Integrated Buildings & Structures

Hello,

I have a question in regards to load application for a building with vertical combinations of lateral force resisting systems. My building is 6 stories with buckling restrained braced frames on levels 2-6 and CMU walls at the base. My question is in regards to the transfer of overturning forces at Level 2. Per ASCE 12.2.3.1: For the design of the lower system, the design coefficients for the lower system shall be used. Forces transferred from the upper system to the lower system shall be increased by multiplying by the ratio of the higher response modification coefficient to the lower response modification coefficient.

My interpretation of this is that I multiply the forces to my lateral force resisting system at the first level by the ratio of my R-values (8/5). This would basically include collector beams at the 2nd level, and shear values into the wall.

Pilasters support the columns of the BRBF at the first level. My tension and compression forces would not need to be multiplied by (8/5) because the pilasters are common to the different framing systems. The design of the pilasters is covered by ASCE 12.2.4 which states structural members common to different framing systems used to resist seismic forces in any direction shall be designed using the detailing requirements of Chapter 12 required by the highest response modification coefficient (R) of the connected framing system.

There is an example in the 2015 IBC Seismic Design Manual which has a similar circumstance to what I am describing. They use the omega for the upper system, however there is no mention of the scaling of forces based on the R-values. The fact they omit it, leads me to my line of thinking. Of course it would be ideal if they just stated the scaling of forces by the ratio of the R-values is not required, but that would be too easy.

Anyone encountered this? Anyone have a different interpretation?

Thanks,

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Spencer Straub P.E., M.ASCE

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​There should be no increase in the omega factor by the ratio of the R-values.  The omega factor is used to ensure safety of the steel framed members but are not required for the shear wall systems.  See ASCE Commentary section C12, page 479, in ASCE 7-10.

The combinations of systems are not unusual especially for air traffic control towers where you have a steel moment frame system above the concrete shaft.

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Dennis Young P.E., M.ASCE
Owner/principle
Young Engineering, LLC
Omaha NE
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​You are correct about multiplying by 8/5ths for forces to design your CMU system in my view. If your entire analysis is done with R=8 then multiplying all the design forces for the CMU system by 8/5ths works. Another way is to run the analysis with R=8 and size all the BRB system. Then run a new model with R=5 and design the CMU system. I would agree that collectors at level 1 should also be at R=5 since they share diaphragm load at that level.

I'm not sure your system is what's intended by 12.2.4. Assuming your pilaster under the BRB column is not connected to a shear wall in any way then that column is not shared with another system.  A good example of 12.2.4 would be a SCBF in one direction and a SMRF in the other that both share a column at one corner. In this case the column is shared by two systems and the design requirements follow the higher R value system.

If 100% of the seismic load into your pilaster comes from the BRB system above, from all EQ load directions, then I think it's part of the BRB system. It's design would need to respond to the demands of the BRB system which are based on R=8 and omega=2.5 for that system.  The design of the pilaster would need to meet 12.3.3.3 covering discontinuous elements. Here the amplified seismic loads would be omega times the forces generated using R=8.

Just one opinion. Good luck.

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Brett King P.E., M.ASCE
Senior Structural Engineer
GHD Inc.
Lake Oswego OR
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