A wood frame shear wall is 20 feet long and must resist 8,000 pounds of lateral force. Using 15/32" structural sheathing with 8d nails at 6" o.c., what is the unit shear?
Correct Answer
A) 400 plf
Unit shear = Total force ÷ Wall length = 8,000 lbs ÷ 20 ft = 400 pounds per linear foot (plf).
Why This Is the Correct Answer
Unit shear is calculated by dividing the total lateral force by the wall length. With 8,000 pounds of lateral force distributed over a 20-foot long shear wall, the unit shear equals 8,000 ÷ 20 = 400 pounds per linear foot (plf). This represents the force per unit length that the wall must resist, which is fundamental for determining if the sheathing and nailing pattern can handle the load.
Why the Other Options Are Wrong
Option B: 600 plf
600 plf would result from incorrectly calculating 8,000 ÷ 13.33 feet, suggesting confusion with wall height or another dimension. This value exceeds the actual unit shear and would lead to over-designing the shear wall system, potentially specifying unnecessary reinforcement or more expensive sheathing materials.
Option C: 300 plf
300 plf would result from dividing 8,000 by approximately 26.67 feet, indicating an error in identifying the correct wall length or possibly confusing the calculation method. This underestimated unit shear could lead to inadequate shear wall design and potential structural failure under lateral loads.
Option D: 500 plf
500 plf would result from dividing 8,000 by 16 feet, suggesting an error in reading the wall length or mathematical miscalculation. This overestimated unit shear would result in conservative but unnecessarily expensive design decisions, specifying stronger sheathing or closer nail spacing than required.
Memory Technique
Remember 'Force Per Foot' - Unit shear = Total force ÷ Length. Think 'F/F = Force over Feet' to recall the simple division formula.
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