Steel framed knee braced mezzanine

[jike]

Per ASCE 7-05, what Table would you use to determine R? Would it make any difference in your selection if mezzanine was independent of the building or if it was structurally tied into the building? What if it derived its stability from the building?

 

[haynewp]

Since nobody else has responded I will try;

If it is attached only at the base of the mezzanine (independent LRFS), then it could have a different period than the main building. I think it is conservative to treat it as a component and add to the main building floor seismic in that case for the main building distribution.

If it is connected to the building at the top and bottom of the mezzanine (dependent LFRS), then I expect it to be more like a typical floor with an R value based on the main building.

If the mezzanine has an independent LFRS, then I would design the mezzanine itself for the lesser R of treating it as a component or if it were a standalone frame with a different R.

 

[Den32]

As long as you are in Seismic Design Category B or C, your best bet is to use "Steel Systems not Specifically Detailed for Seismic" as R=3

Regarding Interaction with the Building - like Haynewp said we have done it both ways (separate system and 'component' of the buidling), depending on the size of the mezzanine. For the 'component' of the building, we put horizontal concentrated loads at building columns (where mezzanine connects to it) and designed the column to resist that load (and added the lateral forces to the building LFRS)

Regarding Knee-brace Systems - we had previously used 'R' value for Eccentrically Braced Frames (b/c that seemed logical) but then read that Knee braced Systems should be treated as "Moment Frames" since the system works based on fixity at the beam-column joint (makes sense). However, I can't remember if I read this in an ARticle or if it is in the code.

 

[jike]

Den32:

I agree with R=3 for moment frames not specifically detailed for seismic. I look at an independent mezzanine as a non-building systems similar to a building. ASCE 7-05, Table 15.4-1 now requires an R = 1 for normal detailing (under AISC 360).

This seems like a large penalty from R=3 to R=1.

Any comments?

 

[kslee1000]

jike:

I don't play numbers, since not familar to the current practice. However, suggest you to think the mezzanine as a building (mainly utilized by human) within the main building with its own seismic characteristics.