AISC 341-02 Seismic Provisions with ASD Design

[sundale]

I have two basic questions regarding the Seismic Provisions for Structural Steel Buildings if one is dumb enough to try an ASD design method.

Question 1: To do a "traditional" ASD design, is it correct to design to ASD load combinations (i.e. 0.7E) and then decrease the load by the prescribed 1.7 multiplier and then increase the load by the appropriate ASD "phi" factors per this document? For example, a column with (simplistically only) a 100 kip (ultimate) seismic force would be designed for:

0.7 * 100 / 1.7 / 0.85 = 48 kips = design load for ASD.

This is what I interpret this document to say and the 0.7 combined with the 1.7 seems too liberal to me. Are they embedding the old (forbidden now) 1.33 stress increase with this?


Question 2: How does one treat the load to capacity ratios that are referenced in this document? For example, for a SCBF, one needs to consider Pu/(phi Py) for the column design forces (E vs Em). How do you do this with ASD and service loads? Does one ratio service loads (not ultimate) to nominal capacities as modified in my first question or do you have to run ultimate load combinations to determine Pu?

I will thank folks in advance for any advice given.

 

[structuresillinois]

sundale:

I too was confused when I first started studying AISC 341 for ASD. I think I've figured it out though. Somebody please correct me if I'm wrong...

As far as I understand, you don't use the ASD LC's when designing for seismic (as in your Question #2). Instead, use the strength LC's (1.2D+0.5L+E) to calculate Pu. Then in order to use the ASD option, follow these steps:

1. Calc Fa (from ASD Green book, based on KL/r)
2. Multiply by member area to determine allowable axial load.
3. Convert to Pn by multiplying by 1.7
4. Multiply by phi factor, which is 0.85 for compression.

So basically, you are comparing Pu to (phi)(1.7)(Fa)(A).

For flexure, (phi*Mn) = (phi)(1.7)(Fbx)(Sx). (Calc Fbx using ASD Green book)

This is how I've interpreted AISC 341-02. I hope this is correct (somebody please correct me if I'm mistaken) because this what I plan to do on the SEII next week!

 

[sundale]

That is how I interpretted this methodology also. Ultimate forces, then allowable stress design with a 1.7 multiplier times a phi factor. This makes total sense to me.

If you read the ASD part of AISC 341, however, it clearly states to use ASD load combinations. Read pages 70 and 71 of AISC 341-02 (SCBF, SMRF). Service loads with a 1.7 boogie factor do not seem right to me, despite what this document says.

The 1.7 nominal conversion implies an ultimate load concept. The minimal written ASD reference states to use ASD service load combinations. This is very confusing.

I'm sitting for the SEII next week also. My study books work all the solutions in LRFD. If you run your loads as ultimate, you get very similar member sizes to the LRFD solution using an ASD design. If you run ASD load combinations, you get very liberal member sizes with an ASD design compared to the LRFD solution.

I e-mailed this question the AISC solutions center a few days ago but this is likely a waste of time and electrons.

 

[mrMikee]

sundale,

I'm curious what you mean with your comment "if one is dumb enough to try an ASD design method."

Regards,
-Mike

 

[sundale]

ASD steel design is a valid (and good) design method. I don't mean to denigrate it at all. It works well and is much simpler than LRFD. I had both undergraduate and graduate courses in the (then) new LRFD method, but was told as a "young buck" engineer to use ASD. I have used it ever since; I LIKE my old and battered green book because it is my one constant in the ever changing sea of code changes.

But let's face it. ASD steel design is from 1989 and the building codes treat it like a bastard stepchild. The academics and AISC have won the war here. The practicing engineers who must know steel, concrete, masonry and wood and do a design with a time budget will eventually need to learn either the more complex LRFD method or the new Unified Method. This is getting off topic.

What I meant by "dumb enough" is to try and fit an ASD steel design into a code and concept (AISC 341-02 and seismic forces) which is totally written and geared for ultimate strength.

 

[mrMikee]

sundale,

Thanks for your response. It appeared you were frustrated with something and I was curious what it was. I appreciate that you took the time to explain your views.

Thanks,
-Mike