Earthquake Hazards: Tips for safety-checking your home and workplace

By Marie Barron, Designer & Lorianna Kastrop, Vice President, The Kastrop Group, Inc., Architects

The October 17th anniversary of the Loma Prieta earthquake and the recent Napa Valley earthquake remind us that we should never take earthquake safety for granted. Below are some suggestions for researching your home or workplace or any building that you spend significant amounts of time in, such as gyms, workshops, etc. The California Building Code is very stringent regarding earthquake safety, but its objective is to prevent catastrophic building failures, such as collapse. It doesn’t prevent building damage, nor can it take into account what has been installed in the building, such as furniture. Therefore, we should not be complacent about buildings, even new well-designed buildings, to protect us in the event of a major earthquake.

Start by finding out what types of buildings you live and work in. In particular, some building types are known to be hazardous in an earthquake—unreinforced masonry buildings and soft-story buildings. The following definitions are from Wikipedia:

“An unreinforced masonry building (or UMB, URM building) is a type of build where load bearing walls, non-load bearing walls or other structures, such as chimneys are made of brick, cinderblock, tiles, adobe or other masonry material, that is not braced by reinforcing beams…One problem is that most mortar to hold bricks together is not strong enough. Additionally, masonry elements may “peel” from the building, and fall onto occupants or passersby outside.

In California, construction of new unreinforced masonry buildings was prohibited in 1933, and state law (enacted in 1986) required seismic retrofitting of existing structures. ..The California law left implementation and standards, up to local jurisdictions. Compliance took many years. As of 2008, most (but not all) of the unreinforced masonry buildings have undergone retrofitting.”

“A soft story building is a multi-story building in which one or more floors have windows, wide doors, large unobstructed commercial spaces, or other openings in places where a shear wall would normally be required for stability as a matter of earthquake engineering design. A typical soft story building is an apartment building of three or more stories located over a ground level with large openings, such as a parking garage or series of retail businesses with large windows.

 

Earthquake Hazards
Soft story partial collapse due to inadequate shear strength at ground level, Loma Prieta earthquake

 

Buildings are classified as having a “soft story” if that level is less than 70% as stiff as the floor immediately above it, or less than 80% as stiff as the average stiffness of the three floors above it. Soft story buildings are vulnerable to collapse in a moderate to severe earthquake a phenomenon known as soft story collapse. The inadequately-braced level is relatively less resistant than surrounding floors to lateral earthquake motion, so a disproportionate amount of the building’s overall side-to-side drift is focused on that floor. Subject to disproportionate lateral stress, and less able to withstand the stress, the floor becomes a weak point that may suffer structural damage or complete failure, which in turn results in the collapse of the entire building.

Soft story failure was responsible for nearly half of all homes that became uninhabitable in California’s Loma Prieta earthquake of 1989, and was projected to cause severe damage and possible destruction of 160,000 homes in the event of a more significant earthquake in the San Francisco Bay Area, California. As of 2009 few such buildings in the area had undergone the relatively inexpensive seismic retrofit o correct the condition.”

In addition to seismic retrofits for soft-story buildings or unreinforced masonry, it is a good idea to have a structural engineer or licensed contractor check that the foundation of your home is in proper condition and that the building is correctly attached to the foundation. The City of San Mateo has listed the following hazard reduction instructions for homes:

“Prescriptive Earthquake Hazard Reduction of Existing Light Wood-Framed Residential Structures Not More Than Three Stories in Height”:

  • Replacement of existing footings and stem walls that are deteriorated, cracked or unreinforced.
  • Anchor bolt installation steps:
    • Concrete should be replaced if it is not fully cured, hardened, and free of cracks
    • Drill holes in the concrete in compliance with the Research Report and manufacturer’s recommendations
    • Drill holes in the wood through existing sill plates with a minimum edge distance of 1 ½ bolt diameters. Anchors or bolts must be placed within 9-12 inches from both ends of all sill plate members
    • Adhesive anchors are necessary for concrete of weaker quality
  • Source: http://www.cityofsanmateo.org/DocumentCenter/View/227

For interior hazard reduction, homeowners and employees should do an initial inspection of their living or working environment and take the following proactive steps suggested by the U.S. government:

Strategies:

  • Fasten shelves to walls.
  • Keep high shelves clear of large or heavy objects. Instead, place them on lower shelves.
  • Store breakable items in low, closed cabinets with latches.
  • Secure hanging items (e.g. framed items, mirrors) to the wall with closed hooks or earthquake putty.
  • Keep heavy or breakable objects clear from beds, sofas, or other places you might be seated, including mirrors, large picture frames, etc.
  • Anchor bookcases, filing cabinets, china cabinets, and other tall furniture to wall studs (not drywall) or masonry with flexible straps that allow them to sway without falling to the floor. It is especially important to secure your water heater, refrigerator, and furnace and gas appliances to the studs and possibly bolting them to the floor, if applicable. A trick to find wall studs or joists is to slide a strong magnet along the surface of the wall. It will hold where there are nails or screws in the studs.
  • Secure expensive electronics (e.g. computers, televisions, microwave ovens) with flexible nylon straps .
  • Brace overhead light fixtures and top-heavy objects.
  • Install flexible pipe fittings, which are more resistant to breakage, to avoid gas or water leaks.
  • Repair any deep cracks in ceilings or foundations, and make sure to get professional help if there are any signs of structural defects.
  • Store weed killers, pesticides, and flammable products securely in closed cabinets with latches and on bottom shelves.
  • Locate safe spots in each room (e.g. under a sturdy table or against an inside wall) that you can get to in the case of an earthquake.

Nonstructural solutions to strengthening your home’s earthquake safety:

  • “Install foundation bolts
  • Brace cripple walls.
  • Reinforce chimneys. (Older brick chimneys are usually not properly braced to withstand earthquakes, and have been a significant source of building damage in recent earthquakes.)
  • Install an earthquake-resistant bracing system for a mobile home.”

Source: http://www.ready.gov/earthquakes

More Strategies:

  • Secure kitchen cabinets with “child-proof latches, hook and eye latches, or positive catch latches designed for boats.”
  • Have flexible connectors on all gas appliances to reduce the risk of fire.
  • Secure televisions, stereos, computer, microwaves and other heavy/costly electrical appliances with flexible nylon straps and buckles for easy removal and relocation.
  • Use hook and loop fasteners or “non-damaging adhesives such as earthquake putty, clear quake gel, or microcrystalline wax” to secure breakables (e.g. lamps, pottery objects, and collectibles) in place so that they do not become deadly projectiles during an earthquake.

Common Building Problems that Present a Danger:

  • Inadequate foundations. Look for damage and check that bolts in the mudsills are no more than 6 feet apart in a single story and 4 feet apart for a multistory building.
  • Unbraced cripple walls. There should be plywood panels connecting the studs of short “cripple” walls.

Source: http://www.cityofsandimas.com/download.cfm?ID=27376

Another source with diagrams outlining some of the content mentioned above: http://www.seattle.gov/emergency/library/mitigation/NS-Guide-is-your-home-protected-update10-25.pdf

By attending to these hazards, you will be better prepared for the next big earthquake and perhaps save lives and property. There is a lot more information available online and from professionals in architecture, structural engineering and construction. If you suspect that you have an earthquake hazard, and you have questions about it, don’t hesitate to contact local professionals for their advice.   As noted above, many of the remedies are relatively inexpensive and would provide not only peace-of-mind, but life-safety protection.

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