CSLB General Building (B)Framing and CarpentryMedium
A double 2x12 Douglas fir-larch beam carries a reaction of 3,200 pounds at its bearing point. The allowable compression stress perpendicular to grain (Fc⊥) for this species is 625 psi, and the beam is 3.5 inches wide. What is the minimum required bearing length to prevent crushing at the support?
- A1.5 inches
- B2.5 inches
- C3.5 inches
- D1.0 inch
Show answer & explanationAnswer & explanation
Correct answer: A. 1.5 inches
Minimum bearing length = Reaction ÷ (Fc⊥ × beam width) = 3,200 ÷ (625 × 3.5) = 3,200 ÷ 2,187.5 = 1.46 inches, which rounds up to a practical minimum of 1.5 inches to prevent crushing of the wood fibers at the bearing surface.
Why the other options are wrong
- B. 2.5 inches provides more bearing than required, though not incorrect for oversizing, it is not the calculated minimum.
- C. 3.5 inches equals the full beam width but far exceeds the minimum needed.
- D. 1.0 inch is insufficient bearing area and would exceed the allowable compression stress.
Minimum Bearing Length
The shortest length of beam support needed to keep bearing stress within allowable compression perpendicular to grain, calculated as Reaction ÷ (Fc⊥ × width).
- Formula: L = R / (Fc⊥ × width)
- Prevents crushing of wood fibers at supports
- Fc⊥ values vary by species and grade
Memory trick: Reaction over stress-times-width tells you how much wood you need.