New AISC Combined Spec
New AISC Combined Spec
(OP)
I just went to a recent seminar where the new AISC Specificaton was introduced - it is a combined spec where ASD and LRFD are mixed in together so you have one spec and two different design methods within.
I thought it was a good solution overall for AISC as they have had all sorts of issues with many engineers just refusing to convert to LRFD.
A couple of observations on it all:
1. One point brought out was that if all the updates over the last few years had occurred in ASD (i.e. if there never was an LRFD) then the ASD spec would be a lot more complex than the current one. So therefore many of the arguments about ASD being so much simpler aren't quite as accurate as we initially thought.
2. In the ASD - ALL equations have changed.
3. Do you think that some engineers will get the two systems mixed up during design? Will this cause problems, errors, collapses?
4. The speaker asked our room of about 150 to raise their hands if they currently use ASD and then if they currently use LRFD - the result was about 80% ASD and 20% LRFD
I thought it was a good solution overall for AISC as they have had all sorts of issues with many engineers just refusing to convert to LRFD.
A couple of observations on it all:
1. One point brought out was that if all the updates over the last few years had occurred in ASD (i.e. if there never was an LRFD) then the ASD spec would be a lot more complex than the current one. So therefore many of the arguments about ASD being so much simpler aren't quite as accurate as we initially thought.
2. In the ASD - ALL equations have changed.
3. Do you think that some engineers will get the two systems mixed up during design? Will this cause problems, errors, collapses?
4. The speaker asked our room of about 150 to raise their hands if they currently use ASD and then if they currently use LRFD - the result was about 80% ASD and 20% LRFD





RE: New AISC Combined Spec
Forgetting seismic for a moment, I wonder how important all the updates to the steel specification really are. Did they mention if there were deficiencies in the ASD spec that could cause unsafe designs even if you followed the spec in every way? Or are they just getting rid of some of the approximations, such as the calculation in ASD for allowable bending stress in unbraced beams. It seems to me that ASD in its currrent form has a sucessful history. I know that in some of the derivations for the formulas in the current ASD spec, approximations were made to make hand calculations easier. I am concerned that todays academics are taking those approximations out, so we can have a "theoretically correct" solution, and that with computers, rigorous equations are not a problem. If so, I strenuously disagree. I read an article where Robert Disque said that while he was still with AISC, he was in favor of the new LRFD spec, but that when he left AISC and started using it in the real world, he didn't like it.
I have found LRFD cumbersome to use for design. For example, designing a crane girder with a cap channel by ASD is relatively easy, but it is a lot harder with LRFD. I am going to order myself another 9th edition ASD manual so I can always have it around, because of the shoddy build quality they used for that book.
RE: New AISC Combined Spec
However, I bit the bullet some time ago and converted to LRFD in steel. Yes, its a bit more difficult, but I felt that after using ASD for so many years, and the fact that all my young engineers were coming into our office with LRFD on the brain, I figured that I'd better "keep up" and learn LRFD. After all, I had been using ultimate design with ACI for years and liked it.
So many engineers I talk to sound just like me when discussing metric, but it really isn't that hard, it isn't that difficult to use once you get it under your belt. And it is more accurate in regards to level of safety.
But I don't condemn any engineer for wanting to stay with ASD and I don't disagree with your points about ASD providing safe designs for years. The combined spec seems to be a nice compromise.
RE: New AISC Combined Spec
RE: New AISC Combined Spec
RE: New AISC Combined Spec
For industrial buildings and industrial facilities (example:mining and gas facilities, pipe racks,etc), I think that ASD would be a good method for design ( I use it) due to the load and resistance factors for this type of buildings are diferents (and sometimes not well defined) from those used in residential, malls or offices buildings.
What do you think about that?
Regards.
EoB.
RE: New AISC Combined Spec
As I mentioned above in my first post, the AISC has indicated that LRFD has numerous other checks and more complexity because it has been the subject of all the research over the years while ASD has just been left alone. AISC thought, incorrectly, that ASD would be abandoned so no new research and development was done to it.
But with the stubborn engineers out there who want to keep ASD, AISC finally saw the writing on the wall and has now updated ASD to correspond to LRFD in the same spec.
That means that the "simple" ASD that everyone likes will be a bit more complex in the new spec.
RE: New AISC Combined Spec
THE REALLY BASIC QUESTION IS DO ANY OF THESE COMPLEXITIES MAKE SAFER STRUCTURES. CAN ANY ONE ANSWER THIS QUESTION.
RE: New AISC Combined Spec
If you are designing a small number of elements or a noncomplex structure, the simple method may be quite appropriate. But if you are mass producing elements or designing a very unusual structure, the complex method could be very useful.
Most codes and specs now provide dual procedures to allow the designer the option. A simple, conservative procedure is allowed with some restrictions on applicability. Alternately, a more complex procedure is allowed without restrictions.
RE: New AISC Combined Spec
Did AISC mention mixing of the 2 methods in design, particularly in lateral components design?
I think everyone will be finding the easiest parts of each method and applying them from this combined book.
RE: New AISC Combined Spec
I guess as long as you are very VERY consistent in your calcs and careful not to mix things up you could probably use, say, ASD for gravity stuff and LRFD for lateral - or something like that.
But it seems that you would be adding a layer of complexity that would just expose you to a higher probability of errors.... is it really worth it?
RE: New AISC Combined Spec
1. DESIGN THE WHOLE STRUCTURE IN LRFD USING RAM STEEL INCLUDING WIND AND QUAKE.
2 OUTPUT GRAVITY REACTIONS TO PLAN w/ AN INCREASE OF 1.7 +/- AND ALLOW FABRICATORS TO DESIGN CONNECTINOS IN ASD- BECAUSE THAT IS WHAT THEY ALL WANT.
3. OUTPUT LATERAL BRACE CONNECTION LOADS IN LRFD AND REQUIIRE FABRICATOR TO DESIGN CONNECTION AND SUBMIT CALCS.
4. DESIGN ALL SIMPLE ADDITIONAL ELEMENTS IN ASD, i.e. MECANICAL FRAMES, SINGLE BEAMS, WIND GIRTS ETC.
NO PROFESOOR WOULD EVER AGREE TO IT BUT THAT IS THE WAY IT WORKS IN MY AREA
RE: New AISC Combined Spec
1. Depending on the size of the structure I use ASD in RAM
2. The lateral brace forces are shown in ASD if design is ASD or LRFD if design is LRFD. But I allow the connections to be designed using ASD (applying a "phi" factor to the LRFD loads) if fabricator wishes. Signed and sealed calc's a must regardless of what method is used. (Most engineers in my area still prefer ASD....me included... .....that is why I allow the connection design as ASD)
RE: New AISC Combined Spec
I think the reason why I along with the other people I know are using ASD for the smaller things, is that we feel more comfortable in the results obtained since we more likely performed the calcs correctly using ASD. This is since it is easier and we have experience with it. And there is also the speed of the CURRENT ASD.
RE: New AISC Combined Spec
In other words if you want to use the ASD option in the new steel code, you're still going to have to learn the LRFD formulas.
RE: New AISC Combined Spec
RE: New AISC Combined Spec
The "young" engineers out of school are learning only LRFD. If you want them to use ASD you need to show them how.....
In my opinion the days of ASD will be numbered once the combined specis out.
We might as well get on the LRFD bandwagon now instead of being forced onto it later.
RE: New AISC Combined Spec
I remember how painful it was to adapt to the IBC, so I sincerely feel sorry for hardcore ASD folks. I think that it would be a lot easier to adapt to LRFD than the IBC.
The LRFD equations are sometimes more complicated but everybody I know owns reputable software that's used for all but the simplest calcs. These simple calcs can be yanked out of AISC tables anyway and it's no harder to compare Z to Zrequired than S to Srequired.
As a counterexample of LRFD difficulty, I don't ever hear ASD folks bragging about the ease of ASD composite beam design versus LRFD!! Not to mention the better results.
In a few short years, 89 ASD will be but a distant memory just like my beloved SBC.
DBD