A 48-inch diameter custom-made circular saw sliced “a precise horizontal cut” through the 600-millimetre stone base of a historic public school in Vancouver separating it from the two storeys of classrooms and one storey of attic space above.
It was a key step of an unusual and challenging project to install a base isolation seismic system to an 1897 vintage building at Vancouver’s Lord Strathcona Elementary School.
The seismic upgrade ensures the building will remain functional after a large earthquake, says John Sherstobitoff, principal of Ausenco, the engineer for the seismic work, which was part of a major renewal project for the public school, the oldest operating school in the city.
The cut allowed Vancouver-based contractor Heatherbrae Builders to lift the three-storey masonry structure up some three millimetres to transfer the building load to the new isolators.
The process was accomplished with flat jacks which are large pancake-shaped devices that are loaded with a water-like fluid to expand the few millimetres needed for the lift and load transfer.
The isolators (about 400 millimetres tall), placed above the jacks, take the brunt of the shake in an earthquake, allowing the building above to remain stable.
Sherstobitoff says prior to making the precise horizontal cut in the base of the building — accomplished by setting the saw blade on a track anchored to the side of the building — shoring above the isolation plane with load bearing posts in the basement was required.
“We then precut into the wall about a third of the way (to control location of cracking at the isolation plane) and then jacked up the isolators to pick up the load before we finished saw cutting through the wall,” he explains.
The jacking process was done in increments and the building was surveyed and inspected throughout the process. At completion, the water was pumped out of the jacks and replaced with epoxy to produce a “rock hard” flat jack to carry the load of the building, says Sherstobitoff.
The flat jacks and isolators were made by U.S.-based Dynamic Isolation Systems.
The lifting process took only about a week, a period where the engineer remained onsite to review and inspect the work.
The unusual job garnered Ausenco the Award of Excellence in Buildings and the Lieutenant Governor’s Award last year from the Association of Consulting Engineering Companies — B.C.
Sherstobitoff says he first proposed the base isolation concept to the school board in 2006 and his firm was involved in various followup design phases for the seismic upgrade of this school.
The design was approved by the school board because it was “cost neutral” compared to a conventional upgrade.
“It was an easy decision to put in a system that was going to preserve heritage and have post-earthquake functionality compared to a conventional upgrade scheme,” he states.
Sherstobitoff says base isolation was cost-neutral partly because the heating system was located in another school board building.
“We didn’t have to isolate a lot of equipment in the basement which would have made it more costly,” he adds.
While the project is the only base isolated building in Canada, Sherstobitoff says the technology has been done around the globe, in many instances to much larger buildings.