ASD vs LRFD

[SteelPE]

This question is related to one I posted earlier:

Going to be working on a project designing connections where the EOR gives the shear reactions in ASD and gives the moment reactions in LRFD. He basically states ASD = 0.8*LRFD and LRFD = 1.5*ASD. In looking at these formulas I would think that the most advantageous method would be convert everything to LRFD because there isn't a large reduction in the other direction. In general, is this a correct statement?

Such a simple question and yet for some reason I am a little confused. I'm so stupid!

 

[connectegr]

The conversions should be equal. In your case a 20% decrease of the factored loads or a 50% increase of the service load does not make sense. In most cases the conversion of connection design allowables (shear, tension, bolts, welds, etc) is 1.5 x the ASD values. I would point out this to the EOR or choose the conservative combination.

Depending on the type of structure LRFD should result in a lighter structure. But, with regard to connection design service loads (ASD connection design) or factored loads (LRFD), should result in the same connection. There are some minor issues with conversion factors (1.4-1.6), so the EOR usually chooses a conservative factor.

You can inform your client that this process is not the most efficient/economical presentation. But, from the fabricator's perspective the final connections will not be much different.

http://www.FerrellEngineering.com

 

[dcarr82775]

Something seems rather wrong in the EOR's specs. I would either design for the worst case, or point out that his conversion factors do not make sense.

 

[BAretired]

SteelPE,

If you convert the ASD shear reactions to LRFD using the relationship LRFD = 1.5*ASD you will be conservative, so I would agree with that tactic.

BA

 

[JAE]

If DL = 20 kips
and LL = 30 kips
The factored LRFD force is 1.2(20) + 1.6(30) = 72 kips
With a phi factor for steel connections at 0.75 the "safety factor" is 72 / 50 / .75 = 1.92

With equal DL and LL you get a safety factor = 1.86
With ONLY DL you also get 1.86 (using the 1.4DL combination)

Not sure how 1.5 works here.

 

[WillisV]

@JAE, LRFD and ASD are calibrated to a 1.5 safety factor when LL is 3.0 times DL - in your example this is obvously not the case hence the difference. See p16.1-261 of the 2010 AISC Commentary.

 

[JAE]

I guess if you ignore the phi factor and just compare the load side of the equation you would get the following:

If DL = 20 kips
and LL = 30 kips
The factored LRFD force is 1.2(20) + 1.6(30) = 72 kips
The "safety factor" is 72 / 50 = 1.44

With equal DL and LL you get a safety factor = 1.40
With DL = 20 and LL = 60 you get 1.5

This is probably how the 1.5 was derived - based upon a ratio of LL to DL

 

[JAE]

WillisV - you caught me before I posted. Thanks.