Verifying, when R = 3 or less AISC SP doesn't apply?

[JeffLam]

Hello,

Just wanted to verify with someone, I designed a structure with R=3 for seismic base shear. Does that mean I do not need to follow AISC seismic provisions? I overheard from someone in this forum where they mentioned if R=3 or less AISC SP doesn't apply. Where is this noted specifically?

Thanks!!
Jeff

 

[jdgengineer]

Assuming you are using a lateral system in ASCE 7 that has an R listed than is 3 or less, and you are permitted to use it in your seismic region you do not need to follow the seismic detailing requirements in AISC 341.

This is stated in AISC 341 in Section 2.1.

"When the seismic response modification coefficient, R, is taken as 3 or less, the structure is not required to satisfy these Provisions, unless specifically required by the applicable building code"

 

[jdgengineer]

Also see IBC Section 2205.2.1. As long as you are in Seismic Design Category A, B, C, and using an R =3 or less, you do not need to follow AISC 341.

 

[JeffLam]

My structure is a platform that is 17 feet high. It has knee braces in all directions.

Can I justify by using R=3 due to ASCE 7-05 that the structure is not specific braced frame?

 

[jdgengineer]

Are you in Seismic Design Category A, B, or C?

Also note -- a knee braced frame is not a brace frame, it is a moment frame, and can be detailed according to the ordinary moment frame provisions if you are in a higher seismic design category.

 

[JeffLam]

Thanks for your quick responses jdge.

The platform is for recycling plants. These recycling plants can be located anywhere in the world...

I've classified them under occupancy category I. I guess I can tell my boss that this platform is to be only located in areas with SDC =A,B, or C ??

 

[jdgengineer]

I'm assuming this is a non-building structure. See Chapter 15 of ASCE 7. Per Table 15.4-1 you are permitted to design as an ordinary moment frame, with various detailing requirements depending on the R you use. If for whatever reason you don't want to detail to AISC 341 (which I believe the requirements are fairly minor) you can use an R =1 for any Seismic Design Category.

 

[SAIL3]

jd....are you sure by using R=1 that one does not have to conform to AISC 341 for cat D,E,F etc??....
by using R=1, what is this assuming about the response of the structure...I thought the seismic loads/response were capacity driven...

 

[jdgengineer]

For a nonbuilding structure, it is shown in the Table in Chapter 15. Not sure if it allowed for a building as I've never done it and not sure why you would.

By using an R=1 you are assuming the structure remains elastic. So under the design earthquake nothing should yield.

 

[jdgengineer]

What are the implications of using an R =1 with no seismic detailing? Well if an earthquake comes along that I weaker than the design level earthquake it's all gravy as the structure can handle the forces. If its a larger earthquake, well then the structure doesn't have any ductility and will likely fail. For a non-building structure i think the codes stance is "eh that's fine". For a buIldin structure you'd want the seismic detailing so the structure has the resiliency to and a stable mechanism.

 

[JeffLam]

I'm trying to design a platform that will used by employees to divide outlying trash from conveyors. I have taken into your considerations of classifying the knee-brace system as OMF and have verified that in AISC SP C-11.

Therefore my R=3.5 per ASCE 7-05 chapter 12.

The thing is, I'm using a conservative base shear of .4W regardless of the formula provided in ASCE 7-05. It was a conservative approach based on a Seismic Zones Map of the United States provided by my company.

The problems I'm running into now is... I'm using W shapes for my columns and since I decided to go with knee braces in both directions, I'm going to have larger columns to satisfy combined moment(both x and y directions) and axial load. I might be forced to go with a CBF in the weaker directions.

 

[PMR06]

How about designing two prototypes: one for SDC ABC and one for DEF? The steel savings in the ABC design might make your product more economical (competitive) in areas of low to moderate seismic.

 

[JeffLam]

PMR, you make a good point. I can always make an intial design for ABC and continue, if my company allows, to design for DEF later on the future.