AISC 9th edition vs. 13th edition 8

My company uses 9th edition, Allowable "Stress" Design and not 13th edition Allowable "Strength" Design. Our work in the petro-chemical, offshore, and general industry usually means that time is more important than saving 10% steel weight. We must order steel often before sizes can be finalized and schedules are always critical. Stress design is crutial to our work. I would like AISC, or another organization, to produce an updated Stress design.

AISC is still pretending that everyone uses and loves Strength design. Using their own publications and magazines such as "Modern Steel Construction, I believe that they have monopolized the conversation. When a question regarding 9th edition is posed, AISC ignores that and says to use 13th ed.

I'd like to know where the typical engineer stands on this matter. What book does your company use? Did you move to 13th edition because you felt you have to because of codes?

And PLEASE, don't try to argue that 13th is just as fast. That argument is settled.

Comments?

 

[Lion06]

We started using the 13th edition about a year and a half ago. I only graduated in 2006 and I was actually taught steel design using the Green Book. Talk about being out of touch. I definitely agree that it's easier to think in terms of stresses and not moments. I also think it's easier to recognize, rather quickly, the efficiency of the section when dealing with allowable stresses as opposed to allowable moments. If your allowable stress is 33ksi on an A992 beam, then it's efficient regardless of the section. If the allowable moment is 300k-ft, that doesn't tell me very much. Is that 300k-ft with an S of 300in3, such that the allowable "stress" is only 12ksi or an S of 109 with an allowable "stress" of 33ksi?

That being said, it is based on the latest research. Why should we not be designing with the latest information?

 

[JoshPlumSE]

Before the 2006 IBC came out I probably saw 90% of the projects that passed through my office were ASD 9th. Now, it's probably down to 50%.... Still surprisingly high considering that it's not even legally allowed anymore.

The 13th edition is definitely NOT as fast as the 9th edition. However, I would say that (other than the new analysis requirements) it is probably the easiest to use strength based code that AISC has released.

No longer do we have to rely on lamda_c or lamda_o or some greek letters that I've never been able to read or pronounce. We're back to use b/tf, h/tw, KL/r, et cetera.... That alone is a real breath of fresh air for me.

 

[weab]

SEIT and JoshPlum,
Thanks for your input. My understanding for strength design is that, besides being the latest, it usually results in lighter structures thereby making it more competitive with concrete. Savings could be up to 10% per beam and less than that on an overall project.

However, a quick stress design benefits my clients more. Additional steel weight is outweighed by getting oil wells online earlier. At 100,000 barrels per day for an offshore platform at $50 per barrel (conservatively), our clients would be happy to save one week.

I think there is room for a dual system, Stress design and Strength design.

Incidently, 9th ed is legal if clients agree.

 

[westheimer1234]

i have worked in oil and gas companies for many years after i graduated and we have always used the green book never lrfd

 

[nutte]

I still don't understand the complaints between "Stress" and "Strength." If it's easier to do stress, then do stress. We're just talking about M/S and P/A here.

 

[jberg]

I was taught the 3rd Ed. LRFD in school, but once I got into industry they quickly taught me the 9th Ed. ASD way. 9th Ed. ASD is much quicker for design (IMHO), but I find the 13th Ed. does a better job of explaining the concepts.

The allowable stress is still in the 13th Ed. formulas, it just takes some algebra to take the strength and relate it to stress on the section modulus or area.

It probably seems odd to rewrite these equations into stress, but I have a difficult time getting a "feel" for strength design.

Joel Berg

 

[marinaman]

I've been a licensed practicing structural engineer for 19 years, and I use the 9th edition.

I agree 100% with being forced to use 13th. I do not like the idea, at all, of taking all of my years of experience in stress, and now trying to redevelope a feel for structures based upon moments and strengths. I believe that it reduces the public's safety in that the "feel" factor is removed.

Good engineers are made as much by art as science. I hear engineers backing the 13th addition from time to time, and almost every time, is by someone from AISC, or in academia, or who doesnt own the engineering business.

I understand both approaches (LRFD and ASD). I can use both. Having done so, ASD is by far my preferred method.

And think about existing structures. Structures with low allowable stresses and shapes that havent been made in years. You absolutely have to have a feel for the stresses to work with these structures.

I think there's a place for both methods and both should be supported.

 

[SteelPE]

Right now we are using what ever code is required by the building code. Some of the jurisdictions I do work in still allow the 89 specification. We are still in the process of verifying computer programs to make sure they apply the 05 spec correctly… and are finding problems which have yet to be resolved.

In college I was taught strength design, When I went to work I was taught allowable stress design. When I went to grad school I had to learn strength design again. Back to work and back to ASD. Then 3 years later AISC came out with the combined spec… now I have to learn a version of strength design again.

At this point it doesn’t really bother me. I find the 2005 spec does a better job of explaining issues that were left out of the 1989 spec.

Most of the older people I work with basically don’t want to learn the new spec and rely on me to make the necessary adjustments to their calcs.

 

[JAE]

I am a firm owner.
I have about 29 years in the business as a structural engineer.
I learned in school based on ASD.
I learned LRFD on my own in the early 1990's.
I've used ASD and LRFD both.
I don't see a significant difference in the resulting sizes unless live load is higher or lower than "typical".
LRFD, or strength design concepts in ASD, are not that difficult to learn and use.
I have a feel for structural behavior whether it is with stress or strength - it really doesn't matter to me.

And finally - I've seen over the years many...many structural engineers resistant to change. Once I worked with an older engineer who was using ACI concrete methods from the late 1950's in 1985. Never bothered to learn strength design in concrete. I swore I'd never let myself get that archaic.

So many of us loose an edge in learning as engineers. We get used to, and comfortable with, certain ways of designing and calculating and after a few years in the business find ourselves sometimes overwhelmed and perhaps even scared of all the new knowledge and technology that comes flying at us from new codes and standards. I can certainly empathise with this. But my view is that we engineers must always keep learning and growing in our field or we become dinosaurs.

 

[weab]

JAE,
I like what you have to say. I fully agree about learning more and keeping up with the latest developments. I wish that engineers would not be forced to get PDH's because they should want to advance themselves, but that's another matter.

I acknowledge that LRFD (and Allowable Strength Design) is more realistic and accurate, though I find myself somewhat gratified that you find little difference in design. Maybe I'm wrong, but I think that the difference from stress to strength design in steel is not as dramatic as concrete design may have been.

I guess my gripe is that strength design offers little difference in steel cost, but makes our engineering costs less competitive so we have to use 9th edition just like all of our competitors in offshore work. The 9th edition is very effective but has been abandoned.

 

[civeng80]

At least you engineers have a choice between WSD and Limit state design.

I for one for brought up with WSD (but now use Limit State design). I have a feel for sections and spans but still cannot quite wrap my brain around Limit state design. If you design the section by a plastic method then surely analysisshould be be a Plastic design and not elsatic procedure.

Under AS 4100 we do not have a choice but Limit state design.

 

[KootK]

I'm sure that I'll get spanked for this but I can't resist...

If you want to abide by the modern codes AND rattle off designs at a healthy clip, how about using some computer software?

RAMS Beam costs a pittance and will see you through the vast majority of your steel beam designs. The output even calculates stresses for you so that you can FEEEEEL the answer!

You don't have to look too hard to see that modern codes often assume that designers will be using computer aided solution methods. Heck, after a few more code cycles, my money says lateral analysis will only be permitted using non-linear dynamic simulation.

Computers. By all means: doubt 'em, check 'em, hate 'em. Just don't forget to use them.

 

[mtu1972]

I am another old timer that still uses ASD (8th Edition) and don't apologize for it. I was taught with the 7th Edition.

I was taught both WSD and USD for concrete as the industry was just making that transition while I was in school, and had no problem using USD in practice.

I learned LRFD during my graduate work (2000+) and understand its concepts. At that time I was not thrilled that the load factors were different than those for concrete. We were carrying factored and unfactored loads through the system for steel design/serviceability and concrete design and saw little benefit in carrying a different set of factored loads for the LRFD design.

Once the load factors were made similar I thought it would be time to finally make the transition, but never quite got there.

I've purchased all of the AISC Manual revisions, read most of them, and really studied the 13th during slow times at work - but still haven't made the switch.

I do agree with Kootenaykid that the use of computer programs in imperative, and I try to model even the most rudimentary structures, even individual columns with more complex load combinations. I still choose the ASD design option out of habit.

When designing individual beams, I do it the old way.



GJC

 

[271828]

Stars for JAE and KootenayKid.

I've designed buildings for 9 years--first 3 years using ASD89 and last 6 using LRFD. During those last 6 years, most projects were complicated and on heinous fast-track schedules. I really don't see the big deal in going from one spec to the other.

This reminds me of how I felt when they took my 99 SBC away and made me use the IBC2000. Not being a seismic expert, I was put-off by having to now deal with MORE terms that I didn't understand: Sds, etc. versus Av, etc. The guys who knew their seismic design didn't care either way. ASD89 vs ASD2005 is the exact same thing. You'll never read something from a steel expert bemoaning ASD89 vs ASD2005. It's always from folks who don't have a particular interest in studying steel, so they've fallen behind.

 

[271828]

"When a question regarding 9th edition is posed, AISC ignores that and says to use 13th ed."

Can you come up with a specific example of this? I've seen it several times, but there was no sinister motivation as you suggest. The motivation was simply that there was not an ASD89 way to approach the problem without the designer inventing his own mutilation of equations that were not intended for the purpose. The 2005 Spec. includes tremendously more situations than does the 89 Spec.

For one example of many, many possible ones, consider a crane beam with cap channel. One could use Sect. F4 of the 2005 Spec. with very little modification. What do you do with the 89 Spec.? I know the answer: use the typical equations in Ch. F. The problem is that these equations were derived for the case of doubly-symmetric I-shapes. You'd have to beat the square peg to fit the round hole to use the 89 Spec. and who knows whether the approach would be accurate.

To make matters worse, the vast majority of 89 Spec. users wouldn't even know that they were using an equation for something other htan what it was derived for. This is because there are so many approximations and constants embedded in ASD 89 equations (in efforts to make them simpler) nobody can look at them and understand the origin. That's why modern specs have such larger equations: they leave the variables and equations as close to the model as possible so the physical interpretation isn't buried and lost. The size of the equation is irrelevant anyway because any sensible engineer will either buy programs or write his own. It's not some diabolical plot from AISC and academics to screw over the other guy, LOL.

 

[civeng80]

Kootengaykid

It is true that computer programs can handle frame problems and beam problems as such.

There were so many little niggling things which could easily be checked or designed by using WSD (which I still use today) e.g. connections, stiffeners, plate thickness etc which in my opinion is crazy to check for Limit states. So much easier and better to check/design using working stress design.

 

[DaveAtkins]

KootenayKid,

I have said this many time in this forum, and I will say it again. In my opinion, computer software should be used for ANALYSIS, not DESIGN. I am more comfortable allowing the computer to tell me what moments, axial forces, and deflections are, while I choose the final member sizes and design the connections.

Our ultimate goal as structural engineers is to design structures which are safe and serviceable. The public should not care if we use allowable stress design or allowable strength design.

I still use ASD, but use LRFD when it is advantageous (for example, adding load to an existing member which has a lot of dead load).

DaveAtkins

 

[271828]

"connections, stiffeners, plate thickness etc which in my opinion is crazy to check for Limit states. So much easier and better to check/design using working stress design."

I do not understand this and would appreciate it if you'd help me to understand.

If I check a double angle connection with ASD89 and ASD2005, the number of design checks is either the same or very similar. Most or all of the equations are about the same--bolt shear, angle shear rupture, angle shear yielding, block shear, welds if it's welded--even that funky eccentric weld to the support side is unchanged--been the same since the 20's unless I have the date off a little.

If I check a stiffener for compressive loads, I have the same design checks to perform--compressive strength, local buckling, welds, etc., regardless of whether I'm using ASD89 or ASD2005.

No offense intended, but I'm getting the feeling that you're not doing all of the design checks because they're not spelled out as clearly in ASD89.

 

[271828]

"I have said this many time in this forum, and I will say it again. In my opinion, computer software should be used for ANALYSIS, not DESIGN. I am more comfortable allowing the computer to tell me what moments, axial forces, and deflections are, while I choose the final member sizes and design the connections."

Dave, I totally understand the part after "while I choose"--I think the blind usage of sizes that are spat out of a program is about the most dangerous common practice today. I have a great example of this that I typed here about a year ago probably. It was a composite bldg designed using the most popular automated system. The engineer had the shored button turned on and ran the design. There were 34' long W12x14 beams and 30'+ girders which were W18x35 or 40. The first bay to receive concrete started to collapse until they jammed a forklift under it to hold it up. If ANYBODY with >2 years of experience had looked at the actual sizes on a full-size strl sheet, this would've been caught.

That being said, you're not saying to not let the program do design *calc* are you? I wouldn't personally go that far. Say you have a large, 5-6 story structure with a couple dozen moment frames of lots of different bay sizes. I don't think you're going to try to get forces for every load combo, find the worst, and go through Ch. E, F, and H calcs for every member. My view is that one should very harshly scrutinize what the program's doing in this area and then use it to do the calcs. Then a HUMAN eye and a highlighter mark goes on every last size to make sure it makes sense. Just my opinion.