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LFRD and ASD 23
- Thread starter lampi2k
- Start date
TehMightyEngineer
Structural
PEinc: Not true if you're under ASCE 7-10. H (lateral earth pressure or ground water) is 1.6 in LRFD, F (well defined fluid load) is added to dead load, and Fa (flood load) goes from 1.0 to 2.0 depending on the zone. So there is a very well designed safety factor for LRFD with fluid and dead loads. You are correct about AASHTO though. I've run into that before where hydrostatic loads controlled the design and we increased the safety factor similar to how ASCE 7-10 does.
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.
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.
TME, thanks; I agree. AASHTO marches to a different drummer. I've done the same with hydrostatic loads on cofferdams for highway projects.
This all emphasizes my point that ASD and LRFD do not always give the same result. It depends.
This all emphasizes my point that ASD and LRFD do not always give the same result. It depends.
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- #24
I brought this subject up years ago concerning academia teaching LRFD exclusively. Although I tend towards ASD (allowable STRESS design) like Ron, I really wish the codes would just pick one and be done with it.
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.
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.
Allow me to amend my last post. You still have to apply a safety factor to the old ASD method, so perhaps the two methods are not that different. Again, I just wish the industry would just pick one. It looks like the universities and codes favor LRFD, so let's just go with that.
- Thread starter
- #27
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
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
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- #32
msquared48
Structural
And just because something is newer does not automatically mean that it is better. That would be a very dangerous assumption.
Mike McCann, PE, SE (WA)
Mike McCann, PE, SE (WA)
Sure, but in this case it is better.
Doug Jenkins
Interactive Design Services
We've discussed LRFD vs ASD several times in the Geotechnical Forums - and being an older fart, I would like to stick with tradition and thank god I no longer work in Canada or the States . . . where the LRFD is being imposed on Geotechs. And where do they get the "factors" from - well, correlating to the old methods . . . so why the change. I can agree with Ron - Whinging is good!
TehMightyEngineer
Structural
About the only thing I could add is I suspect there is some correlation between the plastic design method, the lower bound theorem, and strength design. This suspicion is because plastic design and the lower bound theorem require ductility and verification of each strength limit state to find the controlling design strength. Thus, designing for allowable stress just doesn't seem applicable. Therefor we would have to use the modern allowable strength design and at that point you're basically just doing LRFD only with the more recognizable factors of 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.
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.
I prefer to combine the two methods - using allowable stresses with load factors just to make damn sure the thing never falls down.
Sometimes when I'm in a gray zone I also throw in the phi factors for fun.
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Sometimes when I'm in a gray zone I also throw in the phi factors for fun.
Check out Eng-Tips Forum's Policies here:
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msquared48
Structural
JAE has always been, well, "out there"... ![[laughtears]](/data/assets/smilies/laughtears.gif "[laughtears] [laughtears]")
Mike McCann, PE, SE (WA)
Mike McCann, PE, SE (WA)
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- #39
It's just that these LRFD vs. ASD conversations are definitely beating a dead horse.
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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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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TehMightyEngineer
Structural
JAE: Well, I would agree with you except the AISC presentation I linked to up above really convinced me that if you're using ASD for a large structure you can be forced into stupidly conservative designs because the codes appear to be written with LRFD in mind and then ASD shoved in afterwards. I used to be ASD all the way but that presentation convinced me that, given the original question, LRFD can provide significantly more efficient structures than ASD given very particular conditions. Plus if you're using the direct analysis method of AISC or have any concrete involved then LRFD will likely be simpler in the long run.
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.
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.
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