May 27, 2010

FEATURE | Steel

130 Bloor West wins CISC Steel Design Award

DON PROCTER

correspondent

Adding floors to an existing tower isn’t done often but that’s exactly what the owner opted for when faced with renovating a heritage listed, 14-storey office tower in downtown Toronto.

Successfully pulling the complex job off garnered the engineering/building team a top award at the Canadian Institute of Steel Construction’s 20^th Annual Ontario Steel Design Awards.

The seven new steel-framed storeys were built atop the 50-year-old tower at 130 Bloor West that features a two-floor heritage-listed penthouse residence overlooking the busy high end retail shopping district.

“It’s the kind of project that charges up a structural engineer,” says Brian Stonehouse, project principal, Halcrow Yolles, structural engineer for the project.

HALCROW YOLLES

Steel proved to be the most viable material for the reinforcement work and structural framing

To meet the challenge, the existing structural frame and foundations had to first be reinforced to accommodate the additional loads. The work was complicated by the fact the building had to remain fully occupied with office and retail tenants.

Steel proved to be the most viable material for the reinforcement work as well as the structural framing for the additional floors. To meet the reinforcement agenda, selected columns and beams were strengthened with steel plates to accommodate gravity loads while the moment frame (lateral system) was reinforced with vertical steel bracing, says Stonehouse. Additional vertical steel columns were installed in enlarged footings in the basement.

Areas near the elevator core were cross-braced from column to column over two-floor spans. To deal with increased lateral forces from wind or potential earthquakes, the new taller building required additional vertical braced frames in various configurations such as Xs and inverted Vees from the foundation to the top of the building, says Stonehouse.

The task involved opening up the existing concrete floor slabs and interior walls around the columns to be reinforced. Steel materials were hoisted by crane from the street and passed through openings such as windows and then transported on stretcher-like mobile rigs through offices to location. Block and tackle equipment was used to hoist the heavy pieces into place. The exterior façade remained unblemished throughout the process.

Stonehouse says the work was done in dust-tight enclosures containing sophisticated ventilation equipment to prevent welding fumes from permeating occupied office space. Walters Inc. of Hamilton was the steel supplier/erector. PCL Constructors Inc. was the general contractor and Quadrangle Architects Limited was the architectural firm.

HALCROW YOLLES

The truss erection and isolation pads.

One of the reasons steel proved the better choice than concrete for the additional floors was the amount of shoring and re-shoring required for a poured-in-place concrete job. The shoring would have “considerably complicated” the engineering and construction process, says Stonehouse. “Imagine the logistics of bringing in a system that could support the weight of all that shoring.”

To provide adequate support and eliminate vibration from the subway beneath the building, the seven new floors are supported on four transfer trusses positioned on rubber isolation pads, which separate the new structure from the existing one. The long-span steel trusses supported a tower crane used for construction above the existing penthouse. The lateral and gravity loads created by the new construction are transferred through the isolation pads. The design by Halcrow Yolles was modeled on computer software by Walters Inc. Erection sequences and all detailing was viewed in three dimensions.

The structural steel floors contain concrete on metal decks. The seven new levels feature 14 residences.

Among the advantages of steel over concrete was speed of erection. Steel also proved to be the easiest material to modify to suit tenant needs -- an important factor because none of the new floors has uniform layouts. Steel was cut to size to meet the space needs and floor beams were engineered with openings for water/drainage systems in separate acoustical spaces.

Steel also was the perfect material for the many odd-sized cantilevered balconies and irregular slag edges, says Stonehouse. Its light weight also helped to minimize the reinforcement agenda for the existing building and its foundations.

Few changes were allowed to the heritage penthouse so the loading imposed by the additional floors was transferred to its perimeter and the four transfer trusses (each a storey deep) straddled the penthouse. The trusses include glazing to allow for views of the city.

The ground floor will be reconfigured to provide additional retail space.

The site itself presented a flurry of scheduling challenges. The builder had a nine-hour work window a day, between 10 p.m. and 7 a.m., when the building was unoccupied. But even late-night scheduling was difficult because the noise of jack-hammering concrete floor slabs could disturb neighboring restaurants and bars. Using steel allowed the building conversion to continue while keeping the existing building weather-tight.

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