If an earthquake hits, damage to public structures and infrastructure will be beyond the control of the average citizen, but you do have some control over your individual circumstances in relation to your own home. In our continuing series, we are discussing steps you can take to increase the chance for your family's survival and to reduce damage to your home. The first step is to evaluate your home to see where potential problems exist.
We are familiar with gravity and can understand the need for the foundation and walls of a structure to be strong enough to support the floors and roof. They also need to be strong enough to support the "live load" of a home, meaning the equipment, furniture and people that exist inside it. In Utah, we must figure in a snow load to account for possible weight on the roof during the winter months.
Earthquakes may disrupt a home's gravity load in rare instances where the ground falls away from beneath the home and gravity pulls it down. Typically, the greater danger to a structure comes from the earth shaking and moving from side to side. This produces lateral forces or twisting, which structural elements used for supporting gravity loads have little strength to resist. Seismic upgrades involve strengthening a home's ability to resist these lateral forces.
First, look at the materials used to build your home. Concrete is a mixture of sand/gravel, water and cement. It results in a wall that is strong for holding gravity loads but very brittle and unable to deal well with twisting or lateral loads. Masonry walls (i.e., bricks, cinder blocks, stone, etc.) have similar characteristics. Both these types of wall need steel bars and/or steel mesh inserted into the wall to bond the pieces together so they can work as a unit to resist seismic forces. While building codes now prohibit unreinforced masonry structures, these were allowed before 1970. Therefore, older homes with unreinforced masonry and concrete foundations and walls are most vulnerable to earthquake damage.
Many "brick homes" built after the middle of the last century are really wood frame homes with bricks added to the outside for appearance's sake. This brick veneer may crack and fall off during an earthquake, but it is only decoration; the wood framing is the actual structure of the home. Wood frame structures can usually weather a seismic event better than masonry structures.
The height of a wall or an element of your home is also significant. Generally speaking, the higher something is, the more unstable it will be. An example would be a chimney. Chimneys are often unbraced and very vulnerable to lateral forces. The weight of the falling masonry is a danger to the roof structure or anyone below it inside or outside the home.
Atypical construction methods may also put you at risk. For instance, corner windows, popular in the 1940s, usually have a steel pipe at the outside corner under a steel lintel used to support the wall above. This pipe is not always connected to the steel lintel, which causes a weakness in the wall. Obviously, if the corner of a home fails then walls and at least part of the roof come down with it.
Next week we will talk about methods to strengthen various parts of your home to resist seismic damage.
Architects Ann Robinson, AIA, and Annie Vernon, AIA, are founding principals of Renovation Design Group, www.RenovationDesignGroup.com, a local architectural firm specializing in residential remodeling design.