It’s not uncommon for a major construction project to change after it’s already started. Look at Wolf Point West, which had its 43rd and 45th floor setbacks moved to the 45th and 46th floors, creating two more residences. Or the Skinner West Elementary School addition which is gaining a floor, long after just three were approved by the city.
In the case of Jeanne Gang’s spectacular 95-story Vista Tower going up at 363 East Wacker Drive, even a minor change is of major interest. So you know our Spidey Sense went on red alert when we heard that the 83rd floor is going to be sacrificed in the name of health and safety.
It’s a victim of the Windy City’s… wind. The 83rd floor is going to become a two-story high “void space” known as a blow-through floor that will “disrupt and reduce the wind effects on the building.” Visually, it’s right about where the top segment of the tallest tower bows most outward.
According to city documents, the blow-through floor won’t just lessen the force of wind at the 83rd floor. “The interruption of the regular wind flow along the building will serve to disrupt the wind pressure for many stories above and below the blow-through level.”
And lest you think that we’re hyping up the whole “safety” angle to generate clicks — the way petty much every internet publication does these days — those are the words of the Chicago Department of Planning and Development, not ours:
Prior to the addition of the blow-through floor, acceleration limits, as defined by the International Organization for Standardization, were predicted to be significantly exceeded, indicating performance that would be unacceptable. According to a letter from Magnusson Klemencic Structural and Civil Engineers, this would result in building occupants feeling ill and possibly afraid for their safety.
That’s in a letter from the city to the lawyers for the project, giving the green light for the blow-through floor, as well as reducing the complexity of the glass in some other parts of the skyscraper. 42nd Ward Alderman Brendan Reilly was notified, and gave it the nod.
The hue and cry you hear in the distance is the sound of several hundred architecture students on 35th street shouting, “What about mass dampers?”
That was considered. Tuned Liquid Sloshing Dampers, to be exact. And wind tunnel tests show that the blow-through floor is going to need help from multiple TLSDs to keep Vista rock steady.
What about simply changing the design of the building? Surprisingly, that was considered, too. But the changes didn’t mitigate the wind load enough to make it worthwhile. Though, that’s not to say that Vista isn’t changing.
The rolling stones at city hall see the dead floor and they want to paint it black. All of it. The walls, the core, the ceiling, slow pigeons — everything in the double-height 83rd floor will be black. And, by law, the space cannot be illuminated. So when Vista twinkles in the Chicago skyline at night, she will have a black choker across her neck.
And she’s also getting a little taller. Maximum height is now 1,198 feet, five inches above the Chicago City Datum — just 19 inches short of the maximum allowed by city ordinance.
The height of the ceiling on the highest occupied floor gains a little over seven feet, moving to 1,171 feet, 11 inches.