What You Can Do
This porch on a wood-frame house failed during the 1989
magnitude 6.9 Loma Prieta earthquake. The "red tag"
indicates that this home is unsafe and must not be entered
or occupied. (USGS photo)
The second type of damage is nonstructural. This type of damage does not affect the viability of the structure, but may still prevent people from reoccupying the structure after an earthquake.
Is your house, condo, or apartment strong enough to withstand an earthquake?
The Structural Engineers Association of Northern California contributed to the writing of the new booklet entitled "Putting Down Roots in Earthquake Country." Here is some of the information from this booklet about the safety of homes and other residential buildings and their ability to stay strong in earthquakes.
If you live in a single-family home or duplex...
Answer the quiz below to see if your home is likely to be so badly damaged in a future quake that people might be injured or that it would be unsafe to occupy. If you answer "no" to any of these question, you probably should have a structural engineer, architect, or contractor evaluate it unless it has been strengthened in the last few years. They will check to see if it is strong enough to keep you and your family reasonably safe in a quake. For example:
- Does your home have an adequate number of bolts connecting the "sill plate" to the foundation?
- Is there plywood on the inside surface of the crawl space extending from the sill plate to the base of the floor joist above to prevent the wall studs from collapsing?
- Are there metal brackets connecting the rim joists to the top plates?
- Is the ground floor a large open space lacking interior walls (weak or "soft" story)?
- Are there large openings in the walls of the lower story, such as a garage door, that should be better braced?
- Is your home a hillside house that was not adequately designed to withstand strong earthquake shaking?
What are Common Structural Deficiencies?
The most common structural deficiencies in houses are related to the foundation - the main support system for the dwelling. Houses that are not adequately anchored or bolted to the foundation have the potential to slide off the foundation during the earthquake, severely damaging or destroying the building.
Strengthing your crawl space
What are Common Nonstructural Deficiencies?
Even if a house is structurally sound, items inside the structure might have shifted during an earthquake and caused enough damage to render the house uninhabitable. The most common items that can cause this kind of damage are un-braced water heaters, masonry chimneys, tall shelves and tall file cabinets. Not only can these elements cause damage to the property, any toppling could also cause severe injury.
Fewer than 10% of homeowners have taken steps to retrofit their homes-Is your home bolted to its foundation? If you live in an older building, has it been retrofitted? Is your water heater strapped? Could unsecured furniture or objects fall and cause injury or damage?
How Does One Fix These Problems?
The good news is that these kinds of basic problems in a house can be remedied through easy and inexpensive means. Nonstructural deficiencies, for example, can usually be mitigated (or protected against failing during an earthquake) with the use of simple metal clips and straps bought at a local hardware store.
Structural Engineers examine the
seismic anchorage and bracing of
critical building equipment
Once you determine if your home needs retrofitting, identify problems, prioritize how and when to fix them, and get started! The latest recommendations of structural engineers, contractors, and city building officials who are experts on retrofitting are available on the ABAG web site at http://quake.abag.ca.gov/fixit/.
Strengthening your crawl space
The number of foundation bolts, linear feet of plywood, and floor-to-wall connections (brackets) that are required to seismically retrofit your home varies depending on its size and weight. Remember, earthquakes will find the weak spots in your house. So, if you add bolts but not plywood, you may still have a problem when the ground shakes.
Structural-Safety Quiz for Single-Family Home or Duplex If you live in a single-family home or duplex, the strength of your home depends on when it was built, its style of construction, and its location.
1. When was your home built?
- Before 1960 = 5 points
- 1961-1978 = 3 points
- After 1978 = 1 point
2. How tall is your home?
- 2 or more stories with living area above a garage = 5 points Split level, on a hillside or gentle slope = 6 points
- 1 story, 3 or more steps up to the front door = 4 points
- 1 story, less than 3 steps up to the front door = 1 point
3. How hard is the ground likely to shake under your home?
- Portions of the Bay Area shown as yellow or green in color on the shaking hazard map (page 8) = 5 points
- Elsewhere in the Bay Area = 7 points
TOTAL POINTS =_____
If your home scores 13 or more points on the quiz, you probably should have an structural engineer, architect, or contractor evaluate it unless it has been strengthened in the past few years.
If you live in a condominium or apartment...
Many condominiums and apartments have parking on the ground floor. These weak or "soft" first stories may lean or collapse in an earthquake. Some multistory buildings in the Bay Area can have problems because they were constructed before 1972 of concrete or brick that is inadequately reinforced. Many cities have requirements that these buildings be seismically retrofitted. You are less likely to be killed in a retrofitted building, but you may not be able to reoccupy it after a quake.
As a renter, ask your landlord these questions:
- What measures have been taken to ensure the seismic safety of this building?
- Have water heaters been strapped to the wall studs?
- Can I secure bookshelves and furniture to the walls?
Go to http://quake.abag.ca.gov/fixit/ to take a quiz to see if your apartment building or condominium may need retrofitting. This Web site also has links to information that can help your landlord find appropriate ways to improve the strength of your building.
If you live in a mobile home...
Look under your home. If you only see a metal or wood "skirt" on the outside with concrete blocks or steel tripods or jacks supporting your home, you need to have an "engineered tie-down system" or an "earthquake-resistant bracing system" (ERBS) installed.
An ERBS should have a label on the bracing that says, "Complies with the California Administrative Code, Title 25, Chapter 2, Article 7.5."
Brick chimneys can collapse if shaken...
You can reduce the chance of bricks falling through a sheetrock ceiling in a quake by putting sheets of plywood above ceiling framing. However, "retrofitting" masonry chimneys with bracing or strapping is not an effective safety measure, because they may still fall as a unit when exposed to strong shaking.
Here are some other ideas for residences, offices and other commercial spaces:
- Meet with an insurance agent to determine if special earthquake insurance might be appropriate for your building and/or business.
- Locate utilities systems' shut-off valves and place appropriate tools in each location for shut-off in the event of an earthquake.
- Secure valuable pieces of equipment by bolting them to the floor and secure desk-top equipment like computers with strips of adhesive material such as Velcro.
- Develop an emergency plan. The emergency plan should include training family members and employees to resist the urge to immediately leave the building. Falling bricks, architectural elements or window glass may still present hazards.
- Review with family members and/or employees on a regular basis.
- Have a first aid kit, fire extinguishers in appropriate locations, and a "disaster kit" including drinking water, flashlights, batteries, a radio, and other items.
- Keep duplicate copies of critical family and business documents off site.
- Place a list of emergency numbers at telephone stations and keep a list at home in case the building is inaccessible.
Re-occupying the Building
After an earthquake, city officials, building owners or tenants may be apprehensive about reopening a building because of liability and safety concerns. Conversely, they may wish to reopen as soon as possible to minimize business losses or to demonstrate the rebuilding process.
Damage Assessment: Immediately after the earthquake a process of inspecting structures by building inspectors, structural engineers, architects or qualified volunteers should begin. Each building should be inspected for its potential threat to public safety and tagged to signify its access potential for re-occupancy.
The color codes represent the following:
Red Tag Do not occupy. This does not mean that the building must be demolished; it merely means that a hazardous condition exists. A further detailed evaluation by a structural engineer must follow.
Yellow Tag Limited Occupancy
Green Tag Inspected and approved for occupancy.
If the building is given a red or yellow tag, additional repairs or shoring will be required before the building can be re-occupied. Even a green-tagged building may not be able to reopen until utility services are restored, which may take several days or weeks.
Be sure to consult with a structural engineer before re-occupying a damaged building.
Long Term Repair and Strengthening versus Demolition
One of the most common misconceptions about earthquake-damaged buildings is that they must be demolished or that demolition is required because of an administrative or a funding deadline.
Most buildings, despite damage, may be more economically repaired than demolished and reconstructed. It is important that the building owner not rush into making a decision as funding deadlines approach. They can often be extended. The owner should carefully consider the impacts of the decision in all of its aspects.
Preparation for Aftershocks: Continuing aftershocks are likely to occur after a major earthquake. These aftershocks can further damage buildings weakened or damaged by the initial shock. Where possible, the damage assessment should indicate needs for interim shoring of unsafe elements such as cornices, damaged walls, etc.
Re-entry of Building to Remove Contents; This is often possible even on red-tagged buildings under controlled conditions.
Demolition Alternative -Factors to Consider:
Rebuilding Costs: This alternative involves not only the cost of demolishing the damaged building, but the further costs of building a replacement structure. While certain types of disaster assistance will be available to defray these costs, other disaster-assistance programs, such as those administered by the State Office of Historic Preservation and the National Trust for Historic Preservation, are available only for designated historic structures. These will not be available for replacement structures.
Further, where market conditions are weak, commercial loan funding for replacement structures may be difficult to obtain. It is important to take the time to seek out second opinions of both engineering and building issues from qualified professionals.