Quakes can rock building standards; how will Vancouver fare long

tiffiant

Quakes can rock building standards; how will Vancouver fare?
Every time a tremor hits, new information is discovered


VANCOUVER -- The rubble of three major earthquakes in the last year will give rise to a better understanding of how to protect buildings and lives in Vancouver and around the world.

We have long known that the Pacific Northwest is overdue for a catastrophic earthquake. It could come in one of three deadly forms: the megathrust subduction earthquake, the earth’s most powerful ground movement where one offshore plate springs back upwards; the intraplate quake that erupts deep between two pressing plates; and the shallow, local crustal quake that takes place along the myriad fault lines that make up our local geology.

All three types have been witnessed in recent history.

The 9.0-magnitude Tohoku earthquake March 11 in Japan, which triggered a devastating tsunami, and the 8.8-magnitude Chilean earthquake in February 2010 were subduction earthquakes. The 6.1-magnitude violent shaker that hit Christchurch, New Zealand on Feb. 22 was of the local, short, sharp and shallow variety that erupted along a previously unknown fault.

The 6.8-magnitude Nisqually quake that hit Tacoma and Seattle in 2001 was a sister of the deep intraplate quakes that also shook Seattle in 1985 and 1949.

The three recent ones, Tohoku, Chile and Christchurch, will likely produce modifications to building codes, according to earth scientist Garry Rogers and structural engineer Ken Elwood. Both are members of the National Research Council’s standing committee on earthquake design, which is responsible for the earthquake provisions in Canada’s National Building Code.

“I think these earthquakes will really change the way we look at things,” said Rogers, a scientist with the Geological Survey of Canada, and the leading authority on Canada’s west-coast earthquakes.

Elwood, a professor at the University of B.C. who specializes in earthquake design for concrete buildings, saw first-hand the devastation in Christchurch. He was there for a conference when the quake struck. He sees many similarities between Christchurch and what would happen in Vancouver, which has many unreinforced concrete and masonry buildings built before the 1980s.

“One of our hurdles to overcome is that unfortunately we don’t have a very good inventory of those kinds of buildings,” Elwood said. “We don’t know strictly where we stand in terms of seismic risk.”

In 1991, Vancouver tried to quantify how many buildings were at risk of seismic failure. The study, published in 1995, showed that, of 1,150 buildings three storeys or higher that were built before the city’s first seismic codes were introduced in 1973, 400 were considered to be at “high” or “very high” risk of failure.

On Tuesday, in light of what happened in Japan, New Zealand and Chile, Vancouver council asked for another list of at-risk buildings.

Rogers says the Christchurch earthquake produced shaking that “vastly exceeded” the design criteria for buildings. The Chilean earthquake revealed problems with how large concrete walls in taller buildings are built. The Canadian concrete code for seismic design may now be modified based on research by one of his colleagues, Elwood said.

tiffiant

“We can certainly learn from the damage there in terms of improving the performance of concrete walls here.”

But it is the Tohoku earthquake that could really advance knowledge of building design and how well older buildings can be retrofitted or seismically upgraded. That’s because after the shallow 6.8-magnitude earthquake that severely damaged Kobe in 1995, the Japanese government revised building codes, greatly funded earthquake-science and structural-engineering research, and ambitiously retrofitted older buildings around the nation.

Significant earthquakes always modify building codes, which aren’t really designed to protect a building from failure. All they’re supposed to do is make sure it stays up long enough for the people inside to safely get out.

“Canadian building codes, as with others, are life-safety codes, they’re not no-damage codes,” Rogers said. Buildings that need to be operational post-earthquake, such as police and fire stations and hospitals, are supposed to be built to a much higher standard.

It was the 1906 San Francisco earthquake and resulting fire that spawned North America’s first seismic building standards. Three successive California quakes — the 1971 San Fernando, 1989 Loma Prieta and 1994 Northridge quakes — all resulted in major improvements. Changes arising from the San Fernando temblor ensure concrete buildings built after the mid-1980s are more likely to survive even the most serious tremors.

Over the years Vancouver has taken the Canadian lead in earthquake protection. The Northridge quake, which triggered fires in San Francisco when its water system failed, resulted in Vancouver building a $54-million dedicated saltwater fire-suppression system in hardened steel mains that covers most of the downtown peninsula and large portions of the Broadway corridor.

While the 2001 Nisqually quake didn’t result in new codes, it did show the value of having non-structural items such as parapets and ceiling tiles restrained, according to Will Johnston, Vancouver’s chief building inspector. It was also one reason why Vancouver made changes in 2003 to how it decides a building must be seismically upgraded. In the past, upgrades were triggered based on how much a building’s renovation would cost. Now they’re done on a sliding scale; even a small renovation requires non-structural items to be restrained so they don’t fall on people as they flee.

In 2003, Vancouver also became the first — and only — city to insist new one- and two-family buildings are built to seismic code. Elsewhere, seismic codes apply only to larger buildings. Johnston said Vancouver has always led the country in instituting building-code changes recommended by the NRC.

In 2005, the NRC also changed the minimum standard for how much seismic activity a building must withstand. Scientists and structural engineers use “shaking levels” to assess the potential for damage. A 6.4-magnitude shallow earthquake 10 kilometres away can do as much damage as a 9.0 subduction earthquake 100 kilometres out to sea, Rogers said.

The formulas used to calculate how much shaking a building must withstand will probably come under examination after the most recent quakes.

The quakes in Christchurch and Tohoku were so well recorded in multiple ways that structural engineers and seismologists are getting rich data that was previously unavailable, Rogers said.

“Both New Zealand and Japan have fantastic strong-motion instruments in their urban areas so we have exact recordings of what the ground did,” he said.

“That’s really important to the Ken Elwoods of the world, if we seismologists can [give them the data] and they can put that ground-motion data into their computers and see what happens. That’s where both Christchurch and the Tohoku earthquakes will really change the world.”

tiffiant

Local residents pass collapsed houses that were pushed onto a road in the city of Kesennuma in Miyagi prefecture on March 16, 2011. The official toll of the dead and missing following a devastating earthquake and tsunami that flattened Japan's northeast coast has topped 11,000, with 3,676 confirmed dead, police said. The total number of people unaccounted for in the wake of Friday's twin disasters rose by more than 800 to 7,558, the national police agency said in its latest update