Did you know that in Utah there are little earthquakes all the time? Actually, according to earthquaketrack.com, Utah experienced 16 earthquakes last month and 495 last year.
These are obviously small tremors, usually below magnitude 3.0. However, the probability of a large earthquake along the Wasatch Fault during the next 50 years is 1 in 4 or a 25 percent chance, according to the University of Utah Seismograph Stations. As far as your house is concerned, even the small earthquakes have the potential to cause some damage.
So what can a homeowner do to face the power of Mother Nature? One thing is to discuss the option of earthquake insurance with your agent. The type of construction of your home (framed vs. masonry) and your location are key elements that determine if your house will qualify and how much the premium will be. In addition, if you are planning a renovation, there are a few smart seismic upgrades that are in the “while you are at it” category.
It is important to remember there is no way to completely “earthquake-proof” your house. However, engineers have come up with some extreme and expensive methods to help a building survive an earthquake.
This shows a concrete bond beam after it was poured to add reinforcement to an unreinforced concrete wall as a seismic upgrade. (Annie Schwemmer, Renovation Design Group)
As we have discussed in a previous column, one of these techniques is the use of base isolators, which act as giant springs installed under the foundation of a building. This is the solution which was used on the Utah State Capitol, the Salt Lake City and County Building, and the Salt Lake Tabernacle of The Church of Jesus Christ of Latter-day Saints. The historic significance of these buildings justified the great effort and money expended to retrofit these structures.
This seismic upgrade includes steel reinforcing and forms for a concrete bond beam on top of an unreinforced masonry wall. (Annie Schwemmer, Renovation Design Group)
A seismic upgrade on your home, however, isn’t about saving the structure, but focuses on preventing the structure from collapsing and trapping or killing the people inside.
The intent of an upgrade is to strengthen each part of the house so it resists lateral (sideways or twisting) forces, as well as to provide connections to transfer the forces from one element to another down to the ground where they can dissipate.
Horizontal forces on the chimney need to transfer to the roof structure; roof forces must transfer to the walls, which also receive the forces from the ceiling and floors; the forces from the walls must transfer to the foundation and back into the ground. If any of these parts are weak or poorly connected, the structure is in danger of failure and collapse.
Because the walls take forces from several other building parts, certain walls need to be strengthened. These are known as shear walls since they are meant to absorb a greater share of the lateral or shear forces.
Each building ideally has a minimum of two shear walls that run perpendicular. Strengthening existing walls involves removing the gypsum board or plaster and installing plywood panels over the studs with a required pattern of nailing. The sheet rock or plaster is then replaced, so these walls will not look significantly different from any other wall. A structural engineer can analyze your home to determine which walls will function best as shear walls.
Tying the building together is another weapon against earthquake damage. There are special metal pieces manufactured to connect various building parts. Anchor bolts tie the walls to the concrete foundation, along with metal straps that are used to hold down the floors, ceilings and roofs to the walls. A professional contractor can add these pieces to an existing house, but it often requires opening up the walls, floors and ceilings to physically connect the key structural pieces of the home.
This type of seismic upgrade is generally done in connection with a home remodeling project when the house is going to be torn up anyway.
Masonry buildings are the most vulnerable to earthquake damage. There is no framed wall to connect to the floor or roof, so the approach is to encourage the brittle individual bricks to work together as a “team” instead of individually separating from each other in a collapse.
Adding a concrete bond beam around the top of the wall is one technique. This is a continuous pour of concrete as wide as the masonry wall, about 8 to 12 inches high, with rebar inserted into the existing masonry units to tie them to the bond beam. Since this requires the removal of the roof, it is also done in conjunction with a major remodeling project.
Outside of the more drastic approaches of strengthening shear walls or tying your building parts together, there are some practical steps for homeowners to prepare for an earthquake. Secure large furniture to the wall with metal brackets and fasten smaller objects in place with putty. Install flexible pipe fittings to avoid gas or water leaks, and strap the water heater to the wall to prevent it from tipping.
Of course, the Internet is chock full of information on this subject. Many agencies have addressed earthquake preparedness, including the United States Geological Survey, the Red Cross and the Utah State Government, which has a website called bereadyutah.gov. FEMA has two publications with useful information: “Homebuilders’ Guide to Earthquake Resistant Design and Construction” applies if you are considering building a new home and “Earthquake Safety Guide for Homeowners,” that speaks to retrofitting an existing home.
There is no doubt that we live in earthquake country, so we would be wise to educate ourselves and take whatever steps are feasible for our individual situations. Your home is probably your biggest investment, so take a little time to consider how to best prepare to protect your assets and your family.
Ann Robinson and Annie V. Schwemmer are the principal architects and co-founders of a residential architectural firm focused on life-changing remodeling designs at RenovationDesignGroup.com. Send comments or questions to ask@RenovationDesignGroup.com