Be Prepared - Types of Home Structures and the Earthquake Protection They Need

Foundations
Additions and Remodels
Engineering
By Contractors.com Team April 30, 2021

The planet we live on is a very dynamic place. It is always in a state of change. But with dynamism comes unpredictability. Unpredictability in nature is sometimes good. It can make the sky bright and colorful immediately after a thunderstorm, or make the flowers bloom sooner. But this means that the bad is unpredictable as well. As much as we’d rather not think about it, over a hundred million of us live in areas of the United States that are prone to earthquakes. These seismic events can come at any time. You don’t feel most of them, but some of them might knock over your dishware. An even smaller number of these quakes might even knock down your house. 

But while nature is unpredictable, we don’t have to be. There are many ways that we can prepare ourselves for the next “big one”. Regardless of what kind of house you live in, there are measures you can take to make sure it withstands mother nature’s shaky test. You can make your home earthquake-safe, but being prepared means retrofitting your home’s structure. In this article, we’ll cover building construction types and how a structural engineer can complete an earthquake retrofit. 

Soft-Story Building

Do You Know What Kind of Earthquake Retrofit Your House Needs?

Do You Know What Kind of Earthquake Retrofit Your House Needs?

These buildings are known for having first stories which are structurally weaker than the floors above. They are a type of wood-frame home structure and have large unreinforced openings in their ground floors. This allows these buildings to have large windows, spacious lobbies, and sometimes even parking lots. Unfortunately, these large openings are huge seismic weaknesses. The main forces which bring down a building are lateral (horizontal), and soft-story buildings don’t deal with this well because of their weak ground floors. 

There are over 10,000 such buildings in San Francisco, and experts predict a large earthquake in the Bay Area would destroy 80% of them. This could be prevented with retrofits. One of the easiest ways to strengthen a soft-story building without taking away too much of its space is to reinforce the existing walls. This can be done by adding shear walls, or by reinforcing weak parts of the building with a steel frame. Steel is a good aseismic material because it can bend without breaking, allowing it to resist the horizontal forces of an earthquake. This would bring the strength of the ground floor to 80% of that of upper stories, which is enough for the structure to survive an earthquake. 

Unfortunately, there is a major caveat with soft-story retrofits, and that’s cost. Retrofitting a small soft-story building would range somewhere in the tens of thousands of dollars. A large soft-story building could cost more than $100,000 for a retrofit. Some city governments have tried to incentivize homeowners to retrofit their homes by taking the edge of this cost. If you live in California, you’ll be happy to know that many cities in the Golden State are happy to waive zoning requirements and give tax incentives to homeowners who choose to retrofit.   

Wood-Frame Houses

What Kind of Earthquake Retrofit Do Wood Frame Houses Need

What Kind of Earthquake Retrofit Do Wood Frame Houses Need

Wood-frame houses are the mainstay of American suburbia. They are affordable and can be built quickly. But while soft-story buildings may be the kind of wood-frame structure most vulnerable to earthquakes, they aren’t the only ones. The truth is that all wood-framed houses need some form of retrofitting to be earthquake-safe. This is especially true for homes built before the 1980s. The problem is that a wood-frame home usually isn’t connected to its foundation. Because of this, the seismic forces that shake your home up and down and side to side will cause the home structure to buckle and twist even if the foundation doesn’t move. 

Several things can be done to prepare a wood-frame home for an earthquake. The main thing to do is have your foundation bolted by a structural contractor. This anchors the building’s upper structure to the foundation and keeps it from shifting excessively. There are various ways of doing this, and the cost of the project depends on your home’s design and age. Prices start at $500 for a home sitting on a slab foundation and go up to $5,000 for homes with a full basement. Before foundation work can be done there are some home improvements you’ll need to check off your retrofitting checklist first. 

It is recommended that you strengthen your wooden frame by reinforcing the connections between structural beams. Any wood-eating pests will have to be eradicated, and any structural damage done by them repaired. A home structure compromised by termites isn’t going to do well in an earthquake even with a bolted foundation. Also, you need to check that your home has a strong foundation that is in good condition. A foundation made of reinforced concrete is the best option, as brick or unreinforced concrete foundations are more likely to fall apart in an earthquake. Cripple walls are another concern. Any wood-frame home built before the 1980s will most likely have one, and they will need to be braced with plywood sheathing.  

Tilt-up Homes Built Before the Late 1990s

Tilt-up Homes Built Before 1990s and Earthquake Protection

Tilt-up Homes Built Before 1990s and Earthquake Protection

Tilt-up buildings became especially popular in the past thirty years because they are easy to build affordably. They are called tilt-up buildings because the outer walls of the concrete structure are “tilted up” and held in place until the roof and floors are installed. The problem with these buildings is that they aren’t designed to deal with earthquakes. Tilt-up structures built after the late 1990s, when more strict regulations came into play, are stronger. But buildings built before these regulations are still at risk. The main problem with tilt-up buildings is that their roof-to-wall anchorage systems are not strong enough to withstand quakes. During an earthquake, the roof of a tilt-up structure can easily separate from the wall, causing a partial or complete collapse of the structure. Some states such as California are beginning to require that tilt-up buildings be seismically retrofitted

Fortunately, retrofitting a tilt-up building is relatively simple to do. A licensed contractor can complete an earthquake retrofit of a tilt-up’s structure by strengthening the connections between the walls and the roofing. Additional structural strength can be added by reinforcing the building’s frame with steel beams. As with most retrofits, however, the costs are significant. Tilt-up retrofits cost between $3 and $10 per square foot, while the most economical tilt-up buildings are 5,000 square feet at minimum. Thankfully, many states realize this and are offering tax credits to homeowners for seismic retrofits of their tilt-ups.

Non-Ductile Concrete Buildings

Earthquake Retrofitting Concrete Structures

Earthquake Retrofitting Concrete Structures

Something which is ductile can stretch without losing structural integrity. In other words, it’s pliable, but not brittle. Non-ductile concrete buildings, as their name implies, aren’t good at being pliable. That’s bad news if these buildings stand in earthquake country since a quake can easily cause major structural damage. For this reason, these buildings were basically outlawed in California in 1976. However, over ten thousand of these buildings had been built in California by that point including apartment blocks and office buildings. 

This was part of a nationwide concrete highrise construction boom that began in the 1900s. One thousand of these buildings are in Los Angeles. So it’s no surprise then that the Los Angeles Department of Building and Safety (LADBS) has an earthquake retrofit program that requires seismic retrofitting for these brittle buildings. What makes these buildings “non-ductile” is the presence of concrete instead of steel reinforcement. These concrete highrises just don’t have enough steel rebar to resist the sideways movement caused by quakes. This makes this type of concrete building inflexible, and they’re therefore more prone to major damage or collapse.  

The key to retrofitting a non-ductile concrete building is by adding steel reinforcement to the building’s structure. This will be different for each individual building. For this reason, the owner of the structure will have to hire a contractor to determine the specific steps needed to bring the building up to code.

Written by
Contractors.com Team

Written by Contractors.com Team