Slim Tower Presents Big Challenges

Don Procter

June 2005

Toronto’s tallest residential condo tower, the 51-story One King West nearing completion downtown, is billed as the most slender tower on the planet. For every 11 feet it rises, it is only 1 foot wide.

Being less than 3 feet from an office tower on one side and butt up against a designated historic building on the other posed some unusual challenges for the builders. For starters, the tower had to be structurally tied into the 1913 banking temple without damaging its heritage designated façade or its fanciful plaster and woodwork interiors.

Plaster restoration contractor Iconoplast Designs Inc.’s job called for the restoration and replacement of ceilings, arches, cornices, capitals and columns. The contractor, which has done major restorations throughout North America, including Broadway theaters in New York City, had its hands full at One King West. Owner Jean-Francois Furieri says the method of tying the new tower into the 15-story historic building was ingenious, but it added wrinkles to the restoration job. "Making our work inconspicuous, especially on such a large scale, was tricky,” he said.

To build the ultra-thin tower with minimal sway at its top, the two buildings were joined by a steel structural framework embedded in the concrete shearwalls. To meet seismic code requirements and to reinforce the old building, which carries part of the load of the new building, steel cross-bracing was installed in the stately Edwardian building. The obvious location for the cross bracing was in the ceiling of each floor plate, but because some of the interiors of the building were historically designated by the city, installing the cross bracing without destroying ornate plaster ceilings such as in the three-story grand Banking Hall was next to impossible, explains architect Jamie Rasor, job captain, Carson Downey Architect Inc., the project’s architect.

Consequently, an ingenious method of cross-bracing the floor above the hall was done. It was covered over by a concrete mixture called Isocrete and a 2-inch finish coat. Being four times lighter than conventional concrete, Isocrete was light enough for the floors to support without damaging the ornate ceilings below, Rasor says.

"Our job followed with the replacement of original details and invisible mending,”explains Furieri, a third generation plasterer. "Even though we take the care as preservationists, it is not always possible to salvage the historic fabric.” In some areas castings were made of the original plaster details, such as column capitals that were re-attached to the columns.

On the opposite side of the slim tower, builders had to be equally careful not to damage the neighboring office tower. While precast concrete was initially specified as the cladding system for the tower’s first 18 floors, for various reasons—including price and ease of installation—the design was changed to exterior insulation and finish systems. EIFS contractor Tonar Cement Finishing Inc. was awarded the job.

"We’ve never worked in a space that confined,” says Paul Ribeiro, Tonar’s supervisor. A swing-stage modified to less than 24 inches wide squeezed into the narrow opening. A protective screen and plywood hoarding protected the neighboring building while daily cleaning of EIFS debris accumulating at the base of the structure was necessary because a portion of the area between the two buildings was a passageway for office workers.

The area of EIFS coverage is about 120 feet long, from the 3rd floor to the 18th floor. While the EIFS contractor had worked up to as high as 28 stories on another project, at One King West Tonar the contractor had to set up its outrigger beams for the swing stage at 43 floors high.

Initially, the EIF system was to be a conventional rain screen system, but due to the site’s confines, the specs were changed to a system made by DuRock Alfacing International Limited, called PUCCS. It consists of a patented back-vented insulation board with "puck-like” shapes that form the drainage path, Ribeiro explains. "We determined that this would be a better system for working at this tight site than other rain screen systems.”

About the Author
Don Procter is a free-lance writer in Ontario, Canada.