Seismic R values for mixed moment frame and shear wall elements

[mtnstructural]

Designing a residence with flexible roof diaphragm and one moment frame. All other lateral force resisting elements parallel to the frame and perpendicular to the frame are wood frame shear walls which would normally require R=6.5. The ordinary steel moment frame requires R=3.25. We have multiple parallel shear walls in the same direction and offset in plan from the frame. Is it required to use R=3.25 for all lateral loads in the frame direction or can it be justified to design the parallel offset shear walls for R=6.25 loads and design the frame for loads corresponding to R=3.25?

 

[Luceid]

ASCE 7 Table 12.2.3 says that different lateral systems in the same direction have to use the most stringent criteria between the two. One reason for why they might be doing this is because if you design the wood shearwalls for reduced forces using R=6.5, the moment frame will theoretically started to yield before the wood shearwalls hit their "design force", which is going to shift the lateral stiffness of the overall building, could induce extra torsion, etc. which could push more force to the shearwalls than what you might get from your analysis that assumes that the two systems will both be present and elastic when the design forces are reached. This may be less critical since your moment frame is likely much less stiff compared to your walls anyways, but I think it gives a sense for where they are coming from in the code.

So unfortunately, as far as I know, independent lines can't be used, except in very specific scenarios (I seem to recall something related to nonbuilding structures attached to buildings with less than 25% of the effective seismic weight, like an exterior trellis, could be designed with an independent R-value since these have cantilevered columns which would really hurt something like a plywood shearwall lateral system). This could be justification to bump your detailing up to an intermediate or special moment frame, but given that its a residence, these will come with some cost bumps of course.

Also OMF's should use R=3.5 rather than 3.25, although this was likely just a typo (Based on ASCE 7-16 anyways).

 

[Luceid]

Apologies, I just glanced at Chapter 12 and found an exception that may help you.

ASCE 12.2.3.3 gives some exceptions, which allow you to use the R value for the line of resistance as you say, if the following 3 conditions are met:

1. Risk category 1 or 2
2. Two stories or fewer above grade
3. Light frame construction or flexible diaphragms

This may work out for you in the end!

Note that the diaphragm would still have to be designed assuming the worst case R value.

 

[mtnstructural]

Thank you very much, Luceid. Your insight into the code is very helpful for my understanding. The exception in ASCE 12.2.3.3 appears very reasonable and we do have a two-story residence of light frame construction. The diaphragm design for the worst R-value is very logical. Much appreciated Sir!

 

[bigester93]

r=3.5 is good