Seismic steel connection design with R <= 3 (USA)

[MPIng123]

Hi,

I'm a structural engineer practicing in Canada. I've come across some plans for a project in USA and I have a question for engineers practicing in USA, since I couldn't find a clear answer in the AISC 360 standard.

A little bit of context :

In Canada, for the seismic load, we use two modification factors : Rd and Ro, which, for a conventional steel structure will be 1.5 and 1.3. So, when I design my bracings, the forces will be reduced by (1.5*1.3), similarly to the "R" factor that is used in USA (as far as I understood).

However, the detailer must either design a connection with the force that have been divided by (1.5*1.3) but that connection must have a rupture mode that is ductile, or multiply the forces so that the connections are designed with RdRo = 1.0*1.3. At the end, most detailers will design with the forces multiplied by 1.5. And the engineering drawings will (or at least should) be explicit as if the given forces are with Rd = 1.5 (detailer must multiply by 1.5 for its design), or Rd = 1.0 (detailer can use the given forces directly).

So, let's say I have a drawing from USA that says R=3, and seismic design category "C" in the general notes. On the elevations with bracings I have forces for each load cases. Do I have to multiply by 3 the force from load case "E" in order to design my connections?

Thank you

 

[JAE]

Typically using R=3 in the US means you are bypassing AISC's (and the building code's) seismic requirements.
An R=3 structure is, by definition in the code, a "Steel System Not Specifically Detailed for Seismic Resistance" (excluding cantilevered systems)

Simply design to the given required loads. No seismic rules beyond that.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TehMightyEngineer]

No you do not multiply the connection forces by anything.

It depends on whether you're specifically designing to intermediate or special seismic detailing. Per ASCE 7-10 table 12.2-1, steel ordinary systems in SDC C have no height limitation. You should not require anything beyond the ordinary steel detailing. Thus, AISC 341 (seismic standard) isn't needed and you can just design the connections per the usual AISC 360 standard.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)
American Concrete Industries

https://americanconcrete.com/

 

[wannabeSE]

In the US, ASCE 7 has an overstrength factor, Ω₀, that is used to increase the force to certain connections and elements. This amplified force is only required when the code specifically requires its use. In the case of an R=3 structure described by JAE and seismic design category C, Ω₀ needs to be considered for in collector connections per ASCE 7-10 §12.10.2.1.


An ordinary concentric braced frame and ordinary moment frame are not exempt from seismic detailing requirements in AISC 341 unless they are classified as non-seismic per JAE's comment above with R-3. When "ordinary" systems use R=3.25, then AISC 341 seismic provision must be satisfied. See ASCE 7-10 14.1.2.2.1.

 

[TehMightyEngineer]

Thanks for correcting me on the terms; you're 100% correct that "ordinary" systems has a specific meaning and to use the R > 3 you do need to meet AISC 341.

My intent was to reference the "Steel System Not Specifically Detailed for Seismic Resistance".

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[JoshPlumSE]

Thanks wannabeSE, I was going to point out the same thing about ASCE 7 requiring collector forces to be multiplies by Omega. I'd go a bit further and say that any "discontinuity" areas should probably also have the omega level forces. It's something of an engineering judgment call. But, I'm thinking soft stories which used to have that requirement, but I don't think it's there anymore.