Effects of Earthquake Ground-Shaking on Structures
Most large earthquakes occur in a region where two plates collide (plate tectonic theory). Earthquake effects that can have an impact on structures include: ground shaking in 3-dimensions, soil failures, ground settlement, and seismic sea waves. In constructing buildings to resist damage due to the above effects, one has to balance the extra costs that are incurred with the probability of an earthquake causing severe damage within the lifetime of the structure. Furthermore, the levels of uncertainty are much greater than those encountered in the design of structures to resist other phenomena, i.e. wind and snow. Two factors are used to determine the likelihood of earthquake ground-shaking (or seismicity) at a given location. The first is the historical record of earthquakes. How often have large earthquakes been measured in a specific area during about the last 100 years? The second is the geological record of earthquake effects. This can be done by carefully interpreting the geological landscape. In most areas, building codes have been adjusted to local earthquake conditions and should be referenced.
Building Response to Ground Shaking
The response of a building to an earthquake is dynamic not static. For most design considerations, the loads are considered static. For a dynamic response, the building is subjected to a vibratory shaking of the base. Exactly how a building responds is complex and depends on the amplitude and frequency of vibration along with the material and design of the building. Most of what has been learned comes from "in field" testing. This testing carefully measures the ground shaking and places accelerometers at several positions in a buildings superstructure. Below ground structures are generally less affected by ground movement because the entire structure will move together.
All buildings have a a "natural frequency" associated with them. If you place strain onto the structure of a building and then let it snap back into equilibrium, it will sway back and forth with an amplitude that decays with time. The swaying will happen with a particular frequency called its natural frequency. If the ground shakes with the same frequency as a building's natural frequency, it will cause the amplitude of sway to get larger and larger. Such that, the ground shaking is in resonance with the building's natural frequency. This produces the most strain on the components of the building and can quickly cause the building to collapse. Assuming similar design and materials, a structures natural frequency, fb, can be estimated by
where Ns is the number of stories in the building.
The soil on which the foundation of a building may rest can also undergo changes that effect the soil's ability to support the foundation. Some soils behave in a liquid fashion when subjected to ground vibrations causing their internal resistance to slipping to be greatly reduced. This process is called liquefaction.
General Behavior of Certain Building Materials
Timber structures nearly always resist earthquakes quite well. Timber is a relatively ductile material. That is, it can be stretched, bent, and hammered, without loosing much of its strength. This material will also damp the vibrations fairly well. Damping is important in reducing the resonance and overall strain placed upon the building parts.
Steel is very ductile material used commonly in building. The Transamerica Pyramid in San Francisco, built to withstand earthquakes, swayed more than 1 foot but was not damaged in the 1989 Loma Prieta California earthquake. This is partially due to its steel construction.
Reinforced Concrete achieves ductility through careful limits on steel in tension and concrete in compression. Non-reinforced concrete can be easily sheared in responding to ground movement and is a serious hazard when supporting heavy loads.
Masonry is a very diverse building material. Reinforced masonry behaves similar to reinforced concrete. The boundary between mortar and the masonry unit adds additional failure potential. Non-reinforced masonry possesses little ductility and is not expected to behave elastically as you want material to behave in an earthquake.
To summarize, the characteristics of a building that are important for determining its seismic response are natural frequency, damping, ductility, and stability of resistance under repeated reversals of inelastic strain.
** Most of the information in this handout comes from: "Guide to Application of the 1988 Edition of the NEHRP Recommended Provisions in Earthquake-Resistant Building Design", published by FEMA, September 1990.
For questions or comments regarding these pages contact Dr. Alan Scott / email@example.com / this page was last updated April 05, 2000