Allowable Stress vs. Strength Design
Allowable Stress vs. Strength Design
(OP)
Hi all,
I've got a question regarding load factors. Say I am designing a free standing sign for wind forces and my governing overturning load combo is 0.6D + 1.0W per ASCE 7-05 ASD. I plan on using concrete blocks to resist any wind loads. To determine the amount of overturning force I must resist I factor the DL of the sign assembly and the WL per the above load combo. This would give me a required load to resist and I would place that amount of concrete block to resist (ignoring the DL reduction to the block and treating it as an "anchor").
Now lets say I have a client that uses strength design for load combo's. I would follow the same procedure except using 0.9D + 1.6WL. But, since I have a fixed block weight it would require roughly 60% more block. Unlike material design I don't get an "increase" in weight by using it's "ultimate strength".
I've always seen Strength Design and Allowable Stress as being approximately equivalent. Would I be out of line if I were to bring the required loads back to ASD levels when determining the amount of block needed?
I've got a question regarding load factors. Say I am designing a free standing sign for wind forces and my governing overturning load combo is 0.6D + 1.0W per ASCE 7-05 ASD. I plan on using concrete blocks to resist any wind loads. To determine the amount of overturning force I must resist I factor the DL of the sign assembly and the WL per the above load combo. This would give me a required load to resist and I would place that amount of concrete block to resist (ignoring the DL reduction to the block and treating it as an "anchor").
Now lets say I have a client that uses strength design for load combo's. I would follow the same procedure except using 0.9D + 1.6WL. But, since I have a fixed block weight it would require roughly 60% more block. Unlike material design I don't get an "increase" in weight by using it's "ultimate strength".
I've always seen Strength Design and Allowable Stress as being approximately equivalent. Would I be out of line if I were to bring the required loads back to ASD levels when determining the amount of block needed?






RE: Allowable Stress vs. Strength Design
ASD 1.0/0.6 = 1.67
LRFD 1.6/0.9 = 1.77
You need to reduce the dead load. If it were considered an "anchor" you still want some kind of safety factor.
When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.
-R. Buckminster Fuller
RE: Allowable Stress vs. Strength Design
I bet once you were to adjust your calculations for that 0.6 the amount of concrete blocks required would be very similar from ASD to LRFD.
RE: Allowable Stress vs. Strength Design
A confused student is a good student.
RE: Allowable Stress vs. Strength Design
Let's not start another one. And that topic doesn't have much to do with the original poster's question here.
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RE: Allowable Stress vs. Strength Design
Mike McCann, PE, SE (WA)