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This resource page provides an introduction to the concepts and principles of seismic design, including strategies for designing earthquake-resistant buildings to ensure the health, safety, and security of building occupants and assets.
The essence of successful seismic design is three-fold. First, the design team must take a multi-hazard approach towards design that accounts for the potential impacts of seismic forces as well as all the major hazards to which an area is vulnerable. Second, performance-based requirements, which may exceed the minimum life safety requirements of current seismic codes, must be established to respond appropriately to the threats and risks posed by natural hazards on the building's mission and occupants. Third, and as important as the others, because earthquake forces are dynamic and each building responds according to its own design complexity, it is essential that the design team work collaboratively and have a common understanding of the terms and methods used in the seismic design process.
About half of the states and territories in the United States—more than 109 million people and 4.3 million businesses—and most of the other populous regions of the earth are exposed to risks from seismic hazards. In the U.S. alone, the average direct cost of earthquake damage is estimated at $1 billion/year while indirect business losses are estimated to exceed $2 billion/year.
A. Origin and Measurement of Earthquakes
Plate Tectonics, the Cause of Earthquakes
Earthquakes are the shaking, rolling, or sudden shock of the earth's surface. Basically, the Earth's crust consists of a series of "plates" floating over the interior, continually moving (at 2 to 130 millimeters per year), spreading from the center, sinking at the edges, and being regenerated. Friction caused by plates colliding, extending, or subducting (one plate slides under the other) builds up stresses that, when released, causes an earthquake to radiate through the crust in a complex wave motion, producing ground failure (in the form of surface faulting [a split in the ground], landslides, liquefaction, or subsidence), or tsunami. This, in turn, can cause anywhere from minor damage to total devastation of the built environment near where the earthquake occurred.
Measuring Seismic Forces
In order to characterize or measure the effect of an earthquake on the ground (a.k.a. ground motion), the following definitions are commonly used:
- Acceleration is the rate of change of speed, measured in "g"s at 980 cm/sec² or 1.00 g.
- For example,
- 0.001g or 1 cm/sec2 is perceptible by people
- 0.02 g or 20 cm/sec2 causes people to lose their balance
- 0.50g is very high but buildings can survive it if the duration is short and if the mass and configuration has enough damping
- For example,
- Velocity (or speed) is the rate of change of position, measured in centimeters per second.
- Displacement is the distance from the point of rest, measured in centimeters.
- Duration is the length of time the shock cycles persists.
- Magnitude is the "size" of the earthquake, measured by the Richter scale, which ranges from 1-10. The Richter scale is based on the maximum amplitude of certain seismic waves, and seismologists estimate that each unit of the Richter scale is a 31 times increase of energy. Moment Magnitude Scale is a recent measure that is becoming more frequently used.
If the level of acceleration is combined with duration, the power of destruction is defined. Usually, the longer the duration, the less acceleration the building can endure. A building can withstand very high acceleration for a very short duration in proportion with damping measures incorporated in the structure.
Intensity is the amount of damage the earthquake causes locally, which can be characterized by the 12 level Modified Mercalli Scale (MM) where each level designates a certain amount of destruction correlated to ground acceleration. Earthquake damage will vary depending on distance from origin (or epicenter), local soil conditions, and the type of construction.
B. Effects of Earthquakes on Buildings
Seismic Terminology (For definitions of terms used in this resource page, see Glossary of Seismic Terminology )
The aforementioned seismic measures are used to calculate forces that earthquakes impose on buildings. Ground shaking (pushing back and forth, sideways, up and down) generates internal forces within buildings called the Inertial Force (FInertial), which in turn causes most seismic damage.
FInertial = Mass (M) X Acceleration (A).
The greater the mass (weight of the building), the greater the internal inertial forces generated. Lightweight construction with less mass is typically an advantage in seismic design. Greater mass generates greater lateral forces, thereby increasing the possibility of columns being displaced, out of plumb, and/or buckling under vertical load (P delta Effect).
Earthquakes generate waves that may be slow and long, or short and abrupt. The length of a full cycle in seconds is the Period of the wave and is the inverse of the Frequency. All objects, including buildings, have a natural or fundamental period at which they vibrate if jolted by a shock. The natural period is a primary consideration for seismic design, although other aspects of the building design may also contribute to a lesser degree to the mitigation measures. If the period of the shock wave and the natural period of the building coincide, then the building will "resonate" and its vibration will increase or "amplify" several times.
The soil also has a period varying between 0.4 and 1.5 sec., very soft soil being 2.0 sec. Soft soils generally have a tendency to increase shaking as much as 2 to 6 times as compared to rock. Also, the period of the soil coinciding with the natural period of the building can greatly amplify acceleration of the building and is therefore a design consideration.
Seismic Design Factors
The following factors affect and are affected by the design of the building. It is important that the design team understands these factors and deal with them prudently in the design phase.
Torsion: Objects and buildings have a center of mass, a point by which the object (building) can be balanced without rotation occurring. If the mass is uniformly distributed then the geometric center of the floor and the center of mass may coincide. Uneven mass distribution will position the center of mass outside of the geometric center causing "torsion" generating stress concentrations. A certain amount of torsion is unavoidable in every building design. Symmetrical arrangement of masses, however, will result in balanced stiffness against either direction and keep torsion within a manageable range.
Damping: Buildings in general are poor resonators to dynamic shock and dissipate vibration by absorbing it. Damping is a rate at which natural vibration is absorbed.
Ductility: Ductility is the characteristic of a material (such as steel) to bend, flex, or move, but fails only after considerable deformation has occurred. Non-ductile materials (such as poorly reinforced concrete) fail abruptly by crumbling. Good ductility can be achieved with carefully detailed joints.
Strength and Stiffness: Strength is a property of a material to resist and bear applied forces within a safe limit. Stiffness of a material is a degree of resistance to deflection or drift (drift being a horizontal story-to-story relative displacement).
Building Configuration: This term defines a building's size and shape, and structural and nonstructural elements. Building configuration determines the way seismic forces are distributed within the structure, their relative magnitude, and problematic design concerns.
- Regular Configuration buildings have Shear Walls or Moment-Resistant Frames or Braced Frames and generally have:
- Low Height to Base Ratios
- Equal Floor Heights
- Symmetrical Plans
- Uniform Sections and Elevations
- Maximum Torsional Resistance
- Short Spans and Redundancy
- Direct Load Paths
- Irregular Configuration buildings are those that differ from the "Regular" definition and have problematic stress concentrations and torsion.
Soft First Story is a discontinuity of strength and stiffness for lateral load at the ground level.
Discontinuous Shear Walls do not line up consistently one upon the other causing "soft" levels.
Variation in Perimeter Strength and Stiffness such as an open front on the ground level usually causes eccentricity or torsion.
Reentrant Corners in the shapes of H, L, T, U, +, or  develop stress concentration at the reentrant corner and torsion. Seismic designs should adequately separate reentrant corners or strengthen them.
Knowledge of the building's period, torsion, damping, ductility, strength, stiffness, and configuration can help one determine the most appropriate seismic design devices and mitigation strategies to employ.
C. Seismic Design Strategies and Devices
Diaphragms: Floors and roofs can be used as rigid horizontal planes, or diaphragms, to transfer lateral forces to vertical resisting elements such as walls or frames.
Shear Walls: Strategically located stiffened walls are shear walls and are capable of transferring lateral forces from floors and roofs to the foundation.
Braced Frames: Vertical frames that transfer lateral loads from floors and roofs to foundations. Like shear walls, Braced Frames are designed to take lateral loads but are used where shear walls are impractical.
Moment-Resistant Frames: Column/beam joints in moment-resistant frames are designed to take both shear and bending thereby eliminating the space limitations of solid shear walls or braced frames. The column/beam joints are carefully designed to be stiff yet to allow some deformation for energy dissipation taking advantage of the ductility of steel (reinforced concrete can be designed as a Moment-Resistant Frame as well).
Energy-Dissipating Devices: Making the building structure more resistive will increase shaking which may damage the contents or the function of the building. Energy-Dissipating Devices are used to minimize shaking. Energy will dissipate if ductile materials deform in a controlled way. An example is Eccentric Bracing whereby the controlled deformation of framing members dissipates energy. However, this will not eliminate or reduce damage to building contents. A more direct solution is the use of energy dissipating devices that function like shock absorbers in a moving car. The period of the building will be lengthened and the building will "ride out" the shaking within a tolerable range.
Base Isolation: This seismic design strategy involves separating the building from the foundation and acts to absorb shock. As the ground moves, the building moves at a slower pace because the isolators dissipate a large part of the shock. The building must be designed to act as a unit, or "rigid box", of appropriate height (to avoid overturning) and have flexible utility connections to accommodate movement at its base. Base Isolation is easiest to incorporate in the design of new construction. Existing buildings may require alterations to be made more rigid to move as a unit with foundations separated from the superstructure to insert the Base Isolators. Additional space (a "moat") must be provided for horizontal displacement (the whole building will move back and forth a whole foot or more). Base Isolation retrofit is a costly operation that is most commonly appropriate in high asset value facilities and may require partial or the full removal of building occupants during installation.
The materials used for Elastomeric Isolators are natural rubber, high-damping rubber, or another elastomer in combination with metal parts. Frictive Isolators are also used and are made primarily of metal parts.
Tall buildings cannot be base-isolated or they would overturn. Being very flexible compared to low-rise buildings, their horizontal displacement needs to be controlled. This can be achieved by the use of Dampers, which absorb a good part of the energy making the displacement tolerable. Retrofitting existing buildings is often easier with dampers than with base isolators, especially if the application is external or does not interfere with the occupants.
There are many types of dampers used to mitigate seismic effects, including:
- Hysteric dampers utilize the deformation of metal parts
- Visco-elastic dampers stretch an elastomer in combination with metal parts
- Frictive dampers use metal or other surfaces in friction
- Viscous dampers compress a fluid in a piston-like device
- Hybrid dampers utilize the combination of elastomeric and metal or other parts
D. Nonstructural Damage Control
All items, which are not part of the structural system, are considered as "nonstructural", and include such building elements as:
- Exterior cladding and curtain walls
- Parapet walls
- Canopies and marquees
- Chimneys and stacks
- Partitions, doors, windows
- Suspended ceilings
- Routes of exit and entrance
- Mechanical, Plumbing, Electrical and Communications equipment
- Furniture and equipment
These items must be stabilized with bracing to prevent their damage or total destruction. Building machinery and equipment can be outfitted with seismic isolating devices, which are modified versions of the standard Vibration Isolators.
The principles and strategies of seismic design and construction are applied in a systematic approach that matches an appropriate response to specific conditions through the following major steps:
1. Analyze Site Conditions
The location and physical properties of the site are the primary influences the entire design process. The following questions can serve as a checklist to identify seismic design objectives.
- Where is the location of the nearest fault?
- Are there unconsolidated natural or man-made fills present?
- Is there a potential for landslide or liquefaction on or near the site?
- Are there vulnerable transportation, communication, and utilities connections?
- Are there any hazardous materials on the site to be protected?
- Is there potential for battering by adjacent buildings?
- Is there exposure to potential flood from tsunami, seiche, or dam failure?
Consider mission critical or business continuity threats of seismicity on adjacent sites or elsewhere in the vicinity that may render the project site inaccessible or causes the loss of utilities, threat of fire, or the release of toxic materials to the site. Conduct subsurface investigations to discover loose soils or uncontrolled fill that could increase ground motion. Hard dense soils remain more stable, while solid dense rock is the most predictable and seismically safe building base.
2. Establish Seismic Design Objectives
A performance-based approach to establishing seismic design objectives is recommended. This determines a level of predictable building behavior by responding to the maximum considered earthquake. A threat/vulnerability assessment and risk analysis can be used to define the level of performance desired for the building project. Some suggested seismic design performance goals are:
- Conform to local building codes providing "Life Safety," meaning that the building may collapse eventually but not during the earthquake.
- Design for repairable structural damage, required evacuation of the building, and acceptable loss of business for stipulated number of days.
- Design for repairable nonstructural damage, partial or full evacuation, and acceptable loss of business for stipulated number of days due to repair.
- Design for repairable structural damage, no evacuation required, and acceptable loss of business for stipulated number of days due to repair.
- No structural damage, repairable nonstructural damage, no evacuation, and acceptable loss of business for stipulated number of days due to repair.
- No structural or nonstructural damage, and no loss of business caused by either (excluding damage to tenants' own equipment such as file cabinets, bookshelves, furniture, office equipment etc. if not properly anchored).
Regarding the magnitude of the earthquake it may also be stipulated as "Low," "Moderate," or "Large" as another matrix of grading threat and establishing corresponding building performance goals.
3. Select/Design Appropriate Structural Systems
Seismic design objectives can greatly influence the selection of the most appropriate structural system and related building systems for the project. Some construction type options, and corresponding seismic properties, are:
- Wood or timber frame (good energy absorption, light weight, framing connections are critical).
- Reinforced masonry walls (good energy absorption if walls and floors are well integrated; proportion of spandrels and piers are critical to avoid cracking)
- Reinforced concrete walls (good energy absorption if walls and floors well integrated; proportion of spandrels and piers are critical to avoid cracking)
- Steel frame with masonry fill-in walls (good energy absorption if bay sizes are small and building plan is uniform)
- Steel frame, braced (extensive bracing, detailing, and proportions are important)
- Steel frame, moment-resisting (good energy absorption, connections are critical)
- Steel frame, eccentrically braced (excellent energy absorption, connections are critical)
- Pre-cast concrete frame (poor performer without special energy absorbing connections)
Relevant Codes and Standards
Many building codes and governmental standards exist pertaining to design and construction for seismic hazard mitigation. As previously mentioned, building code requirements are primarily prescriptive and define seismic zones and minimum safety factors to "design to." Codes pertaining to seismic requirements may be local, state, or regional building codes or amendments and should be researched thoroughly by the design professional.
Many governmental agencies at the federal level have seismic standards, criteria, and program specialists who are involved in major building programs and can give further guidance on special requirements.
- Federal Emergency Management Agency (FEMA)
Provides a number of web-based "Disaster Communities," organized around multi-hazard issues, including an Earthquake Disaster Community with major seismic related FEMA publications.
- International Code Council (ICC)
ICC was established in 1994 to developing a single set of comprehensive and coordinated national model construction codes. The founders of the ICC are Building Officials and Code Administrators International, Inc. (BOCA), International Conference of Building Officials (ICBO), and Southern Building Code Congress International, Inc. (SBCCI).
- National Earthquake Hazards Reduction Program (NEHRP)
FEMA's earthquake program was established in 1977, under the authority of the Earthquake Hazards Reduction Act of 1977, enacted as Public Law 101-614. The purpose of the National Earthquake Hazards Reduction Program (NEHRP) is to reduce the risks of life and property from future earthquakes. FEMA serves as lead agency among the four primary NEHRP federal partners, responsible for planning and coordinating the Program.
For definitions of terms used in this resource page, see Glossary of Seismic Terminology .
- American Council of Engineering Companies
- American Society of Civil Engineers
- Building Seismic Safety Council (NIBS)—The Building Seismic Safety Council (BSSC), established by the National Institute of Building Sciences develops and promotes building earthquake risk mitigation, regulatory provisions for the nation.
- Federal Emergency Management Agency (FEMA) Mitigation Division—One of the features of FEMA's site is a map library, containing: GIS mapping products and data for the latest disasters, along with current and prior year disasters and custom hazard maps that can be created by entering a zip code and selecting from a variety of hazard types to help determine disaster risks in any community. In addition, the Mitigation Directorate's Flood Hazard Mapping Technical Services Division maintains and updates the National Flood Insurance Program maps.
- Mitigation Clearinghouse—The Clearinghouse serves to provide a dynamic resource library, thereby improving discovery and accessibility of mitigation related literature.
- Natural Hazards Center—The Natural Hazards Center, located at the University of Colorado, Boulder, Colorado, USA, is a national and international clearinghouse for information on natural hazards and human adjustments to hazards and disasters.
- Seismosoft—providing the earthquake engineering community with access to powerful and state-of-the-art analytical tools since 2002.
- USGS National Earthquake Information Center