Reviving

A multidisciplinary team of consultants tackles the tricky restoration of a pair of iconic Chicago towers.
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From Architectural Record
Joann Gonchar, AIA

Plinth problem

Their repainting strategy allowed team members to preserve much of the project's $9.2 million budget for restoration work at the base of the towers, where they considered the needs more pressing. Here, water would pond and pool on the surface of the plaza whenever it rained, and would then saturate the structure below and its reinforcing material, and cause pavers to crack. Freeze-thaw cycles and deicing materials accelerated the whole process, and by the time Krueck & Sexton was hired, the plaza had undergone at least three previous renovation campaigns. These earlier efforts involved removing the stone pavers, refinishing and reinstalling them, and supplementing them with new material. The end result was a mismatched checkerboard effect that compromised the purity of Mies's almost totally flat travertine plane, or what Sexton likes to refer to as the plaza's "plinthness."

For Sexton and the rest of the consultants, restoring the plaza was not just a matter of finding the right travertine or detailing the joints between pavers - which were naturally very important, requiring trips to a quarry in Tivoli, Italy, and the examination of samples and mock-ups. Proper restoration meant correcting the underlying problem - the plaza's inadequate drainage.

The newly restored plaza incorporates a subtle slope to aid drainage, though it looks like a perfectly flat plinth.

Photo © William Zbaren

 

Water no longer pools on the travertine surface, as it did before reconstruction.

Photo courtesy Krueck & Sexton

Although the problem was easy to identify, the fix was less straightforward. In order to make the plaza shed water, contractors would need to introduce a slope to its surface, and they would need to do so while preserving the impression that buildings sit on a perfectly even and level field of travertine. The solution was to incorporate nearly imperceptible ridges and valleys into the plaza surface. These are much shallower than would be installed in a new construction project, but still effective. The only indication that the plaza is no longer flat is the absence of puddles after it rains, says Bruce Vance, AIA, senior project manager for the restoration's general contractor, Bulley Andrews.

The sloped stone sits on top of an unbonded setting bed and a continuous drainage mat over a waterproof membrane. Underneath this assembly is an extensively repaired and reconstructed structural slab, which is also sloped to help channel the water. To the one existing exposed plaza drain, workers added 15 more concealed below removable pavers.

The plaza's poor drainage also wreaked havoc on the storefront that encloses the lobbies, which suffered from both direct water infiltration and condensation build-up. The problem was exacerbated by the negative pressurization of the buildings' interiors, causing the enclosure system to suck in air and water with it when it rained.

An extensively repaired slab is below the new plaza.

Photo courtesy Krueck & Sexton

 

 

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Originally published in Architectural Record
Originally published in June 2010

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