CSLB General Building (B)Framing and CarpentryHard
A designer compares two floor beam options for the same 14-foot span and load: one in Douglas fir-larch No. 1 and one in Hem-fir No. 1, which has a lower allowable bending stress (Fb) than Douglas fir-larch. If both beams must use the same nominal width and depth, what is the most likely outcome of substituting Hem-fir for Douglas fir-larch?
- ASpecies selection has no effect on allowable span, only spacing does
- BThe Hem-fir beam will automatically qualify for a longer span due to lighter weight
- CThe Hem-fir beam will have identical span capacity since grade name is the same
- DThe Hem-fir beam will have reduced allowable span or require a larger size to carry the same load
Show answer & explanationAnswer & explanation
Correct answer: D. The Hem-fir beam will have reduced allowable span or require a larger size to carry the same load
Allowable stress values (Fb, E) vary by species, not just grade name. Since Hem-fir No. 1 has lower allowable bending stress than Douglas fir-larch No. 1, a beam of the same size in Hem-fir will have a lower allowable span or load capacity, meaning it must be upsized or the span reduced to meet the same design demand.
Why the other options are wrong
- A. Species directly affects allowable stress values and thus span capacity.
- B. Weight is not the governing factor in span capacity calculations.
- C. Grade name alone does not equalize allowable stresses across species.
Species Effect on Allowable Stress
Allowable bending stress (Fb) and modulus of elasticity (E) vary by wood species even within the same grade designation, directly affecting a member's span capacity.
- Same grade name ≠ same allowable stress across species
- Douglas fir-larch generally has higher Fb than Hem-fir
- Substituting a weaker species requires larger size or reduced span
Memory trick: Same grade, different tree, different strength decree.