CHICAGO | St. Regis Chicago (Vista) | 1,191 FT | 101 FLOORS

[chicagodeckerdude]

Originally Posted by Barrelfish

My understanding is that it goes something like this:

• Originally, the narrow parts of each frustrum were going to be even narrower (going from 90ish feet at the widest point down to 70ish feet at the narrowest)
• These got widened so there were fewer difference in floorplans on the wide vs. narrow floors (now more like 90ish vs. 80ish). This brought more usable square footage and also reduced costs (less need to jog plumbinb back and forth, etc.)
• That extra width, combined with the buildings narrow profile + the fact that the middle frustrum isn't directly anchored to the ground (there's a roadway underneath), meant that sway was higher than they wanted during wind tunnel testing
• Therefore, the blow through floor was added to reduce the sway. I think this was after construction was already started, so it was too late to change the design of the tower in a way that would eliminate the need for the blow through

I may be conflating things slightly (maybe the blowthru would have been required regardless of the change in floor plates), but that's the general gist.

That explains a LOT! Thanks!