9 Things your Contractor should know.
Most contractors say they know how to retrofit a house. It seems simple; put in some bolts and nail up some plywood. You don’t need to be a rocket scientist to figure that out. We wish it were so simple. According to this New York Times article as well as this CBS News Report, many, if not most, seismic retrofits are being done incorrectly. This stems from many factors; the primary ones being we don’t have a retrofit building code , and there is no special licensing or testing to make sure your contractor understands what he/she is doing.
The Science of Retrofitting
You can’t do a good job unless you are familiar with the science. This includes metallurgy, tests done in university and government testing laboratories, similar tests done by manufacturers, and the content of old building codes. Also very important is a thorough understanding of how houses fail in earthquakes, based on empirical evidence.
This evidence can be found on the Earthquake Engineering Online Archive at UC Berkeley. It contains a large collection of slides and photographs of earthquake-damaged homes. Understanding how and why houses are damaged enables one to address those specific weaknesses in the most effective way.
Getting a Referral.
Asking for references from non-professionals won’t be much help because all they really prove is that the contractor was friendly and cleaned up after himself. Asking professionals such as members of the American Society of Home Inspectors is a better way to go.
Can your seismic retrofit contractor answer these questions?
1- Does your seismic retrofit contractor know what the Base Shear Formula is and how it would apply to your retrofit?
If you like reading more than watching a video, the same information can be found here. This describes how competent engineers and contractors design a retrofit.
2- Has he read this 180 page book on shear walls?
This is the bible of shear wall construction. Any good retrofit contractor who keeps up on the latest research in seismic retrofit design and practice should be very familiar with it.
3- Does your earthquake retrofit contractor know about the three methods used in soft story retrofits?
This is an important reference because it proves he has experience with them all.
4- Does your seismic retrofit contractor know which organization is responsible for shear wall tests in Canada and the United States?
Answer: The American Plywood Association
This is also very important because their tests and reports are the basis of good shear wall designs that have been tested for their effectiveness.
5- Does your seismic retrofit contractor know which chapter in the 2013 California Existing Building Code applies to home retrofitting?
Answer: Appendix Chapter A3 of the International Existing Building Code.
Even though this document is dated, understanding the material therein is still important.
6- Is your contractor aware of any tests that have been done regarding the performance of old foundations and retrofit foundation bolts in regard to earthquake resistance?
This is critical so that your contractor knows when and when not to do very expensive foundation work.
7- Does your contractor know where the earthquake resistance of old building materials such as old siding, foundations, roofs, flooring, stucco, etc. can be found? If you don’t know how resistant a building already is, it is difficult to pinpoint exactly what it needs.
This is important because many existing connections are already earthquake resistant and there is no point wasting money addressing them.
8- Does he know which Chapter in the 2013 California Building Code contains the seismic provisions for new homes?
Answer: Chapter 23.
This is important because much of this information is used in effective seismic retrofitting
9- Does your seismic retrofit contractor know where many of the code approved seismic provisions are found that cannot be found specifically in the 2013 California Building Code? This part of the code addresses things like the principle of rotation used in soft story retrofits
This is an important reference because it contains information on seismic retrofit design and lateral force resisting systems you will not find anywhere else.