LRFD vs ADS Wood design
LRFD vs ADS Wood design
(OP)
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?
Any thought?






RE: LRFD vs ADS Wood design
DaveAtkins
RE: LRFD vs ADS Wood design
Of course, as steel and concrete use limit states design, it would be nice to use one methodolgy all the time.
Rik
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
Are the load factors appreciably different from concrete or steel?
How about the resistance factors?
Rik
RE: LRFD vs ADS Wood design
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 members, such as the compression of the post at the end of a shearwall, but that is just sharpening the pencil way too much.
RE: LRFD vs ADS Wood design
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.
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
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.
RE: LRFD vs ADS Wood design
For wood, given its inherent material variability, and the fact that we really don't know what the loads will really be, I think that an LRFD approach is just an exercise in academic rigor. The academic community is trying to ram it down our throats, yet they don't have to perform designs to a schedule (often inadequate) and a budget(usually inadequate).
RE: LRFD vs ADS Wood design
Even publications by organizations and the publishers of textbooks usually have several serious errors regarding LRFD and no one attempts to publish a design example from start to finish because of the possible errors that will crop up.
Maybe the engineering community can put a stop to LRFD if we speak with one (more or less) voice.
Comments?
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
Hmmm...cheap labor, no OSHA, unlimited materials, no government restrictions, no schedule except for your own desires (as local god)....sounds like heaven for some folks, like emperors or dictators. And no lawsuits either!
I agree - LRFD is frowned upon right now because its new and requires engineers to learn new techniques that they've grown accustomed to.
But I like CraigICE's quote: "LRFD is the answer to a question nobody asked."
RE: LRFD vs ADS Wood design
I like the quote too. Maybe it's my youth talking
RE: LRFD vs ADS Wood design
Also, I don't think that the advances in structual engineering can all be attributed to an LRFD approach. I think you can be skeptical of LRFD but still in favor of expanding the state of knowledge.
RE: LRFD vs ADS Wood design
DaveAtkins
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
Foremost, nobody should be telling us that we have to stop using design processes that are proven to work. As long as clients are happy (ok, moderately satisfied) and structures are safe, why does it matter whether we used ASD or LRFD to get there?
RE: LRFD vs ADS Wood design
RE: LRFD vs ADS Wood design
It only takes them about three years to shed the LRFD.
sduggan's comment about designing to the nearest in-lb is a good point. I showed a youg graduate how to sum up the weight of the steel framing in the 200' x 200' second floor in a central utilities building and then reduce the roughly 12 sizes to three or four by changing the size not before an 8 lb to 12 lb/ft spread in weights occurred. The difference in framing techniques amounted to about 300 lbs for a 40,000 sf floor. At $1 /lb installed, that $300 spent would have saved thousands in detailing expense and waste due to drop-off in the fabrication shop.
This project had been "value engineered" by the contractor's estimater (sp?). To add insult to injury, they decided not to use the 12" concrete slab as originally designed but went with 4" of concrete over metal deck.
The contractor must have made thousands off of this careful design and I made tens of thousands reinforcing the floor so that my customers could install the tanks that everyone new would be placed there.
Even for concrete design, LRFD is relatively meaningless. It's the Whitney's stress block that makes the difference. And you'll notice that over the last 60 years, concrete shear has gotten more and more conservative and that didn't involve LRFD.
Again, shouldn't this discussion be taken mainstream? How many people would like to read our collective comments and add their voices?
Also, forget about that other improvement to engineering called SI units. The code writing people and government agencies are dropping this elegant system of units in favor of our barbaric imperial system. That means no mks (meters, kilograms, seconds) for awhile either.
The distance between the tips of my thumb and little finger is about 20 cm. That's easy -- Newtons and Pascals, that's not.