huangyhg

asd vs. lrfd?
i'm a young gun starting out his career, and my father (a civil p.e.) taught me how to do all my calcs using the asd method. i recently bought salmon & johnson's "steel structures" 5th ed. and they emphasize lrfd. is this a trend that is starting to be used more and more? i'd like to know which method is more prevalent throughout the country (my guess is asd), so let me know what you guys are using.
- adam
asd was more popular for steel design before the most recent steel spec was released (aisc 360-05). because of the format of the spec and the manual, lrfd is quickly becoming more popular. additionally, the "new asd", is not your father's asd. it is allowable strength design and uses all of the lrfd equations with a safety factor (omega) as opposed to load and resistance factors.
us old guys were never taught lrfd and so we use asd. the new kids on the block use lrfd. having done a few calcs - lrfd gennearlly allows for a smaller section. not sure that is so great. you must know your exact loads with little "wiggle" room.
asd worked for 100 years --- so why challenge that??
you need to know and understand the (old) asd, but fluent in lrfd, it is the trend.
they don't even teach asd in schools anymore (for steel).
lrfd is becoming more and more prevalent. concrete is almost exclusively designed using lrfd.
wood design has attempted to move toward lrfd, but is overwhelmingly designed asd. (the newest specification is a dual format specification, similar to aisc 13th ed.)
i disagree with you mike; the whole point of having load factors is to account for uncertainties in a statistically significant manner. if your dead load is off by 10%, that's ok because you have factored your dead loads by 20%, etc.
in steel, lrfd and asd are calibrated to give nearly the same results at a ll/dl ratio of 3. at higher ll ratios, lrfd generally requires larger sections.

technically, asd and lrfd are now supposed to yield the same result.... so it really shouldn't matter.
i was taught lrfd in school. went to work and had to learn asd. went back to grad school (while working) and had to re-learn lrfd. went to a steel seminar on 360-05 and had to learn the lrfd equations again. no wonder why i get them mixed up from time to time.
now with the new spec i don't think it really matters. use what ever you think is easier.
personally, i still use asd. i think it's easier because you don't have to calculate a different set of loads to figure deflections.... and i don't have to explain everything to my older coworkers.

asd and lrfd are only supposed to yield the same result for a ll/dl ratio of 3 (as mentioned above).
the obvious reason for one need to know old-school asd is simply because it has been used for so many structures, and is still in use. the posibility you will need to back-check on one of them is fairly high.
i prefer the old asd, but i learned lrfd in school.
thanks guys. as usual, some great input. anyone else use salmon & johnson's steel structures? any other recommended readings?
s&j is pretty much considered the "bible" for structural steel design.

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