Formwork Pressure Calculator (ACI 347)
Estimate fresh concrete lateral pressure on vertical formwork using ACI 347 equations, with hydrostatic check and SCC option.
Formwork Pressure Calculator
Results
Design maximum lateral pressure
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Controlling condition
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ACI 347 empirical pressure
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Hydrostatic pressure at base
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Equivalent fluid pressure
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This tool is for educational and preliminary design use only. Always follow ACI 347, local codes, and manufacturer data, and have final formwork design checked by a qualified engineer.
ACI 347 formwork pressure equations
Fresh concrete exerts lateral pressure on vertical formwork similar to a fluid. As the concrete sets, the pressure reduces and may be less than the full hydrostatic head. ACI 347 provides empirical equations to estimate the maximum lateral pressure for normal-weight concrete.
1. Hydrostatic pressure
If the concrete remains fluid over the full height, the lateral pressure at depth \(h\) is:
\( p_\text{hydro} = \gamma \, h \)
- \( p_\text{hydro} \) = lateral pressure at depth \(h\)
- \( \gamma \) = unit weight of concrete
- \( h \) = depth below top of pour
In US customary units:
\( \gamma \) in pcf, \(h\) in ft, \(p_\text{hydro}\) in psf.
2. ACI 347 empirical pressure (US units)
For normal-weight, vibrated concrete with:
- Slump between 4 and 7 in
- Concrete temperature between 50°F and 90°F
- Rate of placement \(R\) between about 5 and 20 ft/h
ACI 347 gives the following maximum lateral pressure:
\( p_\text{ACI} = 150 + \dfrac{900 R}{T + 70} \)
- \( p_\text{ACI} \) = maximum lateral pressure (psf)
- \( R \) = rate of placement (ft/h)
- \( T \) = concrete temperature (°F)
The design pressure is then taken as:
\( p_\text{design} = \min\left( p_\text{ACI}, \; p_\text{hydro} \right) \)
The calculator implements this logic and always reports both values so you can see which one controls.
3. Self-consolidating concrete (SCC)
Self-consolidating concrete can behave like a true fluid for significant heights, often generating pressures close to full hydrostatic. For SCC, many designers conservatively assume:
\( p_\text{design,SCC} \approx p_\text{hydro} = \gamma h \)
The SCC option in this calculator bypasses the empirical ACI equation and uses the hydrostatic head directly.
How to use this formwork pressure calculator
- Select the unit system. Choose US (psf, ft, °F) or metric (kPa, m, °C). In metric mode, the calculator converts internally to the ACI 347 US equation and converts the result back to kPa.
- Choose concrete type. Use “Normal vibrated” for conventional concrete with internal vibration. Use “SCC” for self-consolidating concrete; this will assume full hydrostatic pressure.
- Enter pour height and rate of placement. Height is the total wall or column height. Rate is the average vertical rise per hour.
- Enter temperature, slump, and unit weight. For normal-weight concrete, unit weight is typically about 150 pcf (24 kN/m³). The calculator will warn you if temperature or slump are outside the typical ACI 347 range.
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Review the results. The tool shows:
- Design maximum lateral pressure
- ACI 347 empirical pressure
- Hydrostatic pressure at base
- Equivalent fluid pressure (psf/ft or kPa/m)
Worked example
Suppose you are casting a 12 ft high wall with normal-weight concrete:
- Unit weight \( \gamma = 150 \,\text{pcf} \)
- Rate of placement \( R = 5 \,\text{ft/h} \)
- Concrete temperature \( T = 70^\circ\text{F} \)
- Slump = 5 in, internal vibration
Hydrostatic pressure at the base:
\( p_\text{hydro} = \gamma h = 150 \times 12 = 1800 \,\text{psf} \)
ACI 347 empirical pressure:
\( p_\text{ACI} = 150 + \dfrac{900 \times 5}{70 + 70} = 150 + \dfrac{4500}{140} \approx 150 + 32.1 = 182.1 \,\text{psf} \)
The design pressure is the smaller of the two:
\( p_\text{design} = \min(182.1, 1800) = 182.1 \,\text{psf} \)
The equivalent fluid pressure is \( 182.1 / 12 \approx 15.2 \,\text{psf/ft} \).
Engineering notes and limitations
- This calculator focuses on vertical formwork for walls and columns.
- It does not apply directly to slabs, beams, or inclined forms.
- It assumes normal-weight concrete; lightweight concrete will have lower unit weight and lower hydrostatic pressure.
- For very high rates of placement, very low temperatures, or unusual admixtures, consult project specifications, supplier data, and ACI 347 commentary.
- Always apply appropriate load factors and combinations (e.g., ASCE 7, ACI 318) when designing formwork members, ties, and shores.
Frequently asked questions
Can I use this for columns as well as walls?
Yes. The same lateral pressure concepts apply to columns. However, columns often have higher placement rates and may be more prone to full hydrostatic pressure. Use realistic rates and consider SCC behavior if applicable.
How do I use the pressure in formwork design?
The maximum lateral pressure is applied as a uniform or triangular load on the formwork sheathing and supporting members. Use structural analysis to design studs, walers, ties, and anchors for bending, shear, and deflection under this load, with appropriate safety factors.
What if my slump or temperature is outside the ACI range?
The empirical ACI 347 equation is calibrated for typical slumps and temperatures. Outside that range, the relationship between rate, temperature, and pressure may differ. The calculator will still compute a value but flags it with a warning; treat it as approximate and consider using more conservative assumptions (e.g., closer to hydrostatic).
Does vibration increase formwork pressure?
Internal vibration briefly increases fluidity and can increase lateral pressure, which is why the ACI 347 equation is based on vibrated concrete. External vibration or over-vibration can further increase pressures; always follow good concreting practice.