Explain ASD vs LRFD to a dumb ME 20

[jdog1]

Can someone explain in really simple terms the difference between ASD and LRFD? ASD seems easier but is it going away?

I was reading through the new AISC Steel Construction Manual about the two methodologies and, aside from the different load combinations, was having a hard time seeing a clear distinction between the two.

 

[COEngineeer]

So if I use RISA then I need to do 2 basic load cases? One with Factored load to design the member, and one without factored load to calculate deflection?

Never, but never question engineer's judgement

 

[hokie66]

Wait a minute! While reading all this stuff, I haven't weighed in because I don't have a problem with either approach. But when you start talking about changing Young's Modulus of steel, my ears perk up. Why for? Please explain for us expats who use codes other than AISC.

 

[271828]

"So if I use RISA then I need to do 2 basic load cases? One with Factored load to design the member, and one without factored load to calculate deflection?"

Yes. That would be the case regardless of what program you're using.

 

[JAE]

hokie66,

Check out this previous thread - Per Chapter C of the new 13th Edition AISC Spec, the Direct Design Method uses a reduced EI and EA for the analysis for the purpose of accentuating the Pdelta effects a bit:

thread507-201140

 

[frv]

The reduced Modulus of Elasticity is something that comes into play while checking stability of frames using the Direct Analysis Method.

A lot of threads have broached this subject, but to summarize direct analysis:

1) Use computer software that has p-capital delta and p-lower case delta capabilities (i.e. rigorous second-order capabilities)

2) Reduce the stiffness of all members that contribute to lateral stability (EA and EI)

3) Apply notional loads OR model the frame with an initial out-of-plumbness.

4)Analyze and select compression members with k=1

 

[hokie66]

Thanks, JAE and frv. Just a method, not an affront to our good friend Young.

 

[frv]

I guess not, although the specification calls it the "Modulus of Elasticity" instead of "Young's Modulus"..

It's just a matter of time before we get a corporate sponsor.. "Arby's Modulus"

 

[JAE]

"I'm thinkin' stress/strain"

 

[haynewp]

If I remember correctly, the reduced E accounts for softening of the structure as yield points are reached at some localized areas. Anybody ever done spread of plasticity modeling here?

 

[frv]

haynewp..

the reduced E is to account for residual stresses in the fabrication process. Given these stresses, some parts of the cross-section will yield prematurely. This essentially excludes those parts of the cross section from contributing stiffness to the stability of the structure.

 

[haynewp]

=Softening of the structure

 

[Bagman2524]

STAAD PRo has not yet incorporated the Direct Analysis Method. ELM has gotten much more complicated, so we find ourselves in a difficult position in trying to use AISC-05 spec. There design examples are only a limited help. They need shorter, clearer examples to illustrate both DAM and the revised ELM.

 

[Lion06]

I believe you only apply the notional loads in combination with lateral loads only if the ratio of the second order displacements to the first order displacements are greater than 1.5. If it is less than (or equal to) 1.5 then you only need to apply the notional loads in the gravity combinations.

 

[271828]

Bagman, the ELM has always been complicated. Folks just didn't do it right. ELM iS A LOT harder to deal with than DAM, when ELM is done right.

Be on the lookout for an upcoming AISC Stability DG. It is very large and has dozens, if not hundreds of pages of full bldg examples--actually borders on overkill IMO. It was in the review process at least a year ago and I'm surprised it's not out yet.

 

[Bagman2524]

271828. I agree. I just think AISC has done a poor job transitioning to DAM. The Design Guide you mentioned should have been out two years ago and AISC should be offering some very detailed training on DAM.

 

[271828]

I agree totally and am frankly extremely surprised the DG isn't out. I believe it will be considered a great resource when it's released.

 

[JAE]

I sent an email question to AISC about this design guide and they replied that it is under development, but no specific publication date has been stablished....just "near future" for release.