1 - ASD vs. LRFD

[@gideonakegwure7704]

A very clear explanation, thank you!

[@Innotutorial]

Check out mine as well. "Innotutorial" I explained it in details.

[@billj5645]

Allowable stress design seems easier to me for engineers to understand. We learn how to calculate stresses in materials so it makes sense that we should be able to design an object to have a stress at or below some level of "allowable stress" that is chosen to give the correct factor of safety. However just because a calculated stress has exceeded the limit on allowable stress, does that constitute a failure? Is there ability for the material to redistribute forces. Is the peak calculated stress even that important? For instance we can calculate tension stress in a steel member that has bolt holes in it. If we use hand calculations we get one value of stress. If we size the element for allowable stress in a way that is known from history and tests to work then that should be a good design. Now perform a finite element analysis on that element and find that there is a stress concentration at the bolt holes that significantly increases the stress. Does that now constitute a failure in an object that we know from experience will otherwise work? This is a case of the analysis being too smart for the way that it is applied. Of course now that AISC moved all ASD and LRFD provisions into the same code there aren't any true ASD design equations left, you just ratio down the LRFD calculated results in order to compare with unfactored loads. Someday ASD will be only a method used in school to contrast against LRFD/Ultimate strength design methods for teaching purposes. If you do ultimate strength design, or limit states design, you bypass all of this. You test a sample of the object, discover its limiting state of behavior, and determine a way to calculate its strength. Now apply this design to factored loads and determine if the object is acceptable or not. We don't care what the stress is in the object as long as there is a sufficient factor of safety against the final failure of the object. This makes more sense if the end result is to design an object with sufficient factor of safety against failure. Now that the industry has moved design of most major materials to ultimate strength, it would be helpful to streamline the nomenclature. AISC has LRFD design and they talk about limit states, ACI originally named their method ultimate strength design. Can we settle on one name for everything? And the same thing would apply to loads also- do we call them ultimate strength loads? LRFD loads? factored loads? The problem with factored loads is that the load combinations in ASCE 7 for use with ASD design also contain factors, so in effect design loads for ASD are also "factored". In our office we tend to use "strength level" loads which has less ambiguity. Me being older I am happy with "ultimate" loads even if I'm using LRFD design methods. Whichever method you are using, it is necessary to always label your calculations so that someone looking at the calculations understands if the numbers are ASD level or strength level.

[@mashed-out]

Now AISC uses allowable "strength" design (ASD). This allows them to combine ASD and LRFD into one specification. More confusing than good ole allowable stress design!

[@NARAYAN0512]

U r saying just opposite sir.... LFRD allows to include both load and resistance (i.e. capacity and stress)....

[@samuelthedude]

So I started doing wood framing calcs for my designs. I'm reading through NDS standards it ended up hear trying to understand the different from ASD and LRFD. I was taught and do calcs like Fb = Max M / Section modulas and compare that against the AWC tables for the wood we use. I'm assuming that's ASD method? The NDS section on adjustment factors is really throwing me off.

[@MrWinzip67]

i think there's a mistake, in ASD design, max material stress must be greater than the allowable stress, so material is only working between elastic range

[@TeknikSipil360]

No, it's correct. The max. material stress caused by loading must be less than the allowable material stress.

[@NARAYAN0512]

Dear Sir ! I'm finding some difficulty in correlating your 3rd point with AISC description of LFRD.

[@CMRHM]

Very clear, thanks!

[@meagangarcia5231]

You can save 4 minutes off by skipping to 4:00 and reading the slide.

[@yaldanem]

David garber and david gilmour are my two favourites😁😂

[@الريشة-و9س]

bro, what is the method /program that you use to right this slide

[@habeebjasim705]

Thanks

[@martsgj5621]

ok

[@BrucePaul-d7h]

Hagenes Turnpike

[@yaldanem]

David garber and david gilmour are my two favourites😁😂