Commercial Building Inspector B2Roof Ceiling ConstructionHard
A general contractor is constructing a new commercial building with a roof assembly using precast concrete planks. The planks are 8 inches thick and span 30 feet. The design requires a minimum bearing length at the support walls. According to ACI 318, what is the minimum bearing length required for precast concrete floor or roof members on concrete or masonry supports, unless otherwise shown by analysis?
- A4 inches
- B6 inches
- C2 inches
- D3 inches
Show answer & explanationAnswer & explanation
Correct answer: D. 3 inches
ACI 318 (referenced by IBC) specifies minimum bearing lengths for precast concrete members. For precast concrete floor and roof members on concrete or masonry supports, a minimum bearing length of 3 inches is required, unless a structural analysis justifies a shorter length.
Why the other options are wrong
- A. 4 inches would provide more bearing, but is not the minimum required by code.
- B. 6 inches is a significant bearing length, often used for very heavy loads or longer spans, but not the minimum requirement.
- C. 2 inches is generally too short for adequate bearing on concrete or masonry supports.
Precast Concrete Bearing Length
The minimum required length of support for precast concrete floor or roof members on concrete or masonry supports, typically 3 inches, as per ACI 318.
- Ensures load transfer.
- Prevents crushing at support.
- Can be reduced by analysis.
Memory trick: Precast planks need a solid stand, 3 inches is the rule of the land.