huangyhg

industry practice for steel design
hi,
i was wondering, if it is the current industry practice to use limit state for steel design or are you still using working stress (allowable) method?
neilly davies consulting engineers
in the us, everything is moving to limit states, but asd is still widely practiced; especially by more experienced engineers and those who had experienced engineers as mentors who didn't want to learn lrfd.
in australia limit state is almost universal for structural design but some asd is used by mechanical enginers.
thank you for the response, as you mentioned many of the most senior engineers are still using asd. but is not difficult to switch. lsd started in concrete well before in steel.
the current steel manual (aisc 13th) has asd and lrfd in alternate columns on the same page, (both are approved).
but the 13th edition asd is not truly asd, it is lrfd in disguise.
actually, it really is asd. had there never been an lrfd, this is what the evolution of asd would have looked like. the reason it is so very different than the green book is that there were no real updates to it through all the years that lrfd was being developed. now it is state-of-the-art as it would have been if there were no lrfd.
if you "heard" it on the internet, it's guilty until proven innocent. - dcs
i think the hangup is how the old asd is "allowable stressfff"> design," and the new asd is "allowable strengthfff"> design." and why this causes people headaches is beyond me. the equations are the same, you just multiply your allowable stress by an area or section modulus to get an allowable force or moment.
a pet peeve of mine is how people say "limit state design" to mean "lrfd." asd is a limit state design, too. you check your loads to serviceability and strength limits. where the factor of safety falls does not mean this isn't limit state design.
i've only used lrfd a few times and am a proud owner of the "black book". for the most part, everyone i know uses asd and those of us that have used lrfd at all have noticed that the calculations are much more lengthy with lrfd than is the case with asd. as i typed that last line, i could actually hear the groans of the lrfd people, followed by a "no, it's not."
one of the hangups is that most of us tend to think in terms of stress, not load. just one example is bolts. regardless of size, if you tell me that a bolt has a certain tensile stress, i know whether or not it will be ok. when you tell me that a 2 1/4" a615 gr 75 anchor rod has 190 kips of load, it doesn't click with me whether or not it is ok. it's much easier to remember the allowable stress for commonly used bolts that it is to re
quote:
one of the hangups is that most of us tend to think in terms of stress, not load.
hmm, short of taking a survey, i'm not sure about that.
i'm an asd guy, but i'll defend the groaning lrfd guys. the calculations are more lengthy not because it's lrfd, but because you're using a manual (green asd) that is almost 20 years old. the new and improved methods result in more lengthy calculations. blame that on progress, not lrfd.
look at the equations in the 13th ed. they are basically the same for asd & lrfd. the "asd safety factor" vs. the lrfd strength reduction factor are the main differences. structural eit had it correct. the only real difference between the 13th ed lrfd and asd is on the applied load side of the calculation, the resistance calculations are virtually identical.

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