IBC Seismic Limitations

[SmithJ]

Hello All,

Table 1617.6 in IBC 2000 has some limitations called out with respect to building height and seismic category. Footnotes (h) and (i) for the table refer to some limitations to the use of ordinary moment resisting frames with a dead load limit of 15psf. Can anyone please shed some more light on the requirements of this table and what the code is actually attempting to achieve? Does this apply to buildings with mezzanines?

I am designing a metal single-slope building with a concrete mezzanine at mid-height across half of the floor area. Would this be considered a single story building by the code or not? Does the 15psf floor dead and wall load apply to the mezzanine?

The building is in Seismic Category D and footnote (h) says that "Steel ordinary moment frames ... are permitted in single story buildings up to a height of 60 feet ...". How does this footnote apply to this particular structure? If I can't use ordinary moment resisting frames, would I have to use special or intermediate moment frames? What are the implications of using special moment resisting frames apart from the increased response modification coefficient (R) and more stringent detailing requirements?

Thanks for any help that you can give.

JS.

 

[JAE]

You may have a plan irregularity per Table 1616.5.1. with 50% or more of the diaphragm open. The mezzanine is a story, albeit a partial one.

Also - be sure to check through Table 1616.6.3 for the specific analysis procedure required.

The 15 psf limit would then apply to your mezzanine.

 

[SmithJ]

JAE,

Thanks for your reply. The concrete mezzanine is definitely not going to meet the 15psf limit. Does this mean that OMRF's are out of the question and I need to use SMRF? If I use an SMRF, I assume that the 15psf limit is no longer applicable.

Notice that footneote (h) does not specifically give any guidance with respect to multi story frame systems in Category D. How should one interprete this portion of the code?

Any help with determining plan irregularities? This is my first foray into this and any insight would be great.

Thanks,
JS.

 

[AlohaBob]

See Tables 1616.5.1 & 2. You likely have stiffness and mass irregularities with your mezzanine. CAlculate these and compare. footnote (h) excludes OMF for multistories in category D.

 

[JAE]

OK - you have a multi-story building (2 stories), you probably are in group 4 of Table 1616.6.3 which allows you to use the Equiv lateral force procedure of 1617.4 but with dynamic characteristics included in the model (1617.4.4.5)

With an OMRF, in SDC D, from Table 1617.6, you have a 35 foot limitation in height, and footnote h applies (footnote i is for SDC E and F).

Footnote h limits you to End Plate Moment connections and other limits on weights of walls and roof.

The 15 psf floor issue is for footnote i which is only for SDC E and F.

The above is how I read it.

 

[twells12]

In the IBC 2003, I don't believe OMF will apply to your situation. The 15 psf floor issue is for footnote i which is for SDC D, E and F. (difers from IBC 2000)

I have a very similar situation.(IBC 2003, SDC=D) I can use IMF or SMF that are pre-qualified in FEMA 350. BUT IMF connections are not pre-qualified in FEMA 350. Only OMF and SMF are. Is there any advantage to using IMF versus SMF if there are no pre-qaulified connections?
I have W8 columns which it appears I can only use pre-qual conn. of Bolted Unstiffened End Plate. Is this correct?

 

[Taro]

The IBC OMF is basically the same thing as the FEMA 350 IMF. They are both tested connections that can withstand 2% drift.