Jun
2
By Marney Mutch
Island homeowners concerned about their home's ability to survive a major earthquake are always given the same pat advice: ensure it's bolted to the foundation. In practice, however, wood frame homes require much more than the foundation bolts.
A large number of BC's homes are built on pony walls that collapse well before bolts can even do anything and are the first point of failure. Even though bolts are important, bracing these walls is the first line of defense. This is done by converting the pony walls into shear walls, of which bolts are only one component.
The objective of seismic retrofitting is to keep the pony walls from collapsing and thereby save the house and the inhabitants from injury. To accomplish this, the retrofit must resolve three structual weaknesses. The failure of even one could cause a collapse.
The first and most important requirement is to convert the pony walls into shear walls. This is done by bracing them with plywood made specifically for this purpose, bolting them to the foundation, and attaching this assembly to the floor with tested earthquake resisting hardware.
Once completed, the shaking of the house is transferred into the shear walls and absorbed by the ground.
One illustrious example of how effective seismic retrofits can be occurred quite by chance in San Francisco. Architect Michael O'Hearn had purchased two identical, and adjacent, Victorian homes he intended to retrofit. He had completed the first when the World Series game-stopping earthquake hit in 1989. The retrofit home remained habitable and required only $5,000 to repair. To restore the other cost $260,000.
So far, there are no Canadian building code guidelines pertaining to the seismic retrofit of existing homes. According to Marney Mutch, owner of Victoria-based Shear Seismic Inc. (), a preview of the 2012 BC Buiding Code updates confirmed the absence of any new provisions. For that reason, Shear Seismic Inc. continues to take its lead from California's long-established seismic retrofit code.
"This code was developed based on the findings of structural engineering teams that investigated failures in homes caused by several catastrophic earthquakes; homes built similarly to our own. They have decades of first-hand experience and know what works and what doesn't. They get that it doesn't require a structural engineer to resolve the leading causes of destruction and their building code reflects that. We are very fortunate that we can benefit from that experience."
In terms of cost, retrofits start at about $3,000. An average size home with unfinished pony walls can often be completed in one day. Homes with drywalled basements take a little more work to access the exterior wall and restore the drywall to its original state. Preliminary structural assessments are typically offered free