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LFRD and ASD
23

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

RE: LFRD and ASD

In the end both methods should lead you to roughly the same answer.

RE: LFRD and ASD

It should be LRFD by a nose. But if the design is deflection controlled,it won't matter.

Enercalc lets you switch to either one easily. It's usually pretty close.

RE: LFRD and ASD

I never really liked LFRD. Almost always use ASD

Faith is taking the first step even when you can't see the whole staircase. -MLK

RE: LFRD and ASD

Since the addition of the Black Book (AISC 360-05) ASD/LRFD have been calibrated to have roughly the same results.
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

One of the benefits of LRFD is if you have a project with a lot of interaction between steel and concrete, you already have the ultimate loads. LRFD is our company standard. Design with LRFD will result in more members being controlled by serviceability load cases, ASD will result in more members being controlled by strength load cases, but as other posters have said, the results are about the same.

RE: LFRD and ASD

5
I went to the doctor and learned they are now practicing LRFD in medicine. If you have a sickness, they factor it up to the next higher illness to make sure you are adequately cured. For example, if you have a cold, they will factor it up to pneumonia and treat you accordingly. They say this is to increase the odds that you will be cured based on their studied statistical analysis of illness.

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

4
LRFD is an overcomplicated solution to a problem which does not exist. We ASD'ers lost the war probably 10 years ago, but we keep the flame alive in our hearts.

RE: LFRD and ASD

One thing I recently learned is that when you calculate the effective length per an amplified second order analysis using the effective length method you can actually get some very different results comparing ASD to LRFD with ASD being the more conservative. In the office I've typically used ASD for smaller projects with no concrete design.

Maine EIT, Civil/Structural.

RE: LFRD and ASD

2
I totally ignore LRFD. It makes no sense to complicate a simple solution.

Mike McCann, PE, SE (WA)


RE: LFRD and ASD

There is a good article in the August 2014 STRUCTURE magazine addressing ASD vs. LRFD for joists. It states that when live load is less than three times dead load, LRFD design will produce a lighter joist, just as jdgengineer pointed out above. The article also talks about the advantages of stretching out the joist spacing vs. deck cost, but I don't like going >6' on center in any circumstance with the normal 1 1/2" deep wide rib deck that I normally use.

RE: LFRD and ASD

6
Oh waaa waaa.. I normally don't like to antagonize people like this but seriously. Get with the times. Most other countries have gone to both metric and LRFD. Why do people from the US refuse to follow the lead of everyone else. You sound like my wife when she thinks her way must be the best and won't listen to anyone else.

RE: LFRD and ASD

If ASD and LRFD give the same answer, it is a coincidence. In ASD, there is one allowable stress and, therefore, one safety factor resulting from all of the different load types. In LRFD, different load types have different load factors based on the accuracy/reliability of load values and based on resistance factors of the structural material. For example, a steel beam designed by ASD for a highway project and supporting a large LL may use an allowable bending stress of 0.55Fy which gives a safety factor of 1/0.55 = 1.818. In LRFD, the LL load factor would be 1.75 with a resistance factor of maybe 0.9 which essentially means the safety factor is 1.75/0.9 = 1.94. For other load types known with greater reliability, the ASD & LRFD "safety factors" could be closer. The strong point of LRFD is that it allows a designer to more accurately fine tune "safety factors" according to the reliability of the different load types (wind, water, LL, DL, earthquake, etc.) and different structural materials (wood, concrete, steel, etc.). That being said, I don't like using LRFD. It creates more work for me because I often have to design a project using both ASD & LRFD methods. Seems counterproductive to me.

www.PeirceEngineering.com

RE: LFRD and ASD

LFRD is better. ASD has been calibrated to give roughly the same results, so you have the illusion of still using ASD, but really it's just masking the fact you're design is still based on statistical analysis of probability of failure, based on variability in load and materials.

RE: LFRD and ASD

ASD existed well before LFRD, so it was never "calibrated". A safety factor is a safety factor. Doesn't really matter whether you apply the safety factor to the load or to the strength of the member.

Faith is taking the first step even when you can't see the whole staircase. -MLK

RE: LFRD and ASD

I am tired of being told to "get with the times". If it ain't broke, then don't "fix" it.

Mike McCann, PE, SE (WA)


RE: LFRD and ASD

ASD as allowed by code now is not the same as it used to be, and the safety factors you use now are set up to give similar answers to LFRD. LFRD is simply a better method than guessing a safety factor and going with it.

RE: LFRD and ASD

ASD hasn't really changed. Instead of multiplying by 0.6, it not requires dividing by 1.67. Result is same.

Faith is taking the first step even when you can't see the whole staircase. -MLK

RE: LFRD and ASD

http://media.aisc.org/NASCC2014/140327-N40A.mp4

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

You still need to consider the different load factors for different types of loads. There are too many combinations to make a general statement that the result is the same for ASD & LRFD. As usual, "It depends."
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

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.

RE: LFRD and ASD

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.

www.PeirceEngineering.com

RE: LFRD and ASD

I think 0.6*DL + 1.0*WL actually has a physical meaning, unlike 0.9*DL + 1.6*WL for overturning.

RE: LFRD and ASD

ASD is intuitive and less prone to application error. I sometimes have to use LRFD....but I do so while kicking and screaming....and whining Waaah! Waaah!

RE: LFRD and ASD

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.

RE: LFRD and ASD

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.

RE: LFRD and ASD

(OP)
Thank you all guys,. is ASD more conservative than LFRD?

RE: LFRD and ASD

Quote:

LRFD is an overcomplicated solution to a problem which does not exist.

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

LRFD was rejected/ignored out of hand by the PRACTICING engineering community for over 20 yrs until it was forced on them....wonder why??

RE: LFRD and ASD

Quote (SAILS3)

...wonder why??

I do, why?

RE: LFRD and ASD

Quote:

LRFD was rejected/ignored out of hand by the PRACTICING engineering community for over 20 yrs until it was forced on them....wonder why??

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

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)


RE: LFRD and ASD

Quote:

And just because something is newer does not automatically mean that it is better. That would be a very dangerous assumption.


Sure, but in this case it is better.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: LFRD and ASD

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!

RE: LFRD and ASD

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.

RE: LFRD and ASD

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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RE: LFRD and ASD

JAE has always been, well, "out there"... laughtears

Mike McCann, PE, SE (WA)


RE: LFRD and ASD

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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RE: LFRD and ASD

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.

RE: LFRD and ASD

As JAE pointed out, there have only been a few posts that actually addressed the OP's question. Since we've digressed I've decided that writing a haiku will be just as productive as arguing preference

Allowable Loads
Resistance changing Designs
Factors our Stress now

RE: LFRD and ASD

Well, his question was, "What method is most economical and safe to use in the design of steel structure, is it LFRD or ASD?" (With emphasis added.)

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

Just about everything I do is serviceability controlled. Less time = more money. ASD for the win!

RE: LFRD and ASD

Archie: Yep, I can't think of one reason that makes LRFD safer than ASD except perhaps that one could make a mistake going from an ASD steel design to LRFD concrete design, but that's all I got.

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

I just like to keep it simple, both in the design approach and the construction. Complicating anything increases the possibility of errors. Errors increase costs too. ASD over LRFD. I will be quiet now...

Mike McCann, PE, SE (WA)


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