ASD vs LRFD/USD -- plain english explanation??

Structural Layout Vignette and Multiple Choice

ASD vs LRFD/USD -- plain english explanation??

Postby Katie » Sat Oct 18, 2014 3:25 pm

OK, so here is my thinking on the difference between Allowable Stress Design and Load Resistance Factor Design / Ultimate Strength design. I still have yet to find a simple, plain english explanation of the difference between ASD and LRFD/USD. I THINK it has to do with differences in accounting for a factor of safety. Here is my attempt at an explanation; if anyone can confirm or correct me here that would be great:

-both ASD and LRFD / USD require load combinations, and each has a different set of load combinations. Ballast and Mike-SE's guides go through those. This post is helpful: viewtopic.php?f=8&t=14340&p=52564&hilit=ASD+LRFD#p52564 And Mike's guide is also good for this: http://pegroup.us/areforum/702%20-%20Ch ... 0Loads.pdf

-with Allowable Stress Design you take your yield strength of your material, and you reduce it by some fraction to make the allowable stress smaller than the yield stress. And then you compare that with your actual loads to make sure they're less than that. So the reduction of allowable load is your factory of safety. With ASD you still have "load combinations" but the load combinations are just the service loads, things like just "D+L" instead of multiplying them by some factor to make them bigger (that's LRFD/USD). Ballast book is a little old so teaches Allowable Stress Design for steel, and Kaplan also focuses on ASD for steel. However NCARB does NOT want that method used for steel anymore, so that stinks. I think just Mike's guides will show you the LRFD method for steel (thanks Mike!!). In the exam hopefully we just have to remember to use the LRFD column and not ASD column when looking at charts for member selection.

-with LRFD / Ultimate Strength Design (which are essentially the same idea) instead of getting most of your factor of safety by decreasing your allowable load, you INCREASE your actual loads by the load factoring to make them bigger, and then compare that with the the ultimate strength of the material. You also reduce the (given) nominal strength a little bit too. The load combinations you use are the "ultimate" load combinations....stuff like 1.2DL + 1.6LL or 1.2DL + 1.6LL + 0.5(Lr or S or R), etc. So your factor of safety comes from inflating the actual/nominal value of your loads to be more than they really are. Then the given "nominal strength" of the material is also multiplied by a resistance factor (something less than 1) to knock down the required strength a little bit too....kind of like how ASD does it, but less I think. And then your required strength coming from your inflated factored loads has to be less than the nominal strength times the resistance factor.

-for the test, ASD should be used for wood and masonry. LRFD should be used for steel and USD for concrete.

Hopefully someone can confirm, correct, or expand further on this.

Cheers,
Katie
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Re: ASD vs LRFD/USD -- plain english explanation??

Postby OsHendz » Mon Oct 20, 2014 3:57 pm

I really needed this simple explanation. Thank you
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Re: ASD vs LRFD/USD -- plain english explanation??

Postby olive » Sun Nov 30, 2014 6:29 am

thanks!
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Re: ASD vs LRFD/USD -- plain english explanation??

Postby Patt » Mon Dec 01, 2014 4:42 pm

I'm two weeks out from the exam. Here is how I've come to understand this issue so far. ASD and LRFD use two different sets of load combination formulas. Any question on the exam related to this issue will give the particular load combination formula to be used if one is required. In a sense, from the point of view of mathematical calculations, that's all I need to know about LRFD. As long as I can use a given load combination formula in a problem I'm good. No?
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