One objective of structural engineering is to protect against the catastrophic damage an earthquake can cause. While most earthquakes are small and cause little or no damage, they also can be catastrophic, leading to loss of life and billions of dollars in destruction to buildings, roads, bridges, sewers and the entire built infrastructure of communities.
Across the United States, California is best known for earthquake risk, but several other regions are earthquake-prone: Charleston, S.C., eastern Massachusetts, the St. Lawrence River basin, and the central Mississippi River valley are some that experience earthquakes frequently.
The area at greatest risk for earthquakes in Utah is the Wasatch Front — which just happens to be where most of the population and infrastructure is located. And this means that structural engineering has to include detailed seismic analysis.
Response of buildings
An earthquake or tremor is the result of the release of energy deep underground, which causes seismic waves through the earth’s crust. When the waves reach the surface, they can cause buildings and other structures to wave, in turn.
In 1927, U.S. building codes incorporated standards for resistance to lateral force, which is the result of seismic waves, related to the weight of the building. Since then, building codes have advanced to include requirements that structural engineering plans take into account the height of the building, and response to lateral force including protective elements distributed throughout the structure.
It’s not possible to predict when an earthquake will strike, but it’s a certainty that one will, sooner or later. It’s a matter of time, especially in areas like California and Utah. Part of structural engineering is determining how resistant a structure will be, and what needs to be done to protect it and the people in the structure.
Seismic analysis and structural engineering
Seismic analysis includes a number of practices that calculate how a structure, or landscape, will react to an earthquake.
- Equivalent static analysis looks at a building’s response to lateral forces, assuming the building won’t twist in reaction to seismic waves. It can be used only for low-rise buildings.
- Response spectrum analysis takes into account multiple responses of a structure to an earthquake. Computer analysis looks at not just the lateral motion of the structure, but also the harmonics — similar to a vibrating guitar string.
- Linear dynamic analysis is useful for taller buildings and buildings with non-orthagonal, or irregular profiles. Computer models calculate the structure’s elasticity or stiffness, and make predictions about the response.
- Nonlinear static analysis is sometimes also called “pushover” analysis and is used for structures that will respond in a nearly uniform manner at every point. It applies calculated forces to the computer models that include non-linear properties — that is, doubling the force applied does not cause an equivalent increase in response, but either a greater or a lesser response.
- Nonlinear dynamic analysis combines records of ground motion during earthquakes with detailed models of the planned structure. It considers the structure’s non-linear properties over time, and therefore gives results that are relatively more reliable than other forms of seismic analysis.
Even though nonlinear dynamic analysis is required in many building codes, the actual response of a building to seismic events depends on the intensity of the shaking and movement. Structural engineering uses all the different types of analysis for a complete and reliable assessment.
Seismic analysis is something that no structural engineering project can afford to skip. And in most jurisdictions, it’s part of the building code. Call our civil engineering professionals to discuss the seismic analysis of your next structural engineering project.