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Uplift Load Combination with Wind

Mediumload combinationsupliftwind
At a roof connection the dead-load effect is 18 kN downward and the wind effect is 30 kN upward. Take downward as positive, so D = +18 kN and W = −30 kN. Evaluate the supplied combination U = 0.9D + W (the wind effect is already on the basis this combination requires; no other forces act). What uplift resistance must the connection provide?
  1. A13.8 kN uplift resistance
  2. B46.2 kN uplift resistance
  3. C13.8 kN compression (downward)
  4. D30.0 kN uplift resistance
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Answer: A. 13.8 kN uplift resistance

Equation: U = 0.9D + W. Substitution: U = 0.9(18) + (−30) = 16.2 − 30 = −13.8 kN. Negative means net upward load, so the connection must resist 13.8 kN of uplift. Wrong: 46.2 kN adds the stabilizing dead load as if it were uplift (16.2 + 30); 13.8 kN compression reads the sign backward; 30 kN ignores that dead load offsets part of the wind uplift.
kN kN uplift
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