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Earthquake and Retrofit Terms Glossary

Common Earthquake and Seismic Retrofitting Terms

Adequately braced - Wood framed cripple walls are considered to be adequately braced if they have been strengthened in accordance with the provisions of Chapter A3 of the California Existing Building Code

Anchor bolt - Steel rod threaded on one end that is fixed in concrete (or masonry) to attach a structure to its foundation. (Anchor bolts in new construction are placed in concrete before it cures or hardens. Anchor bolts in existing construction are typically adhesive or expansion anchors).

Chapter A3 - Prescriptive provisions for seismic strengthening of cripple walls and sill plate anchorage of light, wood-frame residential buildings.

CRMP - California Residential Mitigation Program (CRMP) is a joint-exercise-of-powers entity formed by its members, the California Earthquake Authority, a public instrumentality of the State of California (CEA), and the California Governor’s Office of Emergency Services (Cal OES).

Cement - A fine gray powder that produces a bonding paste when mixed with water.

Concrete - A combination of cement, sand, crushed rock, or gravel. It is used for foundations, slabs and sidewalks.

Continuous perimeter foundation - A foundation, typically constructed of concrete or masonry, that is continuous under the exterior walls of a dwelling.

Crawl space - A shallow, unfinished space beneath the first floor of a house without a basement. The crawl space allows ventilation of floor framing and access to pipes and ducts.

Cripple stud - A short stud that is used in a framed timber wall.

Cripple wall - Also known as crawl space wall. A less than full height wood stud wall extending from the top of the foundation to the underside of the lowest floor framing.

Drywall - Also known as wallboard, gypsum board, plasterboard, and by the trade name SheetrockTM, a wall-surfacing material composed of sheets of gypsum plaster sandwiched between a low-grade backing paper and a smooth-finish front surface paper that can be painted.

Earthquake - The sudden movement of the Earth when two large pieces of the Earth's crust, called tectonic plates, suddenly slip is called an earthquake. The release of pressure causes shock waves to shake and roll on the Earth’s surface.

Engineered solution - Under California Building Code Chapter A3 an engineered solution is required for cripple walls between 4-feet and 7-feet. An engineered solution is unique to each house.

Faults/Fault Plane/Fault Lines - A fault is a weak point within a tectonic plate where pressure from beneath the Earth’s surface can break through and cause shaking in an earthquake. Faults can be as short as a few inches or miles long. An earthquake fault rarely follows a straight line. The major fault types include normal, reverse and strike-slip.

Foundation - The part of a building or wall which supports the superstructure. Typically made of concrete or masonry in residential construction.

Low-slope site - A building site with a natural slope of 10 percent or less.

Magnitude - This is the measurement used to describe the total energy released in an earthquake. Scientists also use the moment magnitude scale, which measures the movement of rock along the fault, and accurately measures larger earthquakes, which can last for minutes and affect a much larger area.

Masonry - Building elements constructed of stone, brick, hollow clay tile, concrete blocks, gypsum blocks, or similar materials, or a combination of them.

Plan Set A - A standard plan set, developed by the Association of Bay Area Government (ABAG), available to homeowners and contractors for use as the construction document for the seismic retrofit of a wood frame dwelling. "Plan Set A"  is in accordance with Chapter A3 of the California Existing Building Code. Check with your local building department to see if they have adopted Plan Set A.

Post - A timber member set on end to support a wall, beam, or other structural member. Typically a 4x4 (4" by 4") or 6x6 (6" by 6") in residential construction.*

Prescriptive standards - This prescriptive provision or plan set is a "blueprint" version of a prescriptive ("cookbook") standard for strengthening homes to better withstand earthquake shaking. When approved by the local building official, the plan set may be used to strengthen older homes without the need for costly site-specific plans and design calculations. This plan set provides a low-cost method to help improve an older home's chances of surviving an earthquake.

Raised floor - A wood-framed floor that is supported or raised up off the ground by either wood-framing (such as a cripple wall) or a concrete foundation.

Seismology - Seismology is the study of earthquakes. Scientists who study earthquakes are called Seismologists. When we talk about strengthening a house to make it stronger against earthquakes, it is referred to as a seismic retrofit because it is helping reinforce the house against the seismic waves and shaking of a damaging quake.

Seismic Waves - Earthquake waves travel through and on top of the surface of Earth causing the shaking and vibrations on the ground. Earthquake waves can move 20 times the speed of sound and can be felt a long way away from their point of origin.

Seismograph and the Richter Scale - When the earth trembles, a seismograph/seismometer measures the strength and duration of the earthquake and produces a graphic plotting the event. In the past, the Richter Scale was used to compare earthquakes, calculated in a ten-point scale. Scientists now use the moment magnitude scale, which measures the movement of rock along the fault.

Sill Plate - A horizontal member, or board, laid directly on top of the foundation, to which the cripple wall, or first floor of the building, is connected.

Soft Story - A structure where the first floor is substantially weaker and more flexible than the stories above it due to large openings in the walls or very tall stories. This type of house typically requires an engineered solution due to cripple walls that are over 4 feet and/or wide garage door openings.

Soil liquefaction - The shaking from an earthquake can turn loose soil into a liquid during an earthquake.

Standard Plan - A plan set that includes specifications, details, and instructions for the installation of foundation anchors and cripple wall bracing (for walls shorter than 4'-0" tall). These plan sets are intended for use without the services of a design professional (architect or engineer). Check with your local building department to see if they have adopted a standard plan for seismic retrofits.

Tectonic Plates - Tectonic plates are large, thin layers of the Earth's crust. They stretch, move, slide and grind against each other. When the Earth’s upper mantle become stuck as the plates move past one another, the plates lock together, and pressure builds up. When they finally release, earthquakes occur. Many earthquakes happen on the edges of large sections of tectonic plates.

Tsunami - A giant ocean wave (or series of waves) created by an undersea earthquake, volcanic eruption or landslide.

Unreinforced masonry - A masonry structure that has no steel reinforcing bars embedded in it. The masonry blocks or bricks are connected by mortar.

For more about retrofitting and construction terms visit the Earthquake Brace + Bolt Program Glossary.

For more about the science of earthquakes and earthquake terms visit USGS Glossary.

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