Earthquakes are classified as natural disasters. We can’t eliminate them. Yet, we can and should prepare to reduce the potential damage.
Whether we are building a new house or retrofitting an existing home, there are solutions that we can implement, to shift an Earthquake from a disaster into an unpleasant event.
Note that we are focusing on inland earthquakes, distinguished from tsunamis which create more severe damage through floods.
In this blog, we explore the following:
- How Does an Earthquake Impact the Home?
- Load Path
- Environmental Aspects Impacting Design and Construction Decisions
- Going Above Building Codes
- Earthquake-Resistant House
- Final Thoughts
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 an earthquake occurs, the ground moves, initiating movement in the foundation of the house. For a moment, the other parts of the house: walls, roof, and upper stories remain still. The movement proceeds from element to element. By the time the roof starts moving in the direction of the foundation, the foundation is already moving back towards its initial position, so the roof and foundation are moving in opposite directions. This phenomenon is also true with vertical forces, where opposing up and down movements occur. Opposite movements continue until the ground motion stops, followed by the building elements. If the opposing forces are strong enough, and the structure has weak spots that cannot withstand them, some parts, or all of the house, may displace from the foundation, tilt over, or collapse.
According to FEMA’s Homebuilder's Guide to Earthquake-Resistant Design and Construction (232), a house can remain intact and withstand the forces of an earthquake, if “the load applied on the structure has a path allowing load transfer through each building part down to the foundation and supporting soils.”
The notion is to have a continuous load path throughout the different elements of the house (roof, walls, and 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.
The roof, ceiling, floor, and bracing wall systems are the basic elements resisting earthquake loads. Therefore, FEMA stresses that adequate earthquake performance of a house relies on:
- Adequate strength and stiffness of roof, ceiling, floor, and bracing wall systems.
- Adequate connection between systems to provide a functional load path.
- Adequate connection to the foundation.
Strengthening and connecting elements are important, but a smart design 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:
- Generally, the lighter the structure, the less load impact it suffers. A light roof is favorable, especially if you prefer high ceilings. The second story, as well as partitioning walls, should be as light as possible.
- Avoid irregular house shapes.
- Avoid a soft story - 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.
- Choose adequate materials for elements on each floor. The main material properties to look for in earthquake engineering are:
- Ductility: the capability of a material to bend or deform before it fractures.
- Strength: the ability of a material to withstand forces, such as stress, before it permanently deforms.
- Stiffness: a degree of resistance to deformation. The ability of a material to return to its original shape once the force applied to it is removed.
Building materials such as wood, steel, and glass, have different characteristics: heavy vs. light, stiff vs. ductile. Those characteristics should be considered for use in the proper location and with the appropriate function within the overall house system.
Environmental Aspects Impacting Design and Construction Decisions
Numerous factors may impact design, materials, and construction decisions while making an earthquake-proof building.
Here are the key factors to research or discuss with hired professionals:
- For new or retrofitted homes:
- Does the location of the house have a high risk of earthquakes?
- How does the type of soil impact the specific foundation and materials?
- How does the slope of the land impact the foundation and materials?
- How does the shape of the land impact 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 property?
- Do old telecommunication structures or other utility structures nearby (e.g. old posts) pose a potential risk?
- For retrofitted homes:
- When was the home built? New and stricter building codes were passed during the ’80s. A house built after the ’80s may be more resilient to earthquakes than older houses.
- Does the house have cripple walls 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, which weakens the structure.
- If the house is built on a slope, it probably means that 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? Was the garage strengthened? Was the chimney reinforced? Were the walls (cripple, masonry) strengthened or reinforced?
Going Beyond Building Codes
When building or retrofitting a house, you must comply with your state and local (county and city) building codes. Remember that 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.
Since building code requirements are considered minimum requirements, a house designed and constructed in full compliance with building codes, may still be compromised in an earthquake event.
Earthquakes are addressed in each of the International Code Council (ICC) 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 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 70), 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. Therefore, the average West Coast home built today to comply with I-Codes is about 1.5 times stronger and stiffer than it would have been under the 1988 Uniform Building Code. The extra strength makes the building less likely to collapse or to be red-tagged in a large earthquake. The greater stiffness reduces the probability of damage to 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 the I-Codes, it’s a good start. If not, then you should further Inquire:
- Are your local codes more stringent?
- If not, consider adopting these codes yourself as a minimum, especially if you live in a high-risk area.
As mentioned, to protect its structure and the occupants, a house needs to be designed and built with sufficient 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 reduce potential damage and increase occupants’ protection.
Here are the above-code measures to discuss with your architects and contractors:
- Adopt I-Codes where local codes have not yet adopted them.
- Strength and Stiffness: explore codes that define the measures or factors for strength and stiffness. Discuss with an experienced architect, engineer, or contractor whether you should go above these metrics.
- 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.
There are measures you can apply to your existing home to make it less susceptible to earthquake forces and better protect occupants and belongings. Here are some options to 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.
- Brace chimneys 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.
This is another method that can be defined as “above code”.
Following a chain of moderate magnitude, yet damaging, earthquakes that struck California during the 1980s and 1990s, it was clear that 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 it is assumed 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.
Performance-based design reverses the process. It starts with an explicit definition of the desired performance, then the design is 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.
Economically marginalized communities are more vulnerable to environmental and climate hazards. A house is and should be, the prime asset and haven for families. Unfortunately, some communities cannot afford the cost to increase the resilience of their homes.
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. Echale 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.
Earthquakes are a natural phenomenon, but not necessarily a disaster. Proper planning and execution can mitigate the disaster and yield desired outcomes while saving lives and reducing damage to the house.
To reduce damage from sudden lateral and vertical forces, it is advised to strengthen and stiffen the house’s 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:
- Complying with the latest I-codes, as a minimum.
- Proper planning, design, and construction:
- Knowing the level of risk in your area
- Avoiding irregular house shapes
- Avoiding building on top of soft stories
- Choosing the right materials based on strength, stiffness, and ductility
- Analyzing environmental factors such as slope and type of soil.
- Considering environmental risks such as adjacent structures, trees, and landslides.
- In very high-risk zones and if budget allows, go above code or adopt the latest “performance-based” design method.
KEEP COOL. BUILD RESILIENCE. EAMPACT.