Residential Earthquake Retrofits in BC by Shear Seismic Retrofits

Frequently Asked Questions

Our house was already bolted but doesn’t have the square plates you show. Do they make that much difference?

Mudsill Plates, as they are called, are a great product and were first developed by Harlen Manufacturing following the 1994 Northridge, California, earthquake.  They were invented to mitigate the effects of over-sized bolt holes in mudsills.  Over-sized holes are pretty much found on every existing house because carpenters drilled the holes oversized so that they could adjust the edge of the mudsill and make it flush with the foundation.  However, based on tests done by Harlen, this seriously weakened the effective of the bolts by 50%.  They also discovered that a mudsill plate on a half inch bolt made the bolt about the same strength as a 5/8 bolt.  At that time, based on the 1991 NDS, the bolt went from 840 pounds to 1335 pounds.  Because the strength differential is so minimal between a half inch bolt with a MSP and a 5/8 bolt,, with or without a MSP, very few 5/8 mudsill plates were sold.  We used to have a few of them.  Placed on the mudsill to concrete interface, they would certainly be worth installing, but in retrofit work the underside of the mudsill is not accessible.

Do I need a contractor or an engineer?

As in other fields, contractors and engineers tend to specialize, so if you choose to work with an engineer or contractor, be sure it is one with special training in retrofit design. There is no point in asking a contractor to see his retrofit licensing because special licensing is not required. Pick one that has had specialized training in retrofit engineering principles. With this training, the contractor can also design your retrofit.  Engineer-trained contractors, though hard to find, are usually the best choice. Not only do they understand the design principles, but they also understand the most practical ways to achieve maximum protection.  In almost every case, an engineer-trained contractor will design a retrofit equal to that of an engineer but at a much more affordable price, not to mention that engineer’s fees are $4,000+ for a design before the work begins.

Marney has been mentored in retrofit design by 3 of California’s top seismic retrofitters:  Structural Engineer, Thor Matteson, author of the shear-walling bible “Wood-Framed Shear Wall Construction” published by the International Code Council (www.shearwalls.com); and by Howard Cook, a former FEMA earthquake inspector turned retrofit expert, and Jeff Bailey of Berkeley’s Bay Area Retrofit (www.bayarearetrofit.com).  Every Shear Seismic retrofit is reviewed by each to ensure the most effective and cost-efficient strategies.

How much does a retrofit cost?

The cost to retrofit a home depends on: the way the house was originally built, where it is relative to a fault, the height of the cripple walls, the number of stories, and many other factors. Though costs can vary greatly, a seismic retrofit generally costs between .75% and 1.5% of a home’s value, a far cry from the 10-15% earthquake deductible you won’t end up having fork over. And you’ll have a house for your family to live in afterward.

How can I know whom to choose?

Seismic retrofit technology is continually evolving, and a retrofit contractor needs to understand and apply the most current seismic retrofit research. Customer references can tell you whether he was on time, cleaned up after the job, etc. but cannot tell you if the contractor knows what he is doing. If you are doing a retrofit that involves shear walls, make sure he has, at least, read the book “Wood-Framed Shear Wall Construction” by Structural Engineer Thor Matteson.  Otherwise, the only way you can determine if your retrofit professional will retrofit your house properly, is by educating yourself. Then make sure the contractor explains exactly what he is going to do any why. If he cannot do this, he himself probably does not know what he is doing. This may require that you crawl under your house with him but it is the only way you can know if you are getting what you pay for.

Under what conditions does a structural engineer need to be involved in the retrofit design?

We would need to consult a seismic structural engineer when your home has/is:

  • more than two stories
  • more than four residential units
  • an unusual shape, is very long and narrow, or is shaped like an “L”
  • a two-car or larger garage with living space above
  • a “split-level”
  • a porch recessed under a second story
  • a weak foundation
  • a brick, block or rock foundation
  • is almost touching an adjacent home
  • on a steep hill
  • supported anywhere by posts rather than a wall