NASCLA Accredited Commercial General ContractorConcreteHard
A concrete contractor is inspecting the forms for a structural beam before concrete placement. The forms are constructed from plywood sheathing supported by lumber walers and strongbacks. The plans indicate that the beam is 12 inches wide and 24 inches deep. During inspection, it is noted that the forms for the beam are not adequately braced, and there is a concern about lateral pressure from the fresh concrete. Assuming a unit weight of fresh concrete of 150 pcf and a maximum fluid pressure of 150h (where h is the height of fluid concrete in feet), what is the approximate maximum lateral pressure that the bottom of the formwork must resist?
- A150 psf
- B300 psf
- C450 psf
- D600 psf
Show answer & explanationAnswer & explanation
Correct answer: B. 300 psf
The maximum lateral pressure exerted by fluid concrete is calculated as 150h, where h is the height of the fluid concrete in feet. The beam is 24 inches deep, which is 2 feet. Therefore, the maximum lateral pressure at the bottom of the formwork is 150 psf/ft * 2 ft = 300 psf.
Why the other options are wrong
- A. This would be the pressure at 1 foot deep, not 2 feet.
- C. Incorrect calculation, perhaps for a 3-foot depth.
- D. Incorrect calculation, perhaps for a 4-foot depth.
Lateral Concrete Pressure
The horizontal pressure exerted by fresh, liquid concrete against vertical formwork surfaces, which designers must account for in formwork design.
- Increases with depth of concrete.
- Affected by concrete density, rate of pour, temperature, and admixtures.
- Crucial for formwork stability and safety.
Memory trick: Pressure is depth times weight, for forms to stay straight.