ASCE 41-13 Equation (7-21) and "Controlled By" Clarifications

[KelleyA]

Hello,

I'm currently working on a tier 3 seismic retrofit of an existing 1 story CMU building. When I originally ran through equation (7-21) I used Table 7-3 to obtain a value for my C₁C₂ factor. At the time this got me C₁C₂ = 1.4.

Going through the building though I realize I may have made a bad assumption in my primary components. I assumed that my CMU shear walls would be "Controlled By" shear and used a m-factor of 2.0 for LS and 3.0 for CP. Looking at m values for controlled by flexure though I see my m-factors would be 7.0 for LS and 8.0 for CP, which would raise my C₁C₂ to 1.8. So I have two questions:

1. What defines what the wall is controlled by? Is it a demand-capacity ratio for each type of force at each wall? Or is each wall "controlled by" whichever force you're analyzing at the current time? Ie. when checking flexure you use a m of 7/8 and when checking shear you use a m of 2/3.

2. For equation (7-22) and more specifically (C7-3), can you assume a DCR_max of 1.0 if you plan to retrofit everything so that nothing is failing? If everything is adequate there shouldn't be a DCR greater than 1.0. This would set u_strength = 1.0 and thus C₁C₂ = 1.0. I feel like this is probably not the case as the DCR probably only refers to the primary components as they exist and not how they will be after being retrofitted.

Thank you for your time.

 

[KelleyA]

All,

Just wanted to post I found an answer to item number 1. "Controlled by" is determined by the limit state of the wall. You solve for your flexural capacity for the wall per section 11.3.4.2.1 and your shear capacity of the wall per section 11.3.4.2.2 (Both of which state to use the strength design procedures of TMS 402). From there you back solve the shear force required to reach the flexural capacity of the wall. If that shear exceeds your shear capacity you are shear controlled, if not you are bending controlled.