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Combining Lateral Systems in a Single Moment Frame

Combining Lateral Systems in a Single Moment Frame

Combining Lateral Systems in a Single Moment Frame

(OP)
We are working on a new project in a low seismic area (SDC B) with proposed precast concrete columns in the front of the facility. The proposed lateral system will be precast concrete columns in combination with joist girders/joists used as moment frames.
From Table 12.2-1 out of ASCE 7-10 the design coefficients and factors for seismic force-resisting systems are:
• C.7 Ordinary reinforced concrete moment frames with an R=3
• H Steel systems not specifically detailed for seismic resistance with an R=3

Is there anything in IBC 2015/ASCE 7-10 that would disallow this? From my research it looks like this is allowed per "12.2.3 Combinations of Framing Systems in the Same Direction" as long as we use the worst case requirements of each individual system.

RE: Combining Lateral Systems in a Single Moment Frame

I don't think the intent of the code is to allow a single moment frame with concrete columns and steel joist/joist girder as the beams. In my experience, when the code describes "Combinations of Framing Systems", it is describing the option to have a concrete moment frame (beam and columns) at one location and a R=3 steel system somewhere else in the building supporting seismic load in the same direction. I don't know of any guidelines or seismic criteria that would be satisfied by having two different materials in one moment frame like you're describing. If your seismic forces are very low (which they should be in SDC B), could you create a steel moment frame with 4" thick precast facade pieces wrapping around the column to get the same look with a simpler LFRS? I think you could even make the case that the detailing simplification and constructability would save you money in the long run.

RE: Combining Lateral Systems in a Single Moment Frame

I agree with birneys regarding the intent of the code. I’m not even sure how you design it even if it isn’t specifically disallowed. There is no cross reference between ACI and AISC for seismic design. Another option is to go with a steel moment frame and cast concrete around the columns.

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