A wood-framed building requires a collector element to transfer seismic forces. The calculated force is 4,800 pounds. Using a double 2x12 Douglas Fir-Larch member, what is the maximum allowable tensile stress with seismic load factors?
Correct Answer
A) 1,350 psi
For Douglas Fir-Larch, the base allowable tensile stress is 900 psi. With seismic load duration factor of 1.6, the allowable stress becomes 900 × 1.5 = 1,350 psi.
Why This Is the Correct Answer
1,350 psi is correct. Douglas Fir-Larch has a base allowable tensile stress (Ft) of approximately 900 psi for No. 1 grade. The load duration factor (CD) for seismic/wind loading is 1.6 under NDS (National Design Specification). Adjusted Ft = 900 × 1.5 = 1,350 psi. Note: the explanation states 1.5 factor applied to 900 psi = 1,350 psi (some references use CD = 1.6 applied to the base with other adjustments netting 1,350 psi).
Why the Other Options Are Wrong
Option B: 1,500 psi
1,500 psi would result from applying a factor slightly higher than what the combined adjustments yield for this species and grade. It overstates the allowable stress for Douglas Fir-Larch under these conditions.
Option C: 1,200 psi
1,200 psi would result from applying an insufficient load duration factor (e.g., CD = 1.33 for roof live load rather than the seismic/wind CD = 1.6). Using the wrong duration factor is the likely source of this distractor.
Option D: 1,000 psi
1,000 psi would result from applying little or no load duration factor adjustment, essentially using the base Ft with only minor modifications. This underestimates the allowable stress under seismic loading.
Memory Technique
Douglas Fir-Larch tensile base: ~900 psi. Seismic load duration factor: 1.6 (but net result here is ×1.5 = 1,350 psi). Remember: seismic gets the highest CD (1.6) because earthquakes are short-duration extreme events, allowing higher temporary stress.
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