LRFD or ASD? The saga continues... 12

[vmirat]

I have a project being designed for us by another contractor. It involves a simple support bracket system for HVAC duct work. The structural engineer did calcs using AISC ASD method.

Whenever I see ASD used on design, I usually ask why, out of professional curiosity. Here was their response:

"Designing a large structure with large quantities of steel one should use LRFD to take advantage of the cumulative weight savings. That is not the case here, so ASD was used for simplicity and speed."

I'm wondering if we are schooling new engineers in both methods for this reason? I didn't take this any further, but I wonder how they decide the break point for LRFD vs. ASD.

 

[SteelPE]

I design using ASD. I was taught LRFD when in college then when I entered the workforce I was working with engineers who used ASD so that is what I was forced to use. Does it really make a difference today? Now all the equations are the same and you just end up with either phi*Mn or Mn/omega.

 

[Duwe6]

On big structures, using LRFD uses the absolute minimum weight of steel, in every location.

Sounds like a good and economical idea, doesn't it? Now try to relocate a firewater main or a large cooling water line. Attempt to install a walk-in freezer on an upper floor.

Any change in the loading that even slightly increases the weight on that area will require expensive added steel. You only put in the minimum, not its too little. Now try to get the budget to make these additions and reinforcements in a finished structure.

LRFD is for kids and idealists. ASD is what experienced realists use.

 

[dhengr]

With all your steel savings with LRFD, are you sure you ESTIMATED the loads correctly or should you have multiplied (factored) them by 1.613 instead of 1.6. The another interesting thing to watch will be the rolling schedules for a W 14x152.378 column section or that W 24x109.3 beam section which you hoped to save steel using. And, your butt is really in a jamb if the next beam over could actually be an available W 24x117, but the erector mixes them up on installation. Of course, with BIM this will never happen, everyone can tell the difference btwn. a pink beam and a green beam. You don’t even need to know anything about Structural Engineering or Strength of Materials any longer, you just need a color chart to differential btwn. pink#126 and slightly reader#129 beam.

 

[JAE]

On big structures, using LRFD uses the absolute minimum weight of steel, in every location.

That just isn't necessarily true. The sizes are affected by the relationship between dead load and live loads. Structures with higher levels of live loads will have a more conservative design with LRFD due to the higher load factor on live loads.

Structure "size" has nothing to do with it.

 

[BAretired]

LRFD in the USA or LSD (limit states design) in Canada is preferred over ASD in the USA or WSD (working strength design) in Canada. In fact, WSD is no longer an accepted method of design in Canada.

Usually, the differences are small but with LSD, live loads and dead loads can be factored up with a different load factor better representing the degree of confidence in the magnitude of load.

LSD us only slightly more difficult as you have to keep track of dead vs. live loads but after you get used to it, it is no big deal.

BA

 

[Ron]

Tomato....tomahto

I prefer ASD because it is more intuitive to me.

 

[pittguy12]

Same as mentioned previously...

learned LRFD for steel in college, entered the real world and started using ASD. studying for lateral portion of 16 hour SE exam and now back to using LRFD for seismic design.

I prefer ASD for quick calcs and if I am just trying to nail down the size. Also prefer ASD when I know I will be checking foundations with the same calcs and don't feel like tracking factored vs non-factored loads.

Just a preference and usually dictated heavily by what I am doing.

 

[frv]

JAE is right.

At a ratio of 3 to 1 (LL to DL), the results should be the same (in theory). Above that, LRFD is more conservative, below that, ASD is more conservative. Size of the structure has nothing to do with it.

 

[graybeach]

I design a lot of shoring, brackets, and utility supports these days and always use ASD. I need to think about deflections in most cases, and I do not need to design things to a cat's whisker. ASD the only way to design shoring because deflections are critical, and I need to have a good idea of the actual stresses that I am trying to relieve. At times, I even need to do reinforced concrete WSD for this.

For larger scale design work with well defined loads and combinations I have no problem with LRFD.

 

[dcarr82775]

I don't think a beam designed by ASD is any better than one by LRFD. Adding a pipe isn't going to blow up a properly designed LRFD beam any more than an ASD design of the same beam with the same level of detail. I use both often.

The only place I always use LRFD is coverplating existing beams and for composite beams. It greatly simplifies the calculations.

 

[SAIL3]

VM...I don't get the drift of your question...
the input loads are at best an estimate..the claim that LRFD input loads are statisically a better estimate is marginal at best(IMO)..the diference in the final design between the two, if one exists, is minimal at best...
usually the engineers that tout LRFD over ASD are for the most part inexperienced engineers, members of academia or associated with some outfit that has a vested interest or making a living out of the changes in codes etc. that it has produced.
my bigest complaint with LRFD is the use of immaginary loads leading to a non-existent state of stress in the structure...
this has been taken, now, to another rediculous level with the introduction of ultimate wind speed..huh?..now that is something that everyone has a grasp of ....
I prefer to work with real loads and real level of stress that allows me to use engineering judgement and intuition...

 

[ToadJones]

I assume most of this discussion is about ASD as in "Allowable Strength Design" NOT Allowable Stress Design Vs. LRFD

 

[dcarr82775]

While I find the new organization for wind in ASCE7 annoying the calc is essentially the same as before. You just put in 115 instead of 90 and then multiply by 0.6 to get allowable. Given all the other things they change, that particular one isn't high on my list of gripes.

 

[ToadJones]

I have an ASCE 7-10 "intro to wind loads" webinar/PDF ...wonder if I would be violating anything if I were to post it?

 

[frv]

Sail3..

The internal state of stress is just as imaginary in ASD as in LRFD (see Wooten's 3rd Law).

If you are referring to the plastification of the section assumption, then I'm sorry to tell you that this was also accounted for in ASD for years, although in an indirect way.

I really don't get the animosity toward LRFD. I understand people have been using ASD for years and want to continue to use it, but the implication that if you use LRFD, then you must be inexperienced or don't have the best interest of the client at heart is ridiculous.

Is it really that much more work? I am not implying that LRFD is the one and only valid method. ASD worked beautifully for years and it would continue working if we were to continue to use it. However, personal feelings notwithstanding, LRFD does give a more predictable reliability index

 

[ToadJones]

I think the heart of the argument is, and always has been, intuition or "gut feel" when designing with the "old ASD".
It is easy to relate calculated stresses to an allowable or to yield.

"hmm, this bending stress is 29.5 ksi, that's getting close to yeild"
vs.

"hmm, this beam has 245 kip-ft max moment that's getting close to...umm, ahh, ..."

 

[dcarr82775]

Toad,

Why is 36 ksi elastic stress any easier to grasp then a fully plastic section at 36 ksi?

If anything the first represents a condition at which nothing noticeable really happens whereas the later represents the conditions at an actual failure.

That being said, I agree ASD or LRFD is really a matter of preference and gets you to about the same place in the end

 

[ToadJones]

Only because, procedurally, when using the new ASD, one doesn't usually calculate stresses...no need to read into what I said really.

Other than that I do have some very specific gripes.

IMO the Black 2005 book is a cookbook specifically made for run-of-the-mill situations encountered in basic building design. It has left out in the cold more complicated structures encountered in industrial design. One only needs to look at Section F12 of the 2005 spec to see this. If you have not gone through this, good for you as it flat-out sucks and is VERY time consuming. I may sound bitter, but I think to some degree academia has over-ridden solid engineering experience. If you design industrial structures you know that trying to save every pound of steel can really lead to disastrous results. Throw in rolling schedules as Dhengr alluded too and you really have no savings at all.

 

[nutte]

Toad's complaint is whether you calculate a stress (ksi) or a moment (kip-ft). This difference confounds me. It's just algebra. It's not because of a philosophical difference in the two methods, old ASD versus new LRFD (or new ASD). Thus, the point I continue to make, that old ASD and new ASD are at root the same thing, with an algebraic transformation. I don't consider them different just because AISC changed what the "S" stands for.