Formwork Pressure Calculator
Calculate maximum lateral formwork pressure (psf) using ACI 347-14 standards based on vertical pour rate, mix temperature, and wall height.
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ACI 347-14 Formwork Lateral Pressure Principles
When concrete is placed into vertical formwork, it initially acts as a liquid hydrostatic fluid. As the cement hydration reaction begins, the internal shear resistance develops and the concrete undergoes stiffening (loss of workability). Consequently, the lateral pressure reaches a peak value (P_max) at a certain depth below the top surface rather than increasing hydrostatically all the way to the base.
ACI 347-14 Mathematical Formulations
- Walls with R ≤ 7 ft/hr and placement height ≤ 14 ft:
P_max = C_w × C_c × [ 150 + (9000 × R) / T ] - Walls with R > 7 ft/hr or placement height > 14 ft:
P_max = C_w × C_c × [ 150 + 43400/T + (2800 × R) / T ] - Columns:
P_max = C_w × C_c × [ 150 + (12000 × R) / T ] - Enveloping Constraints:
600 psf ≤ P_max ≤ (w × h)(wherew × his full hydrostatic head).
Frequently Asked Questions
Why does cold concrete temperature increase formwork pressure?
Cold concrete temperatures retard the cement hydration reaction, delaying the stiffening and set time. The mix remains in a liquid, hydrostatic state for a longer duration, exerting higher lateral pressure.
What is the minimum lateral pressure ACI 347 allows for form design?
ACI 347 specifies a minimum design lateral pressure of 600 psf (28.7 kPa), regardless of how slow the pour rate is, to account for impact and vibrator surcharge.
How is form tie spacing related to lateral pressure?
The total tensile force on each form tie equals the lateral pressure (psf) multiplied by the tributary area (tie horizontal spacing multiplied by vertical spacing in feet).