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: Recall working stress method provisions.
In the working stress method, permissible stresses are allowed to be increased when wind or earthquake loads are considered along with dead and live loads.
Step 2: Standard increase allowed.
As per IS codes, when dead load, live load, and wind load act together, the permissible stress can be increased by 40%.
Step 3: Convert percentage to decimal form.
\[ 40% = 0.4 \]
Step 4: Conclusion.
Hence, the correct answer is (C) 0.4.

Answer 2

Another way to approach this is by reasoning about how the permissible-stress increase typically scales with how many additional load cases are being combined together, and checking which of the given options is consistent with that trend for the specific case of dead, live and wind load acting simultaneously.

Working stress design intentionally allows a higher permissible stress whenever an additional, less-frequent load type (like wind) is included together with the basic dead and live loads, because the chance of the peak wind load coinciding exactly with the peak dead and live loads at the same instant is relatively low; codal provisions therefore permit a specific increase for this particular combined case of dead, live and wind load acting together.

1. 0.5: This value is set aside for combinations involving even more simultaneous extreme load types together, and represents a larger overstress allowance than what is codified for the dead plus live plus wind combination specifically.
2. 0.33: This fractional value corresponds to the increase permitted for a different, narrower combination case than the one asked here, and is lower than the specific allowance fixed for the dead, live and wind case being considered.
3. 0.4: This matches the specific codified allowance for the case where dead load, live load and wind load are all considered acting together, representing a 40 percent increase over the normal permissible stress.
4. 0.6: This is an even larger increase than what is codified for this particular three-load combination, and belongs to a different, more extreme loading scenario.

Matching the specific three-load combination described in the question to its codified allowance gives the value fixed for this case.

Therefore, the correct answer is 0.4.