Question

How much percentage increase of permissible stress is allowed when dead load, live load and wind load are considered together in working stress method?

• A. 0.5
• B. 0.33
• C. 0.4
• D. 0.6

Hint:

In working stress method, wind or seismic effects allow an increase in permissible stress to account for their short-duration nature.

Answer 1

The Correct Option is C

Step 1: Working stress design allows a higher permissible stress when less-frequent loads like wind are combined with dead and live load.
Step 2: For the specific combination of dead load, live load and wind load acting together, the codified increase is 40% over the normal permissible stress.
\[ 40\% = \boxed{0.4} \]

Answer 2

A third way to approach this is to recall the specific set of standard percentage allowances used for different combined-load cases in working stress design, and match the three-load case in the question to the correct one among them.

1. 0.5: a fifty percent increase is a larger allowance than what is specified for the dead, live and wind combination in question, and instead corresponds to a different, more extreme combined case.
2. 0.33: a one-third increase is the allowance associated with a different combined-load case than the specific dead plus live plus wind combination being asked about here.
3. 0.4: a forty percent increase is precisely the allowance specified for the case of dead load, live load and wind load acting together, matching the exact combination described in the question.
4. 0.6: a sixty percent increase is too large an allowance for this particular three-load combination and belongs to a different scenario.

Matching the exact wording of the load combination in the question, dead plus live plus wind, to its specific codified allowance value confirms the correct percentage.

Therefore, the correct answer is 0.4.