What method is most economical and safe to use in the design of steel structure, is it LFRD or ASD? thanks
In the end both methods should lead you to roughly the same answer.
It should be LRFD by a nose. But if the design is deflection controlled,it won’t matter.
Enercalc lets you switch to either one easily. It’s usually pretty close.
I never really liked LFRD. Almost always use ASD
Since the addition of the Black Book (AISC 360-05) ASD/LRFD have been calibrated to have roughly the same results.
I typically, use whichever is more convenient for me on the project.
If the project is mainly steel I use LRFD.
If the project is mainly wood with supplemental steel I use ASD.
The differences in the results are related to the live to dead load ratio. I believe that ASD is more conservative when the live load to dead load ratio is <3. If it is higher than 3 than I believe LRFD is more conservative. I might have the exact turning point wrong, but that is the concept.
One of the benefits of LRFD is if you have a project with a lot of interaction between steel and concrete, you already have the ultimate loads. LRFD is our company standard. Design with LRFD will result in more members being controlled by serviceability load cases, ASD will result in more members being controlled by strength load cases, but as other posters have said, the results are about the same.
I went to the doctor and learned they are now practicing LRFD in medicine. If you have a sickness, they factor it up to the next higher illness to make sure you are adequately cured. For example, if you have a cold, they will factor it up to pneumonia and treat you accordingly. They say this is to increase the odds that you will be cured based on their studied statistical analysis of illness.
Then, to keep from over-curing you, they will reduce the dosage of the drugs, usually by eighty to ninety percent. More for certain types of people.
LRFD is an overcomplicated solution to a problem which does not exist. We ASD’ers lost the war probably 10 years ago, but we keep the flame alive in our hearts.
One thing I recently learned is that when you calculate the effective length per an amplified second order analysis using the effective length method you can actually get some very different results comparing ASD to LRFD with ASD being the more conservative. In the office I’ve typically used ASD for smaller projects with no concrete design.
There is a good article in the August 2014 STRUCTURE magazine addressing ASD vs. LRFD for joists. It states that when live load is less than three times dead load, LRFD design will produce a lighter joist, just as jdgengineer pointed out above. The article also talks about the advantages of stretching out the joist spacing vs. deck cost, but I don’t like going >6’ on center in any circumstance with the normal 1 1/2" deep wide rib deck that I normally use.
Oh waaa waaa… I normally don’t like to antagonize people like this but seriously. Get with the times. Most other countries have gone to both metric and LRFD. Why do people from the US refuse to follow the lead of everyone else. You sound like my wife when she thinks her way must be the best and won’t listen to anyone else.
If ASD and LRFD give the same answer, it is a coincidence. In ASD, there is one allowable stress and, therefore, one safety factor resulting from all of the different load types. In LRFD, different load types have different load factors based on the accuracy/reliability of load values and based on resistance factors of the structural material. For example, a steel beam designed by ASD for a highway project and supporting a large LL may use an allowable bending stress of 0.55Fy which gives a safety factor of 1/0.55 = 1.818. In LRFD, the LL load factor would be 1.75 with a resistance factor of maybe 0.9 which essentially means the safety factor is 1.75/0.9 = 1.94. For other load types known with greater reliability, the ASD & LRFD “safety factors” could be closer. The strong point of LRFD is that it allows a designer to more accurately fine tune “safety factors” according to the reliability of the different load types (wind, water, LL, DL, earthquake, etc.) and different structural materials (wood, concrete, steel, etc.). That being said, I don’t like using LRFD. It creates more work for me because I often have to design a project using both ASD & LRFD methods. Seems counterproductive to me.
LFRD is better. ASD has been calibrated to give roughly the same results, so you have the illusion of still using ASD, but really it’s just masking the fact you’re design is still based on statistical analysis of probability of failure, based on variability in load and materials.
ASD existed well before LFRD, so it was never “calibrated”. A safety factor is a safety factor. Doesn’t really matter whether you apply the safety factor to the load or to the strength of the member.
ASD as allowed by code now is not the same as it used to be, and the safety factors you use now are set up to give similar answers to LFRD. LFRD is simply a better method than guessing a safety factor and going with it.
ASD hasn’t really changed. Instead of multiplying by 0.6, it not requires dividing by 1.67. Result is same.
You still need to consider the different load factors for different types of loads. There are too many combinations to make a general statement that the result is the same for ASD & LRFD. As usual, “It depends.”
Load factor for a water load is 1. Therefore, if designing for a water load only, there will be very little or no safety factor using LRFD. Whereas, for a live load of the same magnitude as the water load, the load factor would be 1.75 per AASHTO. The answers cannot be the same. That’s a big difference to consider (1 vs. 1.75). I would not design with a LF of 1 when water is the major, critical load.
PEinc: Not true if you’re under ASCE 7-10. H (lateral earth pressure or ground water) is 1.6 in LRFD, F (well defined fluid load) is added to dead load, and Fa (flood load) goes from 1.0 to 2.0 depending on the zone. So there is a very well designed safety factor for LRFD with fluid and dead loads. You are correct about AASHTO though. I’ve run into that before where hydrostatic loads controlled the design and we increased the safety factor similar to how ASCE 7-10 does.
AASHTO vs ASCE 7 is a little bit apples and oranges though as AASHTO uses different phi factors than ACI, AISC, and so on.
TME, thanks; I agree. AASHTO marches to a different drummer. I’ve done the same with hydrostatic loads on cofferdams for highway projects.
This all emphasizes my point that ASD and LRFD do not always give the same result. It depends.
I think 0.6DL + 1.0WL actually has a physical meaning, unlike 0.9DL + 1.6WL for overturning.
ASD is intuitive and less prone to application error. I sometimes have to use LRFD…but I do so while kicking and screaming…and whining Waaah! Waaah!