Design and Engineering of Formwork Systems

Module 3: Design and Engineering of Formwork Systems

1. Formwork Design Principles

Proper formwork design ensures structural integrity, dimensional accuracy, and worker safety. It requires a thorough understanding of construction drawings, load distribution, and alignment standards.

• Interpretation of Shop Drawings & Formwork Layouts

• Shop Drawings: Detailed fabrication plans showing formwork dimensions, material specs, and connection details.

• Formwork Layouts: Site plans indicating formwork positioning, bracing points, and pour sequences.

• Key Considerations:

• Check for conflicts with rebar, embeds, and utilities.

• Verify concrete cover and edge chamfers as per structural drawings.

• Bracing, Alignment, and Tolerances

• Bracing Requirements:

• Diagonal and horizontal braces to resist lateral concrete pressure and wind loads.

• Adjustable props (e.g., Acrow props) for vertical support under slabs.

• Alignment Standards:

• Use laser levels or total stations for precision (±3mm tolerance for high-rise cores).

• Plumbness tolerance: Typically 1:500 (e.g., 10mm deviation per 5m height).

• Deflection Limits:

• ACI 347 mandates max deflection of L/360 (or L/240 for exposed concrete).

• Example: For a 3m beam form, max allowable deflection = 8.3mm (L/360).

2. Structural Calculations

Formwork must withstand fresh concrete’s hydrostatic pressure, construction loads, and environmental forces.

• Concrete Pressure Formulas (ACI 347)

• For Walls:

• P = 150 + (9000 × R/T)

• P = Lateral pressure (psf or kPa)

• R = Concrete pour rate (ft/hr or m/hr)

• T = Concrete temperature (°F or °C)

• Maximum Pressure: P ≤ 150h (where h = fluid concrete height in feet).

• For Columns:

• Higher pressure due to rapid filling: P = 150 + (43,400/T) + (2800 R/T).

• For Slabs:

• Lower pressure (static load dominates): P = γ × h (γ = concrete unit weight, h = slab thickness).

• Deflection Limits for Formwork Materials

• Plywood Sheathing:

• Max deflection L/360 under total load (e.g., 19mm thick plywood at 300mm spacing).

• Steel Walers/Beams:

• L/400 deflection limit to prevent visible concrete bulging.

• Support Props:

• Check buckling capacity using Euler’s formula for long props.

Practical Implications & Failures

• Case Study: A wall formwork failure due to exceeded pour rate (R) without adjusting bracing.

• Solution: Use real-time pressure sensors in high-risk pours.

Tools for Design & Verification

• Software:

• ETABS/SAP2000 for load analysis.

• PERI CAD for modular formwork planning.

• Manual Checks:

• Always cross-verify software results with ACI hand calculations.

Key Takeaways:

1. Design Accuracy prevents blowouts and rework.

2. ACI 347 formulas govern safe concrete pressure estimates.

3. Deflection limits ensure aesthetic and structural quality.