Comprehensive Guide to Wall Footings (Strip Footings): Design, Calculation, and Best Practices

 

Introduction to Wall Footing (Strip Footing)

Wall footing, also known as strip footing, is a fundamental component in building foundations, particularly for load-bearing walls. This type of footing distributes the weight of the structure evenly to prevent excessive settlement and ensure stability. Wall footings are typically used in residential and light commercial buildings where the load from the structure is relatively uniform and spread over a continuous length.

Components and Purpose

Components:

1. Concrete: The primary material used for wall footings due to its strength and durability.
2. Reinforcement Bars (Rebar): Embedded within the concrete to provide additional tensile strength.
3. Soil: The bearing medium that supports the entire footing structure.

Purpose:

• Load Distribution: Spread the load from the wall to a larger area to reduce pressure on the soil.
• Stability: Prevent differential settlement which can lead to structural issues.
• Support: Provide a stable base for walls to rest upon, enhancing the overall integrity of the building.

Types of Wall Footings

1. Simple Wall Footing: A continuous strip of concrete placed directly under the load-bearing wall.
2. Stepped Wall Footing: Used on sloping ground, where the footing steps down at intervals to maintain level with the soil grade.
3. Combined Wall Footing: Used when two columns are close enough that their individual footings would overlap.

Design Considerations for Wall Footing

1. Load Calculations:

• Determine the total load from the structure (dead load and live load).
• Calculate the load per unit length of the wall.

2. Soil Bearing Capacity:

• Conduct a soil test to determine the bearing capacity.
• Ensure the footing size is designed to not exceed this capacity.

3. Footing Dimensions:

• Width: Generally 2 to 3 times the width of the wall it supports.
• Depth: Should be sufficient to avoid frost action and to reach stable soil strata.

4. Reinforcement Design:

• Rebar placement and spacing must be designed to handle tensile forces.
• Typically involves longitudinal bars and stirrups.

Steps to Design Wall Footing

Step 1: Determine Load Requirements

• Calculate the dead load (weight of the wall, floor, roof, etc.).
• Calculate the live load (temporary loads such as people, furniture, etc.).
• Sum up these loads to get the total load per unit length.

Step 2: Assess Soil Bearing Capacity

• Perform geotechnical investigations.
• Obtain the soil bearing capacity from the site.

Step 3: Calculate Footing Size

• Width (B): Using the formula $𝐵=\frac{𝑄}{{𝑞}_{𝑎}}$ where $𝑄$ is the total load per unit length and ${𝑞}_{𝑎}$ is the allowable soil bearing pressure.
• Depth (D): Generally 0.5 to 1 meter but must consider local frost depth and soil conditions.

Step 4: Design the Reinforcement

• Longitudinal Reinforcement: Determine the number and size of rebar required based on bending moments.
• Transverse Reinforcement: Design for shear strength to avoid cracking.

Step 5: Check Stability and Safety

• Ensure the footing is safe against sliding and overturning.
• Verify the factor of safety is adequate (typically 1.5 to 3).

Example Calculation

Assumptions:

• Total load per unit length (Q) = 100 kN/m
• Allowable soil bearing capacity (q_a) = 200 kN/m²

Calculation:

1. Width (B) Calculation:

   $$$$𝐵=𝑄𝑞𝑎=100 kN/m200 kN/m2=0.5 m

2. Depth (D) Determination:

   • Considering frost depth and load factors, assume a depth of 0.6 meters.

3. Reinforcement Design:

   • Assuming bending moment and shear forces, design appropriate rebar (e.g., 12 mm diameter bars at 200 mm centers).

Conclusion

Wall footing, or strip footing, is a critical element in foundation design, ensuring load distribution and structural stability. By following systematic design steps—load calculation, soil assessment, dimensioning, reinforcement design, and stability checks—engineers can create safe and effective footings tailored to specific building and soil conditions. Proper design and construction of wall footings contribute significantly to the longevity and safety of structures.