Footing Design Steps as per IS Code (IS 456: 2000)

The Indian Standard Code (IS 456: 2000) provides comprehensive guidelines for designing footings, ensuring that the structure is safe, durable, and cost-effective. Below are the steps for designing footings as per the IS code. Footing design is a crucial aspect of structural engineering, ensuring that the load of a building is safely transferred to the ground.

Steps for Footing Design as per IS 456: 2000

1. Understanding the Structural and Soil Parameters

Before starting the design, gather the following essential information:

• Load Details: Understand the nature and magnitude of the loads coming from the structure, such as dead loads, live loads, wind loads, and seismic loads.
• Soil Properties: Determine the soil’s safe bearing capacity (SBC), which is critical for deciding the size and type of footing.
• Permissible Settlements: Identify the permissible settlement limits to ensure that the foundation can withstand long-term service without excessive settlement or differential movement.

2. Choosing the Type of Footing

Depending on the load distribution, soil conditions, and type of structure, select the appropriate type of footing:

• Isolated Footing for individual columns.
• Combined Footing when two columns are closely spaced.
• Strip Footing for supporting walls or a series of columns.
• Raft or Mat Footing for heavy loads or weak soils.
• Pile Footing for transferring loads to deeper soil layers.

3. Calculation of Loads on Footing

Calculate the total vertical load acting on the footing, which includes:

• Dead Load (DL): The weight of the structural elements (columns, beams, slabs, etc.).
• Live Load (LL): Loads that vary with occupancy, such as people and furniture.
• Other Loads: Wind, seismic, and snow loads, if applicable.

Total Load = Dead Load + Live Load + Other Loads

4. Determining the Footing Size

Use the formula for the base area of the footing to determine its size:

A= P*1.1/qsafe ​

Where:

• (A) = Area of the footing,
• (P) = Total load on the footing,
• (qsafe) = Safe bearing capacity of the soil.

Ensure the selected area is sufficient to distribute the loads without exceeding the soil’s SBC.

5. Check for Bending Moment

The design of the footing must account for bending moments, which occur due to the loads acting on the foundation. Calculate the ultimate bending moment (M) at the column face using the formula:

M= P/8×(distance between the column face and footing edge)^2

Check the footing’s ability to resist this moment by designing appropriate reinforcement using IS 456: 2000 guidelines.

6. Shear Strength Check

The footing needs to be checked for shear strength to ensure that it can resist the vertical forces acting on it. Two types of shear need to be considered:

• One-way shear (Beam Shear): Checked at a distance of d (effective depth) from the face of the column. The design shear stress, ( \tau_v ), should not exceed the permissible shear stress based on the grade of concrete.
• Two-way shear (Punching Shear): Checked around the perimeter of the column. The critical section for punching shear is located at a distance of d/2 from the column face.

Punching Shear= P/ Perimeter of critical section × Effective depth ​

Ensure the shear stresses are within the allowable limits as per IS 456.

7. Design of Reinforcement

Once the bending moment and shear strength checks are done, calculate the reinforcement required:

• Main Reinforcement: Determine the amount of steel required to resist the bending moment in the footing. The steel should be provided at the bottom of the footing slab. The area of reinforcement is calculated using the formula:
  As​=M/(0.87×fy​×d)
• (As) = Area of steel required,
• (M) = Ultimate bending moment,
• (fy) = Yield strength of steel,
• (d) = Effective depth of the footing.
• Distribution Reinforcement: This is placed perpendicular to the main reinforcement to ensure proper load distribution. The amount of distribution reinforcement is typically 0.12% to 0.15% of the concrete cross-sectional area.

8. Check for Development Length

Ensure that the provided reinforcement bars have sufficient development length as per IS 456: 2000. The development length (Ld) is essential to ensure that the stresses in the reinforcement are fully transferred to the concrete.

Ld = fy/4×τb ​

Where:

• (fy) = Yield strength of steel,
• τb​ = Bond stress between concrete and steel.

Ensure that the available length of the bars is greater than the development length to prevent slippage.

9. Check for Uplift and Sliding (If Applicable)

If the footing is subjected to lateral loads or uplift forces (common in cases of wind or seismic loads), check the stability against:

• Sliding: The horizontal forces acting on the footing must not exceed the frictional resistance between the footing and the soil.
• Uplift: Ensure that the weight of the footing and the soil above it is sufficient to resist any uplift forces.

10. Detailing of Footing

After designing the reinforcement, prepare the detailed drawings, including the following:

• Footing Dimensions: Overall length, width, and depth of the footing.
• Reinforcement Layout: Indicate the size, spacing, and placement of both main and distribution bars.
• Cover to Reinforcement: Ensure the reinforcement has sufficient cover, typically 50 mm for footings, to protect the steel from corrosion and fire.

11. Safety Factors

In accordance with IS 456: 2000, apply safety factors to account for uncertainties in material properties and load estimations. The following safety factors should be considered:

• Partial Safety Factor for Materials: 1.5 for concrete and 1.15 for steel.
• Partial Safety Factor for Loads: 1.5 for dead loads and live loads.

12. Final Design Check

Once all the checks are performed, review the entire design to ensure compliance with the following:

• All load combinations as per IS 875 have been considered.
• The footing design adheres to the limits for bending, shear, and deflection.
• Safety factors are appropriately applied.
• The footing layout meets the construction and practical site requirements.

13. Conclusion

The design of footings as per IS 456: 2000 requires careful consideration of the loads, soil properties, and structural requirements. Proper checks for bending moment, shear, reinforcement design, and development length are essential for ensuring the foundation’s safety and stability. Following the steps outlined above will result in a well-designed footing that meets all safety standards and structural demands.