explain asd vs lrfd to a dumb me

huangyhg

explain asd vs lrfd to a dumb me
can someone explain in really simple terms the difference between asd and lrfd? asd seems easier but is it going away?
i was reading through the new aisc steel construction manual about the two methodologies and, aside from the different load combinations, was having a hard time seeing a clear distinction between the two.
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the traditional difference is using service load stresses against an allowable stress for asd or using factored load forces against a maximum strength for lrfd.
asd has changed from allowable stress design to allowable strength design.  there used to be different equations for asd and lrfd (they were completely different specs), but now there is one equation for design strengths that are divided by a safety factor, omega, for asd and multiplied by a strength reduction factor, phi, for lrfd.
the only real difference now is that asd provides a constant factor of safety for all designs regardless of load types while lrfd provides a higher factor of safety on the loads that are less well defined (1.6 ll factor compared to a 1.2 dl factor).  at a ll/dl ratio < 3, asd is more conservative, at a ll/dl ration = 3, it is a wash, and at a ll/dl ratio > 3 lrfd is more conservative.
any additional questions, please ask.
it really boils down to how you apply a safety factor to the design of   
it really boils down more to a statistical probability of failure.
in asd you treat dead and live loads equally. for example, if you have a case where your dead to live load is 1 to 1 (say 200 kips dead and 200 kips live) the safety factor would be exactly the same as if your dead to live ratio was 1 to 3 (say 100 kips dead and 300 kips live) given the same total load.
lrfd recognizes the inherent unpredictability of loads and assigns a much higher "factor of safety" to live loads (we increase them by 1.6), whereas it recognizes that dead loads are most likely much closer to what you calculate (we only multiply dead loads by 1.2).
by the same token, lrfd also recognizes the uncertainty of different failure modes. for example, flexural capacity of a concrete beam is fairly predictable; therefore we count on 90% of the theoretical value. shear in concrete, on the other hand is much less predictable; therefore, we only count on 70% of the value. i believe asd also recognizes this to some extent, by applying different factors of safety  to different failure modes, but i only really worked asd for a few months, so i don't remember.
when all is said and done, you design so that the decreased member capacity with which you rely exceeds the capacity required by the factored loads.
in asd you take the total loads that you expect on the structure and apply a single factor of safety to the   
as jae mentioned above, another reason that certain failure modes are "penalized" more harshly than others is because we want failures to be ductile. a beam that's failing due to excessive flexural stress tends to give a lot of warning (you see walls cracking and doors not closing, etc.)
a shear failure on the other hand is brittle. unless you actually see the beam and see some shear cracks forming, you really wouldn't be able to see anything going on in your structure before it fails (assuming it was over loaded). as a result, you go the extra mile to avoid these failures and you do so by reducing their theoretical capacity by a higher amount than you do for ductile modes of failure.
a significant difference is that you can look at a structure as a complete item and apply a more uniform factor of safety (not to be confused with a safety factor) to the overall structure.  this you can do with lrfd.  lrfd gives a better appreciation of the manner of failure and loading.
dik
i hate how they cram lrfd down your throat in school.  asd is easier to use and generally more conservative.
abusemantpark-
they "cram" lrfd down your throat because all the major codes are moving this way. as dik mentioned, it provides a more uniform (and consequently reliable) factor of safety.
as for more conservative, that entirely depends on the dead to live ratio. as i mentioned in my previous post, asd and lrfd were calibrated at a specific dead to live ratio (i believe 1 to 2). at this ratio the true factor of safety is the same. if your dead to live ratio is lower than 1 to 2 (that is, more dead load) then yes, asd is more conservative. however, at anything above this ratio, lrfd is more conservative (as it places more uncertainty on live load).
i'm not knocking asd- i know it's been used for many years quite successfully. but lrfd is not entirely without merit.
i hadn't read structural eit's post- i'll take his word for it and say the ratio is 3 to 1.
frv, strleit is correct about the breakeven being at 3.  i think this is in part 2 of the manual.  
i like to think of it as large l/d --> use asd because it's more economical.  then again, i'm a metal bldg systems guy at heart, lol.
jae:
i take it that the .9 resistance factor allows for the milked cow condition?
mike mccann
mccann engineering