Advanced Composition, 6th period
Earthquakes have been occurring in California for many thousands of years. They only have really become problematic in the last 150 years when the state has become much more populated. The results of earthquakes can be devastating. Roadways, pipelines, and buildings collapse. Personal belongings are damaged. Structural failure in an earthquake is a common, devastating occurrence that can be prevented by proper building technique, and that can also be reduced in likelihood by simple retrofits. For example, by building with the proper materials the building is less likely to be damaged or collapse. Having an appropriate foundation with proper reinforcement for ground conditions is also critical to a building’s survival.
The consideration of building materials must be selected at the time when buildings are constructed. Selecting the proper materials is crucial to the earthquake stability of a building. Of all the choices of structural materials, unreinforced brick or concrete is by far the worst choice. Brick walls are not very strong on their own, and tend to buckle when the lateral (horizontal) forces of the earthquake occur (Johnson 76). Even if they do not buckle, they often send loose bricks flying off the house, which can be dangerous for people and property such as cars outside. These unreinforced brick buildings are the cause most of non-highway earthquake deaths (Yanev 78). Concrete, although technically different from bricks displays many of the same characteristics. Neither one of them can handle the lateral forces of the earthquake. Concrete, as is brick, is not ductile; it has no flexibility to it, rather it is brittle (Johnson 80). Therefore, the concrete walls are likely to break apart and fall over when upset by an earthquake.
Reinforcement of concrete and brick buildings does help their structural stability, however they still amore likely than wood or steel framed buildings to be damaged in an earthquake (Perkins 10). Concrete and brick are both reinforced in similar ways by adding steel bars inside. Brick is put up with two slightly separated layers of bricks. Between the layers steel ties connect the bricks, and the leftover space is filled with concrete. Filling any hollows in the brick or concrete is important. Otherwise the purpose of reinforcement is being defeated, because the bricks are still loose, and mortar, a structurally weak material, is the only thing holding them together (?? ??). Similar to in brick walls, steel rods are placed inside. However, in concrete walls it must be done at the time of construction, before the concrete is formed. This helps to hold the concrete together even if it cracks, and to hold the walls up. To provide adequate support the rods must run both horizontally and vertically (Yanev 105). Horizontal supports keep sections of the wall from falling over, particularly a whole side of a building. The vertical supports help prevent the wall from collapsing, such as having the middle fall out from under the top.
Steel framed buildings, such as skyscrapers, on the other hand are generally much more stable in an earthquake. Steel is more ductile, and can be stretched or bent without breaking. Damage is typically limited to localized damage (Yanev 108). Most of the damage that happens to steel framed buildings is caused by not having connections between the horizontal and vertical framing elements. Skyscrapers, which are almost always steel, are considered among the safest places to be in an earthquake (Johnson 80). Earthquake resistant features are usually installed at added cost, because the risk of loss is large enough that it is worthwhile to protect the owner’s investment. Skyscrapers are also usually designed to withstand wind quite well. Although the forces from an earthquake come from a different place, inside rather than outside, they withstand earthquakes well as they are designed to have enough flexibility (Johnson 80). However, there is a smaller disadvantage about skyscrapers. They cause the length of the earthquake to be extended inside the building, due to the flexibility. In the Loma Prieta earthquake, the 10 second earthquake lasted for 80 seconds in a 32 story building (?? ??). The other widely used technique for preventing the skyscrapers from collapsing is called seismic isolation. The technique, which can also be used on other types of buildings, is to keep the building from moving with the ground. By adding layers of rubber and steel to the foundation the stress on the building in an earthquake can be reduced to one tenth of what it would be without (Brown 54).
Another good choice for building materials in earthquake country is wood. This is in part because these types of buildings have a greater structural stability and are much less likely than brick to collapse in an earthquake (Moley). Also, similar to steel, the wood has good ductility. For these reasons, wood framed houses are the most commonly built houses in California.
However, in wood framed buildings, there are other important variables affecting the structural stability. These would include architecture. Boxy one story houses are generally most structurally stable. However, having less bracing and support, split-level and irregular shaped houses have less stability (Helfant 32). The biggest problem in these buildings is there is not enough support for upper floors. This is particularly a problem over garages. The garage door leaves a large gap in the framing, where cross-bracing cannot occur. Without the cross-bracing, the buildings are less able to withstand the lateral movement in earthquakes. The same type of problem, where the support for the upper floor collapses, often also occurs in apartments with ground floor parking garages that have small supports, and houses on stilts.
Although houses on stilts may look nice, they are skinny and can easily snap in an earthquake. Plywood sheathing can help prevent this. By having plywood attached to the piers in many places (the more nails, the better), the force of the earthquake must be much greater to break the house off the stilts (Yanev 130). Regular houses need similar bracing in the crawl spaces under the house. Cripple walls, the walls in the crawl space, should be reinforced by nailing plywood to the framing. Adding plywood is an inexpensive fix that can have a huge economic return in an earthquake. The building is significantly less likely to fall over if the cripple walls are reinforced (National Research Council 10). The reinforcement significantly adds more shear strength to resist the lateral forces in most earthquakes. On the exterior of a house, stucco backed with mesh is another way to provide shear support. However, stucco siding should not be used as a replacement for plywood sheathing. If the stucco cracks it loses almost all of its shear strength (Helfant 32). Having both stucco and plywood sheathing is a good choice for earthquake resistance.
Another important consideration in preventing a wood-framed house from destruction is having the house securely connected to the foundation. This is usually done with anchor bolts. Many older houses do not have any anchor bolts or if they do, they are inadequate. Anchor bolts are easy to add. An average homeowner with basic experience in using drills and wrenches can install them. To install them anchor bolts are dropped into a hole drilled in the foundation, going through the mudsill, the bottom part of the wood framing. There is a nut on the top of the bolt, which is tightened with a wrench, causing the diameter of the bottom of the bolt (which is in the concrete) to increase. The lower part of the bolt is kept in place with tension. Removal, without undoing the bolt, would require several tons of pressure (Helfant 18). Using another type of bolt, epoxy anchor bolts can also increase this strength. They are more expensive and are more difficult to install, but using them provides greater earthquake resistance.
One common reason houses are destroyed is the quality and type of the foundation is not appropriate. Having a brick foundation is bad in an earthquake, because besides not being able to hold anchor bolts, the individual bricks tend to break apart, leaving the house sitting on rubble (Helfant 20). Continuous wall foundations, made out of reinforced concrete provide much more stable support, and are good for most houses. For weak or unstable soils other types of foundations that may be more beneficial. On many unstable soils, having a drilled pier foundation, which has concrete or steel pilings going deep into the ground, can prevent uneven settling of a house that can occur in an earthquake (Yanev 117). For landfills, which have unique problems, a “floating” foundation is best. If not properly compacted, the soil in landfills can have devastating effects such as turning into quicksand or dust flurries (Halacy 109). For example, the Marina Lagoon area in San Francisco was made with rubble from the 1906 quake. A few years later sand was added to make the ground usable for the 1915 Panama Pacific Exposition (Ward 8). Later houses were built there, many of which were destroyed or damaged in the Loma Prieta quake. Many of the homes in the Marina district could have been saved if they had been built with a “floating” foundation (Yanev 117). This is done by having the foundation made out of a layer of reinforced concrete floating on top, not attached with piers or foundation walls. Floating foundations do not have the problem of falling into the quicksand like conditions that can be formed.
With all these things that can be done to prevent damage, it is amazing that so much is not done. There are many ways to make buildings earthquake resistant. Many of the things that prevent earthquake damage must be done when a building is built, such as proper selection of materials, and design of the foundation, and building layout. However, some of these techniques, like adding anchor bolts and plywood sheathing can be added later. Not adding these simple, cost effective measures is like asking to have one’s home collapse in an earthquake. When shopping for a house, it is recommended to consider the earthquake resistance of potential homes. No one knows when the next big one will strike. Having a properly built and reinforced home is the first step in earthquake safety.
Brown, Billy Walker and Walter R. Brown. Earthquakes. Massachusetts: Addison Wesley, 1974.
Halacy, D.S. Jr. Earthquakes. New York: Bobbs-Merril Co. Inc., 1974.
Helfant, David Benaroya. Earthquake Safe: A Hazard Reduction Manual for Homes. Berkeley: Builders Booksource, 1989.
Johnson, R.T. “The Lessons of Loma Prieta,” Popular Science vol. 236 no. 3. March 1990, pp. 74-79.
Moley, Kathy. On Shaky Ground: Living With Earthquakes on the North Coast. Humboldt University, 1995. Online. Available http://glinda.cnrs.humboldt.edu/earthquakes/shaky_ground.html 1 June 1999.
National Research Counsel. Practical Lessons from the Loma Prieta Earthquake. Washington DC: National Academy Press, 1994.
Perkins, Jeanne B. On Shaky Ground. Association of Bay Area Governments, 1987.
Ward, Peter L. et al. The Loma Prieta Earthquake of October 17, 1989. US Geological Survey, 1989.
Yanev, Peter. Peace of Mind In Earthquake Country. San Francisco: Chronicle Books, 1991.