ASD & LRFD

[EIT2]

I am considering attending a two-day seminar at the University of Wisconsin for Structural Steel Connections.

Among the suggested materials to bring, are the AISC ASD or the AISC LRFD. I do not have a copy of the LRFD, nor do I intend on purchasing it, at this time. Should I?

Just what is a/the primary difference between the two? (Is there a simple answer?) If there IS a great difference between the two manuals, I can't imagine the seminar instructors having the time to provide separate examples, referencing both manuals, for many of the discussion topics.

Is the LRFD an inevitably, imperative, future (or current) resource?

What is YOUR guess as to the ratio of ASD/LRFD users?

What is your preference?

Any other comments?

Thank you all for your time.

 

[dik]

I think the LRFD Manual was available as a free download from the AISC; not sure if that's the case... If it is, I'd download it and print off a copy... hope you have a duplex laser printer... there's a lot of pages...

I've been using limit states design in steel for 30+ years... our class was the first one to use limit states as the main design methodology... for both concrete and structural steel. It's not a bad method... and based on a little better rationalization...

 

[EIT2]

dik,

I have never heard of limit states design. It sounds suggestively similar to allowable stress in principle.

Will to elaborate on what it is and how it differs from ASD and/or LRFD?


Thank you!

 

[EIT2]

dik,

I have never heard of limit states design. It sounds suggestively similar to allowable stress, in principle.

Will to elaborate on what it is, and how it differs from ASD and/or LRFD?


Thank you!

 

[dik]

It's very similar to LRFD... use of material factors as well as loading factors...

 

[Lutfi]

I can use both. However, I prefer the ASD and I ma not the only one. AISC is not having a great deal of success in converting 100% to the LRFD. That is why the ASD manual is till available.

The Canadians and Europeans had better success that America.

 

[SlideRuleEra]

In my opinion, both ASD and LRFD have advantages and disadvantages. LRFD probably is slightly more economical when designing NEW common structures (say buildings & many bridges) that meet typical requirements (say building codes). Once you get into the following situations ASD may have more advantages:
1. Industrial facilities that have unique structural requirements.
2. Changing a structure's use - members may have to be analyzed because of changed loading conditons.
3. Evaluating the condition of deteriorated structures - they may still be satisfactory but you often must consider members to have reduced capacity. Overall I see ASD as more versital but for many engineers the economics of LRFD are truly more valuable.

In the bigger picture, LRFD will most likely prevail. Then designing in concrete (ultimate strength) and steel will have a similar theoretical approach.

Also take a look at thread507-68864

 

[BigH]

SlideRuleEra: Yes, I remember the slipstick too. But, I hope that you are wrong with the LRFD when it comes to the geotechnical aspects of the foundation design. This has been discussed in this forum extensively under "Safety Factor Questioned . . . "

 

[TpaRAF]

While the LRFD promises more (materially) economical designs, it really depends on the loading size. With bridge design, the LRFD designs are less economical for medium and small span lengths.

From a design point of view, the LRFD formulae tend to be too complex, and design using LRFD takes more engineering time than ASD designs. I'm refering to the AASHTO bridge codes; the AISC LRFD design code is somewhat more complex than the ASD code.

For example :
AASHTO LFD / ASD distribution factor : DF= S / 5.5
AASHTO LRFD distribution factor :
DF= 0.075+(S/9.5)^0.6*(S/L)^0.2*(Kg/(12*L*ts^3))^0.1
Kg= n(I+A*eg2)

Here's a article discussing the two methodolgies you may find useful :

http://www.gostructural.com/edindexASD.html

Primarily, it depends on your clients. In my case the state DOT adopted LRFD, so that's what we must use. If your client doesn't care, you can select which code to design by. I think about 40% of the state DOT's have adopted LRFD or plan to in the near future.

I hope this helps with your evaluation! RAF