Load and Resistance Factor Design (LRFD)

[shimpundu]

Why LRFD in geotechnical engineering work? Can someone, please, show me the advantages.

 

[Focht3]

Advantages?! There aren't any!

Check out this link:

http://216.239.33.104/search?q=cache:LBaFjrA3qHYJ:

http://www.eng-tips.com/gviewthread...507/qid/56224+BigH+Focht3+LRFD&hl=en&ie=UTF-8

Sorry the URL is so long - but it contains an excellent discussion that was deleted from one of the Structural Engineering fora of [green]Eng-Tips[/green] for some reason. But Google still had it! (Thank God!)

I'd love to hear your comments -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

As far as LRFD and ASD go...I've got a few comments. First, I was a little surprised by Focht3's seemingly very quick response to a question with so many sides like this by saying, "Advantages?! There aren't any!"...Don't you know that geotechs are forbidden from using such extreme words/phrases?? Ha Ha! Don't let John Bachner catch you using an extreme phrase like that. Hey, just kidding there! Maybe it's ok when not writing a report or something. It's strange because I tend to not use extreme words or phrases even in my personal life now. Some situations are very serious-see below:

My wife; "Hon, the car only has an 1/8 of a tank of gas left. Will we make it to the next city in 55 miles?"
Me; "Well Hon, we MAY make it."

I’m sorry, I digress in a feeble attempt to be funny.

Surely there must be something good about LRFD when applied to geotechnical engineering, right? I suppose there are also downfalls too. Both sides were discussed in the link given.

To me, the bottom line is that the design parameters we give to structural engineers to use, or our own interpretation of the behavior of a soil-structure system has to pass the final step; that is, we need to be confident enough to put our name on the design in the end and to uphold the standard of care in the development of the design with the end design usage in mind. The method used to do that apparently varies quite a bit from person to person.

Personally, I've always believed that because soil parameters vary so widely, relative to other construction materials, that a concept such as LRFD would lend itself quite well to soil mechanics. A more statistical approach, or a probability-based approach makes sense to me. As opposed to ASD where the factor of safety tends to be applied at the end of a design and it acts as a single blanket reduction factor for all parameters involved regardless of how confident we are in each. The factor of safety is very subjective.

If we had a way to deal with soil parameters independently, it seems as though that would make the most sense (unless, of course, if this method adds lots of additional time or effort to the work). We can usually estimate within a reasonable range the unit weight of a soil, particularly if we construct a quick phase diagram. On the other hand, hydraulic conductivity is much more elusive and with our best measurements and estimates the actual hydraulic conductivity insitu may vary substantially from what we thought it was. A calculation such as critical gradient in a dewatered cellular excavation, in which both parameters must be considered (unit weight and hyd. cond.) could be treated appropriately with an LRFD approach...in theory. In reality, no one will balk at adding 3 more feet of sheet pile in the ground so that we can make "extra sure" there won't be a problem with boiling at the base. Because of the reality of the situation, slicing the resistance factors so closely may be a waste of time, when it is economical just to add a little extra in the way of sheet pile.

I also believe that just about any soil analyses (for example allowable bearing pressure, i.e. settlement and bearing capacity, or slope stability, etc.) will benefit much from using a type of parametric analysis. Duncan's method mentioned in one of the other posts in the link given is very nifty. It’s always been beneficial for me to observe which soil parameters are affecting the output the most and how. Of course, the results you get are only as good as the level of knowledge you have in inputting parameters and checking different scenarios. What is the “probability” of unacceptable performance using this design over that design?” The question of probability, quantitative or even qualitative, can’t really be satisfactorily answered without a parametric study. Yet, this is usually the type of question the client is most interested in asking!

I look forward to comments!

 

[BigH]

MRM - I'm tuckered out on LRFD or whatever; however, I give you some for your comments on doing a parametric study. I've used such since I started in 1975 - especially for slopes. See how the behaviour is for a range of undrained shear strengths, or settlements against footing sizes, etc. Then choose the most appropriate one for the job at hand. E.g., do a slope stability analysis with basal clay at Su = 10, 15, 20, 30 kPa. Based on the geo information, Su=15 might be the most appropriate - so you know the estimated FS at this value. Still, you can see how sensitive your FS is to the Su value.

Good point to be made - geotechniques is much fuzzier than steel/concrete and we shouldn't lose sight of this - our analyses techniques far surpass our modeling attempts, I opine.

 

[Focht3]

(My first post in this thread was a tongue in cheek attempt at humor. I guess it didn't work...)

I have LRFD burnout, too. To me, the biggest problem is that it turns the allowable bearing capacity into a commodity, like concrete compressive strength. Something that can be "controlled." Soil behavior just doesn't fit into that category.

Have you ever tried to get a structural engineer to use a partial F.S. that is different than what s/he is accustomed to using for "bearing capacity"? Some get downright indignant about it. Site variability - and its potential impact on building performance - seems to be lost in the translation from geotechnical engineering to structural engineering.

Perhaps the answer to this problem lies in the application of Mike Duncan's statistical approach to the geotechnical side of the problem, then letting the structural engineer (and/or owner) pick the factor of safety s/he wishes to apply based on the predicted failure rate/risk of failure. In a way, I already do that as a part of my practice with respect to treating expansive soils.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[MRM]

Focht3 & BigH,
I see what you mean. Good point regarding the potential for LRFD to be misused when applying "resistance" factors to soil parameters.

Perhaps it may become tempting for someone, who doesn't understand all the variables involved, to apply the same factor of say 0.8, to a phi value, undrained shear strength value or what have you....no matter what the site conditions are, where the site is in the world, the fine details of the soil, loading conditions, testing methods, etc. This could potentially be very dangerous!

Making something that is complicated by nature cookbook=lots of potential problems! Good reminder you fellas had.

BigH, the slope stability scenario you gave was a great example of where a parametric study really shines!