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.

 

[271828]

271818-
Not sure what you are getting at....

Sorry, I forgot this part. I was referring to:

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.

Maybe I'm missing something: What are some situations for which the green book gives guidance for a subject not covered in the black book. How does the industrial designer take a step backward in going from the green book to the black book?

 

[ToadJones]

Can't argue what you say.
However, for the situations I described earlier, these provisions are not very reasonable. Allowable stress was more practical.

I could post a sketch of the shapes I deal with, and you'll understand. Determining Fcr would burn 1/2 of the job budget.

 

[vmirat]

So, my original post was intended to illustrate that there are folks out there using both methods, with ASD (Allowable Strength Design) being used for "simple" structures and LRFD used to take advantage of possible cost savings in steel for larger projects. The person of whom I quoted is not the first that I've encountered. It seems to be fairly common. As such, from a practical standpoint, should we be teaching entry level engineers both methods, since the colleges are only teaching LRFD? In addition, how do you teach the new engineer when to use ASD and when to use LRFD? What's the break point?

 

[ToadJones]

As for:
"...If F12 didn't exist, the engineer would hopefully compute Mn for an unsymmetric member exactly as currently shown in F12...."

Is this really what an engineer would do?

Or would an engineer make some practical assumptions about the stresses at various points in the cross-section and maybe break that section into portions or components that would be carrying these loads and stresses and proportion them to accordingly?

Perfect example of Wootens Third Law

 

[Duwe6]

Some of these gentlemen seem to be missing the $$$ part, caused by the d@mn&ble clients. A properly designed structure done to LRFD is at, or very close to its limit state. Any added/relocated liquid-filled pipe over about 6"NPS will require adding bracing, plating the bottom of beam flanges, adding round-bar bar-joist reinforcements. I have done ALL of these.

Unless the client can actually leave the design alone from the time the Engineer-of-Record gets released to start calculating loads, and the end of that buildings useful life 50+ years later], the small savings in the cost of extra steel tonnage is overwhelmed by the costs of any modification to that building.

On a Car Plant designed by the major Detroit car plant designer, somebody left out wind bracing for the Paint Bldg exhaust stacks. This building being in Alabama, the bracing was VERY necessary to keep the stacks from crumpling in a 50mph wind. The solution was to add guy-cables. Simple, except the bar joists for that portion of the roof were already maxed out. A 6-10 man crew on overtime & doubletime [all the Straight-Time hours were already budgeted] worked for about 4 weekends 10-12 hours per day, to add enough bracing to allow those 1/4" diameter guy cables to be installed.

Then we did the same thing for the frame assembly area. Due to an inprocess revision, the catwalks above the robot welding cells were moved. More bracing, both sides of every attachment point of those catwalks.

Just barely enough has never been a decent design philosophy, and to me LRFD = just barely enough for today's loads; God help the next guy/gal.

 

[nutte]

Duwe6, if that building had been designed to ASD, I bet the same thing would have happened. The probem there was designing everything to the gnat's tail end. I'm not convinced that for the majority of buildings and designers, a building designed to LRFD would be appreciably different from one designed to ASD. Sure, theoretically you can use lighter members when the LLL ratio falls below 3:1, but in practice, are the results going to be that different? It boils down to one's preference.

 

[JAE]

I agree with nutte on that one. The examples that Duwe6 provided have nothing to do with LRFD vs. ASD. The equations are exactly the same - just the positioning and weighting of the safety factors are different. It is all up to one's preference - both methods are valid and fine.

 

[TLHS]

I don't use the american codes much, but I do touch old codes relatively often that are ASD.

Honestly, I don't think the choice makes any realistic difference at this point. Neither is particularly hard to use. I like LRFD because I feel like I carry through a better sense of my load cases, but that's really just the way I do it more than anything.

You can do simple things with both and you can do complicated things with both. Unless you're designing way closer to the line than I would you're going to end up with approximately the same amount of steel.

If you're doing a simple thing in LRFD you can generally eyeball your governing case at the beginning, do your combination in a line at the beginning and then just carry the combined load through.

It's honestly just preference. You can get a sense of the assumed (somewhat pretend) stresses in both.

The only annoying issue is when you have to change half way through for some reason, which is why I'm happy that geotech stuff is moving towards LRFD in Canada now.

 

[archeng59]

Based on my experience, LRFD typically seems to provide less steel if you are sizing the members for strength only. If you size members for servicability, it seems either LRFD or ASD gives you the same member based on what is needed for deflection and lateral drift criteria.

 

[BigH]

After reading all this - anyone wonder why geotechs don't, as a whole, want to change the way we have been analyzing for years?

 

[JennyNakamura]

While there may be technical points regarding which method is more conservative than the other, and several others as mentioned above, from a designer's perspective I don't understand what the big deal is? Once you get used to them they are both just as easy to work with and many folks can switch back and forth with out even thinking about it. Kind of like the whole metric vs. imperial debate - doesn't matter to me...

 

[dcarr82775]

Metric!?!?! You take that back, those are fighting words

 

[pittguy12]

I agree with Jenny. I prefer ASD but can easily switch back and forth. With the 13th edition of the steel code, this is relatively easy as you are presented with both methods side by side.

 

[271828]

A little off the subject, but I'm surprised there isn't more praise for AISC including both ASD and LRFD. ACI doesn't do that. This question is so obvious that it's certainly been asked and answered before: What do you guys who dislike LRFD do when you design a concrete building?

AASHTO doesn't give ASD and LRFD in one document. I haven't checked the NDS lately, but last item I checked, they didn't either. Does anybody else besides AISC?

 

[271828]

After reading all this - anyone wonder why geotechs don't, as a whole, want to change the way we have been analyzing for years?

After reading all of this, and numerous similar threads in the past, I have a different thought: Perhaps AISC should've killed ASD quickly. I can't see how there would've been MORE criticism. LOL

I need to go back and study the history of ACI 318 to see how that conversion to USD went.

 

[frv]

NDS does offer both in one document.

As told to me by my steel professor a few years back (shortly after the 13th Ed. manual came out), AISC didn't originally intend to have a combined spec. They wanted everyone to adopt LRFD exclusively. However, even after the 3rd edition of LRFD, many engineers continued to use the old green book. AISC thought that the combined manual would serve as a transition. I suspect they are now stuck with the dual manual for the foreseeable future.

The problem is that the engineers using the green book still don't like the 13th edition because it has been converted to "strength" as opposed to stress. The argument being that regardless of member size, they could get a feel for how loaded the member was by comparing the actual stress to the yield. Seems like a valid point, but oh well.

 

[abusementpark]

ASD is easier to use since you only have to deal with one set of load combinations for strength and service limit states. Also, it is simpler to only have to factor one side of the design equation.

As someone else mentioned, serviceability often governs the design of most beams and frames, so the economic penalty isn't really significant. I guess it the effect on the design of gravity columns could be measurable, but I don't like penny-pinching columns anyways.

 

[msquared48]

For all my simplified steel design, I still use the 8th edition from 1980 - WSD - and it makes a lot of sense. Couple that with preferring USD over WSD for concrete design. I still have to modify the WSD steel loads to design the footings.

I can understand the competitive nature between the ASD and LRFD approaches for saving material, but when you do a lot of remodels and additions as I do, I do appreciate extra capacity beyond what is needed for flexbiity of use, whether it was then, or now.

We have had a lot of discussions here regarding PEMB's and how they are designed to the line for reasons of cost, and the point always made is that they cannot be added onto easily due to the tailoring of material useage. Every concept has it's advantages and drawbacks, but the one concept that always rings true to me is "Pay me now, or Pay me later."

I guess you have to look at the future to try to envision how a building might be used, if it will ever be modified at all. But who can precdict that? Most likely, it will be modified at some point in it's lifespan, and that is where I prefer the conservative approach.

I guess I'll have to update myself with the newer ASD and LRFD. My son uses both. I'm just an anachronism.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[fancypants]

TMS 402-08 Masonry code has both ASD and Strength Design also. I also grew up using LRFD. Got into the real world and found it heavy in ASD. Learned to use both and can switch easily also.

I do like ASD because for me, I can do the load combo's in my head much easier when doing quick checks. One only has to multiply by 0.75, 0.6 or 0.7 in some of the cases. Dead + Live, Dead + Wind, .6Dead + Wind are all simple for the brain cells. What I am saying is i can take 75% of something in my head faster then I can add 60% etc...

I also find that carrying service loads to the foundation is a much better prospect with ASD.

Neat conversation.

 

[RFreund]

If you are doing hand calcs predominantly than ASD is easier and faster. You don't have to go back and forth for serviceability and strength.
Also if you're doing hand calcs than your probably not trying to pin the tail on a gnats ass so the savings issue really isn't there.
However if you really need to sharpen your pencil or if you are using a fancy computer where it is easy to go back and forth between strength or service levels, then either is fine.

EIT

http://www.HowToEngineer.com