1 × 0.00616225 /1

= d²/1/0.00616225

= d²/162.27 ≈

= d²/162 kg/m (say)

So the Formula for unit weight of steel in kg/meter is = d²/162

Unit Weight of steel bar per feet = d²/(162.27 x 3.28084) (1 meter = 3.28084 feet)

= d²/532.38 ≈

= d²/533 (say)

= d²/533 kg/feet

By using the formula d²/162 (where d is the diameter of the steel bar), we can determine the unit weight of the steel bars, providing the weight in kg per meter. The formula d²/533 yields the weight in kg per foot.

Below are the unit weights for various steel bar diameters:

Diameter (mm) | Unit Weight (kg/m) |
---|---|

6 | 0.222 |

8 | 0.395 |

10 | 0.617 |

12 | 0.888 |

16 | 1.58 |

20 | 2.469 |

25 | 3.858 |

32 | 6.32 |

40 | 9.876 |

These values represent the weight of the respective steel bars per meter. If you require the weight per foot, simply use the formula d²/533.