The Complete Guide to Combined Footing BBS (Bar Bending Schedule)
A combined footing is a type of shallow foundation used to support two or more columns in a single, integrated base. It's a critical engineering solution when columns are too close together for individual footings to be feasible. The design and reinforcement detailing of a combined footing are significantly more complex than for an isolated footing, as it must handle the loads from multiple points. Creating an accurate Bar Bending Schedule (BBS) for a combined footing is a vital step, and a Combined Footing BBS Generator can be a helpful tool for preliminary estimation.
This guide will explore the purpose of combined footings, their unique reinforcement patterns, the principles behind calculating cutting lengths for their steel, and how our simplified generator can provide a baseline BBS for a typical rectangular combined footing.
Why and When Are Combined Footings Used?
Engineers opt for a combined footing in several specific scenarios:
- Closely Spaced Columns: When two columns are so close that their individual isolated footings would overlap, a single combined footing is the practical solution.
- Property Line Constraints: If a column is located right at the edge of a property line, there isn't enough space to provide a symmetrical isolated footing. In this case, the exterior column's footing is combined with the adjacent interior column's footing.
- Variable Soil Conditions: To bridge over a localized pocket of weak soil under one column.
Reinforcement Detailing in a Combined Footing
The reinforcement in a combined footing is more complex because of its bending behavior. It acts like an inverted beam spanning between the columns.
- Bottom Reinforcement (Main Mat): The soil pressure pushes the footing upwards, causing it to sag between the columns. This creates tension at the bottom. Therefore, a heavy mesh of **longitudinal** (along the longer direction) and **transverse** (along the shorter direction) bars is provided at the bottom.
- Top Reinforcement (Negative Moment Steel): Directly under the columns, the footing experiences a "hogging" or negative bending moment, similar to a continuous beam over a support. This creates tension at the **top** of the footing. Therefore, a mesh of top reinforcement is required in the longitudinal direction, concentrated in the area around the columns.
The Challenge of Combined Footing BBS
The primary challenge for a BBS for combined footing is that the exact amount and placement of the top and bottom steel depend on a detailed structural analysis. The bending moment is not uniform. However, for a simplified estimation, we can assume a typical reinforcement pattern.
1. Cutting Length of Bottom Bars
These are typically straight bars with 90° hooks at the ends.
Longitudinal Bar Length = `(Footing Length - 2 × Cover) + (2 × 9D)`
Transverse Bar Length = `(Footing Width - 2 × Cover) + (2 × 9D)`
2. Cutting Length of Top Bars
Top bars are also typically straight bars placed longitudinally over the columns.
Cutting Length = `(Footing Length - 2 × Cover)`
3. Number of Bars
The number of bars is determined by their spacing.
No. of Longitudinal Bars = `(Footing Width / Spacing) + 1`
No. of Transverse Bars = `(Footing Length / Spacing) + 1`
Frequently Asked Questions (FAQ)
What is the difference between a combined footing and a raft foundation?
A combined footing supports two or a few columns in a single line. A raft (or mat) foundation is a very large, thick slab that supports *all* the columns of the entire building. Rafts are used when soil bearing capacity is very low or column loads are extremely high.
Is the reinforcement the same for a rectangular and a trapezoidal combined footing?
No. A trapezoidal shape is used when the load on the exterior column is much higher than the interior one. The reinforcement detailing for a trapezoidal footing is more complex to match the varying width. This calculator is designed for a simpler rectangular combined footing.
Why is this calculator for estimation purposes only?
Because the exact reinforcement (number, diameter, and exact length of top bars) depends on a structural analysis performed by an engineer using software like SAFE or STAAD.Pro. The loads on the columns and their spacing critically affect the bending moment diagram, which dictates the reinforcement. This tool provides a BBS based on a *typical* reinforcement pattern, which is great for initial costing but should not replace a formal design.
Conclusion
Combined footings are a clever solution to common foundation challenges. Their design and reinforcement are a step up in complexity from isolated footings, requiring an understanding of both positive and negative bending moments. A detailed Bar Bending Schedule is essential for the correct execution of this complex reinforcement. Our free combined footing BBS generator provides a valuable starting point for estimating the steel required, helping you to plan and budget for your foundation work with greater insight.