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# Rethinking Displacement Calculations: Unveiling Inconsistencies in the O86 Code and Seeking Solution

## Rethinking Displacement Calculations: Unveiling Inconsistencies in the O86 Code and Seeking Solution

(OP)
The O86 code provides a 4-part formula to calculate displacement.

Total storey drift should account for:
shearwall bending deflection;
shearwall shear deflection;
shearwall panel nail-slip;
hold-down elongation at the anchor;

this entire formula must be then re-multiplied by the RdRo factors (if RdRo=5.1 as opposed to 1.3)
This means a hold-down deflection of 5mm provided by Simpson Strongtie will be over evaluated by this calculated requirement to be 15.5mm when in fact, Simpson Strongtie demonstrated that the hold-down will very likely have broken off at that point, which makes this formula inconsistent with reality and also gravely exxagerates deflections which doesn't help with allowable storey drift.

Thanks
Replies continue below

### RE: Rethinking Displacement Calculations: Unveiling Inconsistencies in the O86 Code and Seeking Solution

I design in the US, not Canada, but our codes are somewhat similar. We have the 4-part deflection equation for shear walls as well, and while we don't have RdRo, we do have Cd for seismic design - the deflection amplification factor - which serves roughly the same purpose.

I suspect the solution you seek is to use a different shear wall arrangement that stays within the limits of the code.

You have to take another step back and look at how you're arriving at the loads going into the shear wall. As I understand it, your RdRo value is also used the way we use our Response Modification Factor (R). The more ductile a system, the lower the design base shear for the building. So for us, our R for a shear wall system is 6.5. So we're essentially taking the maximum considered earthquake, and then saying that due to the ductility of the system enough of that energy will be dissipated such that the building will be strong enough if designed for forces considering that reduction. So long as your LFRS is designed well, strength failures won't necessarily be your problem. Stability will be. It's only through the ability to deform that these systems can survive these events without being designed for crazy high forces. So if you have a brittle failure mechanism hiding in your ductile seismic force resisting system (tensile rupture of a steel rod, for instance)....that's not a problem with the code, it's a problem with your design. You need to proportion that so it's not a brittle failure mode at the strength OR deflection required to be considered.

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