LFRD and ASD
LFRD and ASD
(OP)
What method is most economical and safe to use in the design of steel structure, is it LFRD or ASD? thanks
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RE: LFRD and ASD
RE: LFRD and ASD
Enercalc lets you switch to either one easily. It's usually pretty close.
RE: LFRD and ASD
Faith is taking the first step even when you can't see the whole staircase. -MLK
RE: LFRD and ASD
I typically, use whichever is more convenient for me on the project.
If the project is mainly steel I use LRFD.
If the project is mainly wood with supplemental steel I use ASD.
The differences in the results are related to the live to dead load ratio. I believe that ASD is more conservative when the live load to dead load ratio is <3. If it is higher than 3 than I believe LRFD is more conservative. I might have the exact turning point wrong, but that is the concept.
RE: LFRD and ASD
RE: LFRD and ASD
Then, to keep from over-curing you, they will reduce the dosage of the drugs, usually by eighty to ninety percent. More for certain types of people.
RE: LFRD and ASD
RE: LFRD and ASD
Maine EIT, Civil/Structural.
RE: LFRD and ASD
Mike McCann, PE, SE (WA)
RE: LFRD and ASD
RE: LFRD and ASD
RE: LFRD and ASD
www.PeirceEngineering.com
RE: LFRD and ASD
RE: LFRD and ASD
Faith is taking the first step even when you can't see the whole staircase. -MLK
RE: LFRD and ASD
Mike McCann, PE, SE (WA)
RE: LFRD and ASD
RE: LFRD and ASD
Faith is taking the first step even when you can't see the whole staircase. -MLK
RE: LFRD and ASD
Great presentation, well worth watching the whole thing. However, skip to 1:07:20 where he compares LRFD to ASD with 2nd order effect per AISC and ASCE 7. Blew my mind how different they were.
Maine EIT, Civil/Structural.
RE: LFRD and ASD
Load factor for a water load is 1. Therefore, if designing for a water load only, there will be very little or no safety factor using LRFD. Whereas, for a live load of the same magnitude as the water load, the load factor would be 1.75 per AASHTO. The answers cannot be the same. That's a big difference to consider (1 vs. 1.75). I would not design with a LF of 1 when water is the major, critical load.
www.PeirceEngineering.com
RE: LFRD and ASD
AASHTO vs ASCE 7 is a little bit apples and oranges though as AASHTO uses different phi factors than ACI, AISC, and so on.
Maine EIT, Civil/Structural.
RE: LFRD and ASD
This all emphasizes my point that ASD and LRFD do not always give the same result. It depends.
www.PeirceEngineering.com
RE: LFRD and ASD
RE: LFRD and ASD
RE: LFRD and ASD
Has anyone considered blending the two methods into one? Isn't there a way to modify the AISC load factors of LRFD to get the same end result? One reason I like ASD is that you only deal with the AISC load factors. LRFD requires a load factor and strength reduction factor. The old blue 7th edition manual was nice and simple (yes, I'm pining over the good old days).
Perhaps the real question is: are we more concerned about saving a few dollars on member size or designing a structure that we feel more confident will stand up to the loads over 20 years? AISC 7 has been roundly criticized for evaluating loads such as wind to the nth degree, and to what end? Remember the old adage: measure with a micrometer, mark with a crayon, and cut with a chainsaw. I guarantee you the construction contractor is still cutting with a chainsaw.
RE: LFRD and ASD
RE: LFRD and ASD
RE: LFRD and ASD
I disagree. It's an imperfect and partial solution to a problem that does exist.
The problem is how to make the best use of higher strength materials, without increasing the risk of failure or serviceability problems.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: LFRD and ASD
RE: LFRD and ASD
I do, why?
RE: LFRD and ASD
Because people get set in their ways, and tend to think the way they have always done it is the best way, even when it isn't.
That's why.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: LFRD and ASD
Mike McCann, PE, SE (WA)
RE: LFRD and ASD
Sure, but in this case it is better.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: LFRD and ASD
RE: LFRD and ASD
That said, it's only a suspicion. I don't know enough about plastic design concepts to know if strength design is really required for it. My understanding is such but I bet someone much smarter than I can correct me if I have assumed incorrectly.
In the end I think the standards such as AISC, ASCE, ACI, and so on are being developed more and more for LRFD with ASD (strength) being cheated in. Thus,there will probably be more and more discontinuities between LRFD and ASD in the specification, with LRFD being what the specifications are preferred to be used with. As a young engineer not influenced by habit I prefer ASD but, in working with the modern codes, I will probably start used LRFD a lot more as we get further along in the code cycles.
Maine EIT, Civil/Structural.
RE: LFRD and ASD
Sometimes when I'm in a gray zone I also throw in the phi factors for fun.
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RE: LFRD and ASD
Mike McCann, PE, SE (WA)
RE: LFRD and ASD
http://www.deepexcavation.com/en/unconservative-aa...
RE: LFRD and ASD
They never convince anyone to leave their favorite.
The original question was not which is preferred - but rather which method provides more economy or more safety for steel design.
I think the question was answered pretty well at the beginning but as usual descended into which is preferred.
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RE: LFRD and ASD
However, I certainly concede that for any reasonably small project it's completely a wash and will always boil down to user preference and thus these discussions rarely come up with "new" information.
BigH: Your link comes up with a 404 error.
Maine EIT, Civil/Structural.
RE: LFRD and ASD
Allowable Loads
Resistance changing Designs
Factors our Stress now
RE: LFRD and ASD
Several people addressed the economics of it and Ron addressed the "safe to use" issue. As he pointed out, ASD is less susceptible to user error and surely that's of consequence to those of us who are highly susceptible to such...
Here's my haiku:
Dead load plus live load
equals total load. Unless
factors phi it up.
RE: LFRD and ASD
RE: LFRD and ASD
That said, if you look at the original question saying "what method is BOTH the most economical AND safe to use in the design of a steel structure..." I would say LRFD would be the answer. It is safe as far as I can tell. Maybe not "safest" but it is safe. It's also the most efficient unless you have a very large live load relative to your dead load. Thus, I would say the only reasonable answer, however against the grain it is, would be LRFD.
Maine EIT, Civil/Structural.
RE: LFRD and ASD
Mike McCann, PE, SE (WA)