Steel Special Moment Frame on Concrete Retaining Wall & Pilasters

[Charred]

I have a unique project where the SFRS will be steel special moment frames. (SDC D and architectural constraints do not allow for braced frames. On one side of the building, the finish grade is extremely high and will require a heavy concrete retaining wall. I planned to have the SMF columns at that side be supported by reinforced concrete pilasters. See attached elevation. However, in reviewing ASCE 7-10 Section 12.2.5.5 states 'Where a special moment frame is required by Table 12.2-1, the frame shall be continuous to the base.' Does this requirement mean I have to take the SMF columns all the way to the ground floor like the other columns? Or can I designate the top of the pilasters as the base in this condition? If I have to take the SMF columns to the base, I would need to modify the architect's floor layout and push the wall far enough away so my columns can pass by.

 

[phamENG]

Check the commentary. SMF are only required if you exceed 160 feet in height. You're significantly less than that. Per 12.2.5.5, you can discontinue them and put them on a "more rigid system" so long as you adequately consider vertical combinations of R, Cd, and Omega per 12.2.3.1. Sounds to me like you're ok.

I'd encourage others in earthquake country to chime in and verify my interpretation is correct - though I've studied and done some seismic design, the bulk of my lateral designs are controlled by wind provisions.

 

[JoshPlumSE]

Take a look at the 2016 version of ASCE-7.... I think the section makes what phamEMG was saying a lot more clear. Between the new version of the code and the old commentary, you're probably able to discontinue the SMF above the base.

Note: I've worked on an OSHPD project where we ran into this issue. It sounds ridiculous, but the way we got around it was by having an SMF frame embedded inside our wall. The wall is going to receive the full load, but we had to comply with that provision....

 

[Charred]

I appreciate the input guys. I have the second printing of ASCE 7-10 and it does not include the expanded commentary on Seismic - which I also have, but forgot about until pharmENG suggested it.

Josh - detailing for that sounds like a nightmare. Continuous reinforcing and meeting all of ACI's requirements around an embedded steel moment frame...