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Transition from ASD to LRFD

Transition from ASD to LRFD

Transition from ASD to LRFD

(OP)
I CONFESS!!! I am ASD ONLY!!!

Wow!! I feel so much better getting that out!

I’ve done nothing but ASD.

I don’t want to change! But, I will if I must.

1. In your opinion:
a. (In the next 5 years) WILL we be exclusively LRFD? The trend seem inevitable.
b. Is LRFD “better”?
c. ASD pros and cons
d. LRFD pros and cons
e. Will we continue to have a choice?
2. What recommendations can you offer to make a (smooth) transition from ASD to LRFD?
3. Any other ASD vs. LRFD comments?

The only time I’ve been challenged to use LRFD is when sizing and specifying post-installed anchors – I will address that topic in another post.

Thank you!

RE: Transition from ASD to LRFD

I'm almost perfectly balanced between ASD and LRFD in terms of what I use. I like LRFD but use ASD for wood or very simple designs. Overall, the switch is fairly painless when you realize it's just moving the safety factor from the capacity to the demand side of the equation.

That said, if you're talking about going from Allowable Stress Design (AISC green book) to the Allowable Strength Design & LRFD codes then I both highly support this but also feel you will find a lot of things you miss about "stress" design versus "strength" design. Even though I'm a young engineer I've used both and prefer "strength" design even though it's harder to get a "feel" for what's happening under the hood in the numbers.

I'll answer assuming you mean "strength" design but if you're talking about going from 9th edition AISC to 13th/14th edition AISC then the answers become much more complex.

1.
a) No, I highly suspect we'll see the codes that are already LRFD only (concrete) stay LRFD only and the other codes will adopt a balanced approach similar to AISC's combined spec. This definitely appears to be the trend in wood, masonry, & steel. Perhaps after 5 years or so we will get towards only LRFD but I doubt it will be universal; most likely it will just be more common to use LRFD than ASD.

b) Not in my opinion. I believe it's more efficient but just because something results in more efficiency doesn't make it better. I can call out a perfect stitch weld but it will take 3 times longer to design and 3 times more work for the fabricator. LRFD is a tool and has benefits and weaknesses like all tools. In the day and age of computers where load cases can be automagically generated and checked I find LRFD to be just as easy to use as ASD. Plus, if you only have a D+L load case then it's hardly even more work when hand calculating the load. Where LRFD shines, and what it's made for, is large buildings where getting the most efficiency can save big $$. Thus, it is "better" for many situations, and equal to ASD in others. Only last note; ASD and LRFD hopefully result in similar "safe" results with some slight variation for the LRFD load cases. However, the equivalent length method of second order analysis has a weird "glitch" in the ASD provisions that will, in some situations, give you very different results than LRFD. Obviously direct analysis is the recommended method these days and is far superior in my mind but it's worth mentioning that the conversion between ASD and LRFD is not perfect in the various specs.

c) Some pros: Simple to run load cases. No worries if you need to check service load cases (deflection, foundation soil pressure, etc.) or strength load cases (capacity).
Some cons: Often forced to convert everything to LRFD when you go to codes that use LRFD only (typically foundations). Some weirdness in AISC.

d) Pros: More efficient designs. Easy to apply with computers. Lateral loads are calculated at strength levels in ASCE 7-10 so potentially more accurate. More "recommended", if not more common.
Cons: More steps in design. Demands calculated are not "real" and thus it's harder to get a "feel" for the true capacities for members.

e) For the near future, I believe so.

2. Read some LRFD examples. It's a fairly painless transition. Just know that your "feel" for capacities will be off, so it will take some getting used to.

3. Try it out on smaller projects first and then ramp up to bigger ones.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: Transition from ASD to LRFD

It's worth nothing that "ASD vs LRFD" shows up like once every 4 months on engineering forums. There's a lot of previously discussed info out there.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com

RE: Transition from ASD to LRFD

I thought LRFD was adapted for concrete design since concrete did not have an accurately defined yield point.
Kind of like glass, which is designed to the statistical likelihood of a flaw, which is its weak point.

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