Formwork Pressure Calculator
Element Type
5 ft/hr
°F
pcf
Concrete Type
ft
Design Pressure793 psf
Max Lateral Pressure (ACI 347)793 psf
Full Liquid Head Pressure1,800 psf
Cw (Unit Weight Factor)1.00
Cc (Chemistry Factor)1.00
Recommended Tie Spacing: 45" o.c.
Based on 3,000 lb snap tie capacity with 12" tributary width
This formwork pressure calculator computes the maximum lateral pressure that fresh concrete exerts on wall and column forms using the ACI 347R-14 equations. It applies the unit weight coefficient Cw and chemistry coefficient Cc to a pressure that grows with pour rate and falls with concrete temperature, then caps the result at the full liquid head w·h. From the design pressure it also suggests a tie or snap-tie spacing.
Formula
p = Cw·Cc·(150 + 9000·R/T) for R ≤ 7 ft/hr, capped at w·h
- p
- Maximum lateral pressure on the form (psf)
- Cw
- Unit weight coefficient (1.0 for 140-150 pcf concrete)
- Cc
- Chemistry coefficient: 1.0 normal, 1.2 for self-consolidating concrete (SCC)
- R
- Rate of placement, ft/hr (walls above 7 ft/hr use the extended ACI formula)
- T
- Concrete temperature (°F); w·h is the full liquid head with density w and height h
How it works
- Choose wall or column and enter the pour rate R (ft/hr), concrete temperature T (°F), concrete density (pcf), and element height (ft).
- The tool sets Cw from the density (1.0 for typical 145-150 pcf concrete) and Cc from the mix type (1.0 normal, 1.2 for self-consolidating concrete), then evaluates the ACI 347R formula for your pour rate.
- The lateral pressure is capped at the full liquid head w·h, reported as the design pressure, and converted into a recommended tie spacing assuming a 3,000 lb tie capacity over a 12-inch tributary width.
Worked example
A wall poured at 5 ft/hr with 70 °F concrete at 150 pcf and a 12 ft form height.
- Cw = 1.0 (150 pcf is within 140-150) and Cc = 1.0 (normal concrete).
- Since R = 5 ≤ 7 ft/hr: p = 1.0 × 1.0 × (150 + 9000 × 5 / 70) = 150 + 642.9 = 792.9 psf.
- Full liquid head = 150 × 12 = 1,800 psf, so the design pressure is min(792.9, 1,800) = 793 psf.
- Tie spacing = 3,000 / (793 / 12) ≈ 45 in on center.
Maximum lateral pressure ≈ 793 psf, well below the 1,800 psf liquid head, giving a recommended tie spacing of about 45 inches.
Frequently asked questions
- Why does lower temperature raise formwork pressure?
- Cold concrete sets more slowly, so it stays fluid longer and acts like a liquid up to a greater depth before it stiffens. Because temperature T sits in the denominator of the ACI formula, a lower T produces a higher lateral pressure for the same pour rate.
- When does the full liquid head govern?
- For tall forms filled quickly or at very low temperatures, the ACI formula can predict a pressure higher than the simple hydrostatic head w·h. The lateral pressure can never exceed full liquid head, so the calculator caps the design pressure at w times the height.
- Why is self-consolidating concrete treated differently?
- Self-consolidating concrete (SCC) is highly fluid and develops little internal arching, so it can exert close to full hydrostatic pressure. ACI applies a chemistry coefficient Cc of 1.2 for SCC, increasing the predicted form pressure by 20%.
- How is the tie spacing estimate derived?
- The tool assumes a representative 3,000 lb working tie capacity acting over a 12-inch tributary width and divides that capacity by the pressure per inch of width. Actual spacing must use the specific tie and form system ratings from the formwork supplier.