Industry Practice for Steel Design

[neillydavies]

Hi,
I was wondering, if it is the current industry practice to use limit state for steel design or are you still using working stress (allowable) method?

Neilly Davies Consulting Engineers

http://www.neillydavies.com.au

 

[frv]

In the US, everything is moving to limit states, but ASD is still widely practiced; especially by more experienced engineers and those who had experienced engineers as mentors who didn't want to learn LRFD.

 

[sdz]

In Australia Limit State is almost universal for structural design but some ASD is used by mechanical enginers.

 

[neillydavies]

Thank you for the response, as you mentioned many of the most senior engineers are still using ASD. But is not difficult to switch. LSD started in concrete well before in steel.

Neilly Davies Consulting Engineers

http://www.neillydavies.com.au

 

[civilperson]

The current Steel Manual (AISC 13th) has ASD and LRFD in alternate columns on the same page, (both are approved).

 

[Lion06]

But the 13th edition ASD is not truly ASD, it is LRFD in disguise.

 

[swearingen]

Actually, it really is ASD. Had there never been an LRFD, this is what the evolution of ASD would have looked like. The reason it is so very different than the green book is that there were no real updates to it through all the years that LRFD was being developed. Now it is state-of-the-art as it would have been if there were no LRFD.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[nutte]

I think the hangup is how the old ASD is "Allowable Stress Design," and the new ASD is "Allowable Strength Design." And why this causes people headaches is beyond me. The equations are the same, you just multiply your allowable stress by an area or section modulus to get an allowable force or moment.

A pet peeve of mine is how people say "limit state design" to mean "LRFD." ASD is a limit state design, too. You check your loads to serviceability and strength limits. Where the factor of safety falls does not mean this isn't limit state design.

 

[DerChad]

I've only used LRFD a few times and am a proud owner of the "black book". For the most part, everyone I know uses ASD and those of us that have used LRFD at all have noticed that the calculations are much more lengthy with LRFD than is the case with ASD. As I typed that last line, I could actually hear the groans of the LRFD people, followed by a "No, it's not."

One of the hangups is that most of us tend to think in terms of stress, not load. Just one example is bolts. Regardless of size, if you tell me that a bolt has a certain tensile stress, I know whether or not it will be ok. When you tell me that a 2 1/4" A615 GR 75 anchor rod has 190 kips of load, it doesn't click with me whether or not it is ok. It's much easier to remember the allowable stress for commonly used bolts that it is to remember the allowable load for every given size. It's not that big an issue to calculate it but that's more time. And as everyone would agree, more time means more money and I don't see our fees increasing.

It's interesting to note how even with my limited use of the LRFD methods, I've found that deflection controlled my beam designs...which led me to a heavier beam...which took me back to something similar, though still slightly smaller, in size that I would have picked through use of AISC 9th edition. Also interesting to note is all the talk about "bouncy floors".

 

[nutte]

One of the hangups is that most of us tend to think in terms of stress, not load.

Hmm, short of taking a survey, I'm not sure about that.

I'm an ASD guy, but I'll defend the groaning LRFD guys. The calculations are more lengthy not because it's LRFD, but because you're using a manual (green ASD) that is almost 20 years old. The new and improved methods result in more lengthy calculations. Blame that on progress, not LRFD.

 

[KBVT]

Look at the equations in the 13th Ed. They are basically the same for ASD & LRFD. The "ASD safety factor" vs. the LRFD strength reduction factor are the main differences. Structural EIT had it correct. The only real difference between the 13th Ed LRFD and ASD is on the applied load side of the calculation, the resistance calculations are virtually identical.

 

[JLNJ]

ASD for me, and it's more than just a code-formula thing.

I like to think in terms of where my loads fall in relation to a safe (allowable) stress condition. It just makes more sense to me than thinking in terms of comparing factored loads to an ultimate state condition.

 

[vmirat]

If you're designing an entire building that includes a steel structure with concrete foundations, then I have found it easier to use LRFD throughout the entire design process, since you have to use LRFD for the concrete. Otherwise, if you use ASD for the steel, you have to calculate your loads based on two different code mandated load combinations and then make sure you don't mix them up throughout the process.

 

[Lion06]

vmirat-
You still need service loads for deflections. You can't get away from the service load combinations.
Surely you don't check drift with 1.6*50-year wind.

 

[LobstaEata]

In reading Nutte's comment about the more lengthy calculations for LRFD being the result of progress, I was reminded of a quip from one of my professors over 25 years ago.

"The evolution of an Engineer begins with a passion for learning more and more about less and less until absolutely everything about nothing is known."

That said, LRFD brings us one step closer to our ultimate goal.

 

[vmirat]

StructuralEIT,

I would agree that serviceability is a unique case. Even if you used ASD, you would still have a separate load case for serviceability, as you have pointed out for wind. However, you could do wind with the LRFD factored loads by adjusting the allowable drift, depending on your local jurisdiction. Where I am at, there is no local code for wind drift serviceability, so I could offset the LRFD factored loads by setting the drift limit higher.

The point I was making was that you could do your steel design and concrete design using the same load combinations, especially when you get to foundation design.