Calculating the number of shuttering props required for a construction project is crucial for ensuring the safety and stability of concrete formwork. This process involves a combination of engineering principles, load assessment, and practical considerations, rather than a single simple formula.
Understanding Shuttering Props
Shuttering props, also known as scaffolding props, adjustable steel props, or acrow props, are temporary vertical supports used to hold formwork (shuttering) in place until the concrete gains sufficient strength. They bear the weight of fresh concrete, formwork materials, and live loads during construction.
Key Factors Influencing Prop Calculation
The precise number and spacing of shuttering props depend on several critical factors:
- Slab/Beam Thickness and Dimensions: Thicker and wider concrete elements exert greater loads, requiring more or stronger props.
- Span of the Slab/Beam: Longer spans mean more distributed load over a larger area, influencing prop placement.
- Height of Shuttering: Taller structures often necessitate stronger props or additional bracing to maintain stability.
- Type of Formwork Material: Plywood, steel, or plastic formwork have different stiffness and load-spreading capabilities.
- Load-Bearing Capacity of Props: Each prop type (e.g., light-duty, heavy-duty) has a specified safe working load (SWL).
- Concrete Density: The type of concrete (normal weight, lightweight) affects the total weight.
- Site Conditions: Ground stability, wind loads, and accessibility can influence prop arrangement.
- Construction Methodology: How concrete is poured (e.g., pump, bucket) can create dynamic loads.
General Approach to Calculating Shuttering Props
While a structural engineer typically performs the detailed design, here's a general methodology for estimating prop requirements:
Step 1: Determine Total Load
The total load on the props consists of:
- Weight of Fresh Concrete:
Volume of Concrete × Density of Concrete - Weight of Formwork:
Area of Formwork × Weight per unit area of formwork material - Live Load/Construction Load: Typically assumed as a standard value (e.g., 2.5 kN/m² or 250 kg/m²) for workers, equipment, and material storage during pouring.
Step 2: Calculate Shuttering Area (Related Calculation)
Before supporting, you need to know the area being supported. The shuttering area for various structural components like columns, beams, and slabs is fundamentally calculated by multiplying the peripheral length of the component by its depth (or height). For instance, when determining the shuttering area for a column, you would first establish its cross-sectional dimensions. If one of these dimensions is, say, 0.50 meters, this value contributes to defining the column's perimeter, which is then multiplied by the column's height to get the total shuttering area.
- For Slabs:
Total Area = Length × Width - For Beams:
Shuttering Area = (2 × Beam Depth + Beam Width) × Beam Length(for sides and bottom) - For Columns:
Shuttering Area = Peripheral Length of Cross-section × Column Height
Step 3: Determine Prop Spacing and Quantity
This is the most critical step and often relies on design standards or manufacturer's recommendations for prop spacing based on the calculated load.
- Assess Load per Unit Area: Divide the total load by the area being supported to get the load per square meter (kN/m²).
- Refer to Span Tables/Design Charts: Manufacturers of formwork systems provide tables indicating safe prop spacing for different slab thicknesses and loads. These are typically derived from engineering calculations considering beam deflections and prop capacities.
- Calculate Number of Props:
- Once the required spacing is known (e.g., 1.0 m × 1.0 m grid), you can calculate the number of props needed for a specific area.
Number of Props = (Total Area / (Prop Spacing Length × Prop Spacing Width))- Always round up to the nearest whole number and add a contingency (e.g., 5-10%).
Example: Simplified Slab Prop Calculation
Let's assume you have a concrete slab of:
- Dimensions: 10 m × 5 m = 50 m²
- Thickness: 0.2 m (200 mm)
- Concrete Density: 2400 kg/m³ (24 kN/m³)
- Formwork Weight: 0.2 kN/m²
- Live Load: 2.5 kN/m²
- Prop Safe Working Load (SWL): 20 kN per prop
- Weight of Concrete:
50 m² × 0.2 m × 24 kN/m³ = 240 kN - Weight of Formwork:
50 m² × 0.2 kN/m² = 10 kN - Total Live Load:
50 m² × 2.5 kN/m² = 125 kN - Total Load:
240 kN + 10 kN + 125 kN = 375 kN - Required Number of Props:
Total Load / Prop SWL = 375 kN / 20 kN/prop = 18.75 props- Always round up: 19 props.
This simple calculation gives a minimum number. In practice, props are arranged in a grid with specific spacing to ensure even load distribution and prevent localized failure. If the engineer specifies a 1.0 m x 1.0 m grid for this load, you would need (10m / 1m + 1) * (5m / 1m + 1) = 11 * 6 = 66 props. This demonstrates why specific spacing determined by design charts is critical over a simple total load division.
Types of Shuttering Props and Their Use
Different props offer varying load capacities and adjustability:
| Prop Type | Typical Material | Load Capacity (approx.) | Application |
|---|---|---|---|
| Adjustable Steel Prop | Steel | 10 kN - 40 kN | Most common for slabs, beams, and columns |
| Heavy-Duty Prop | Steel | 40 kN - 100 kN+ | Heavier slabs, multi-story buildings |
| Aluminium Prop | Aluminium | 10 kN - 40 kN | Lighter weight, easier handling |
| Timber Props (Scaffolding) | Timber | Variable | Traditional, less common for modern formwork |
Important Considerations for Safe Prop Installation
- Base Plate and Head Plate: Ensure props have proper base and head plates for stable contact with the ground/slab below and the formwork above.
- Sole Plates: Use timber sole plates under props on soft ground to distribute the load and prevent sinking.
- Verticality: Props must be installed perfectly vertical to achieve their full load-bearing capacity.
- Bracing: Horizontal bracing between props might be necessary, especially for tall structures, to prevent buckling.
- Reshoring/Backpropping: For multi-story construction, lower floors may require reshoring to support the weight of newly poured concrete above.
- Inspection: Regularly inspect props for damage, correct installation, and stability during and after concrete pouring.
Conclusion
Calculating shuttering props is a critical engineering task that must prioritize safety. While the shuttering area calculation provides the extent of the formwork, the prop calculation involves assessing the total weight to be supported and ensuring adequate prop strength and spacing. Always consult with a qualified structural engineer for detailed design and approval of shuttering and propping systems to ensure the safety and integrity of the structure.
