For AISC, which method, LRFD, or ASD, should one use ? ? 2

[bertyboy]

For AISC, which method, LRFD, or ASD, should one use ?

I am a Structural Engineer in the oil and gas industry, in Aberdeen, Scotland, U.K.
In this industry we use some of the American codes, e.g. the AISC codes.

ASD is the allowable stress design. I have the 'green book', 9th edition, dated 1997.
Has this method been replaced by LRFD ?
Presumably this is a Load Factor Design method ?
When did this method come out ?
Is ASD still valid ?
Which of these 2 codes is the most commonly used code, in the U.S.A. ?
In terms of column design ( combined compression, and bending ), is there any difference between the methods in these 2 codes, aside from load factoring ?

 

[whymrg]

In US we use ASD as well as LRFD. US building code allows to use either one.

It is better to use method that you are familiar with.

LRFD is more advanced, so you might get smaller sections for the same load.

On another hand, California requires that schools and hospitals to be designed using ASD, since it has larger Factor of Safety.

Good luck.

 

[Lutfi]

I am old school. I am well versed in the ASD. It works fine for me. I only use LRFD only if my clients direct me to use it.

LRFD never picked up the momentum in the US. I am not sure why. There are several factors one of which is the engineering graduates of the late 70s and early 80s,were not educated in the use of LRFD. As humans we become comfortable in what we use and refuse to change. Another reason is ASD has performed well for steel design in the US, which made lots of engineers ask why change now.

AISC attempted repeatedly to encourage the use of LRFD and the success rate is not as they desired.

I maintain that once recent college graduates center the market, they will force the market to use what they have been educated and trained on.

Bottom line, use the method that you feel comfortable with and one that you can substantiate results.

 

[FM1]

bertyboy
ASD is always valid. LRFD (load resistance factor design) is the new method and is slowly begining to find its way in since, as whymrg mentioned, can lead to some savings in material.

I dont know if you are at the offshore or onshore end of the oil industry, but the API code for offshore platforms (the latest I worked with 1993) does not (did not) recomend LRFD or LFD.

In column design, as long as you apply the appropriat factors to loads and material strength the idea is the same. A book by Salmon and Johnson titled "Steel Structures, design and behaviour" is a good source. I am sure there are many others.

 

[Joecandraw]

ASD is perfered in the industry for many reasons. It is known to all the steel detailers and there software is set up for it. More over LRFD is not allowed in many areas because it has a building safty design factor that is marginal. In other words ASD is safer all around. In Areas with high winds, earthquakes or costal storms I would not use LRFD do to the weaker safter margins. Example for 30 cents you can use a 3/8 shear tab verses a 1/4 shear tab for most shear tabs. A 3/8 shear tab will take almost 1/3
more twisting in a high wind. Thus is it safer. Also a better weld will develop when the shear tab is welded. Both items increase the proformence of the building package.

 

[HgTX]

What some people call a weaker margin others call a more efficient design...

The codes establish minimums. Nothing stops you from doubling the size of everything just to be safe. The question is how safe is safe enough? If LRFD yields smaller members (which, overall, it is known to do), does that mean it's dangerous, or that ASD was overconservative?

The main reason ASD is preferred is because it is familiar, period. (Same reason the US still hasn't gone metric, but the metric threads are over in another forum.) It was also preferred because it was simpler, but the upcoming Grand Unified AISC code makes things equally messy for both methods; from what I can see only the factors are different. AISC has a review copy on their website at

http://www.aisc.org

.

There was another thread (perhaps in another forum) that had some pretty good ASD vs. LRFD discussion but I can't find it now. Perhaps someone with more patience than I have can locate it. (Is the keyword search function going to be fixed?) Anyway, in that thread someone pointed out that it's mostly LRFD being taught in the schools, and that it will be used more and more often as engineers enter the workforce who learned on that method.

That's not to say I don't love my Green Book.

Hg

 

[JAE]

Joecandraw - I have to respectfully disagree with you. LRFD is not, and never has been, significantly less safe than ASD.

One proof - for a structure with a very high amount of live load relative to dead load, the safety factor on the design would approach 1.6/0.9 = 1.777 for flexure while in ASD it is 1/0.6 = 1.666.

LRFD is simply a better way to weigh the differing variabilities in loads and resistance in a statistical manner. ASD is simply a selected safety factor regardless of the variability of the applied loads or resistance. So in once sense, ASD is LESS safe in that it doesn't consider these valid variabilities.

Both LRFD and ASD have been fully adopted in the International Building Codes in the US and in fact, AISC will soon present its new combined LRFD/ASD specification where ASD has been finally upgraded to reflect current research and knowledge. This will make ASD a bit more cumbersome to use but granted, many US engineers prefer to stay with what they've used for the majority of their careers.

I agree with Lutfi that over time, LRFD will become more dominant as the engineers who use ASD will retire.

 

[CAOranger]

There is big resistance from the building industry in the US for embracing LRFD, not becasue LRFD is unsafer. Lutfi has explained why.

ASD's safety factor is not necessarily a "safety factor", it could turn out to be an "unsafety factor", because ASD treats all types of loads as one type which has been proven to be wrong based on statistical studies.
For regular beam design, it does not matter whether you use ASD or LRFD, but if you have unusual situations ASD manual can not help you. Example, if a contractor asked you to evaluate of an existing plate bending in strong axis, what do you say to the contractor? That I do not how to do it? There are many structures (existing or new), ASD will prove to be unsafe, but in fact they are not.

The thing to remember is that ASD or LRFD specifications (green manual and silver/blue manual) are not ***legal*** documents. These specifications are adopted by the building codes by reference with some modifications.
For seismic design, code provisions for ASD are becoming similar to LRFD provisons. For drift calculation, you can not use ASD load combinations, but you are allowed to use ASD for design.

Prevalence of ASD in US is comparable to the prevalence of imperial system (inch, pound, kips, mph... system vs SI system) -- because of resistance from the building industry to change. My bosses never opened LRFD manuals in their whole lives and they are in the code committee. See!

However, you should be following the method that is common to your industry. Building structure are different than other structures -- if you ask a building engineer to design a car chasis [using ASD ], you will be getting a humvee!

 

[Lutfi]

CAOranger did a good job explaining.

On positive note, LRFD resolved the long standing debate regarding the use of 33% stress increase (which in my opinion was abused by many engineers). When using factored loads we totally do away with the need to increase stresses by 33%.

It bothered me to no end that engineers were increasing allowable stress by 33% when they had dead load and wind loads acting!

 

[bertyboy]

Thanks for all your comments.
The reason why I am so interested is because I am writing a spreadsheet, to do AISC ASD combined bending and compression design.
You have all convinced me that the ASD design has still got a very good future.
my opinions are as follows :-
1) Both our steel and reinforced concrete codes in Britain are Load Factor ( or as we say limit state )I have never liked these methods. It seeems to me to be ridiculous to go to all the effort to have different load type factors, when the materials used have such a large strength variation, especially concrete.
2) I dont think their is sufficient difference in the load factors to justify them, e.g. :-
a) Dead + Live = 1.4Gk + 1.6Qk
b) Dead + Live + Wind = 1.2Gk + 1.2Qk + 1.2Wk

Basically I believe that it has become unecessarily complicated ( numerous load combinations ), and I don't believe that it has succeeded in saving much material.

 

[pba]

Bertyboy,

As you will know, the UK's allowable stress steel design code - BS449 has not been withdrawn.

I've used the UK ultimate limit state code - BS5950 for all of my career and have only checked other peoples calulations done to BS449.

I take issue with your comment on the magnitude of the partial safety factors. 1.4Gk+1.6Qk can be VERY different to 1.2(Gk+Qk+Wk). Also don't forget 1.4(Gk+Wk) and 1.0Gk+1.4Wk....

The main point is, we work out the effects of the different ULS load combinations then design for the worst case. In many cases the worst case is blindingly obvious and then there is only one case to consider!

 

[TFL]

pardon my ignorance what does gk, wk, and qk mean? obviosly they are live, dead etc. I assume they are british terms.

 

[pba]

tfl,
Yes you are correct Gk and gk are dead load, Qk and qk are imposed (live) load and Wk and wk are wind loads.

The capitalised terms are the total loads and the lower case versions imply contributing loads.

If anyone out there knows why G and Q are used please let us all know!

 

[TFL]

ok so there is no sense to the letters g and q they are just arbitray. i think a prefer dead and live.
thanks for the education

 

[Focht3]

Here's one of the LRFD/ASD discussion threads:

http://www.eng-tips.com/viewthread.cfm?qid=67167

Part of the problem has been that all of the "players" haven't been at the discussion table when USD/LRFD were developed. Geotechnical engineers, in particular, have problems with LRFD - see the thread above for a more complete discussion.

Until AISC recognizes the very real and serious limitations in LRFD - and moves to deal with them by dialogue, not fiat - resistance to LRFD will continue. Some may be reluctant to use LRFD because of their comfort with ASD. That's not my reason -





Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[HgTX]

What provisions of the AISC Specification for Structural Steel Buildings are inappropriate for geotechnical engineering? The closest thing I can think of would be design of sheet piling, and that's not addressed in that spec.

The question had to do with AISC, not AASHTO (or whoever covers piling, foundation, and retaining wall design).

Hg

 

[VTPE]

It is important to understand the probability involved in developing the "allowable stress" factors in ASD. The allowable stress was developed to protect against probable inconsistencies in design in (3) main instances. First, inconsistencies occur in assumptions made in the development of the equations engineers use to determine the applied stresses. Engineer's tend to make rectangals out of parabolic stresses. Second, inconsistencies occur in the fabrication of the elements in the structure. For example, residual stresses and out of plumbness occur in every peice of steel. Finally, inconsistencies in construction. Anyone who has seen a building built realizes a contractor can make decision the engineer may not know about. The bottom line is, once a building is erected, the "Factor of Safety" is gone. LRFD allows for the reduction in steel stress based on the above probabilities while allowing the engineer to make educated judgements on the loads applied, thereby giving the flexibilty and predictability required to do a good analysis. Any new engineering done on the existing steel should be done with limit states design. One final comment, the ASD Manual has not been updated since the end of the eighties. There has been significant research done, especially in the areas of composite design and connections that is not present in the ASD manual. The discussion about LRFD giving smaller sections is not really relevant. Stronger steels have given rise to smaller sections and the need for both design methodologies to not only check strength, but serviceability. More often, for both designs, deflection and vibration analysis will control.

 

[pmkPE]

I do industrial type work and prefer the ASD. When LRFD first came out I went through every example in the ASD and LRFD to see what the differences were between the new code and ASD. The only thing I learned was that it took many more calculations to come up with the exact same answer in LRFD as were given in the ASD examples. From my perspective, clients are looking for reduced rates on our part so the addition of more calculations to get the same answer is not cost effective.

From what I also remember at the time that LRFD came on the scene for steel design, savings of a few percent were being realizied on large commercial type projects. However, in industrial allpications there are too many situations where we want there to be additional safety factors that combined with non-symmetrical, low redundant type configurations did not warrant in my opinion the use of LRFD.

 

[vmirat]

It seems to me that this is an issue of economics versus public safety. If a member is designed using both methods and one method results in a smaller member (i.e. less cost), should we go with that member size? Or do we have an ethical obligation to provide the safest structure possible (i.e. go with the bigger member)? The US building codes currently leave the method of design up to the engineer. That puts the liability on us. I'm inclined to use both methods and pick the larger member. I wish AISC would just pick one and be done with it.

 

[PEinc]

State highway departments and the FHWA are requiring design by LRFD. There is no choice.