Boundary Zones for Core Walls

[Luceid]

For tall rectangular cores where the effective flange effectively allows it to be designed as a full section per ACI 318-19, does the entire core become a complete boundary element all the way around, if the boundary element length (c-0.1*lw or c/2) is longer than the thickness of the "flange" wall (as shown below)?



I can understand the rationale - if the flange is in fact all in compression, then all of the bars in that flange need to be protected against buckling by being tied, concrete core needs to be protected, like a column taking heavy compression. But this seems crazy - and I'm having a hard time finding precedents that I am confident in. An SOM paper for CTBUH only has boundary zones at the intersecting corners, not along the entire flange.

I've looked at past discussions here and can understand how you could end up with two-way BE zones at the four wall corners if you do the P-M design for each wall independently, rather than as a net rectangular core. The general advice seems to be that there is an advantage to designing the full core, but because of load reversal, two directions of EQ, is the full core just a boundary element?

 

[HTURKAK]

I have copy and pasted commentaries of relevant clauses;

R18.10.6.3 By this procedure, the wall is considered to be acted on by gravity loads and the maximum shear and moment induced by earthquake in a given direction. Under this loading, the compressed boundary at the critical section resists the tributary gravity load plus the compressive resultant
associated with the bending moment. Recognizing that this loading condition may be repeated many times during the strong motion, the concrete is to be
confined where the calculated compressive stress exceed a nominal critical value equal to 0.2fc'. The stress is to be calculated for the factored forces on the section assuming linear response of the gross concrete section.....

R18.10.6.4 The horizontal dimension of the special boundary element is intended to extend at least over the
length where the concrete compressive strain exceeds the critical value . For flanged shapes, L-shapes, and C-shapes, the calculation to determine
the need for special boundary elements should include a direction of lateral load consistent with the orthogonal combinations..