June 27, 2008

HALSALL ASSOCIATES

Two 35-ton steel Pratt trusses were used to replace an old concrete column in the Rogers Sportsnet studio building.

Ontario Region Steel Design Awards

Rogers Sportsnet’s Isabella Tower judged both simple and innovative

DON PROCTER

correspondent

Typically, trophy winners of the Canadian Institute of Steel Construction’s (CISC) Ontario Region Steel Design Awards have illustrated an innovative engineering scheme using large volumes of steel. Not so for one of the winners this year.

Two 35-ton steel Pratt trusses comprised the bulk of the steel required to net structural engineer Halsall Associates Limited a CISC prize for its retrofit at the Rogers Sportsnet’s Isabella Tower in Toronto.

“The type of work and the thought that went into this project was more important than the tonnage of steel,” says Guillermo Gabrielli, Halsall project manager.

The job called for the replacement of a major concrete column with ground floor, 18-metre steel trusses carrying the service gravity load of the 12-storey building and its roof.

The project was sparked by the fact the old concrete column was blocking camera shooting angles in the main studio of Rogers Sportsnet.

“One of the reasons I was happily surprised when we won the award is that they mentioned it was a very simple concept but it was carried through in a precise way,” says Gabrielli. “The idea was to support the building without disturbing the floors above so the occupants did not even know this happened.”

The trusses proved a perfect means to the end goal of ensuring the floors above didn’t move once the new support structure was in place, he points out.

“Coming up with the method for doing this was a distinctive engineering feature.” Both the design and building teams played key roles in its success.

The steel trusses were chosen over other solutions for the transfer structure — such as a post-tensioned reinforced concrete beam — partly because they were quicker to install and less expensive, says Halsall engineer Afshin Ebtekar.

There were other advantages to the steel design. It allowed the architectural firm Ware Malcomb to take advantage of the steel as an architectural feature in the lounge area above the television studio and in the master control room.

Ebtekar adds that the trusses allowed room for mechanical/electrical systems and future flexibility for utilities through open spaces between the big steel members.

The project required measuring the behavior of the steel during preliminary loading and unloading operations. Determining the exact load the existing column carried involved a complex testing and monitoring system for the new transfer structural system.

The bottom and top elements of each truss were prefabricated in shop and brought into the building through an opening in the exterior curtainwall, says Gabrielli. Each half truss was moved into place in one work shift.

When the trusses were bolted at each side of the column, two box girders connecting the bottom chords of the trusses were used to support jacks during preloading. The work was done with the last jacking operation of the trusses to the precise load which was determined by data calculated from a monitoring program and a strain/stress analysis algorithm. The analysis incorporated long-term behavior (including creep and shrinkage) of reinforced concrete. Only when the trusses were finally secured in their deformed preloaded state was the concrete column finally blade-cut and removed.

Benson Steel Limited supplied and installed the steel. Halsall created “very detailed” drawings at the bidding stage to ensure accurate pricing, says Gabrielli. “Along with elaborate drawings, we provided notes describing the procedure for general work, monitoring and stages for the preloading of the trusses.”

The project started in October 2006 and was completed in June 2007. The second phase of the $11-million retrofit, primarily the architectural fit-out, was recently completed.

While it wasn’t the biggest job in Halsall’s portfolio, Gabrielli says it was “truly unique. You get one of these once in your lifetime, or maybe twice if you are lucky.”

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