Commercial Building Inspector B2Footings and FoundationsEasy
An inspector is reviewing the formwork for a concrete footing on a commercial building project. The plans specify a footing width of 36 inches and a depth of 12 inches. What is the minimum required thickness of the wood planks used for the footing formwork, assuming standard construction practices for lumber?
- A1/2 inch
- B1 1/2 inches
- C3/4 inch
- D1 inch
Show answer & explanationAnswer & explanation
Correct answer: B. 1 1/2 inches
For concrete formwork, especially for footings, dimensional lumber of 2x material (nominal 2 inches, actual 1 1/2 inches) is commonly used to withstand the pressure of wet concrete and maintain shape.
Why the other options are wrong
- A. This thickness is generally insufficient to resist the pressure of wet concrete without excessive deflection or failure.
- C. While thicker than 1/2 inch, 3/4 inch (plywood) is generally used for sheathing, not typically for the primary planks forming the footing sides due to deflection concerns.
- D. One-inch nominal lumber is typically too thin for footing formwork, risking blowouts or significant deformation under concrete pressure.
Footing Formwork Thickness
The minimum thickness of lumber or plywood used to construct forms for concrete footings, designed to resist concrete pressure.
- Typically uses nominal 2-inch lumber (1 1/2" actual) for planks.
- Must withstand hydrostatic pressure of wet concrete.
- Prevents deformation and ensures correct footing dimensions.
Memory trick: Formwork's strength prevents concrete's messy plight, ensuring the footing's dimensions are just right.