LRFD vs ASD

[ThomasH]

Hi

I work occasionally on projects in the US. I say that because otherwise the following question might seem very odd, perhaps even stupid.

For steel, which design method is prefered today?

I have heard the explanation that for steel ASD is the standard while for concrete load factors are used. The explanation was that the load factors for steel and concrete are not the same so this is to avoid mistakes.

I have started to read the codes but some guidance would be greatly appreciated.

Thomas

 

[dcarr82775]

Personal preference, but LRFD is more common. The latest AISC manual is a dual standard so it covers both ASD and LRFD in the same book. Just go with LRFD IMHO

 

[fegenbush]

LRFD is common enough in the US for you to use, especially so if you are more familiar with it.

 

[ThomasH]

Thanks

I have the AISC standard and like you say, it covers both. And since we normally work with Eurocode load factors are well known.

We'll see what happens but then ASD is not the obvious choice.

Thomas

 

[BamaStrucPESE]

I use ASD for steel, as do most engineers I work with. 99% of the work I do is industrial, not sure if LRFD is more common on commercial projects.

 

[Lomarandil]

It's definitely industry specific. Construction engineering is almost entirely ASD. Power (transmission and substations) was LRFD, albeit with their own factors and combinations.

 

[delagina]

when someone says ASD it usually means the old green book ASD 9th edition allowable stress design. ASD (allowable strength design) is very rarely used.
Main reason some still use ASD is because it's easier for some because they've been using it for a long time and don't want to change their excel connection program, etc...

 

[humanengr]

The preferred method for structural steel design is LRFD. ASD would have been eliminated several years ago, but has been kept primarily for convenience of those who do not want to "switch" to LRFD and for software conversion.

Most AISC provisions, particularly seismic design, are written based on LRFD. When possible, they provide factors to approximate getting back to ASD levels.

The safety factors (LOAD FACTORS) are also more logical when using LRFD load combinations, as compared to ASD where all load types are essentially given the same safety / load factor.

 

[XR250]

@delagina - "ASD rarely used"

Most engineers I know still use it.

I use ASD exclusively as 95% of my designs are controlled by serviceability and they are not very large projects.
It is faster not having to do the loads twice. I can see the benefit on larger jobs, however.

 

[Lomarandil]

In further defense of ASD, when you are interfacing significantly with products (shoring towers come to mind) with published "Safe Working Loads" or with allowable bearing pressures in geotech, doing the design in LRFD just doesn't make as much sense.

I'm a young engineer, in an office of young engineers. While I think there is a copy of the green book floating around, that's not the cause of our choice.

 

[ThomasH]

Hi again

My interpretation is that there is no consensus regarding which to use. There is a PE involved so we'll also discuss it with him. I just wanted some background and I got that.

Thanks a lot

Thomas