The primary formula for calculating the formwork area, particularly for rectangular footings, focuses on the perimeter of the base multiplied by the height of the concrete element.
What is the Formula for Formwork?
The formula for calculating formwork area varies depending on the shape and type of the concrete element being cast. For a common rectangular footing, the formula to determine the formwork area for its vertical sides is:
Area of Formwork = 2 × (Footing Length × Footing Height) + 2 × (Footing Width × Footing Height)
This can be simplified as:
Area = 2h(a + b)
Where:
arepresents the length of the footing.brepresents the width of the footing.hrepresents the height or depth of the footing.
This formula calculates the area of the vertical surfaces that the formwork will directly contact.
Understanding Formwork Calculation
Formwork is a temporary mold or casing into which fresh concrete is poured to cast it into the desired shape. Calculating the precise area of formwork is a crucial step in quantity surveying and cost estimation for any construction project. Accurate calculations ensure material efficiency, reduce waste, and provide a clear basis for labor costs associated with formwork erection and dismantling. The construction sector often deals with vast amounts of diverse and evolving data, which can be fragmented across a project's lifecycle, making accurate and consistent data management, especially for calculations like formwork area, vital for project success and cost control.
Formwork Formula for Rectangular Footings
For a standard rectangular footing, which is a common foundation element, the formwork is typically required for its four vertical sides. The top surface is usually open for pouring, and the bottom rests on the ground, thus not requiring formwork.
Let's break down the formula components:
2 × (Footing Length × Footing Height): This calculates the area of two opposite longer sides of the footing.2 × (Footing Width × Footing Height): This calculates the area of the two opposite shorter sides of the footing.
Example:
Imagine a rectangular footing with the following dimensions:
- Length (
a) = 2.0 meters - Width (
b) = 1.5 meters - Height (
h) = 0.6 meters
Using the formula:
Area = 2 × (2.0 m × 0.6 m) + 2 × (1.5 m × 0.6 m)
Area = 2 × (1.2 m²) + 2 × (0.9 m²)
Area = 2.4 m² + 1.8 m²
Area = 4.2 m²
Therefore, 4.2 square meters of formwork would be needed for the vertical sides of this footing.
Formwork Calculation for Other Concrete Elements
While the basic principle of calculating the contact area remains similar, the specific formulas adapt to the geometry of different concrete elements:
- Beams: Formwork is typically needed for the bottom (soffit) and two sides.
Area = (Beam Width × Beam Length) + 2 × (Beam Depth × Beam Length)
- Columns: Formwork is needed for all four vertical sides.
Area = 2 × (Column Length × Column Height) + 2 × (Column Width × Column Height)- For square columns, this simplifies to
4 × (Side Length × Column Height).
- Slabs: Formwork is usually required for the bottom (soffit) and the edges. Edge formwork depends on whether the slab is cast monolithically with beams or as a standalone element.
Area (Soffit) = Slab Length × Slab WidthArea (Edges) = Perimeter × Slab Thickness(for standalone slabs)
- Circular Elements (e.g., Columns, Tanks):
Area = π × Diameter × Height(for vertical cylindrical surfaces)
Practical Insights and Best Practices
Accurate formwork calculation extends beyond just geometric formulas. Several practical aspects influence the final formwork requirements:
- Overlaps and Laps: Formwork panels often require overlaps at joints, which adds to the total material needed.
- Cut-offs and Waste: Cutting formwork materials to size inevitably leads to some waste. A waste factor (e.g., 5-10%) is often added.
- Reusability: The number of times formwork can be reused significantly impacts material costs. Steel or plastic formwork is highly reusable, while timber has limited reuse.
- Surface Finish Requirements: Different finishes may require different types of formwork materials (e.g., marine ply for smooth finishes).
- Supports and Bracing: While not part of the contact area, the cost and material for supporting formwork (props, walers, ties) must also be considered.
- Openings: Deduct areas of openings (e.g., for pipes, ducts) within the formwork area.
Summary of Common Formwork Calculations
| Concrete Element | Primary Formwork Area | Formula Type (General) | Key Variables |
|---|---|---|---|
| Footing | Vertical sides | Perimeter × Height | Length, Width, Height |
| Beam | Bottom (Soffit) + Sides | Length × (Width + 2×Depth) | Length, Width, Depth |
| Column | All four vertical sides | Perimeter × Height | Length, Width, Height |
| Slab | Bottom (Soffit) + Edges | Length × Width + Perimeter × Thickness | Length, Width, Thickness |
Understanding these formulas and the practical considerations ensures efficient material procurement, accurate budgeting, and effective project management.
