Steel Required for Beams, Columns, Slabs and Foundation 

In the realm of construction, the use of steel is indispensable for ensuring the structural integrity and durability of various components such as slabs, foundations, beams, and columns. Steel’s exceptional tensile strength, flexibility, and bonding ability with concrete make it an ideal reinforcement material, complementing concrete’s compressive strength.

Accurate calculation of steel quantities is crucial for maintaining the load-bearing capacity and overall stability of structures. This blog explores the importance of steel calculation for slab in construction, delving into the methods of steel calculation for slab, beams, and different structural elements to ensure robust and long-lasting buildings.

 

Slabs

In construction, slabs are flat, horizontal structural elements made of concrete, commonly used in floors, ceilings, and roofs. They distribute loads uniformly to supporting beams, columns, or walls. Slabs can be categorised into different types, one-way or two-way, based on their reinforcement and load distribution patterns. Two-way slabs transfer loads primarily in one direction, while two ways distribute loads in two perpendicular directions. Slabs can be cast in situ (on-site) or prefabricated (precast). Proper slab design and construction ensure structural integrity, load-bearing capacity, and resistance to cracking and deflection. Slabs also provide insulation, fire resistance, and soundproofing.

 

Why is steel Used in Slabs?

Steel is used in concrete slabs primarily for its tensile strength, which complements the compressive strength of concrete. This combination creates a composite material that can handle various loads and stresses. Here are the key reasons why steel is used in slabs:

Tensile Strength: Concrete is strong in compression but weak in tension. Steel reinforcement (rebar) provides the necessary tensile strength, preventing cracks and structural failure.

Flexibility and Ductility: Steel’s ability to bend without breaking (ductility) allows it to absorb and distribute loads more effectively, enhancing the slab’s overall durability.

Bonding with Concrete: Steel forms a strong bond with concrete, ensuring that the two materials act together as a single unit, which is crucial for the load-bearing capacity and structural integrity.

Thermal Compatibility: Steel and concrete have similar coefficients of thermal expansion, meaning they expand and contract at similar rates with temperature changes, reducing the risk of cracking.

Fire Resistance: Steel reinforcement within concrete slabs provides better fire resistance than other materials, contributing to the structure’s overall safety.

Construction Efficiency: Steel rebar can be easily fabricated and installed, speeding up the construction process and ensuring precision in reinforcing slab designs.

 

Calculating Steel Quantity for a Slab

The quantity of steel required for a slab is determined by several factors, including the slab’s dimensions and the steel’s density. The basic formula for estimating steel quantity in a slab involves the following steps:

1. Determine the Volume of Concrete: First, calculate the volume of concrete used in the slab. This is done by multiplying the length, width, and thickness of the slab.
2. Apply the Steel Quantity Formula: The quantity of steel typically required for a slab is around 1% of the total volume of concrete. This percentage can vary based on design specifications and structural requirements.
3. Calculate Steel Weight: To get the weight of the steel reinforcement, multiply the volume of concrete by 1% and then by the density of steel, which is approximately 7850 kg/m³.

 

Example Calculation

For a practical example, let’s assume the volume of concrete in a slab is 7850 cubic meters. Using the formula:

• Quantity of Steel = 1% of the Volume of Concrete

So,

• Quantity of Steel = 0.01 * 7850 = 78.50 kg

This means for a concrete slab with a volume of 7850 cubic meters, approximately 78.50 kg of steel is needed.

 

Steel For Foundation

Foundations are the critical base upon which any structure stands, providing essential support and stability. Using steel in foundations is vital for enhancing their load-bearing capacity and resisting tensile forces. Reinforced cement concrete (RCC) is commonly employed for foundation construction, incorporating reinforcing steel bars (rebars) to improve tensile strength and overall durability.

 

Calculating Steel Quantity for Foundation

To accurately determine the steel required for a concrete foundation, follow these steps:

1. Measure Dimensions: Measure the length, width, and thickness of the foundation in meters.
2. Calculate Volume: Multiply these measurements to get the volume of the foundation in cubic meters.
3. Apply Steel Percentage: Multiply the volume by 0.5% (a standard percentage used for foundations).
4. Factor in Steel Density: Multiply the result by the density of steel, which is 7850 kg/m³, to get the total steel quantity in kilograms.

 

Example Calculation for Foundation

Let’s consider a concrete footing with the following measurements:

• Length: 4 meters
• Width: 2.5 meters
• Thickness: 0.3 meters

Step-by-Step Calculation

1. Calculate the Volume:
   Volume = Length x Width x Thickness
   Volume = 4 m x 2.5 m x 0.3 m = 3 cubic meters
2. Calculate the Steel Quantity:
   Steel required = Volume x 0.5% x Density of steel
   Steel required = 3 cubic meters x 0.5% x 7850 kg/m³
   Steel required = 0.015 x 7850 kg/m³
   Steel required = 117.75 kilograms

Therefore, for a concrete footing with the specified dimensions, approximately 117.75 kilograms of steel would be required.

 

Beam & Column

Beams are horizontal members that carry vertical loads, transferring them to vertical supports like columns. They are crucial in supporting floors, roofs, and walls, ensuring the structural integrity of a building by resisting bending forces.

Columns are vertical members designed to transmit loads from the structure above to the foundation below. They primarily bear compressive loads and are integral to maintaining the building’s stability, resisting buckling and bending.

These components are often made of reinforced concrete to provide the necessary strength and durability for various construction applications​.

Calculation for Beams:

Beams are structural elements that primarily resist loads applied laterally to the beam’s axis. The main reinforcement in beams typically includes both longitudinal bars and stirrups.

Steps to Calculate Steel Quantity for Beams:

1. Bar Bending Schedule (BBS): Prepare a BBS, listing all the shapes and sizes of bars from the design drawings.
2. Count the Bars: List and count all the bars as per the BBS.
3. Calculate Cutting Length: Use the formula:
   Cutting length of bar = (length of member – cover on both sides) + development length.
4. Unit Weight Calculation: The unit weight of the bars can be calculated using:
   Weight (kg/m)=d2162\text{Weight (kg/m)} = \frac{d^2}{162}Weight (kg/m)=162d2​
   where ddd is the diameter of the bar in mm.
5. Total Weight: Multiply the number of bars, cutting length, and unit weight to get the total steel quantity.

Example Calculation for a Beam:

• Beam Dimensions: Length = 6 m, Width = 0.3 m, Depth = 0.5 m
• Main Reinforcement Bars (Diameter 16 mm):
  Number of bars = 4, Cutting length = 5.8 m (assuming 0.1 m cover on both sides) Unit weight = 162162\frac{16^2}{162}162162​ = 1.58 kg/m Weight of bars = 4 * 5.8 m * 1.58 kg/m = 36.688 kg
• Stirrups (Diameter 8 mm):
  Number of stirrups = 10, Cutting length per stirrup = 1.6 m Unit weight = 82162\frac{8^2}{162}16282​ = 0.395 kg/m Weight of stirrups = 10 * 1.6 m * 0.395 kg/m = 6.32 kg
• Total Steel Required: 36.688 kg + 6.32 kg = 43.008 kg

 

Calculation for Columns:

Columns are vertical structural elements that transfer loads from slabs and beams to the foundations. The reinforcement in columns consists of longitudinal bars and ties.

Steps to Calculate Steel Quantity for Columns:

1. Bar Bending Schedule (BBS): List all shapes and sizes of bars from the design drawings.
2. Count the Bars: Note the number and type of bars required.
3. Calculate Cutting Length: Use the formula:
   Cutting length of bar = (height of column – cover on both sides) + development length.
4. Unit Weight Calculation: Use:
   Weight (kg/m)=d2162\text{Weight (kg/m)} = \frac{d^2}{162}Weight (kg/m)=162d2​
5. Total Weight: Multiply the number of bars, cutting length, and unit weight to get the total steel quantity.

 

Example Calculation for a Column:

• Column Dimensions: Height = 3 m, Width = 0.4 m, Depth = 0.4 m
• Main Reinforcement Bars (Diameter 20 mm):
  Number of bars = 6, Cutting length = 2.8 m (assuming 0.1 m cover on both sides) Unit weight = 202162\frac{20^2}{162}162202​ = 2.47 kg/m Weight of bars = 6 * 2.8 m * 2.47 kg/m = 41.496 kg
• Ties (Diameter 10 mm):
  Number of ties = 10, Cutting length per tie = 1.6 m Unit weight = 102162\frac{10^2}{162}162102​ = 0.617 kg/m Weight of ties = 10 * 1.6 m * 0.617 kg/m = 9.872 kg
• Total Steel Required: 41.496 kg + 9.872 kg = 51.368 kg

 

Conclusion

Steel is a vital component in the construction of slabs, foundations, beams, and columns due to its tensile strength and compatibility with concrete. Proper calculation of steel quantity ensures structural integrity and durability. For slabs, the steel quantity is typically 1% of the concrete volume. In foundations, around 0.5% of the volume is used, while beams and columns require specific bar bending schedules (BBS) to determine the exact steel needed based on design specifications. Accurate calculations involve considering dimensions, reinforcement types, and steel density, ensuring a robust and long-lasting structure.