December 12, 2010

Metrodome collapse - would be no surprise to roof's designer

Three years ago, BC Place stadium experienced a roof failure.

As CBC Vancouver's design columnist, I spoke with one of the designers of BC Place's roof, Horst Berger. As a partner in the engineering firm of Geiger Berger, they designed several air-supported roofs including Minneapolis's Metrodome.

Here's the original piece:

"Original BC Place roof engineers split over design issues


(This Vancouver by Design column originally aired On The Coast on CBC Radio One in Vancouver)

January 9, 2007 - Vancouver - One of the original partners in the design company behind BC Place's roof told Vancouver By Design he left the company precisely because of a dispute over the design of such structures.

Horst Berger was one half of Geiger Berger - the company which designed the stadium's air supported roof; it collapsed last week after tearing.

Described by local engineers, as "daring, innovative, and pre-eminent" in the field of fabric roofs, Berger ended his partnership with David Geiger in 1983, the same year BC Place was completed.

Berger, 80, spoke to VBD by phone from his residence in New York state. When told about the tear and subsequent deflation, Berger said, "I'm not surprised."

"The air-supported structure depends on a mechanical system and that is it's Achilles heel."

Membrane roofs like the one on BC Place require pumped in air to keep the dome inflated and further require heaters to melt the snow.

Berger says, "You can not possibly design it [an inflated roof] to counteract a snow load. And I'm a conservative engineer. I want my design to last a thousand years."

But Geiger Berger's inflated roofs are entirely dependent on building owners following maintenance and operation protocols to keep the them up.

"And that's why I broke up the partnership," says Berger.

According to tent designer and engineer, Gery Warner of Tentnology, a large tent design firm in Vancouver, engineers specialized in the field of membrane or fabric roofs considered David Geiger, who is now deceased, as the "air-supported guy and Horst Berger as the tensile structure guy."

In tensile design, cables are used to hold the membranes up. Air-supported membranes use cables to keep the roof from billowing up - this cheaper technology was actually the brainchild of David Geiger.

But Berger says, "The structures are too unstable. It has to be air-tight to perform. Once there is a tear, it loses stability and the roof fails."

"That's why you haven't seen new ones being built."

After his breakup with Geiger, Berger started his own engineering firm with a focus on tensile structures like the sail-like roof of Canada Place in Vancouver, which Berger designed while at Geiger Berger. Berger points to the Denver International Airport, which he designed, as an example of the comparative robustness of tensile design. Denver has experienced record snowfalls this winter.

It is still unclear why BC Place's roof has failed though snowload and high-winds are possible causes.

Technically speaking, the present deflated position of BC Place's roof makes it now a tensile structure. However, in that position the roof will not be able to used its heaters to melt any accumulating snow and as of Tuesday night, the forecast is for up to 5 to 15 cm of snow.

Geiger engineer Kris Hamilton, based in Bellingham, Washington, says the roof in its inverted condition can sustain a snowload of 12 pounds per square feet. Under the building code, structures in Vancouver are required to endure snowloads greater than three times that amount.

BC Place general Howard Crosley told CBC Radio reporter Terry Donnelly the roof will not be damaged by the expected snowfall."

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