New map from the US Geological Survey showing probable areas of seismic activity.
[The recent earthquakes in Japan and Ecuador are yet another warning that it's time to prepare for the inevitable. It turns out that seismic activity in the US isn't just a California phenomenon, but also occurs in the Northwest, Midwest, and even Washington, D. C. -- remember the magnitude 5.8 quake that damaged the Washington Monument on August 23, 2011! Find preparedness plans, like the seven step approach shown here, and other comprehensive earthquake info at the US Geological Survey where you can download their very informative brochure Living On Shaky Ground. In this post, contractor David Jacknin offers some basic advice for building a home that is better prepared to withstand a major seismic event. -- Editor Dan Gregory]
New homes built in earthquake prone areas are subject to
rigorous building codes. These codes are designed to protect the occupants from
loss of life in a seismic event. It is
advisable to exceed these minimum standards and take precautionary
measures to minimize earthquake damage. Here are general guidelines to consider and specific actions you can take to get your house ready for a seismic event.Site Selection – Know
your soil type.
Houses built on bedrock
will generally minimize the damage from an earthquake while houses built on
sedimentary soils, fill or clay will encounter the maximum earthquake
displacements. When building in an earthquake zone, consult a soils engineer before proceeding.
house to the mudsill.
Your house needs to be connected to the
mudsill. This is important to prevent sliding and
uplift that occurs with both vertical and lateral forces.
very important. In California, these are
installed every four feet and connect framing members to the foundation. The idea is to have a continuous connections
from the foundation all the way to
the roof rafters (hold-down image courtesy Mcvicker.com
).Install shear walls.
These are your principal
means of offsetting horizontal/lateral
forces. They are most important in the cripple wall area (below the first
floor). Shear walls are created by covering
your exterior walls in plywood and nailing the bottom, top plates and every
stud 6” on center (see your local building code). This creates a plywood gusset that resists
lateral forces. The thickness of the plywood, nailing pattern and size of the nails are
specified by your local building codes.
Shear walls should be placed on all four corners of the
house for equal strength in each direction. They should be added around all openings such as doors and windows. Your engineer or architect can help you with
details (image courtesy Association of Bay Area Governments
or remove chimneys.
are very unstable and can crash through your roof and ceiling. Consider bracing or removing the masonry chimney and replacing with a metal flue. Though expensive, this measure
save lives as well as save considerably on repair costs.
Split level and houses built on steep
risks from weak connections between floors and between sections of the
house. The house may be well connected
at the foundation level while an upper floor or section of the house may move
independently from the rest of the house. Connections can be improved by using mechanical fasteners and plywood gussets to unite overlapping floors and wings of the house.Roof Connections.
In keeping with the principal of
connecting all framing members from foundation to roof, it is important to connect rafters to wall
top plates. Hurricane ties and other
mechanical connectors are available for this purpose (example shown below courtesy Simpson Strong-Tie
). Reinforce Large
Finally, if your house
includes large openings such as openings around garage doors, your engineer may recommend such measures as
a moment frame, which is a steel structure that
offsets lateral forces. All such measures will help offset the lateral and uplift forces that houses encounter in a seismic event.
architect, engineer and local building department have the tools to help you plan
and design for such an event.