While many of us consider an earthquake to be a natural disaster, we can and should see it as a natural phenomenon and prepare to eliminate the disaster. Properly building a new house or retrofitting an existing one, can make the difference between an Earthquake being a disaster and simply an unpleasant event.

Note: in this blog post, we are focusing on inland earthquakes vs. a tsunami in which the damages it creates (e.g. flood) go beyond the damage of an inland earthquake.

We’ll explore the following:

• How Does an Earthquake Impact the Home?

• Load Path

• Environmental Aspects Impacting Design and Construction Decisions

• Earthquake-Resistant House

• Going Above Building Codes

• Spotlight

• Final Thoughts

• References

How Does an Earthquake Impact Our Homes?

During an earthquake, the ground experiences sudden, random, patterns of movements that apply forces on the house. In general, the two major forces are: 

• Lateral Forces - Side to side, also referred to as “shear” or “raking” forces

• Vertical Forces - Up and down, also referred to as “uplift” forces 

As the earthquake occurs, the ground starts to move, forcing the foundation of the house to move first. At that moment, the walls, roof, and upper stories try to remain still. By the time the roof starts to move in the direction of the foundation, the foundation is already moving back towards its initial position and the roof and foundation are moving in opposite directions. This phenomenon is also true during vertical forces, where opposing up and down movements occur. These opposite movements go on until the ground motion stops and the building movements slow down until they stop. If the forces have been strong enough, and the structure has weak spots that cannot withstand the forces (or loads), then parts, or all of the house, may displace from its foundation, tilt over, or collapse.

Load Path

In order for a house to remain intact and properly withstand the forces of an earthquake, “a load applied at any point on the structure must have a path allowing load transfer through each building part down to the building foundation and supporting soils.” (FEMA 232)

The idea is to have a continuous path of the load throughout the various elements of the house (roof, walls, floors). As the house moves, loads are applied to each element in turn or simultaneously. Each element needs to be strong enough to observe that force and pass it on, through the connections of the elements, all the way down to the foundation, or the ground. A weak element or connection that isn’t properly designed or constructed to withstand those loads will fail and may cause additional loads on other elements causing further damage.

According to FEMA, the roof, ceiling, floor, and bracing wall systems are the basic elements resisting earthquake loads. Therefore, adequate earthquake performance of a house relies on:

• Adequate strength of roof, ceiling, floor, and bracing wall systems.

• Adequate stiffness of roof, ceiling, floor, and bracing wall systems to limit deformation.

• Adequate connection between systems to provide a functional load path.

• Adequate connection to the foundation. 

Strengthening and connecting elements are important, but designing smart will also improve your home’s resilience and help create the desired continuous path of loads. 

Here are a few examples of such a design:

• Overall, the lighter the structure, the less load impact it will suffer. A light roof is favorable, especially if you prefer high ceilings. The floor of a second story, as well as partitioning walls, should be as light as possible

• Avoiding irregular house shapes. 

• Avoiding soft stories - a story that has dramatically fewer walls or supporting pillars than the floors above, including large windows. Avoid building a second story above a garage, due to its large opening and traditionally weak walls. 

• Choosing the right materials for each element on each floor is pivotal. The main properties of materials in earthquake engineering are:

  • Ductility: ductility is the characteristic of a material to bend or deform plastically before it fractures.

  • Strength: strength is the ability of a material to withstand forces, such as stress before it permanently deforms.

  • Stiffness: stiffness of a material is a degree of resistance to deformation, i.e. the ability of that material to return to its original shape once the force applied to it is removed. 

There are different types of wood, steel, glass with different characteristics (heavy, light, stiff, and ductile) that should be used in the proper location to have the right function in the overall house system.

Environmental Aspects Impacting Design and Construction Decisions

There are numerous factors that may impact design, materials, and construction decisions when building or retrofitting a house to make it earthquake-proof.

Here are the key factors that you may want to research or raise and discuss with the professionals you hire:

1. For both new homes and retrofitted homes:

   • Is the area considered an earthquake risk zone?

   • What is the type of soil and how it impacts the type of foundation and materials

   • The slope of the land and how it impacts the type of foundation and materials

   • The shape of the land and how it impacts the shape of the house (narrow rectangular, multi-story, do you need to build on top of the garage, etc.)

   • Is there a risk of potential landslides near the house?

   • Do old telecommunication structures or other utility structures nearby (e.g. old posts) pose a potential risk?

2. For retrofitted homes:

   • When was the home built? During the ’80s, new and stricter building codes were passed, therefore a house built after the ’80s may be more earthquake-proof than older houses.

   • Does the house have cripple walls sitting on the foundation? If so, they need to be properly braced with code-compliant sheathing and bolting.

   • Is the home built on a slab-on-grade or a raised foundation? A raised foundation (pier and beam), means the house is probably sitting on wooden poles that don’t run deep into the ground and the structure may be relatively weak.

   • Is the house built on a slope (e.g. hill)? If so, it probably means all, or part of it sits on a raised foundation, which may not be strong enough.

   • Were reinforcement connections established (foundation to floor and walls, roof to walls, etc.)? Is there a second story above the garage and was the garage strengthened? Is the chimney reinforced? Were the walls (cripple, mansory) strengthened or reinforced?

Earthquake-Resistant House

When building or retrofitting a house, you must comply with your state and local (county and city) building codes. Building codes are a set of standards that specify the minimum, baseline requirements (materials, design, measurements, etc.) to protect the health, safety, and welfare of building occupants and communities.

Building codes become laws when states and municipalities adopt them and make them mandatory. Once a law, building codes regulate many aspects of your house, mainly focusing on design, construction, materials, and alterations. Yet, adopted building codes are still regarded as minimum standards, they do not guarantee best in class performance, efficiency or quality.

Instead of creating and maintaining their own codes, most states, and local jurisdictions adopt the model building codes developed and maintained by the International Code Council (ICC). The ICC’s family of International Codes includes:

• International Building Code (IBC): the foundation of the complete family of ICC codes.

• International Residential Code (IRC): minimum regulations for one and two-family dwellings and townhouses using prescriptive provisions.

• International Existing Building Code (IEBC): refers to existing buildings and applies to projects which intend to modify an existing building. This includes repairs, alterations, relocation of buildings, additions, and change of occupancy. 

• There are other I-codes with specific, focused subject matters such as fire, energy, plumbing, and green construction, which we cover in other blog posts.

The International Code Council was established in 1994, with the goal of developing a single set of national model construction codes. The ICC revisits and publishes new editions of its codes every three years. ICC has brought together three different organizations that have developed three separate sets of model codes throughout the U.S.: Building Offi­cials and Code Administrators International, Inc. (BOCA), International Conference of Building Officials (ICBO), and Southern Building Code Congress International, Inc. (SBCCI).

Seismic Codes

Since building code requirements are considered a minimum requirement, a house, its occupants and its contents still may be compromised in an earthquake event, even if it was designed and constructed in full compliance with building codes. Earthquakes are addressed in each one of the abovementioned codes - IBC, IRC, and IEBC - all working towards ensuring buildings can cope with the forces applied to the structure during an earthquake. 

The I-Codes also follow and reference the latest technical standards, including the ASCE/SEI 7, minimum design loads for buildings and other structures, and ASCE/SEI 41, seismic evaluation, and retrofit for existing buildings.

According to the National Institute of Building Sciences (NIBS, see page 24), historically “building codes have gradually increased the required strength and stiffness of new buildings to resist earthquakes, along with numerous improvements to structural details. Building strength and stiffness increase on the order of 50% every 30 years in the higher-risk areas in the western United States. Thus, the average West Coast building built today to comply with I-Codes is about 1.5 times as strong and stiff as it would have been under the 1988 Uniform Building Code. The greater strength makes the building less likely to collapse or to be red-tagged in a large earthquake. The greater stiffness makes it less likely to suffer damage to many architectural elements such as walls and windows.”

The dedicated sections in these codes are the best available guidance on how buildings should be designed and constructed to limit the damage caused by earthquakes. Complying with recent codes can, and will, increase the resilience of your home and may save lives. 

If your local codes adopted these I-Codes that is a good start, if not, then you should try to figure out:

1. If your local codes went above these codes - great.

2. If not, then you should look at adopting these codes yourself as a minimum, especially if you live in a risk zone area.

Going Above Building Codes

New Construction

As mentioned, to protect occupants as well as the structure, a house needs to be designed and built with enough strength and stiffness to properly perform during an earthquake. According to FEMA, “actual earthquakes can generate forces considerably higher than those used for code-prescribed design.”

Based on the risk zone and personal preferences, there are above-code techniques that can be applied to further enhance a structure to obtain minimal damage and best protect its occupants and their belongings.

Here are some “above code” measures to discuss with your architects and contractors:

1. Adopt I-Codes where local codes have not yet adopted them, or no code is currently required.

2. Strengthening and Stiffness - explore codes that define the measures or factors for strength and stiffness. Discuss with an experienced architect, engineer, or contractor if you need to go above these metrics.

3. Designing strength and stiffness to both vertical and lateral movements. Many times focus is on vertical, although lateral loads pose the same if not higher risks, therefore, adding strength and stiffness to lateral movement can be beneficial. 

Retrofitting

There are many measures you can take to make your house less susceptible to earthquake forces and better protect occupants and your belongings. Here are some options you can explore:

• Replace unreinforced masonry or deteriorating concrete foundations with reinforced concrete.

• Add steel frame and structural sheathing to a soft-story wood-frame.

• Secure frame to the foundation with anchor bolts.

• Inspect exterior masonry walls periodically for cracks and reinforce them; in the case of chimneys, brace them to the roof structure.

• Check for loose roof tiles and properly anchor roofing material to a braced roof frame.

• Strap water heaters to the building frame.

• Secure bookshelves to walls with screws or straps.

• Secure light fixtures and fans to ceiling joists. 

• Secure bookcases to wall studs.

• Strap computer monitors and televisions to walls or desks.

• If your home is heated by natural gas, use flexible pipe connections for gas appliances. 

• Install a seismic actuated gas valve, which shuts off the gas during severe earthquakes.

• Manufactured homes should be tied-down and anchored.

Performance-Based Design

This is another method that can be defined as “above the code”.

Following a chain of moderate magnitude, yet damaging, earthquakes that struck California during the 1980s and 1990s, it was clear a new approach to seismic design was required. Based on historical data, adhering to codes is a prescriptive design where the contractor ensures compliance and there is an assumption that the structure is capable of meeting the performance expectations of the building code, e.g. saving lives. 

However, more often than not, the performance capability of the design is not evaluated. In contrast, performance-based design reverses the process and starts with an explicit definition of the desired performance, and the design is explicitly evaluated to demonstrate that the required performance can be achieved. Based on those criteria, the design process starts. 

Performance-based design is done by highly skilled professionals. FEMA P-58 methodology is the leading performance-based seismic design procedure which provides stakeholders the following benefits that help them make better decisions:

• Communication of usable metrics such as repair cost, repair time, environmental impacts, and potential casualties. 

• Performance is defined as a probability, providing designers some levels of protection against the unpredictable outcome of earthquakes.

Spotlight

Economically marginalized communities are also more vulnerable to environmental and climate hazards. A house is, and should be, the prime asset and safe haven for families, and therefore, it needs to be resilient, which many times comes with a cost some communities cannot afford.

Here is a story of a company that decided to make an impact. Actually, multiple impacts in one go. More than 30 years ago, social housing company Ecoblock International founded Échale, which leverages the power of communities and innovative home-building methods to build and retrofit sustainable homes for communities in need.

In this project, Échale set out to construct an earthquake resilient, adobe-based building system that uses local materials such as soil and is produced on-site, making it energy efficient with a low carbon footprint. Échale educates the community members on how to produce these adobe-based blocks, as well as how to build their houses with such materials. The company also claims they pay community members who are involved in the production of the Ecoblocks and buildings. 

Summary

Earthquakes are a natural phenomenon, not a disaster. Proper planning and execution can mitigate the disaster and yield desired outcomes, while saving lives and reducing damage to the house. 

Earthquakes apply sudden lateral and vertical forces on the house. To reduce damage to the house, it is advised to strengthen and stiffen its elements (roof, walls, foundation, and connections) and design a continuous load path that will move the load back to the foundation.

Hiring a professional to properly design and construct your house is key. Fundamentals of meeting desired outcomes are:

1. Complying with the latest I-codes, as a minimum.

2. Proper planning, design, and construction:

   1. Knowing the level of risk in your area

   2. Avoiding irregular house shapes

   3. Avoiding building on top of soft stories

   4. Choosing the right materials based on strength, stiffness, and ductility

   5. Analyzing environmental factors such as slope and type of soil.

   6. Environmental risks such as adjacent structures, trees and landslide.

3. In very high-risk zones, and if budget allows, going above code when building a new house or retrofitting, or adopting the latest “performance-based” design method.

KEEP COOL. BUILD RESILIENCE. EAMPACT.

References

• Earthquake Building Codes | FEMA.gov

• ICC - International Code Council - ICC (iccsafe.org)

• MS_FullSet_FactSheets.pdf (ymaws.com) National Institute of Building Science

• FEMA P-58 (atcouncil.org) Development of Next-Generation Performance-Based Seismic Design Procedures for New and Existing Buildings

• ÉCHALE – Building with families for families

• Eco-friendly homes that are earthquake-resistant