By Ann Robinson and Annie Vernon
Remember there is no practical way to completely “earthquake-proof” your house. But engineers have developed some extreme and expensive methods to equip a building to ride out an earthquake. One of these techniques, which was actually used on the Utah State Capitol building and the LDS Tabernacle in Salt Lake City, is base isolators, which act as giant springs installed under the foundation. The isolators are actually a new footing beneath each column or structural support.
This concrete beam was poured at the top of an original 1920s unreinforced double-wythe masonry wall to help tie all the exterior walls together so they would be less likely to tip over in an earthquake. In this particular case, a new upper level.
A seismic upgrade on your home, however, isn’t about saving the structure, but, more important, 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 forces as well as provide connections to transfer the forces from one element to another down to the ground where it can dissipate.
Horizontal forces on the chimney need to transfer to the bracing and 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 to the ground. If any of these parts are weak or poorly connected, then the structure will fail and collapse.
Since 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 two shear walls that run perpendicular to each other. Strengthen these walls by removing the gypsum board or plaster and installing plywood over the studs with a required pattern of nailing. The sheet rock or plaster is replaced, so these walls do not look any different from any other wall. A structural engineer can analyze your home to locate your shear walls.T
This steel tie was installed to connect the exterior walls and roof structure together. With the roof and walls working together, the combined structure is much stronger than any of the pieces would be alone.
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 requires opening up the walls, floors, and ceilings to connect the actual 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.
As we discussed previously, masonry buildings are the most vulnerable to earthquake damage. There is no framed wall to connect to the floor or roof so you need to tie the brittle individual bricks together to work as a ‘team’ instead of individually separating from each other in a collapse. One technique is adding a concrete bond beam (about 8″ to 12″ high) around the top of the wall. Since this requires the removal of the roof, it is also done in conjunction with a major remodeling project.
Short of these rather drastic options, you can take some practical steps to prepare for an earthquake. Secure large furniture to the wall with metal brackets and fasten smaller objects in place with putty. Strap the water heater to the wall to prevent it from tipping.
Earthquake insurance can ease the financial burden, but nothing can pay for the loss of a loved one. Analyzing your home may lead to some action that will strengthen your structure or at least “earthquake proof” your family’s emergency plan. As always, we welcome your home architect design questions at firstname.lastname@example.org.