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

Structural Layout Vignette and Multiple Choice

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

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.

-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
Katie

Posts: 49
Joined: Tue Oct 01, 2013 9:03 pm

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

I really needed this simple explanation. Thank you
OsHendz

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Joined: Wed Apr 30, 2014 1:37 pm

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

thanks!
olive

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Joined: Sat Nov 29, 2014 10:22 am

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

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?
Patt

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Joined: Sat May 31, 2014 9:34 am

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