Bookmark this site!

Mr. Handyman How To Guide

Seismic Retrofitting for
Earthquake Damage Prevention

 

Objective
It is the objective of this whitepaper to address three key areas regarding the potential for catastrophic earthquake damage to woodframe houses in Seattle neighborhoods:

1. New research findings from the U.S. Geological Survey and the University of Washington which reveal increased risks of earthquake damage in the Seattle area,
2. An overview of the types of damage likely caused to Seattle woodframe houses during a future earthquake and
   Methods for substantially reducing these damage risks.

Older Houses and Damage Risk
Although woodframe buildings are generally among the safer types of construction when it comes to damage from earthquakes, they are still vulnerable. For example, 46,000 homes were damaged in the Los Angeles area after the 1994 California Northridge earthquake and 23,000 homes were damaged in the San Fransisco Bay area after the 1989 Loma Prieta earthquake.

Particularly vulnerable are woodframe houses built before 1965, which includes the majority of houses in especially vulnerable areas of Seattle.

New Data Reveals Heightened Earthquake Risks to Seattle Buildings
In early October 2007 the United States Geological Survey, working together with scientists at the University of Washington released new data showing that much of Seattle is at a higher level of risk from earthquake damage than originally believed.

According to this new analysis, several neighborhoods are particularly at risk from severe shaking damage caused by an earthquake including:

• Queen Anne
• Magnolia
• Interbay
• The University District
• Montlake
  Note: refer to pinkish to red areas in Figure 1 below.

Using this new data, the USGS has constructed a Seattle Urban Seismic Hazard Map (Figure 1) which is intended to help illustrate how earthquake threats in Seattle vary “ . . . according to geology, soil type, proximity to a fault and other factors” (Seattle Post-Intelligencer, October 12, 2007).According to the P-I article:
Tim Walsh, chief hazards geologist for the state Department of Natural Resources, said the Geological Survey's theoretical model used to create the map also revealed some surprises for what's known as the Seattle Basin . . . . "Using this model, they were also able to show how the seismic waves" -- the destructive forces in a quake -- "get trapped in the Seattle Basin so that the duration of the strong ground shaking, in certain areas, will be much longer than earlier predicted," Walsh said.

 

Figure 1

Damage to woodframe houses during earthquake is generally caused by lateral motion of the earth beneath a house, with which the house, or parts of the house, cannot keep up. The most probable types of damage include.
The wooden part of the structure sliding off its foundation (Figure 2)
The pony (or cripple) walls “racking” and collapsing and/or (Figure 3)
Separation between the pony walls and the upper part of the structure.

 

Figure 2 Home moved off the foundation wall (photo from Applied Technology Council)

 

Figure 3 Collapse of the short wall between the first floor and the foundation (photo from Applied Technology Council)

According to the City of Seattle’s Office of Emergency Management, these vulnerabilities are due to the following structural shortcomings in older woodframed houses:

• The foundation below the exterior walls is discontinuous, non-existent, or in poor condition.
• The connections between the home’s exterior walls and the underlying foundation are absent or inadequate

The short wall between the foundation and the first floor, commonly called the pony wall or cripple wall, is not braced with structural panels to prevent collapse during earthquake ground shaking (Home Earthquake Retrofit Series, Seattle Office of Emergency Management, 9/23/99)

Home Retrofitting Decreases Risk
Fortunately, many woodframe houses in Seattle lend themselves to structural retrofitting which may reduce the risk of damage.

To this end, the Seattle Office of Emergency Management has adopted a set of retrofitting recommendations known as the Standard Home Earthquake Retrofit (SHER) Planset. The planset:
Encourages the understanding and use of a standardized strengthening method,
Expedites building permits for projects qualifying to use the standard plan and
Promotes home earthquake retrofit projects.

This standardized approach provides a straightforward method of seismic retrofitting for houses that conform to certain specifications. Approximately 50% of Seattle houses lend themselves to this approach. Many of the remainder may also be retrofitted using the standards outlined in the SHER planset but may require additional engineering or architectural input.

In brief, the SHER planset specifies the following steps for effectively securing woodframe houses to their foundations:

1. Anchor the sill plate to the foundation wall;
2. Strengthen the short walls between the foundation and the first floor, called pony walls, with approved wood structural panels, also called sheathing;
3. Connect the pony walls to the floor framing above and to the sill plate below with metal framing clips; if no pony wall is present, connect the floor joists directly to the sill plate.

This process is particularly important for houses built prior to 1965:
In most cases, homes built before 1965 in Seattle, Everett, and Tacoma lack foundation anchor bolts. The bolts that were installed generally do not meet today’s requirements for bolt size, spacing, and condition. Homes built between 1965 and 1975 may have foundation bolts, and those built after 1975 generally will have foundation bolts. (Home Earthquake Retrofit Series, Seattle Office of Emergency Management, 9/23/99)

Overall Benefits of a Home Retrofit
The SHER planset outlines the following benefits from retrofitting a woodframe house (excerpted from Home Earthquake Retrofit Series, Seattle Office of Emergency Management, 9/23/99):

Safer Homes
Improved safety is the primary concern of the Standard Home Earthquake Retrofit Plan. The plan defines cost-effective minimum requirements to address the common structural weaknesses described above. Implementing the plan will result in stronger homes that can better protect the lives of building occupants during ground shaking and more likely provide safe shelter after the ground shaking stops.

Lower Repair Costs
After the [Bay Area] Loma Prieta, California, earthquake of 1989, the average cost to repair houses damage due to structural weaknesses described above was $25,000 to $35,000 [Retrofitting costs are substantially less expensive].

Less Damage to Utility Connections
Gas, power, and water service connections can break if there is earthquake-induced displacement between the building and the ground. The shifting of the building’s structure off of its foundation may damage power lines and cause breaks or leaks in gas and water pipes.

Availability of Home Retrofit Loans
Lenders recognize the importance of protecting property investments, and some have developed special loans for homeowners who retrofit using the Standard Home Earthquake Retrofit Plan minimum standards.

Improved Chance of Obtaining Earthquake Insurance
Many insurance companies require that the common structural weaknesses noted above be strengthened as one condition for obtaining home earthquake insurance. Completing these strengthening measures does not guarantee that earthquake insurance will be available. Some insurance companies choose not to offer earthquake insurance at all, and some may have unique qualification standards.

 Planning a Seismic Retrofit
Mr. Handyman provides retrofitting services for houses which conform with the standard SHER planset requirements. Mr. Handyman retrofitting services include the following:
Completing a home assessment checklist to determine if your home qualifies to be retrofitted using the standard plan.

Prepare an “earthquake retrofit plan” including an annotated site drawing.
Apply for/obtain a building permit from the Seattle Department of Planning and Development based on the earthquake retrofit plan.
Execute retrofit plan:

1. Anchor the sill plate to the foundation wall;
2. Strengthen the pony walls between the foundation and the first floor;
3. Connect the pony walls to the floor framing above and to the sill plate below or connect the floor joists directly to the sill plate.

While Mr. Handyman can help you with all of the above, we recognize that some people like to do some of the work themselves. If you have the time, steps 1 through 3 above are areas which, with a bit of study, you can likely accomplish yourself. Please give us a ring to discuss what aspects of your retrofit project you would like our aid with and we would be happy to accommodate your needs.

About Mr. Handyman
Mr. Handyman is a professional home repair and maintenance service providing repair and carpentry services to home and business owners. Mr. Handyman bills primarily on a time and materials basis so our clients pay only for the services they get. We are bonded, licensed and insured and we run thorough background checks on all our technicians to ensure the security of our customers. Our technicians have at least 10 years of experience, primarily in carpentry. Finally, our technicians are direct employees (not contractors), which ensures their focus on your project and excellent quality control in general.

Mr. Handyman of Seattle is part of a national franchise based in Ann Arbor, Michigan and is one of seven Puget Sound-area franchisees who provide service to all Seattle, the Eastside, Snohomish, Kitsap, Skagit, Whatcom and Pierce Counties.  You can read more about us at www.mrhandymanwa.com.