BBS Preparation for Combined Footing Structural Elements

Combined footings are used to support two or more columns when the space between them is limited, or the soil bearing capacity is low. These footings distribute the loads from multiple columns to the underlying soil. Preparing the Bar Bending Schedule (BBS) for combined footings ensures proper estimation of steel reinforcement and aids in efficient construction.

Components of Combined Footing Reinforcement

• Main Reinforcement: Bars placed in the longer direction of the footing to resist bending moments.
• Distribution Reinforcement: Bars placed perpendicular to the main reinforcement to distribute loads evenly.
• Edge Reinforcement: Additional bars along the edges to resist shear forces.

Example of Combined Footing BBS

Given:

    Footing dimensions: 3m x 1.5m x 0.6m
    Cover: 50mm
    Main bar diameter: 16mm
    Distribution bar diameter: 12mm
    Spacing of bars: 150mm c/c
    

Step 1: Calculate Number of Bars

    Effective length = Total length - 2 x Cover
                    = 3000mm - 2 x 50mm = 2900mm
    
    Effective width = Total width - 2 x Cover
                   = 1500mm - 2 x 50mm = 1400mm
    
    Number of main bars = (Effective width / Spacing) + 1
                       = (1400 / 150) + 1 = 10 bars
    
    Number of distribution bars = (Effective length / Spacing) + 1
                                = (2900 / 150) + 1 = 20 bars
    

Step 2: Calculate Bar Length

    Length of each main bar = Effective length + 2 x Cover
                           = 2900 + 2 x 50 = 3000mm or 3m
    
    Length of each distribution bar = Effective width + 2 x Cover
                                   = 1400 + 2 x 50 = 1500mm or 1.5m
    

Step 3: Total Steel Quantity

    Total length of main bars = Number of main bars x Length of each main bar
                             = 10 x 3 = 30m
    
    Total length of distribution bars = Number of distribution bars x Length of each distribution bar
                                      = 20 x 1.5 = 30m
    

Best Practices

• Ensure proper overlap for bars if the length is insufficient.
• Follow design specifications for bar placement and spacing.
• Verify concrete cover to protect reinforcement from environmental effects.

Preparing an accurate BBS for combined footings helps in estimating the required steel quantity and ensures the footing's structural integrity.