UBC quake simulation tests retrofits, underscores need to upgrade older buildings
It took about 40 seconds before the effects of the simulated earthquake on the wooden structure were audible: snapping, cracking, popping.
It was the extraordinary sound of nails coming out of the plywood exterior walls of the demonstration, one-room school, and also the sound of the plywood ripping apart between two windows, and the screws pulling away from the drywall.
The wood structure itself, at the height of the two minutes of shaking, swayed side-to-side like a tree in a storm. It was hard to imagine somebody being inside and remaining unhurt.
Modelled after the 9.0 Tohoku temblor in Japan in 2011 that created a devastating tsunami that killed thousands, the test at the University of B.C.’s Earthquake Engineering Research Facility was meant to simulate a large subduction quake off the West Coast, the so-called “Big One,” that would hit Victoria and Vancouver Island, explained Martin Turek, manager of the facility.
Scientists estimate a 10-per-cent probability of the Big One hitting in the next 50 years.
Even on the third test Wednesday on the large shake table, at 120-per-cent beyond what the province’s building code requires, the structure remained largely intact.
However, on inspection after the test, consulting structural engineer Graham Taylor, who has been part of the UBC earthquake research team for nearly 20 years, said such a structure would likely be “red-tagged,” meaning it could no longer be inhabited.
The one-room school had fared well at the 70- and 1oo-per-cent test levels.
That’s good news.
But given that the one-room building, with weights on top to simulate a second floor, was built to demonstrate how a retrofitted school building would perform, it’s also a warning of the damage that would be expected of older buildings that haven’t been upgraded.
Taylor noted that a wooden structure built, for example, in the 1950s would have come apart at one of the two lower-level tests. That’s because they don’t have seismic hold-downs, large bolts that connect the walls to the foundation, and also don’t have the bracket that provides lateral support for walls. The older buildings also aren’t built with half-inch plywood on the outer walls, positioned and nailed optimally to withstand the shaking.
A study released earlier this year, headed by UBC structural engineer Carlos Ventura, director of the university’s quake research centre, found that nearly 4,000 buildings in Victoria are at-risk of complete damage from a major temblor.
Vancouver is at less risk from the Big One off the coast of the Island, because of distance, but is also susceptible to earthquakes on land and in the Strait of Georgia, at shallow and deep levels.
“What we’ve seen here today, we pushed it harder beyond what the code is requiring, and it did very well. So, there’s a huge benefit in upgrading buildings,” said Taylor. “Conversely, when we see this level of damage with a highly engineered structure, and this level of damage would take place with a very low level of shaking for a vulnerable structure, it’s all the more reason we have to think seriously about fixing them (the older buildings) as quickly as possible.”
The tests are also providing information that will help engineers more quickly assess buildings. That’s because the test results, combined with information being collected on soil in specific areas, will pinpoint the most-vulnerable buildings.
That information — showing which buildings are the most vulnerable — will also help underpin the argument for seismically upgrading buildings, said Graham.
The province and municipalities have been slow to act on pro-actively addressing the seismic risk of older buildings. An investigation by Postmedia News, published in 2016, revealed that the City of Vancouver had failed to create a proactive plan to reduce the seismic hazard of the city’s older private buildings despite identifying a need to do so more than two decades ago.
Of more than 1,100 buildings included in a seismic-risk assessment by the city in 1994, hundreds appeared to have had no seismic upgrades, the Postmedia examination also discovered.
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