Seismic Retrofit for Wood-framed buildings:
Prior to 1980, building codes did not require builders to secure houses to their foundations. This does not mean that every house built before 1980 is “unsecured”, only that it was not a requirement. If your home is not properly secured, it may be at increased risk of “slipping” off the foundation during a major earthquake. Retrofitting involves bolting your home to its foundation and providing shear/pony wall strength (if applicable). The goal is to increase the structural integrity, but does not mean that your home is “earthquake proof” (there is no such thing). Retrofitting does increase the chance that your home will withstand the shaking and be intact following an earthquake. The good news is that wood framed construction, which is the primary building material in our area, performs well during earthquakes. If you are considering retrofitting your home, you may want to check out a class offered by the Seattle Office of Emergency Management and the Department of Planning and Development. The class is offered for free and teaches DIY homeowners and also contractors. It’s not rocket science but rather something anyone with a desire to perform the work can do.
Here’s the link:
There are two (generally speaking) ways to fasten the house to the foundation. One is the “bottom-up” approach in which you go into the crawlspace (below floor level) and install the necessary hardware and plywood (if pony walls exist). The “bottom-up” approach is the one that Seattle has been using for the last decade.
The other approach is a “top-down” approach in which hardware is installed directly above the first floor level and extends down to the foundation. There are pros and cons to both approaches.
A very effective way of performing a seismic retrofit is to simply install hold-downs in all of the corners of the house. This can only be performed with a poured stem wall (there are a few exceptions to this). This “top-down” approach consists of the following:
- Install Simpson HDU2 in all corners of house.
- Fasten HDU2 to double studs with ¼”x3” Simpson SDS screws.
- Use 5/8” diameter galvanized threaded rod to epoxy into poured stem wall.
- Use roto-hammer and ¾” drill bit diameter.
- 6” minimum embedment depth into stem wall is required.
- Use Simpson CNW-5/8 coupler nut if extension of rod is required.
- Anchor bolts are not installed.
To provide a fully-engineered seismic retrofit typically costs between $750 and $1,500. All engineering design, drawings, and details will be provided so that a home-owner or contractor could perform the retrofit. This is assuming that a basic floor plan is provided as well as pictures of all four sides of the building as well as inside the crawlspace (if applicable).
It typically costs the home-owner between $500 and $5,000 to do the retrofit installation themselves. The cost for a contractor to do all of the work is generally between $7,000 and $12,000.
David Bradley, P.E., is listed on the following web-site as having completed the seismic retrofit training at the UW extension:
Seismic Retrofit – Concrete or Masonry buildings:
Seismic retrofit on masonry or concrete buildings can range from the simple installation of steel members to complete steel lateral reinforcing systems. Recently there have been many innovative developments with the retrofit of unreinforced concrete buildings such as the installation of carbon fiber wrap to the inside of the buildings walls. Steel “X-bracing” is always an option and has been effectively used for years. In addition, a “hybrid” approach can also be used in which sheathed wood framed walls can be installed and integrally fastened to the existing structure. The bottom line is that one size does not fit all when it comes to the seismic retrofit of unreinforced concrete buildings.
Therefore the best seismic retrofit solution is highly variable and depends on many factors.