Mixing Lateral Force Resisting Systems

[Mohzus]

Hi everyone,

When using different lateral resisting systems on a given storey, the Canadian code specifies for the base shear reduction factor (Rd, Ro) of the smallest system be used. So for example, a storey containing wood-based conventional shear walls and a CLT panel would have an R value of 5 and 3, respectively, and the 3 value would be used for that storey (not considering the storey above). Just for clarity, let's say the structure is at the maximum height for light-wood frame at 6-stories.

Three questions:
1. Could I keep the R value of 5 for conv. shear walls by detailing the CLT panel to not engage as much load (less rigid connections, seismic separation)?

2. More of a theory based question, since the smallest system reduction values (CLT - R=3) must be used, then reducing the base shear with these values, does the buck stop there and design would continue as normal? Is there any consideration that the conv. shear walls are expected to yield being that their design load would have been much lower than introducing CLT into the mix?

3. What are some general comments and considerations when mixing systems? For rigidity and deflection distribution for example.

 

[hardbutmild]

I'm not that familiar with Canadian codes or timber to be honest, but these are my thoguhts, maybe they'll help.

My first and most important question is displacement control. I'd expect this type of structure to be sensitive more to the displacements than to the force capacity. Because of that I do not see the importance of making the R so large. Usually, I tend to use lower values unless necessary.

1. I'd expect that you could use R=5 if most of the load and energy dissipation is expected to go to the conventional walls. BUT, what is the point of using CLT if you do not engage it? If you don't get the rigidity from them, why use them?

2. I don't understand this question. You design the whole system to R=3. Conventional walls will have to yield, they just won't need to yield "as far" as they would have if R were 5.

3. I'd say that rigidity and deflection would be my largest worry in this example. Other than that, I'd say that general rules for seismic design apply - capacity design, diaphragms (might be even more important in mixed systems)...

 

[EngDM]

If you mix systems you need to assign loads based on stiffness, unless you can ensure a flexible diaphram. In reality wood based diaphrams are somewhere in between, so your CLT might be "too stiff" per say and attract more load. Typically I try to avoid mixing wood SFRS and other SFRS on the same storey, as the load assignment gets pretty involved.