LFRD and ASD

Structural Engineering general discussion
#1

SOURCE https://www.eng-tips.com/viewthread.cfm?qid=369499

QUESTION
lampi2k
What method is most economical and safe to use in the design of steel structure, is it LFRD or ASD? thanks

REPLIES
jayrod12
In the end both methods should lead you to roughly the same answer.

manstrom
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.

Bagman2524
I never really liked LFRD. Almost always use ASD

jdgengineer
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.

steellion
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.

BUGGAR
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.

glass99
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.

TehMightyEngineer
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.

spats
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.

jayrod12
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.

PEinc
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.

canwesteng
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.

Bagman2524
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.

canwesteng
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.

Bagman2524
ASD hasn’t really changed. Instead of multiplying by 0.6, it not requires dividing by 1.67. Result is same.

PEinc
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.

TehMightyEngineer
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.

PEinc
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.

FlashSet
I think 0.6DL + 1.0WL actually has a physical meaning, unlike 0.9DL + 1.6WL for overturning.

Ron
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!

#2

@JAE Any thoughts?

#3

ASD stands for “A Simpler Design”. That’s what I prefer and use.

#4

We were the first class to be introduced to Limit States design which has evolved into LRFD. This was for both Reinforced Concrete and Steel. I’ve been using it since 1965 and use ASD for almost nothing. There is a uniformity to LRFD that is missing from ASD.

Unfortunately over the years there has been a movement to use limit states for all materials and in order to get similar results using the same load factors, the code guys have screwed with the material property factors rather than address serviceablity issues more correctly. Steel used to have load factors of 1.25 and 1.5 for dead and live loads and concrete used to have 1.5 and 1.8 for loads respectively. They merged them to use 1.25 and 1.5 for both and fudged the calculations to match. There is a marked difference for reinforced concrete columns, for example, when using LRFD. They’ve attempted to use LRFD for masonry. They go through ‘hoops’ to statistically determine material strengths then ‘arbitrarily’ throw a ‘safety factor’ of 3 onto the results… LRFD is still a work in progress. For steel design, I nearly always use ‘plastic’ design (an LRFD method) for anything continuous and have for nearly 50 years.

Dik

#5

I feel strongly both ways.

Depends on whether I’m sipping a beer or a scotch.

#6

JAE:

Dik might have something to say about that… :shushing_face:

#7

Being that I went to engineering school in Florida, our brains were pretty much fried from the heat (though it could have been the cheap beer). My structural professors apparently felt sorry for us (or their brains were equally fried) so we were only introduced to LRFD in concrete…everything else was ASD. In my career, I have done little in reinforced concrete design for buildings…done a lot in materials and pavements. Most of my design work has been in steel and aluminum, both of which are clearly amenable to ASD in every respect. I still use it, though I have done quite a few LRFD designs in both steel and aluminum. Wood has always been ASD for me.

I have a bit of heartburn over the serviceability disconnect in both, but seems a bit worse in LRFD (but then, the heartburn might also be beer driven, though the quality of the beer has gone up a bit).

In any case, I will use ASD until the codes tell me I can’t anymore.

#8

Anyone for WSD now?

How about USD?

Am I showing my age?

#9

USD was the beginning of limit states… or LRFD.

Dik

#10

sipping is for wimps…

Dik

#11

For part of my first 10 years, I worked with RJC in Canada. They were one of the top reinforced concrete consultants in Canada at the time, IMHO. They are still at or near the top.

It’s not beer driven… the disconnect is there, and it pains me that engineers don’t fix this obvious shortcoming.

Dik

#12

You haven’t seen me sip.

#13

OK… when a person is thirsty… it takes too long to sip… but, sure…

Dik

#14

There is one area where LRFD is superior to ASD in terms of simplicity: CMU. ASD has a tried and true method, but LRFD uses the same concepts we use in concrete with some modifications, so it’s easier to run calculations.