Allowable Stress vs. Strength Design

[CptKlunk1]

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?

 

[manstrom]

I don't think you have it right. Those load combinations should both give you about a similar safety factor for overturning.

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

 

[jayrod12]

In doing your ASD calcs shouldn't you be multiplying the weight of the concrete blocks as 0.6 as well? Counterweighting with a concrete block is NOT an anchor, it is a counterweight.

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.

 

[medeek]

From what I understand about LRFD you are utilizing two safety/load factors vs. one with ASD. The added complication of LRFD has never made much sense to me, at least not for wood design.

A confused student is a good student.

 

[JAE]

medeek - this site has had many (too many) threads on the merits of LRFD vs. ASD.
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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[msquared48]

Yea. Don't get me started.

Mike McCann, PE, SE (WA)