LRFD or ASD? The saga continues...
LRFD or ASD? The saga continues...
(OP)
I have a project being designed for us by another contractor. It involves a simple support bracket system for HVAC duct work. The structural engineer did calcs using AISC ASD method.
Whenever I see ASD used on design, I usually ask why, out of professional curiosity. Here was their response:
"Designing a large structure with large quantities of steel one should use LRFD to take advantage of the cumulative weight savings. That is not the case here, so ASD was used for simplicity and speed."
I'm wondering if we are schooling new engineers in both methods for this reason? I didn't take this any further, but I wonder how they decide the break point for LRFD vs. ASD.
Whenever I see ASD used on design, I usually ask why, out of professional curiosity. Here was their response:
"Designing a large structure with large quantities of steel one should use LRFD to take advantage of the cumulative weight savings. That is not the case here, so ASD was used for simplicity and speed."
I'm wondering if we are schooling new engineers in both methods for this reason? I didn't take this any further, but I wonder how they decide the break point for LRFD vs. ASD.






RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
Sounds like a good and economical idea, doesn't it? Now try to relocate a firewater main or a large cooling water line. Attempt to install a walk-in freezer on an upper floor.
Any change in the loading that even slightly increases the weight on that area will require expensive added steel. You only put in the minimum, not its too little. Now try to get the budget to make these additions and reinforcements in a finished structure.
LRFD is for kids and idealists. ASD is what experienced realists use.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
That just isn't necessarily true. The sizes are affected by the relationship between dead load and live loads. Structures with higher levels of live loads will have a more conservative design with LRFD due to the higher load factor on live loads.
Structure "size" has nothing to do with it.
RE: LRFD or ASD? The saga continues...
Usually, the differences are small but with LSD, live loads and dead loads can be factored up with a different load factor better representing the degree of confidence in the magnitude of load.
LSD us only slightly more difficult as you have to keep track of dead vs. live loads but after you get used to it, it is no big deal.
BA
RE: LRFD or ASD? The saga continues...
I prefer ASD because it is more intuitive to me.
RE: LRFD or ASD? The saga continues...
learned LRFD for steel in college, entered the real world and started using ASD. studying for lateral portion of 16 hour SE exam and now back to using LRFD for seismic design.
I prefer ASD for quick calcs and if I am just trying to nail down the size. Also prefer ASD when I know I will be checking foundations with the same calcs and don't feel like tracking factored vs non-factored loads.
Just a preference and usually dictated heavily by what I am doing.
RE: LRFD or ASD? The saga continues...
At a ratio of 3 to 1 (LL to DL), the results should be the same (in theory). Above that, LRFD is more conservative, below that, ASD is more conservative. Size of the structure has nothing to do with it.
RE: LRFD or ASD? The saga continues...
For larger scale design work with well defined loads and combinations I have no problem with LRFD.
RE: LRFD or ASD? The saga continues...
The only place I always use LRFD is coverplating existing beams and for composite beams. It greatly simplifies the calculations.
RE: LRFD or ASD? The saga continues...
the input loads are at best an estimate..the claim that LRFD input loads are statisically a better estimate is marginal at best(IMO)..the diference in the final design between the two, if one exists, is minimal at best...
usually the engineers that tout LRFD over ASD are for the most part inexperienced engineers, members of academia or associated with some outfit that has a vested interest or making a living out of the changes in codes etc. that it has produced.
my bigest complaint with LRFD is the use of immaginary loads leading to a non-existent state of stress in the structure...
this has been taken, now, to another rediculous level with the introduction of ultimate wind speed..huh?..now that is something that everyone has a grasp of ....
I prefer to work with real loads and real level of stress that allows me to use engineering judgement and intuition...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
The internal state of stress is just as imaginary in ASD as in LRFD (see Wooten's 3rd Law).
If you are referring to the plastification of the section assumption, then I'm sorry to tell you that this was also accounted for in ASD for years, although in an indirect way.
I really don't get the animosity toward LRFD. I understand people have been using ASD for years and want to continue to use it, but the implication that if you use LRFD, then you must be inexperienced or don't have the best interest of the client at heart is ridiculous.
Is it really that much more work? I am not implying that LRFD is the one and only valid method. ASD worked beautifully for years and it would continue working if we were to continue to use it. However, personal feelings notwithstanding, LRFD does give a more predictable reliability index
RE: LRFD or ASD? The saga continues...
It is easy to relate calculated stresses to an allowable or to yield.
"hmm, this bending stress is 29.5 ksi, that's getting close to yeild"
vs.
"hmm, this beam has 245 kip-ft max moment that's getting close to...umm, ahh, ..."
RE: LRFD or ASD? The saga continues...
Why is 36 ksi elastic stress any easier to grasp then a fully plastic section at 36 ksi?
If anything the first represents a condition at which nothing noticeable really happens whereas the later represents the conditions at an actual failure.
That being said, I agree ASD or LRFD is really a matter of preference and gets you to about the same place in the end
RE: LRFD or ASD? The saga continues...
Other than that I do have some very specific gripes.
IMO the Black 2005 book is a cookbook specifically made for run-of-the-mill situations encountered in basic building design. It has left out in the cold more complicated structures encountered in industrial design. One only needs to look at Section F12 of the 2005 spec to see this. If you have not gone through this, good for you as it flat-out sucks and is VERY time consuming. I may sound bitter, but I think to some degree academia has over-ridden solid engineering experience. If you design industrial structures you know that trying to save every pound of steel can really lead to disastrous results. Throw in rolling schedules as Dhengr alluded too and you really have no savings at all.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
It must be because I was taught LRFD, but I always calc the allowable moment in ASD instead of the stress so my mind always thinks in moments
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
I guess I disagree that old and new ASD are, at the root, the same thing. New ASD is Limit State Design, right?
RE: LRFD or ASD? The saga continues...
Look closely, did they really provide guidance? Or, did they say "we added this section and it is basically worthless so if you run into this situation, do a research project".
I assume most structural engineers have had to analyze unsymmetrical shapes. Unfortunately for me, it comes up quite often in analyzing large built-up crane runway girders. Procedures for this analysis were laid out nicely in AIST Technical Report 13 which was last updated in 2004 and still used 1989 ASD. Obviously, TR 13 needs to update procedures. Meanwhile I dont have the time to develop special software for determining the critical buckling stress of wildly unsymmetrical shapes.
I really don't want to get into an argument. I use new ASD regularly, but there are situations where applying it are nearly impossible. I don't think everyone has had to deal with this, so they figure its "all good".
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
By Limit State Design, I assume you mean that it permits moments greater than that achieved when the extreme fiber is at the allowable stress. Old ASD did the same thing; they just hid it, as frv alluded to. That's why for compact wide flange shapes, you're allowed to take Fb to 0.66*Fy, greater than the regular 0.6*Fy.
RE: LRFD or ASD? The saga continues...
I guess I am just sour grapes
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
But, the decision can also represent a generational gap. Prior to the early 90's the only option was the AISC ASD Manual. However with the introduction of the blue LRFD manual, AISC made a commitment to develop only the one code. Universities began teaching only LRFD and as a result all research was applied only the the LRFD manual and the two subsequent editions. In addition to the experience of our aging designers many industrial firms had significant investment on their ASD software. In the worst case ASD would provide a conservative result, which was little reason to make a change. Larger architectural engineering firms were repeatedly told the of the economic advantages of using LRFD. Many designers also began to see the ASD manual as antiquated. As young engineers advanced in their careers, their LRFD roots became more influential in their firms. Also the LRFD is the primary design principal in concrete and the only design option in many of the countries. In early 2000's AISC changed their direction, it was determined that in the interest of promoting steel it was a good idea to support both design methods. The research and development of the previous 10 years was applied to ASD and the result is the 13th edition manual.
In most cases the selection of method is a matter of choice, and comfort level. The only problem we see is when designers try to combine the two. As connection designers we find projects with both service and factored loads.
http://www.FerrellEngineering.com
RE: LRFD or ASD? The saga continues...
http://www.FerrellEngineering.com
RE: LRFD or ASD? The saga continues...
Right on the geotechnical Load Factor Resistance Design, LRFD, Limit States design, call it what you will. In Canada it has only been limit States Design for steel and concrete for 25 years, good god I need a holiday. But the geotechnical crowd was only brought in to limit states 5 years ago, so you carried factored and unfactored loads where ever you went, dabbling between the codes and standards with metric, and then the contractors with Imperial units, we also have two official languages in Canada. Go figure.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
in contrast, the ACI code(IMO) has presented the LRFD concept in a much more clear, concise and logical manner...something a practicing engineer can apply efficiently....
frv...I don't know what a "predictable reliabilty index" indicates, is it a theoretical measurement?....
it's all a mute point, anyway, since the horse is out of the barn or the money is out of my wallet(which is really the name of the game)..
my basic gripe with LRFD and it's load combinations is that it gives a ficticious load/stress level in the stucture, a state never reached in reality...so in the real world the structure remains in an elastic state(seismic excluded)at a much lower stress level than indicated by by the LRFD load combinations...
as a practicing engineer, if I know the real load/stress level in a structure I can use my engineering judgement based on that...without that, I am compelled to to go the route of the cookbook or "lego-set" engineering approach that is becoming more common nowadays...
RE: LRFD or ASD? The saga continues...
Not sure what you are getting at....
By specifically providing provisions for "unnsymmetrical shapes" and then subsequently providing very little information or procedures on how to design these shapes, AISC has made a mockery of itself and completely side stepped the issue (one that they created).
The Green Book didn't do this.
It's as if the academic folks posed a problem to themselves that they couldn't solve and then rather than tackle it, they skipped class and went for an early afternoon ale.
RE: LRFD or ASD? The saga continues...
For some more information on LRFD and specifically the reliability index, see the commentary to B3.3, p.16.1-214 to 216 of the AISC 13th Ed. Manual.
As far as the "fictitious" state of stress, as I mentioned before, the stress level is just as fictitious using ASD (again, see Wooten's 3rd Law).
RE: LRFD or ASD? The saga continues...
While LRFD may lead to more economical designs from a strength standpoint, it just makes it more likely that serviceability criteria like deflection or vibration will control.
While using stress design may feel more "real", the new ASD and LRFD are both based on plastic capacity. They were developed because it was determined that limiting the steel to a fraction of its yield stress was overly conservative. There is a lot of plastic capacity that can be tapped into for the controlling-case gravity and lateral loads.
RE: LRFD or ASD? The saga continues...
This is the part I struggle with intuitively.
Where is this capacity?
RE: LRFD or ASD? The saga continues...
Section F12 provides a codified basis for evaluate a member other than those covered in F1 through F11. It's for rare situations, hence its location.
If F12 didn't exist, the engineer would hopefully compute Mn for an unsymmetric member exactly as currently shown in F12. However, someone such as a calc reviewer could accuse him of not providing a design per the legally adopted specification.
I don't understand "couldn't solve." F12 is for unknown sections. Just about any section that might be needed, in other words. How can the academic community, Specifications Committee, or anyone short of God preemptively supply the solution? It's assumed that the engineer will compute Fcr using whatever means he chooses (FEA, manual calcs, elastic buckling solution from a book, etc.), and then use F12 to get to Mn.
RE: LRFD or ASD? The saga continues...
Maybe I'm missing something: What are some situations for which the green book gives guidance for a subject not covered in the black book. How does the industrial designer take a step backward in going from the green book to the black book?
RE: LRFD or ASD? The saga continues...
However, for the situations I described earlier, these provisions are not very reasonable. Allowable stress was more practical.
I could post a sketch of the shapes I deal with, and you'll understand. Determining Fcr would burn 1/2 of the job budget.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
"...If F12 didn't exist, the engineer would hopefully compute Mn for an unsymmetric member exactly as currently shown in F12...."
Is this really what an engineer would do?
Or would an engineer make some practical assumptions about the stresses at various points in the cross-section and maybe break that section into portions or components that would be carrying these loads and stresses and proportion them to accordingly?
Perfect example of Wootens Third Law
RE: LRFD or ASD? The saga continues...
Unless the client can actually leave the design alone from the time the Engineer-of-Record gets released to start calculating loads, and the end of that buildings useful life 50+ years later], the small savings in the cost of extra steel tonnage is overwhelmed by the costs of any modification to that building.
On a Car Plant designed by the major Detroit car plant designer, somebody left out wind bracing for the Paint Bldg exhaust stacks. This building being in Alabama, the bracing was VERY necessary to keep the stacks from crumpling in a 50mph wind. The solution was to add guy-cables. Simple, except the bar joists for that portion of the roof were already maxed out. A 6-10 man crew on overtime & doubletime [all the Straight-Time hours were already budgeted] worked for about 4 weekends 10-12 hours per day, to add enough bracing to allow those 1/4" diameter guy cables to be installed.
Then we did the same thing for the frame assembly area. Due to an inprocess revision, the catwalks above the robot welding cells were moved. More bracing, both sides of every attachment point of those catwalks.
Just barely enough has never been a decent design philosophy, and to me LRFD = just barely enough for today's loads; God help the next guy/gal.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
Honestly, I don't think the choice makes any realistic difference at this point. Neither is particularly hard to use. I like LRFD because I feel like I carry through a better sense of my load cases, but that's really just the way I do it more than anything.
You can do simple things with both and you can do complicated things with both. Unless you're designing way closer to the line than I would you're going to end up with approximately the same amount of steel.
If you're doing a simple thing in LRFD you can generally eyeball your governing case at the beginning, do your combination in a line at the beginning and then just carry the combined load through.
It's honestly just preference. You can get a sense of the assumed (somewhat pretend) stresses in both.
The only annoying issue is when you have to change half way through for some reason, which is why I'm happy that geotech stuff is moving towards LRFD in Canada now.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
AASHTO doesn't give ASD and LRFD in one document. I haven't checked the NDS lately, but last item I checked, they didn't either. Does anybody else besides AISC?
RE: LRFD or ASD? The saga continues...
I need to go back and study the history of ACI 318 to see how that conversion to USD went.
RE: LRFD or ASD? The saga continues...
As told to me by my steel professor a few years back (shortly after the 13th Ed. manual came out), AISC didn't originally intend to have a combined spec. They wanted everyone to adopt LRFD exclusively. However, even after the 3rd edition of LRFD, many engineers continued to use the old green book. AISC thought that the combined manual would serve as a transition. I suspect they are now stuck with the dual manual for the foreseeable future.
The problem is that the engineers using the green book still don't like the 13th edition because it has been converted to "strength" as opposed to stress. The argument being that regardless of member size, they could get a feel for how loaded the member was by comparing the actual stress to the yield. Seems like a valid point, but oh well.
RE: LRFD or ASD? The saga continues...
As someone else mentioned, serviceability often governs the design of most beams and frames, so the economic penalty isn't really significant. I guess it the effect on the design of gravity columns could be measurable, but I don't like penny-pinching columns anyways.
RE: LRFD or ASD? The saga continues...
I can understand the competitive nature between the ASD and LRFD approaches for saving material, but when you do a lot of remodels and additions as I do, I do appreciate extra capacity beyond what is needed for flexbiity of use, whether it was then, or now.
We have had a lot of discussions here regarding PEMB's and how they are designed to the line for reasons of cost, and the point always made is that they cannot be added onto easily due to the tailoring of material useage. Every concept has it's advantages and drawbacks, but the one concept that always rings true to me is "Pay me now, or Pay me later."
I guess you have to look at the future to try to envision how a building might be used, if it will ever be modified at all. But who can precdict that? Most likely, it will be modified at some point in it's lifespan, and that is where I prefer the conservative approach.
I guess I'll have to update myself with the newer ASD and LRFD. My son uses both. I'm just an anachronism.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: LRFD or ASD? The saga continues...
I do like ASD because for me, I can do the load combo's in my head much easier when doing quick checks. One only has to multiply by 0.75, 0.6 or 0.7 in some of the cases. Dead + Live, Dead + Wind, .6Dead + Wind are all simple for the brain cells. What I am saying is i can take 75% of something in my head faster then I can add 60% etc...
I also find that carrying service loads to the foundation is a much better prospect with ASD.
Neat conversation.
RE: LRFD or ASD? The saga continues...
Also if you're doing hand calcs than your probably not trying to pin the tail on a gnats ass so the savings issue really isn't there.
However if you really need to sharpen your pencil or if you are using a fancy computer where it is easy to go back and forth between strength or service levels, then either is fine.
EIT
www.HowToEngineer.com
RE: LRFD or ASD? The saga continues...
LRFD will save you money only if your LL to DL ratio is less than 3 and serviceability is not the controlling factor. It has nothing to do with small or large. LRFD isn't any less conservative and you can just as easily modify it.
I really don't understand the argument about designing something to a "gnats ass". If, using ASD, you design whatever member for a unity check of 1.01, then you're going to have just as difficult a time adding anything to it as an LRFD-designed member that has a unity check of 1.01.
Importantly, the other option is just as true. If your unirty check in LRFD is 1.1, it is just as easy to add some load to the member as an ASD member designed to 1.1
RE: LRFD or ASD? The saga continues...
Not that I would want to go around designing this way.
EIT
www.HowToEngineer.com
RE: LRFD or ASD? The saga continues...
You get about the same answer. I can make certain conditions work for LRFD and not ASD, and the opposite is also true for other loading conditions. A gnats ass is a gnats ass and if you think otherwise you are fooling yourself.
I also don't agree with having a better 'feel' for something with ASD. Having a better feel with respect to an arbitrary reference capacity value (say 0.66Fy) at which absolutely nothing actually happens is no better that having a feel for an inflated load relating to a value at which something might actually happen (i.e. failure)
RE: LRFD or ASD? The saga continues...
Ref ASCE 7-05, Appendix C - Serviceability Considerations: Although this Appendix is not a mandatory part of the standard, it does provide guidance for design of serviceability. The load combinations suggested in the commentary of the Appendix C are not the same as the load combinations given in Chapter 2. Moreover, where allowable stress design is used , the drift is computed using the strength level seismic forces. The return period for wind could be different for strength(ASD) and serviceability. Therefore, you still have to write separate load combinations for checking serviceability. Given an option there will always be preferences, but the number of load combinations should not be the criterion in choosing between ASD and LRFD.
RE: LRFD or ASD? The saga continues...
The problem is twofold. First, while 20 years pass and AISC ignores most (yes most) structural engineers desire to use stress design, the code has advanced. That's a good thing. However, the formulas have become out of control and few formulas appear to be what we remember. So what some of us attribute to LRFD is actually just code advancement. Has anyone really looked at formula F4-8? Really!? In the end saving up to 5% of steel is not important to many of us. Saving 5% of cost for something that is only 10% of the overall project cost is not worth anyone's time. Let's face it, opening a building/structure/facility one day earlier pays for the extra steel in my projects.
The second problem is also AISC's making. They get us interested again by reintroducing new ASD. They either think we won't notice that stresses are not used, or they actually believe that we just didn't want factored loads. We use factored loads in concrete design.
In the end, I find that the black book is only better because it addresses issues not addressed in the green book. It however does not provide a better end steel product, just longer formulas. Anyone want to try to convince me that there isn't a shorter formula F4-8 that is 99% as good.
RE: LRFD or ASD? The saga continues...
Surely, I'll be pointed to Eq. F2-6, so I'll go ahead and answer. That Lr is tabulated in Part 3 for every standard shape, so who cares if it's a couple inches long on the page? If you're in F2, and are dealing standard shapes, you're good to go. If you're using doubly symmetric built-up shapes, you're in a special area and don't care if the equation is large.
Besides, [nerd hat on]I timed myself punching Eq. F2-6 through my calculator three times and finished each try in around 25 sec. with no mistake.[/nerd hat off] LOL
Those short ASD89 equations came at a price: clarity.
Take Eq. F1-7 and F1-8 for example. The older Salmon and Johnson texts included derivation of those. There were so many steps and approximations to go from the theoretically exact elastic buckling moment that the new equations bear no resemblance to what they stand for. I wonder how many times they've been misapplied over the years. Huge numbers, I'm sure.
Here's another one: 0.66Fy vs 0.6Fy. Earlier in this thread, someone typed "I guess I disagree that old and new ASD are, at the root, the same thing. New ASD is Limit State Design, right?" which revealed that he or she didn't know what the 0.66Fy was all about in the 1989 Specification. I've heard numerous people reveal that over the years.
RE: LRFD or ASD? The saga continues...
Also, who are the most recent graduates and those of us who are more experienced(old)..
I generally agree with frv and weab about all this focus on saving 5% of steel weight(if it really exists) when other factors in a project can blow this 5% savings away...steel is cheap in comparion..
Before history is revised, the fact is that the majority of practicing engineers rejected the use of LRFD for many years.
If there was a clear, distinct advantage in using LRFD over the old ASD, these engineers, me included, would have embraced it.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
I'm not living in academia.
Engineers (people in general) just don't like change.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
Economics aside, I would tend to lean towards this: If I am very concerned about quantifying unique probabilities of loading and material behavior, ductility and post-yield strength, I would rather use LRFD.
If the structure is not critical for progressive collapse or overloading, ASD all the way. With that said, AISC has unified the two methodologies pretty well. I doubt my preference really matters in that case.
"Structural engineering is the art of modelling materials we do not wholly understand into shapes we cannot..."...ah...screw it, we don't know what the heck we are doing.
RE: LRFD or ASD? The saga continues...
You surely must not evaluate existing structures in your work.
Limit States were also built-in, which means we didn't have to check 3 equations but rather just determine which equation applied and use it.
I also suspect that most posting here do most of their work on new steel buildings and use software for design and dont actually go thru the equations anyway.
RE: LRFD or ASD? The saga continues...
It's a very narrow minded approach/argument.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
The way I see it, there are three main differences between ASD89 and the 2005 or 2005 Manuals/Specifications:
1. The 2005/2010 Specification provides codified equations for many, many situations that are not addressed in the 89 Specification.
2. The 2005/2010 Specification leaves equations closer to their original forms, such as leaving some J's and Cw's in there rather than simplifying as in the 89 Specification Ch. F. This makes it easier to see what's in the equation. Because everybody uses computers today, I fail to see why this is a problem. Heck, we've had programmable calculators for a long, long time.
3. The 2005/2010 Specification is a LOT better organized.
Say you had a column that's an I-shape, but not a hot-rolled shape. Do you know where the flexural-torsional guidance is located in the 89 Spec? (Commentary to Ch. E) How about slender compression element stuff? (Appendix B) In other words, if you had just about any column out of the ordinary, you had to pull together pieces and parts from the Specification, an appendix, and the Commentary.
ALL those are in Ch. E of the 2005/2010 Spec., where they belong.
To keep you from having to look at them if you don't want to, the FTB and slender element parts are tucked away after the routine parts in Ch. E.
Those who do regular types of designs will be pulling values from tables almost every time. In the Specification, those regular design provisions are right up front where they're quick to find. If the pages behind E3 in your Ch. E offend you, you're welcome to tear them out LOL.
If someone finds himself in Sections E4, E7, F4, or F5 (a handful of the sections that people seem to point at as examples), he's probably in some specialty area. If so, then he's an expert on those provisions anyway, and doesn't care about the complexity. If a non-specialist finds himself in there occasionally and the calcs take a little while, then is it that big of a deal? Is this the only time in most SEs lives that they have such a thing happen? Of course not.
This entire issue is completely obvious to me. People will naturally fight against almost any change. That's logical, seriously. If you successfully designed a building in 1990 with the 89 Spec., that building is OK, so what's the point in changing methods? That's a good enough reason to have a personal wish or desire that the steel timeline would've frozen itself in 1989.
However, I don't think anybody has any real leg to stand on in saying that the 89 Spec./Manual is superior to the modern ones, unless you consider ad hominem attacks or genetic fallacies as valid.
RE: LRFD or ASD? The saga continues...
I give up man....You're 100% right.
I use the unified code every day.
For the specific problems it causes me, please refer to the following website:
www.cranerepairengineer.com
I am in no way affiliated with this company, but occasionally do the same type of work.
Please read, at your leisure of course, through the sections referring to "AIST Torsion", "SDC Torsion" "Torsional Warping Constant", "Plastic Section Modulus" and all of the excellent and extensive PDF's available on the site. This will clue you into just exactly how difficult a task using parts of the new code has become for those of us without a personal relationship with Dr. Galambos and an extra 500 hrs per year for research.
I don't mean a word of this facetiously either.
Excerpt from website:
The analysis of unsymmetrical built-up shapes is the most difficult part of the new AISC Code to understand. The Fcr appearing in equations F12-3 and F12-4 represents two different types of buckling/critical stress. There is no formula for either one of the Fcr's except that the code requires that they be "determined by analysis". The Fcr in Chapter F should not be confused with the Fcr noted in other section(s) of the Code dedicated to the critical stress due to flexural-torsional buckling.
The Code generalizes the critical stress resulting from all buckling modes as "elastic buckling stress". As stated in the Code Commentary: "the stresses are to be limited by the yield stress or the elastic buckling stress. The stress distribution and/or the elastic buckling stress must be determined from principles of structural mechanics, text books or handbooks, such as SSRC (Galambos, 1998), papers in journals, or finite element analyses". Much as we tried per AISC recommendation, SDC has not been able to acquire any literature dealing with "lateral torsional buckling" for unsymmetrical sections. In lieu of referencing papers in journals, our last resort is to honor the AISC yield stress limit by keeping all the calculated stresses below the yield stress. SDC is undertaking major modification of our automated crane girder design tools including the following:
Accept user-defined "effective girder component" based on width-thickness ratio of each element up to the AISC non-compact limit.
Use "gross section properties" to calculate flexural shear- and all torsion-related stresses.
Use "effective section properties" per AIST guideline to calculate flexural fiber stresses.
For unsymmetrical sections, use SRSS combination of all ASD (bending plus warping normal) fiber stress with (flexural horizontal shear plus pure torsion plus warping torsion) shear stress and then limit the SRSS value to the smaller of: (a) material allowable stress of Fy / Ω or 0.6 Fy and (b) flexural torsional buckling stress.
This interim scheme has the concurrence of Prof. Galambos.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
Unsymmetric shapes can buckle in various modes, but that fact was not invented by the authors of F12. If people didn't have similar difficulties prior to F12, then does that indicate that they didn't realize that these different modes and checks needed to be made?
I have a criticism of F12 and some parts of Ch. F, a technical writing error (IMO) that I think makes it a LOT harder to understand quickly.
Look at Ch. E, which is clear. One computes Fe which is an idealized elastic buckling stress. Purely linear elastic, no residual stresses, no initial imperfections, etc. This is apples-to-apples to an elastic buckling stress one would get from a finite element analysis. That little "e" makes it pretty clear, I think. Then one goes to subscript cr for Fcr which is for either elastic or inelastic. I'm pretty sure Ch. E is consistent everywhere.
Now look at Ch. F. Now we have subscript "cr" on idealized elastic buckling stresses such as in Eq. F2-4, F4-5, F5-3 and 4 (even worse), and so on. The Ch. F authors (different people?--I don't know) do not keep the "e" elastic buckling stress clearly separated from the "cr" which is buckling stress for inelastic or elastic buckling.
They really need to have a little "e" on every idealized elastic buckling stress everywhere in the Spec. and Manual, and a little "cr" on a buckling stress which can be inelastic or elastic. "e" and "cr" subscript then banned from every other use anywhere in the document. I think people would get it pretty quickly without being steel code reading experts.
This is going the right direction, although slowly. The 13th Ed. had a "cr" in the single plate shear connections part in a place that had nothing to do with stability. (wretch LOL) At least that's gone now.
RE: LRFD or ASD? The saga continues...
Maybe I'm the weird one, but it seems to me that more transparency is better. If AISC would've jumbled all of the provisions together, then I could see the problem. They put the more esoteric ones behind the more commonly used ones.
RE: LRFD or ASD? The saga continues...
RE: LRFD or ASD? The saga continues...
There are three kinds of buckling stresses in the AISC Spec: idealized elastic buckling stresses (like Fe in Ch. E), design level elastic buckling stresses (like Fcr=0.877Fe in Ch. E, adjusted to fit data, etc.), and inelastic buckling stresses (like Fcr=Fy*0.658^(Fy/Fe) in Ch. E).
Whether we're talking about flexural buckling, lateral-torsional buckling, local buckling, or any other kind of buckling, these three usually exist, and have for a long time. They need to be kept straight, or the provisions will be confusing.
Idealized elastic buckling stresses should always have an "e." Design level buckling stresses can be inelastic or elastic, and should always have a "cr" subscript.
Ch. E is clear on this. Ch. F isn't, and I think that's an impediment. It's not the issue in your particular example, but I think it's a problem in general.