LRFD Design Method for steel structure

[Uzair162]

As LRFD Design method is the method that is mostly used in place of ASD Design Method for steel structure design. But i want to know is there any case or condition in which ASD method is better than the LRFD method?
If yes, then what is it?
Specifically for steel structure design...

 

[dik]

Other than for serviceability issues, LRFD has it thumbs down... much more uniform application of engineering design. I was with the first group at university to use limit states design, when it was in its infancy... back in 1965. With steel and concrete design, other than for historic reasons, nothing was done using ASD.

Dik

 

[tmschrader]

Hello....... I just searched this about 2 weeks ago and there is a good thread that cuts through the generalities and compares when each method is more or less conservative. I should have tagged the thread, but have not done that here yet. There is a bunch and I do not recall which one.

I found the google search at the very top gave more revelant results then the search icon next to the forum label. Maybe someone can confirm this possible conclusion from only one observation. LRFD and ASD.

Part of the choice has to do with how sure your are of the Max loads.

I have been around awhile and have mostly used ASD from my green AISC manual. Which does not even have LRFD in it.

 

[JStephen]

The 9th Edition ASD is considerably simpler, but that is an issue with old vs new, not ASD vs LRFD. In the newer code, both are practically identical, except you multiply by a factor here rather than dividing by a factor there.

Also note that "ASD" is somewhat ambiguous, it refers to allowable stress design in the 9th Ed, but the term allowable strength design is used in the newer code.

 

[271828]

Define "better."

I can think of one highly competitive specialty area that routinely deals very high ratio of roof live load to dead load. The 1.2D+1.6Lr sends them toward to ASD. All of their competitors will, so they'd better do it if they don't want to produce heavier structures and go out of business.

 

[Ron]

ASD is simpler, more intuitive, has fewer steps and is less prone to error. Neither is "better"....it's just what you prefer. I use both, for aluminum and steel design, but if given a choice will almost always use ASD.

 

[mtu1972]

I have slightly different memories regarding this topic. Took my first concrete design class in 1971 and they were still teaching both WSD (Working Stress Design) and USD (Ultimate Strength Design) because they weren’t sure USD would catch on. With ACI 318-71 the load factors were 1.4DL and 1.7LL.

Steel design class in same year was ASD (Allowable Stress Design) per AISC 7th Edition.

Took advanced design classes for both materials in Sr. year and again graduate level classes in 1973. Both were taught the same way the previous classes were.

Started work in 1974 and did concrete design with USD and steel design with ASD for many years. Footings were typically sized with allowable bearing pressure. Building superstructure typically steel, basement and foundations with concrete. Carried two sets of loads down through building.

Concrete stayed the same for many more years. Steel added the LRFD option in 1986 but used 1.2DL and 1.6LL load factors. As I was still doing most of my work with hand calculations it would have added a third set of loads to keep track of. Not for me.

I believe it was 2002 when ACI was revised to the 1.2DL and 1.6LL factors.

By then even I was using a computer, so it was a lot easier to use the LRFD option for the whole project. Footing sizes were still typically done with unfactored loads.


gjc

 

[Once20036]

If your goal is to design a single beam and make it as light as possible,
LRFD will provide lighter steel sizes if your 3*DL > LL.
If 3*DL = LL, the results from ASD/LRFD should be numerically equal.
If 3*DL < LL, ASD will provide lighter sizes.

When you get into the realities of doing steel design for an entire structure, using repetitive sizes rather than optimizes sizes, etc - I`m not sure that this effect is significant.

 

[dik]

I have slightly different memories regarding this topic. Took my first concrete design class in 1971 and they were still teaching both WSD (Working Stress Design) and USD (Ultimate Strength Design) because they weren’t sure USD would catch on. With ACI 318-71 the load factors were 1.4DL and 1.7LL.

This was back in '65, and the text was by Ferguson (sp?), almost no ASD, and I seem to recall the load factors were 1.5 and 1.8, if memory serves. My first job in 1970 used USD for the design of the columns at Lions Manor in Wpg. The partner in charge thought it was a great idea. A lot of stuff was designed using ASD then.

Dik

 

[Archie264]

Also note that "ASD" is somewhat ambiguous, it refers to allowable stress design in the 9th Ed, but the term allowable strength design is used in the newer code.

Which I don't appreciate as the two methodologies are different but are both described as ASD. To me it smacks of sneakiness: an attempt to change things without people realizing it. Or, at least one must concede this: it was definitely a change and if the goal had been to avoid confusion they wouldn't have called it ASD.

 

[mtu1972]

My 1st text was by Winter, et al. My Ferguson text was 1973 edition which was probably for my grad school class.

I have read that 1.5 & 1.8 load factors were used earlier but they were 1.4 & 1.7 when I took the first class.

With slide rules we had 3 significant figures and keeping track of both loads and factored loads was a giant pain.

gjc