How to Improve Your Home's Earthquake Resistance
The following is an excerpt from When Technology Fails: A Manual for Self-Reliance, Sustainability, and Surviving the Long Emergency by Matthew Stein. It has been adapted for the Web.
Ask anyone who has been through a major earthquake, such as the Loma Prieta or Northridge quakes, and they will tell you that a serious quake can be a terrifying event. Often the main differences between structures that pulled through in relatively good shape and those that got the “bulldozer remodel” (dig a hole with a bulldozer, push the house into it, start over) were whether the building was of older construction and the contractor was sloppy with shear nailing of the building’s exterior plywood siding or there was insufficient use of “hurricane clips” and “Simpson Strong-Ties” to properly brace the frame.
Shelly and Phil Rodgers were in their home in California’s Santa Cruz Mountains when the Loma Prieta quake struck. The epicenter was about 7 miles from their home. The house shook violently and all of their cupboards opened, throwing every dish, jar, can, bookcase, television, and appliance to the floor. Phil said that the house floors undulated like a snake, appearing to change elevation by more than a foot in different parts of the house as the quake shook through. They were not able to leave the house until the earth stopped moving. Because their car keys and shoes were still inside, Phil had to brave the aftershocks and wade through broken glass to retrieve keys and shoes so they could attempt the drive to town to pick up their kids. He brought a chainsaw with him, which was needed to cut large limbs that had fallen across the road.
On their way to town, they passed the spot where a neighbor’s house should have been. It had been built on tall pylons overlooking the hillside. When the quake struck, it slid off the piers and down the canyon. The two occupants on the first floor managed to crawl out the door moments before it took off, but their son, who was sleeping on the second floor, went for the wildest ride of his life. He miraculously rode through it uninjured, as the first floor disintegrated and the roof split away and to the side.
Shelly and Phil’s home was of post-and-beam construction with massive timbers. Their contractor had paid careful attention to all the exterior plywood shear nailing and steel frame ties. Remarkably, not one window was broken and the house suffered no structural damage, even though many of their personal items were destroyed (they cleaned up their dishes and pottery with a shovel).
When an earthquake strikes, the most damaging motion is usually not the vertical but the horizontal component of the earth movement. The mass of the building tends to want to stay in one place as the earth moves side to side. The resulting sideways forces on the walls, which tend to push the walls into a diamond shape rather than
a rectangle, are referred to as “shear forces.” As the mass and height of a building increase, the walls will be subject to greater shear forces in an earthquake.
Improving Earthquake Resistance
Recently, through the use of modern analytical tools and by studying what worked and did not work in actual earthquakes, seismic design has made a lot of progress. Although there is no way to “earthquake-proof” a structure, there are numerous ways to increase its earthquake resistance.
At some level of seismic severity, all structures will fail. However, good seismic design can dramatically increase the earthquake resistance of a structure. A few factors that affect the earthquake resistance of a building are described below.
- Mass and height. The heavier and more massive the walls and ceilings, and the taller the structure, the higher the shear loads will be in an earthquake. Tent-like structures, such as yurts and tipis, have almost no mass. They will usually survive massive earthquakes without failure, but if they do collapse, they are so light that they will probably cause little damage.
- Flexibility. If a structure can flex and “give” without failure, it can absorb seismic energy and ride out quakes that might damage or destroy other structures. Fiberglass composites have high strength-to-weight ratios and exceptional flexibility, making them a prime candidate for earthquake- resistant structures that are lightweight, strong, and flexible.
- Masonry, earth, and concrete. These materials are fairly strong in compression, but notoriously weak in tension. The properly engineered use of reinforcing steel, bond beams, and structural roof diaphragms can drastically improve the earthquake resistance of these types of structures. Most of the stress in the walls is a downward compression from the weight of the building. In an earthquake, shear loads put sections of the walls in high tension, which is the kind of load that masonry and earth handle poorly. Consequently, older masonry and earthen buildings often collapse in moderately strong earthquakes.
- Low-tech reinforcing. One low-tech way to improve the earthquake resistance of traditional earthen structures is to incorporate load-bearing post-and-beam reinforcement into the walls and to thread strong rope through the wall centers to help hold them together (instead of collapsing) when a quake cracks the earthen walls. Another low-tech reinforcing material is bamboo. Some varieties of bamboo are extremely strong and resilient.
- Length-to-width ratio. Slender masonry or earthen walls are prone to fracture. Massive, thick walls will usually do better in an earthquake. Sometimes external buttresses are added to building corners to provide extra support.
- Isolation design. In the movies, you have probably seen someone attempting to pull a tablecloth out from under a table of dishes. If all goes well, the dishes stay in place as the tablecloth slides from under them. However, if the cloth does not slip smoothly under the dishes, they all fall over. Some modern skyscrapers employ huge bearings under the foundation to allow them to stay somewhat stationary while the earth moves underneath, similar to the action in the tablecloth trick. Massive rubber blocks or concave bearing supports act to bring the building back into alignment with the ground once the earthquake is over. These design features are called “isolators” because they try to isolate the earth movements from the building.
- Chinese low-tech seismic isolation. Because so many buildings in China are made of earth with little or no structural reinforcement and most of China is without access to high-tech building materials, low-tech solutions to seismic isolation for active earthquake zones have been developed. Chinese engineer Li Li noted that cracks in the eroded base of an old farmhouse’s rammed earth walls allowed that particular farmhouse to slide on its earthen foundation and survive the 1960 quake that destroyed all other buildings in the village of Tuquiao in Jilin province. Based on Li Li’s recommendations, numerous buildings that artificially simulate this “cracked wall” design have been erected at a variety of sites. Terrazzo plates are placed, smooth side up, on top of a prepared foundation trench. A thin layer of sand is spread on the plates, then another layer of terrazzo plates is placed smooth side down on top of the sand. A concrete foundation is poured on top of
the second layer of terrazzo plates, and then the building is constructed as usual. The sand layer acts as a “poor man’s” seismic isolation bearing. Engineering shake table tests on large models and ground blasts next to test buildings have yielded excellent results, but the true test will come when the next large earthquake strikes China.
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