waiving the special moment frame 1

[structural87]

Hello,

Does the ASCE7 or IBC anywhere mention that instead of using the special moment frame, we can use the ordinary or intermediate moment frame if we use a lower value of R and therefore, design for higher forces ? Does anyone know if such thing is allowed by the code or not ?

 

[phamENG]

Sure. If your structure qualifies to use OMF or IMF...or even not detailing for seismic at all...then you don't have to use a SMF. It's all in ASCE chapter 12. What kind of building? What SDC? How tall is it?

 

[structural87]

actually it is SDC D, 2 stories PEB building so i should use the special moment frame since ordinary is only applicable to the one story. however, the contractor is proposing to use a lower R and design for higher forces cz in PEB building, achieving a compact section is hard to achieve.

 

[phamENG]

No, just designing for higher forces isn't an acceptable option. There are a couple threads on this here...one from earlier this week...that get into the ideology behind the R factors, methods of design, and why they are so important.

In short, it comes down to a reliable, ductile response. It's not the load level that ensures that, it's the detailing requirements for the various levels of moment frames, braced frames, etc.

 

[HTURKAK]

Refer to Table 12.2-1 Design Coefficients and Factors for Seismic Force-Resisting Systems

Steel ordinary moment frames is not allowed for SDC D,E and F..

STEEL MOMENT-RESISTING FRAME SYSTEMS for SDC D can be
- SMF
- special truss moment frames ( height limited to 160 ft)
- Steel IMF ( height limited to 35 ft)