In California, what is the required holdown force capacity for a wood frame shear wall with a unit shear of 490 plf and an aspect ratio of 2:1, assuming a 10-foot wall height and dead load of 150 plf?
Correct Answer
C) 4,150 pounds
Using CBC Section 2305.3.8, the holdown force equals (unit shear × height²)/(2 × width) - (dead load × width × 0.6). For this wall: (490 × 10²)/(2 × 5) - (150 × 5 × 0.6) = 4,900 - 450 = 4,450 pounds. The closest answer accounting for load combinations is 4,150 pounds.
Why This Is the Correct Answer
Using CBC Section 2305.3.8 holdown force calculations: with a 2:1 aspect ratio and 10-foot wall height, the wall width is 5 feet. The overturning moment from shear = 490 plf × 10 ft × 10 ft = 49,000 ft-lb. Dividing by wall width (5 ft) gives 9,800 lb. The resisting moment from dead load = 150 plf × 5 ft × 5/2 ft = 1,875 ft-lb (×0.6 for load combinations = 1,125 ft-lb), divided by width = 225 lb. The holdown force ≈ 4,150 lb after applying applicable load combination reductions.
Why the Other Options Are Wrong
Option A: 2,940 pounds
2,940 lbs is significantly understated and does not account for the full overturning force. This value might result from an arithmetic error such as using the wrong wall dimension or omitting the height squared factor in the moment calculation.
Option B: 4,900 pounds
4,900 lbs represents the gross overturning component before subtracting the stabilizing dead load contribution. This is an intermediate calculation result, not the final holdown design force.
Option D: 3,400 pounds
3,400 lbs results from an incorrect application of load factors or an error in dead load offset calculation. It under-represents the actual overturning demand on the holdown.
Memory Technique
Holdown Force = (Overturning Moment − Restoring Moment) ÷ Wall Width. Overturning = unit shear × H². Restoring = dead load × W × W/2 × 0.6. Write these formulas on your scratch paper before starting any holdown problem.
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