ASD vs. LRFD? 3

[aswierski]

I'm a young gun starting out his career, and my father (a civil P.E.) taught me how to do all my calcs using the ASD method. I recently bought Salmon & Johnson's "Steel Structures" 5th Ed. and they emphasize LRFD. Is this a trend that is starting to be used more and more? I'd like to know which method is more prevalent throughout the country (my guess is ASD), so let me know what you guys are using.

- Adam

 

[Lion06]

ASD was more popular for steel design before the most recent steel spec was released (AISC 360-05). Because of the format of the spec and the manual, LRFD is quickly becoming more popular. Additionally, the "new ASD", is not your father's ASD. It is allowable STRENGTH design and uses all of the LRFD equations with a safety factor (omega) as opposed to load and resistance factors.

 

[MiketheEngineer]

Us old guys were never taught LRFD and so we use ASD. The new kids on the block use LRFD. Having done a few calcs - LRFD gennearlly allows for a smaller section. Not sure that is so great. You must KNOW your exact loads with little "wiggle" room.

ASD worked for 100 years --- so why challenge that??

 

[kslee1000]

You need to know and understand the (old) ASD, but fluent in LRFD, it is the trend.

 

[frv]

They don't even teach ASD in schools anymore (for steel).

LRFD is becoming more and more prevalent. Concrete is almost exclusively designed using LRFD.

Wood design has attempted to move toward LRFD, but is overwhelmingly designed ASD. (The newest specification is a dual format specification, similar to AISC 13th ed.)

I disagree with you mike; the whole point of having load factors is to account for uncertainties in a statistically significant manner. If your dead load is off by 10%, that's OK because you have factored your dead loads by 20%, etc.

In steel, LRFD and ASD are calibrated to give nearly the same results at a LL/DL ratio of 3. At Higher LL ratios, LRFD generally requires larger sections.

 

[SteelPE]

Technically, ASD and LRFD are now supposed to yield the same result…. So it really shouldn’t matter.

I was taught LRFD in school. Went to work and had to learn ASD. Went back to grad school (while working) and had to re-learn LRFD. Went to a steel seminar on 360-05 and had to learn the LRFD equations again. No wonder why I get them mixed up from time to time.

Now with the new spec I don’t think it really matters. Use what ever you think is easier.

Personally, I still use ASD. I think it’s easier because you don’t have to calculate a different set of loads to figure deflections.... and I don’t have to explain everything to my older coworkers.

 

[Lion06]

ASD and LRFD are only supposed to yield the same result for a LL/DL ratio of 3 (as mentioned above).

 

[kslee1000]

The obvious reason for one need to know old-school ASD is simply because it has been used for so many structures, and is still in use. The posibility you will need to back-check on one of them is fairly high.

 

[swivel63]

i prefer the OLD ASD, but i learned LRFD in school.

 

[aswierski]

Thanks guys. As usual, some great input. Anyone else use Salmon & Johnson's Steel Structures? Any other recommended readings?

 

[frv]

S&J is pretty much considered the "bible" for structural steel design.

 

[kslee1000]

Ask your dad for first hand response. To me, it is an essential for everyday structural steel design tasks. It covers almost every topics you would encounter in this arena, and bridges the gap in between academic and code.

 

[Lion06]

I have S&J 4th and 5th ed. I also find Blodgett's "Design of Welded Structures" extremely useful.

 

[youngstructural]

Omar Blodgett is the man...

All of a sudden I feel like I should be wearing inappropriate amounts of gold. *sigh*

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[swivel63]

that blodgett book is great.

 

[Ron]

ASD is a much more intuitive design method. When LRFD started getting popular about 15 years ago, I looked at it as a way to rationalize underdesign by ASD (when evaluating existing structures or distressed structures). I agree with Mike the Engineer...it often allows a smaller section.

I use both methods, more often ASD, but occasionally LRFD for aluminum analysis.

 

[kslee1000]

Innovation in engineering takes time to stick, many have failed. The sucessful one usually proven its worth by simplicity, savings, and does something not quite possible if without. The best example is the transition from WSD to USD for reinforced concrete design.

Did someone mention the school no longer teaches ASD? It's a surprise and disappointing decision. Not wise, at least for the time being.

 

[DaveAtkins]

For concrete and masonry design, USD makes sense because deflection is generally not an issue. You can use factored loads for design, and you don't need to recalculate loads to check deflection (because you don't check deflection).

For steel and wood design, ASD makes sense because deflection generally controls. You use unfactored loads for design and you then check deflections using those same loads.

DaveAtkins

 

[JAE]

I use the PTGIW* method.






*Pray to God it Works

 

[kslee1000]

I don't want to argue the advantage and disadvantage of a certain method compared to the other. Whenever there is a drastic change like this (ASD to LRFD), you always have pros and cons, both with valid view points and reasons. The results remain to be seen. However, would like to caution those who are reluctant to accomodate inevitable change: do not fall behind the curve, if you want to stay in the game long enough to tell the story to another generation of young engineers about today's change. See you there.