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?

 

[dik]

I'm an old timer, too, and our class was the first year (1965) to use limit states design, we never took a course using working stress design. My only exposure to working stress was originally with masonry design and for the last couple of decades, I've used limit states design for this.

Dik

 

[JoshPlumSE]

I think the point of KootenayKid's post was that some of the arguments against LRFD fall apart if you are using software to ASSIST you in your design.

The main point being a response to someone who said that they have a good "feel" for the efficiency of the beam based on the stress, but not based on the moment capacity.

Obviously, if you are using a computer program then the program is going to report BOTH the bendings stresses in the beam and the capacity of the beam.

As others have pointed out, those arguments only really hold true for member design. Not connections. But, I for one, have thought that the seismic connection design has always been more rational and straight forward for LRFD or strength based loads. The ASD seismic detailing provisions were always IMHO more complex or convoluted.

Josh

 

[mtu1972]

While I've heard numerous horror stories about gargabe in - garbage out, I do use the design functions on my computer software. Having designed buildings for so many years - many with only hand calculations (remember Moment Distribution and the Portal Method) I fee that I have the experience to discern when something doesn't "look right".

That may not be the fact with someone relatively new to structural design.

But when time is so critical to schedule and with all of the intricacies mandated by the Codes, in terms of load conditions, it makes the most sense to take advantage of the design functions. We provide typical connection details and only draw special details where they are really warranted.

It is imperative that a "Sanity Check" is done before any design drawings are issued, but whether the design is done by hand or in the computer should be irrelevant.

GJC

 

[weab]

"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 think this may be a case of LRFD not being as intuitive as ASD89. Perhaps some checks that can be dismissed by observation (or a quick calc)in ASD cannot be as such in LRFD -OR- it could be a lack of experience in LRFD. I'm inclined to believe it's a combination of both.

 

[271828]

I don't quite understand. Say I have a bolted double-angle connection with typical geometry. Any connection designer has a feel for which limit states won't control, such as bearing at the support-side bolt holes if the support is thick. If bearing's much stronger than bolt shear in ASD89, it is also that way in ASD2005. To my knowledge, the list of design checks is the same or similar, so that's also no difference.

Folks very naturally dislike digging into the new Manual in large part because it's organized differently. LOL, here's a good one along those lines. A couple of weeks ago, I needed to go back to the 7th Ed. ASD Manual and try to find the bearing check and "poison bolt" stuff. (I was told that the concept was used back then.) After several minutes of searching, I gave up and couldn't wait to put it back on the shelf--very frustrating. Nothing wrong with the Manual--I just don't know where things are and didn't want to know badly enough to keep at it.

 

[abusementpark]

"However, a quick stress design benefits my clients more. Additional steel weight is outweighed by getting oil wells online earlier. "




I've got to challenge this statement. Do you really think you are saving a significant amount of time by using the 9th edition?

I can only speak based on my experience, but majority of the time the computer is doing the code check which takes no virtually no time, and the stuff that I do by hand in 13th edition takes very little time. AISC has done an excellent job incorporating tables and design aids for beam, columns, connections, composite action, studs, etc. I just don't see logistically why it should take you any longer if you are using the ASD method of the 13th edition. It is just a game of units!

 

[Ussuri]

"However, a quick stress design benefits my clients more. Additional steel weight is outweighed by getting oil wells online earlier. "

Thats certainly interesting. The Client I am currently working for (one of the big ones) does not allow the use of WSD/ASD for new steel designs. It is all LRFD, but not to AISC but API.

 

[swearingen]

"Incidently, 9th ed is legal if clients agree."

I imagine that this is rarely true. If the state requires you to use the 13th, the client has no say-so.

I can't speak for the offshore folks, but is it really true that there is no jurisdiction for codes out there? My offshore experience is from the early '90s and the 9th was still accepted widely...


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[miecz]

The saying went "if you build a better mousetrap, people will use it." In this case, I'd say that the consensus is clear. Engineers didn't use LRFD until we were forced to, by lawyers, of all people. If LRFD was indeed a better design tool, I think it would have been embraced by the community, like all the other good developments in our industry. I am insulted by those who imply that I have no interest in learning.

 

[271828]

miecz, if the last sentence of my first post is what you're insulted by, then I offer my apologies.

I was just trying to say that some are interested more in other subjects: concrete, loads, wood, marketing their business, getting off at 5 and going home to their family (good goal IMO), whatever. They *don't* have an interest in learning how to design steel using the latest knowledge. That's not an insult, but a fact. Folks with a particular interest in steel, in my experience, never complain about the 13th Ed.

I have no interest in learning the latest way to design wood. LOL, if someone tries to make me, I might be on here whining about it too!!

That being said, you're typing about which was "first" not which is "best." I'll not try to say the 13th Ed. is more accurate--I could, but that's the harder one to argue because people point out the lack of precision in real design. The 13th Ed. IS a lot better, IMO, because it covers tremendously more cases than did the 9th Ed.

 

[miecz]

271828

Thanks, but I wasn't referring to your or any particular post. so, no apologies are necessary. I do think that people who have resisted LRFD have been unfairly criticised as, uh, stuck in a rut. I think that engineers are inherently curious people who will give new ideas a fair shake. In this case, I think the community gave LRFD a look and said, no thanks. When the drivers saw the response, they figured out a way to force the change.

The LRFD debate has been going on in this forum for some time. I believe we have a problem, but the problem is not the strength method, it's all the load cases that are mandated by ASCE7. It used to be that the engineer made a judgement as to the governing load case, and performed hand calculations. With so many required load cases, hand calculations take too long. As we switch to computerized calcs, I believe we lose our feel for how things work. In the end, I see a net loss.

 

[271828]

Can't argue with any of that.

LOL, then again, the folks who sleep with an ASCE 7 under their pillow would say that you and I are both stuck in a rut!

This argument will probably be taken to a new level in the next couple of decades with probabilistic design. For example, in floor vibrations--one of my specialties--almost every new paper seems to be heading that direction. Saying it's OK or NG won't be adequate soon--one will have to say there's a 0.2% (or whatever) chance of problems.

 

[miecz]

Perhaps all of our Contract Drawings should include a statement that we design according to a probabilistic design method that recognizes that there is a small chance of failure, and that our liability is limited to the replacement cost of the prints.

 

[JAE]

miez - I respect all that you say. But I would suggest that there are a lot more folks out there "embracing" LRFD than you think. And as time goes on - and more students are trained in LRFD in school (vs. ASD), I think ASD will go bye-bye. I could be wrong - but that is the trend I see.

 

[HgTX]

@ 271828:

I have a tangential and possibly stupid question. What is "poison bolt"?

I tried looking it up and all I got was a pile of role-playing game stuff. I gave up after about the 10th game-related term I excluded from the search. Who knew I'd ever use "-wyvern -dragon" at work?

Hg

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[WillisV]

Poison bolt refers to the old practice of if you have a line of bolts, taking the minimum strength of bolt bearing and tearout at any of the bolt locations and applying that minimum strength to all bolts in the line for the total connection capacity.

Newer methodology lets you take the controlling limit state of each bolt and sum them (for instance the bolt nearest the edge might be controlled by tearout and the interior controlled by bearing).

So "poison" bolt means that one devious bolt near the edge would screw the whole connection capacity.

 

[HgTX]

Ah, thanks!

Hg


Eng-Tips policies: faq731-376

 

[miecz]

JAE

It's the law. Like Prohibition (Metrication, Hitler?), we'll never know how many went along willingly. Some people just obey the law. I'm pretty isolated, so I have no idea.

 

[JAE]

Well, OK, maybe the word "embrace" is not accurate.

 

[weab]

I don't know about laws in your state. I just wrote a design spec last week in which the client (a Fortune 500 mining firm) specifically allows ASD89. Our state has certain requirements for working on state work. I'm not aware that it applies to private work. Are you saying that my client is breaking the law? or my company?

Also, I am getting the feeling that those of us who prefer stress design are getting unfairly characterized as slow to learn. LRFD is better, about 5% better at most. I just want a stress design updated for 2009, a stress design that will produce mostly the same sizes.

Ussuri - Unless I miss my mark, our offshore clients (two of the biggies) still use API stress design (not LRFD), but I'll double check this.