Monday, January 25, 2010
Renovating? Upgrade home's earthquake resistance
By Ann Robinson and Annie Schwemmer
Architecture is an interesting combination of art and science. In this column we usually feature the more visual side of our profession, but physics also has a lot to do with a remodeling project.
How a house is constructed and what makes it structurally sound is every bit as important as how beautiful it looks.
Lately we have been watching the devastation of the earthquake in Haiti, and even though it is thousands of miles away, we are reminded that there is a real possibility that this sort of thing could someday occur much closer to home.
According to the Utah Guide for the Seismic Improvement of Unreinforced Masonry Dwellings, the probability of a large earthquake occurring along the Wasatch Fault during the next 50 years is about 1 in 5 — or a 20 percent chance.
That is reassuringly low. But it doesn't mean that smaller earthquakes can't do damage to your home.
Utah experiences about 700 earthquakes every year, with about 13 measuring 3.0 or above on the Richter scale. A moderate, potentially damaging earthquake has a magnitude ranging from 5.5 to 6.5, and one occurs in Utah about every seven years.
Earthquake-related or not, the Utah Geological Survey says geological processes (landslides, rock falls, ground cracks and liquefaction, for example) damage hundreds of houses in Utah every year.
With that said, we would like to recommend some ideas for seismic renovations. If you are already doing a remodeling project, you might as well protect your home and your family with a few additional upgrades.
While there is no practical way to completely "earthquake-proof" your house, 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 and the City and County Building 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.
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 (sideways or twisting) forces as well as to 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. Strengthening these 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 replaced, so these walls do not look any different from any other wall.
A structural engineer can analyze your home to determine which walls will function best as shear walls.
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.
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 together to work as a "team" instead of individually separating from each other in a collapse.
One technique to accomplish this is to add a concrete bond beam (about 8 to 12 inches 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.