ASD vs. LRFD, Who Decides? 5

[vmirat]

I just bought the fifth edition of Salmon/Johnson/Malhas "Steel Structures - Design and Behavior." This text book is focused on the LRFD method. In the preface, they state, "This modern philosophy of design [LRFD], discussed only briefly in one section of the second edition, is moving toward being the predominant approach to design."

There have been many posts to this forum about ASD vs. LRFD. Several people have commented in this forum that they were taught LRFD in school but that their engineering firm uses ASD. It seems apparent that academia has made the decision that LRFD is the best method and therefore the only method to be used. The question is whether academia should be driving this train or the industry? There is a form of social engineering (pardon the pun) going on here if academia is making the decision for us as opposed to the code committees.

Some have commented that, as far as the 13th edition of the steel manual goes, the results are pretty much the same regardless of which method you use. If so, then why the push for one method over the other in the academic world? Since both methods are related to code compliance, perhaps academia shouldn't teach either one and leave the code issues to on-the-job training or offer a separate course that covers all the code aspects.

My own personal desire is for one method for all materials: steel, concrete, wood, masonry, etc. I don't really care which one it is, as long as it's consistent. But, in my opinion, the decision as to what system is used should be made by the industry, not the teachers.

 

[kslee1000]

271828:

I don't "disagree", nor I "agree". You have read my post correctly. If I am still in practice today, there will be no doubt that I will stay with the trend, but be glad to have the knowledge of something that is fading (similar to the time from WSD to USD for RC).

While we are lucky to have walked from A to B, for the fellows start directly from B, would they be missing something that is originated from A? I won't make comment on something I don't have full knowledge on, but to openly discuss with reservation of doubts. Remember plastic design method?

 

[vmirat]

hokie66,

I'm a civil/structural engineer with DoD. I served six years active duty and took a job with the government. It's been a very interesting career, going places most people would never go.

I agree that the issue of codes in the classroom are inevitible, but I would argue that they belong in a class by themselves, and discussed in a broader sense versus biasing to one method or the other. As many posters to this site have stated, the use of ASD for steel is still very much alive and forcing LRFD-only on future engineers does a dis-service to them. It's kind of like the metric system. My structures class at VMI was taught purely in metric. In my opinion, it's not up to academia to determine how the industry works, their just suppose to prepare us for it.

 

[HgTX]

My steel design class was taught in ASD. The next year they switched over to LRFD, and I was a teaching assistant, so I had to learn it too--on my own, since I wasn't a student in the class.

IT WASN'T THAT BIG A DEAL.

Hg

Eng-Tips policies: faq731-376

 

[271828]

...but I'd argue that the current steel design curriculum does a better job than if it was modified as you suggest.

I believe that ASD will evaporate completely in the next few years. Our job, right now, is to prepare engineers for the long haul, not the next 2 or 7 years.

There is a very good reason why there are not two steel classes, one focusing on behavior and one on codes: nobody would take two undergrad classes. If we took that approach, then everybody would take the code-oriented class because the student would be thinkig short-term. Now we have a bunch of technicians who don't know squat about steel behavior.

Sorry, but in my opinion, the two steel design classes I'm familiar with (they're all about the same at the major universities, I *think*) are about as optimized as they can get in the sense of teaching the max amount of behavior along with enough Spec. provisions so that the students are as useful as a new guy can be.

I've actually found this entire subject to be pretty funny over the last couple of years. This stuff goes both ways. Professors who've never designed (a crime in my opinion) might fall into the trap of thinking they know how designers operate. Similarly, when I retired from design to be a teacher, I thought I knew it all about how teachers should be doing everything. I see that every time a thread like this comes up around here. The thing that goes both ways is: "it's a lot harder than it looks" LOL.

 

[nutte]

27128: Perhaps you have it backward? You teach students one way, and they have to learn a second way at some point in their career. If they learn ASD now, they'll be prepared for the offices that use ASD. Ten years from now, if ASD is gone and LRFD is the norm, they're in a much better position to pick up a new method. The alternative is teach them something they won't use now (LRFD), forcing them to learn another method (ASD) right out of college, when they have little experience.

Of course, if they go to an office that uses LRFD, they're out of luck.

I would prefer to see the universities teach both methods. Light gage steel design is the only class I had where they taught us both. Now with the 13th edition, we do have parallel equations and methodologies, so this should be an attainable goal.

It will be interesting to see if you're right about ASD dying in ten years. The biggest disadvantage in my mind, which has been mentioned in this thread and others, is having to run both cases anyway for serviceability. Sure, you can provide a more reliable factor of safety (abstract as that is), and you might save a few pounds on a beam here and there. In practice, who's desigining that close? Would not the vast majority of designs come out the same regardless of the method used?

 

[nutte]

Minor correction to above post: you said "next few years," not "10 years."

 

[BliMan]

The universities should also teach the 'three legged stool' where you spend a ton of design time on joint geometry, web crippling and code check, but liability and lack of client interest keeps you in the office after your plans go out for rubber-stamp code approval, then off into the field, and later you read in the young inspector's notes that your slip critical joints were 'hand tight' so 'no problem'; the steel delivered to the site 'appears to be Chinese origin', and the welder wasn't certified for those full-pens, but don't worry, he scarfed the inclusions out and did a heavy cover pass on the second try to make up for the heat stress warpage. % )
That's why ASD makes more sense, than saving on steel weight.

 

[vincentpa]

ACCURATE SAFETY FACTOR?????!!!!???!?!?!

In what universe?

Unless we have very specific design information from a manufacturer for a specific loading, we can never have an accurate safety factor because we cannot accurately determine loading! Even dead load is estimated for most design purposes, usually well over-estimated. Live load... forget about it! Accuracy is a misleading word to use. Accuracy depends on quality information on which to base a design. Since we cannot accurately determine the loads, we cannot accurately determine a safety factor. Therefore the arguement that LRFD produces a more accurate safety factor is moot. In fact it is laughable; something that could only be debated in the world of academia. Also, if you are designing to a gnats behind, you are doing a disservice to everyone. There is fat built into every design... I hope.

271828,
If you are correct in that the ASD spec will disappear I will make a prediction. If the ASD code disappears, no engineers will use LRFD. The engineering community will use the 89 green book or the 13th Edition ASD. The engineers will complain to AISC until AISC reinstates ASD... again.

If AISC removes ASD in the future, it will only go to prove my theory that intelligence and foolishness are often mutually exclusive. (BTW 271828 that was not directed at you.)

 

[vincentpa]

Bliman,
Right on! Not only have most professors never designed, they've never been to a jobsite. Even with inspectors there, the structure will not be built as designed. Put fat into it! Going onto a jobsite will put the fear of God in you.

 

[ash060]

ASD in Steel design is dead all ready. In you look in the AISC 360 2005 spec or 13th edition black book, and find the definiton of ASD; it will not say Allowable Stress Design. It will say Allowable Strength Design.

There is not one mention of Fb or Fv in that book. There is no 0.66Fy.

The code has switched to only LRFD; there is not two sets of equations, just two different sets of safety factors.

Just go to page 2-7 in the Black Book and they will tell you that there is no allowable stress design just limit state checks both use the same principles.

So you can use either. I mean the spec itself rarely refers to ASD or LRFD except in the beginning of the chapters where they list the safety or phi-factors.

For example look in chapter E of the spec. They only mention safety factors in one section E1, and no where else. Equations are the same for LRFD and ASD.

 

[nutte]

ash060, there is no difference between old ASD and new ASD, even if AISC says, "but the S is different." They are the same, merely updated to reflect what we know today.

They don't say Fb = 0.66 Fy. They say Mn = Fy Z, and omega = 1.67. Z is about 1.1 S, and 1/1.67 = 0.6, so Z / omega is the same as the old "0.66 S". An algebraic rearranging of equations does not constitute a new design method.

 

[ash060]

No mechanics has not changed but the old method calculated with stress the new method uses forces.

I can't take an applied moment to a beam and divide it by "Z" and get the stress at the extreme fiber. I have to divide by "S" to get the stress which I compare to Fb. In the plastic case I know the stress right away, it is Fy.

The process is different. Allowable stress compares stresses, and allowable strength compares forces.

Z is not new. We have know about inelastic behavior before we started writing steel specs.

Testing and mechanics helps make specs and testing is all most always done up to ultimate. I mean they don't load the beam a little bit and stop, they load it until it breaks.

 

[JAE]

vincentpa, the term "accurate safety factor" isn't a very proper phrase to use.

It sort of misses the point of LRFD. LRFD doesn't claim to have "accurate" safety factors....just more rational ones.

What LRFD does do is look at the different loads (dead, live, etc.) and attempts to weigh their variability in real life....basically what you then state emphatically when you say "...dead load is estimated for most design purposes, usually well over-estimated. Live load... forget about it"

You basically are making the case FOR LRFD by stating the fact that some loads are more understood than others. ASD doesn't even address this.

There have been measurements of materials over the years where we do have a pretty good understanding of what a structure's dead load is as well as its variability.

As for live loads, there have been studies over the years that have attempted to pin down various live load values in actuality (in place on a floor) and also attempts at measuring LL variability (do a search for Bruce Ellingwood as he has done a lot of research in reliability over the years).

I think you have a good point in that live loads are pretty loosey-goosey in real terms. But I don't agree with you when you state all that and then imply that ASD is better....or just as good...or we are foolish to use LRFD.

LRFD provides a similar level of safety but also takes into account live load uncertainty.

I grew up on ASD in school and used it for the first 12 years or so of my work experience. I learned LRFD on my own in 1993. So I think I'm one of those engineers you suggest won't use LRFD if ASD goes away. I do use LRFD and actually prefer it now.

I would second the opinion given above by HgTX - "IT WASN'T THAT BIG A DEAL." and it isn't that big of a deal to learn it.

 

[vmirat]

JAE,
Going back to the intent of my original post, what do you think about academia teaching one method exclusively? Should the college professor be the one making the decision for the industry?

 

[JAE]

If I was the student....or if I was hiring a student out of school, the "good" student would know both. We do get called upon at times to review other's work, past calculations, and past designs. We should, as good engineers, be able to decipher either method.

Since AISC has combined the two in one specification, it seems to me that they could easily incorporate both methods.

Do they have the right to choose one over the other? I would think they do if they have the internal control over the curricula. However, it would seem to me that they'd be under pressure to produce good engineers coming out of university and most would (or should) decide that both methods are necessary.

Now as time goes on, and if one method becomes more prevailent, then perhaps they will eventually abandon one over the other.

 

[271828]

Are some of you suggesting that ASD89 be taught in 2009? If so, then which half of the following topics should be deleted to make room for it? Columns, braced beam flexure, shear strength, base plates, bearing plates, unbraced beams, deflections, beam-columns, bolted tension connections, welded tension connections, tension members, and composite beams.

For the record, I learned LRFD in school, then was forced to use ASD89 for 3 years, then switched back to LRFD for 6 years before I became a professor. Honestly, for someone who will study, it's not a big deal either way.

vmirat, I can tell you with 95%+ assurance what would happen if I mixed ASD2005 with our current curriculum. The upper half of the class would trivially go back and forth between the two methods. The bottom half of the class would struggle figuring out when to factor the loads, otherwise would find the issue to be trivial. The end result would almost certainly be that the inclusion of ASD added nothing to the course and muddied the water a little for the weaker students. This muddying would take away their attention from more important aspects like behavior.

As for the question of whether profs should decide the curriculum, that's an easy one: No, not alone anyway. We have industry advisory boards who give their input, and it is very often used to make internal decisions about the curriculum.

Also, I can't speak for other profs, but I spend a LOT of effort to try my best to help the students develop what I *think* will serve them best in industry. I don't know anybody who uses the engineering classroom to try and ram something down industry's throat.

Why is it that some design guys want profs to respect their experience while not offering the same in return? Imagine what would happen if a prof who'd never been in an engineering office tried to supervise the design of a building--total disaster! Likewise, I can tell you all that after 9 years designing, I thought I knew what profs should be doing but then found that I was completely unprepared for the task of designing and teaching a class. How 'bout not automatically assuming we haven't thought about why we're doing what we're doing?!

 

[271828]

"However, it would seem to me that they'd be under pressure to produce good engineers coming out of university and most would (or should) decide that both methods are necessary."

I agree totally. Looking back to my old school, Virginia Tech, all 5 steel courses were taught with LRFD exclusively. I'd hazard a guess that nobody in his right mind would claim that VT's steel design curriculum is lacking.

If we're talking about quality, why not get to the *real* issue behind quality: sorry teachers who get tenure and stay there forever. If universities focused on hiring and keeping the best teachers instead of focusing 90% of hte hiring decision on research, then the teaching quality would go way up. If some of you guys just have an activist itch that needs scratching or are spoiling for a fight, then I humbly suggest you go find some non-trivial issues like this to work on.

 

[271828]

"If you are correct in that the ASD spec will disappear I will make a prediction. If the ASD code disappears, no engineers will use LRFD. The engineering community will use the 89 green book or the 13th Edition ASD. The engineers will complain to AISC until AISC reinstates ASD... again."

Vincent, what tree did you scramble out from behind?

(JOKING! I remember your affinity for hyperbole from a thread like this from a year or two ago. I like it.)

The IBC no longer references the green book, so engineers will not be using it forever.

The 13th Ed. ASD is almost exactly like LRFD. If someone insists on using Mn/Omega instead of phi*Mn, then it'll be for the short term. After about 4 min. of using Mn/Omega, the guy will realize that he might as well use LRFD. Then a spec. or two later when ASD goes away (my prediction, not AISC's, to my knowledge), nobody will care.

 

[271828]

nutte: "Perhaps you have it backward? You teach students one way, and they have to learn a second way at some point in their career. ...
Of course, if they go to an office that uses LRFD, they're out of luck."

LOL, I have no clue if that makes sense. What's the current percentage of offices that use ASD89, ASD2005, and LRFD2005? Again, are you saying ASD89 or ASD2005?

"It will be interesting to see if you're right about ASD dying in ten years. The biggest disadvantage in my mind, which has been mentioned in this thread and others, is having to run both cases anyway for serviceability."

Sure, but I think this is trivial also. I designed bldgs for 9 years and used LRFD for 6 of those. I barely noticed the difference between ASD and LRFD in this regard.

"Would not the vast majority of designs come out the same regardless of the method used? "

I believe that is true. The main differences are in areas like metal bldgs. ASD is a lot more economical for those due to the high L/D ratio, so the MBMA guys use ASD2005 for a good reason.

 

[271828]

Well, it's been fun, but it's getting late and I need to save some energy to screw over some industry guys tomorrow as I plan the Fall Semester's Steel class.

LOLOL, joking of course!! It's fun being the lone representative of academia in here. You guys are a tough crowd.