huangyhg

lrfd vs ads wood design
i am just curious as to how many people use asd or lrfd for wood design. i am a new graduate and i have learned asd wood design, but i think that lrfd may be more efficient as well as less cumbersome.
any thought?
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i use asd, but then i am an "older" engineer (just turned 43 yesterday). one advantage to asd is that you don't have to use two different sets of loads to determine stresses and deflections, as you do with lrfd.
daveatkins
i use asd for wood. having not looked at the lrfd manual for wood, i think asd would be less cumbersome. i agree with dave, only one set of loads in asd.
of course, as steel and concrete use limit states design, it would be nice to use one methodolgy all the time.
rik
lrfd makes sense for steel and concrete, but is just an additional "bookkeeping" problem in wood. when the code-writers devised the factors for wood lrfd, all they did is calibrate back to asd, and in most cases, you end up with the same size.
i use asd for wood design. have not attempted lrfd yet.
agree with sacreblue...wood lrfd design is a straight conversion from asd with no noticable efficiencies.
to those who have looked at the wood lrfd,
are the load factors appreciably different from concrete or steel?
how about the resistance factors?
rik
rday,
me, i don't recall - i attended a wood lrfd seminar years ago, and we decided then it was a "lost cause". i suppose it may appeal to someone needing an exact analysis of combined gravity load/wind/seismic
lrfd is the answer to a question nobody asked.
it became a part of life when we started designing concrete with the whitney's stress block. it doesn't make much sense for wood because deflection governs much of the time and wood quality varies.
where steel is concerned, you might save 5% of your framing weight but check your floors for vibration.
where concrete is concerned, multiply the sum of your loads by about 1.55 and you'll be within 5% of the lrfd answer. 5% doesn't mean much -- i designed a house in guam for 170 mph gusts and a few years later guam saw a typhoon of 170 mph sustained winds. seismic zone ii stoped in a straight line at the oregon/california border about 25 years ago. now western oregon is zone iii per '97 ubc and parts of the coast are zone iv.
hence, when we don't know what the loads will really be, and nothing gets built the way we design it anyway, fine tuning loads with lrfd only makes sense to people who don't do civil-structural engineering for a living.
craigice: i agree with you 100%.
no matter what we think of the discrepancies asd is being phased out. if you've looked at the new aisc manual you know that asd is now gone for steel, to use "asd" you just back out to service load level after calculating nominal strengths. ncees is going to change the se1 exam from asd wood to lrfd wood (although they promised they would change by 2003).
i'm 27 years old and am in the "grey" area. i have to know both methods because: 1) i work under older engineers that won't even discuss methodologies developed after 1965, 2) i work on alot of international jobs and nobody but the u.s. seems to use asd anymore. most universities are teaching lrfd (because those professors are the ones mainly writing the new codes), so it seems like once the now ~30 year old engineers retire we'll be totally on lrfd.

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