13th edition steel manual 2

[Lion06]

I wanted to get some opinions on the new steel manual. I recently attended a seminar on the new manual where it was compared and contrasted to the 9th edition ASD manual. It was mentioned in the seminar that the new ASD stands for Allowable STRENGTH Design, not Allowable STRESS Design. The speaker was pushing for the movement of everyone from ASD to LRFD and his resonning was that the new ASD is really just a variation of LRFD.
I disagree with that characterization somewhat and wanted to get other opinions. The new manual starts both ASD and LRFD with a nominal moment strength of Mn (which for an adequately braced beam with a compact section = Mp). For ASD it applies a FS of 1.67 This is the same FS applied to Fy in the 9th edition ASD to get Fb=0.60Fy . The difference between Mr and Mp is that Mr uses Sx and Mp uses Zx, so at first it might seem that the new ASD has a leg up. However, the 0.60Fy is increased by 10% to account for the plastic moment strength of a beam with a compact section that is adequately braced. The only benefit I see to the new ASD is that instead of using a generic 10% increase they are allowing you to increase by the ACTUAL shape factor (Zx/Sx), which is very slightly more economical as most shape factors do not exceed 1.15 and a great deal are in the 1.12 range. Using this you end up reaching virtually the same allowable stress, you just take a different road to get there.
I haven't had an opportunity to investigate the case of inadequate lateral bracing yet.
I would appreciate any opinions that you all have.

 

[JedClampett]

I think the push is due to the fact that the ASD hasn't been updated for about 25 years. LRFD has all the advanced technology in it. But no one liked the method. So this was the compromise. You can design in either method, you'll get the same answer (almost), and have all the latest and greatest code knowledge.
My opinion is that I basically didn't have to learn a new steel code for my whole 30 year career. It was nice while it lasted, but sooner or later, engineering marches on.

 

[haynewp]

"The speaker was pushing for the movement of everyone from ASD to LRFD and his resonning was that the new ASD is really just a variation of LRFD."

The speaker may be right. I don't have a copy of the manual yet and the spec just got adopted in my area. However, somebody that helped write the new spec told me you when designing by ASD by the new spec, you are really using all the principals of the LRFD approach. He said it is not really ASD, but LRFD meant to look like ASD to make those engineers dedicated to using ASD happy.

 

[WillisV]

LRFD: Pu < (Phi)Pn

ASD: Ps < Pn / Omega

It is as simple as that. The equations and methodology used to calculate Pn (or Mn etc etc) are exactly the same.

 

[JStephen]

The real differences in use are just a matter of what you're used to- something different just doesn't seem right.

The new code is new and improved, but also includes stuff that wasn't in the old one. More comprehensive, more complicated. You win a bit and lose a bit.

 

[ctcray]

As one who learned LRFD but uses ASD, I always thought the nicest part of ASD was not having to keep track of two versions of each load, one for strength and one for serviceability.

 

[Lion06]

I am not sure my question came across properly. Based on the new ASD, you get an allowable strength. By maxing out that strength, you get a maximum allowable stress. For the case I mentioned above, it is virtually identical to the old ASD. The only difference is you are increasing the allowable Fb, of 0.60Fy, by the ACTUAL shape factor instead of a generic 1.10 shape factor that will conservatively cover all cases.
I didn't intend to debate the merits of ASD vs. LRFD. My only point was that I am not sure the new ASD is really the same as LRFD as was suggested (for the reasons mentioned above).

JedClampett-
I believe the only way you get the same answer (most of the time) is if the LL/DL ratio = 3. If it is greater than 3, then LRFD is actually less economical. This is often not the case, but....

 

[bjb]

I went to one of those seminars too. The point they are making is that the same equations govern the strength of the steel no matter which method you are using. LRFD and ASD now really just differ in how the applied loads are handled.

I dislike the LRFD load combinations and am more comfortable using ASD combinations. My response to the fact that LRFD results in a more uniform reliability is, "so what".

I am using the the 13th edition manual now, but I have to admit I find Chapter C very hard to understand. The speaker at the seminar I attended said that AISC wants to get away from the concept of column effective length, and wants the direct analysis method of Appendix 7 to become the way everyone designs moment frames.

I wonder if AISC is splitting hairs in the name of academic rigour. They can pry my green book out of my cold dead fingers!

 

[minorchord2000]

bjb is exactly correct in my view. I use the new manual now. I especially like the examples that come with the manual. They are on CD. I did find numerous errors but at least AISC is moving the right direction. LRFD was a flop when it came out and the only ones that used it were the new engineering students coming out of school since that is what they were taught. But we had to re-train all of them in the ways we do things in our office. Whenever AISC folks came to our SEA meetings, they heard a rash of resistance from the engineers in the meeting whenever AISC pushed for LRFD. It is a compromise and I applaud this effort to appease both armed camps.

 

[swearingen]

Where LRFD really shines is when the LL/DL ratio falls. This is why AASHTO has been using a form of it for a long time - bridges are a classic case of a structure with a low LL/DL ratio.

When LRFD came out, it was actually calibrated to ASD to specifically come out with the same answers when the LL/DL ratio is 3. At that point (around 1980), ASD was frozen in time and research continued in terms of LRFD. This last manual just brings ASD up to the state of the art. I've heard from an AISC official that ASD would look about the same as it does in the 13th manual had there never been LRFD.