Dual Lateral Force Resisting System, Flexible Diaphragm, Different R values 1

[NCStructuralEng]

I am working on a high end 3 story apartment building. Footprint is long and skinny (70'x300'). So in the longitudinal direction seismic may control. This is wood bearing walls, with wood shear walls. On one side I have plenty of shearwall and also 3 masonry stair and elevator cores. On the other side i have windows...lots of windows. In 300' I bearly have 39' of usable shearwall. My question is...because i have a flexible diaphragm and the load to each shearwall line is attributed by trib area alone; then may i use a different R value for the wood shearwalls on the front of the building (6.5) and another R value for calculating the load on the CMU stair cores on the back side of the building (2) in a different shearwall line. I would usually conservatively go with the lower R value and design the shearwalls accordingly, however, because of the lack of shearwall length, the loading isn't feasible to resist with a wood shearwall. Thanks in advance for your input.

 

[msquared48]

I have heard of varying R for different directions, but never in the same direction. I've always taken the worse R value too.

With the rigidity of the CMU walls being much greater, you are approaching a "C" shaped structure for your wall systems, and your center of mass will be different than your center or rigidity for the walls. I suspect that in the long direction of the building, due to the length of the diaphragm, it will behave more like a rigid than a flexible diaphragm. Definitely flexible across the short direction.

Mike McCann, PE, SE (WA)

 

[JAE]

I suspect that in the long direction of the building, due to the length of the diaphragm, it will behave more like a rigid than a flexible diaphragm

I agree with this. You would have a rigid diaphragm per the definitions on ASCE 7 and then treat the building as an eccentrically loaded system.

You would not use different R values for the same direction - you use the smallest R for dual or mixed systems.

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