What's the difference between ASD and LRFD in Structural Design?

In this video, Trevor will be highlighting the differences between ASD (Allowable Stress Design), and LRFD (Load and Resistance Factor Design). This video will be taking a look at their respective equilibrium equations (Factor of Safety vs. Design Factor) as well as examine the different load combinations.
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[@OtherwiseKnownAsB]

Coming from Australia where AS1170 uses only LRFD, and having NOT been taught the difference at university, that helped a lot with my understanding. You hear these two terms quite a lot in structural engineering. Thanks Trevor.

[@SkyCiv]

Great point! Different countries might only use a single provision for structural design

[@dmontenegro]

This was incredibly helpful, thank you!

[@SkyCiv]

Thanks Daniel, glad you liked it

[@armandogilmayorca2057]

Your video was very useful ti understand both design methods. Thanks!

[@oimpe]

Nice Video! I use your Wind Software really appreciate you all's work and product!

[@husseinrahimi5357]

Dear Trevor
Thank you for your nice video.
Would you explain me the philosophy of increasing the loads and decreasing the resistance in LRFD design method . If we are at the elastic zone in HOOK curve or pass to plastic zone in LRFD method ? . What is the main concept by increasing the loads if we do not pass to plastic zone, or it is virtual and we remain in elastic zone. I can’t catch the beauty of LRFD if don’t understand the logic of assumes in LRFD . Please clarify it me if possible. My in advance gratitude.
H Rahimi

[@affanahmedkhan7362]

thanks

[@SkyCiv]

You're most welcome!

[@sixharmonieslhbf2044]

I sometimes dont understand the advantage of ultimate state design over serviceability state, particularly when you notice that the ultimate stress divided by the average of the FOS equals the service stress. How do you feel and maybe I am over simplifying !

[@SkyCiv]

Thanks for the question! In general it would be disadvantageous to over-simplify and use that approach, since there is a lot of statistical data that goes into the load and design factors generated for LRFD design. If you average the FOS and use that to compare LRFD and ASD effects, you are taking away the specificity of the inputs that makes up the design factors applied in LRFD.

[@thanaamarabeh525]

Thanks a lot! but which is the most economic technique ASD or LRFD and why?

[@TomBrooklyn]

LRFD because it reduces overdesigns.

[@carultch]

The essential difference seems to be that LRFD applies safety factors on the front-end of the calculation directly to the criteria that specifies the loads applied. ASD by contrast, applies safety-factors on the back-end, when you determine the working load at the member level, and select a member that supports this load with a safety factor. Am I correct in this understanding? Should you ultimately get the same result, regardless of which strategy you choose?
One big problem, is any time I try to look up the safety factor required, a lot of times all I get is the definition of "safety factor". A statement of the obvious. The more important question, is how do you know what the safety factor is required to be?

[@nathan87]

Yes, I must admit I've never been clear where the real difference in "philosophy" is at all. Surely LRFD is just a more detailed version of the same thing.

[@thebarbarian493]

Check AISC-360-16 LRFD for detailed safety factors and how differentiate for each loading type and check under LRFD

[@thebarbarian493]

For your first comment i may say
Asd decreases material stregth by factors about 1.5 and requires safety roughly 2 for joint checks While keeping loads almost as applied
LRFD decreases material strength slightly as compared to ASD and requires 0.75 safety for joint checks
However increase dead and live loads incredibly by 1.2DL and 1.6LL and so on....

[@nikpatel7352]

I think MFD is better in efficiency in legjmans

[@sixharmonieslhbf2044]

PS dont click on the Logan bot