The main objectives of earthquake engineering are:
Understand the interaction between buildings or civil infrastructure and the ground.
Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure.
Design, construct and maintain structures to perform at earthquake exposure up to the expectations and in compliance with building codes
A properly engineered structure does not necessarily have to be extremely strong or expensive.The most powerful and budgetary tools of earthquake engineering are vibration control technologies and, in particular, base isolation.
Seismic loading
Seismic loading means application of an earthquake-generated agitation to a structure. It happens at contact surfaces of a structure either with the ground [6], or with adjacent structures , or with gravity waves from tsunami. Seismic loading depends, primarily, on:
Anticipated earthquake's parameters at the site
Geotechnical parameters of the site
Structure's parameters
Characteristics of the anticipated gravity waves from tsunami (if applicable).
Ancient builders believed that earthquakes were a result of wroth of Gods and, therefore, could not be resisted by humans. Nowadays, the people's attitude has changed dramatically though the seismic loads, sometimes, exceed ability of a structure to resist them without being broken, partially or completely.
Due to their mutual interaction, seismic loading and seismic performance of a structure are intimately related.
Seismic performance
Earthquake or seismic performance is an execution of a building's or structure's ability to sustain their due functions, such as its safety and serviceability, at and after a particular earthquake exposure. A structure is, normally, considered safe if it does not endanger the lives and wellbeing of those in or around it by partially or completely collapsing. A structure may be considered serviceable if it is able to fulfill its operational functions for which it was designed.
Basic concepts of the earthquake engineering, implemented in the major building codes, assume that a building should survive The Big One (the most powerful anticipated earthquake) though with partial destruction
Seismic performance evaluation
Engineers need to know the quantified level of an actual or anticipated seismic performance associated with the direct damage to an individual building subject to a specified ground shaking.
The best way to do it is to put the structure on a shake-table that simulates the earth shaking and watch what may happen next.[citation needed] Such kinds of experiments were performed still more than a century ago
Another way is to evaluate the earthquake performance analytically.
Seismic performance analysis
Seismic performance analysis or, simply, seismic analysis is a major intellectual tool of earthquake engineering which breaks the complex topic into smaller parts to gain a better understanding of seismic performance of building and non-building structures. The technique as a formal concept is a relatively recent development.
In general, seismic analysis is based on the methods of structural dynamics. For decades, the most prominent instrument of seismic analysis has been the earthquake response spectrum method which, also, contributed to the proposed building code's concept of today.
However, those spectra are good, mostly, for single-degree-of-freedom systems. Numerical step-by-step integration proved to be a more effective method of analysis for multi-degree-of-freedom structural systems with severe non-linearity under a substantially transient process of kinematic excitation.
Research for earthquake engineering
Research for earthquake engineering means both field and analytical investigation or experimentation intended for discovery and scientific explanation of earthquake engineering related facts, revision of conventional concepts in the light of new findings, and practical application of the developed theories.The National Science Foundation (NSF) is the main United States government agency that supports fundamental research and education in all fields of earthquake engineering. In particular, it focuses on experimental, analytical, and computational research on design and performance enhancement of structural systems. The Earthquake Engineering Research Institute (EERI) is a leader in dissemination of earthquake engineering research related information both in the U.S. and globally.
Research programs
The NSF Hazard Mitigation and Structural Engineering program (HMSE) supports research on new technologies for improving the behavior and response of structural systems subject to earthquake hazards; fundamental research on safety and reliability of constructed systems; innovative developments in analysis and model based simulation of structural behavior and response including soil-structure interaction; design concepts that improve structure performance and flexibility; and application of new control techniques for structural systems.
NSF also supports George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) that advances knowledge discovery and innovation for earthquakes and tsunami loss reduction of the nation's civil infrastructure, and new experimental simulation techniques and instrumentation.
NEES comprises a network of 15 earthquake engineering experimental equipment sites available for experimentation on-site or in the field and through telepresence. NEES equipment sites include shake-tables, geotechnical centrifuges, a tsunami wave basin, unique large-scale testing laboratory facilities, and mobile and permanently installed field equipment.
NEES Cyberinfrastructure Center (NEESit) connects, via Internet2, the equipment sites as well as provides telepresence, a curated central data repository, simulation tools, and collaborative tools for facilitating on-line planning, execution, and post-processing of experiments.
Earthquake simulation
The very first earthquake simulations were performed by statically applying some horizontal inertia forces based on scaled peak ground accelerations to a mathematical model of a building . With the further development of computational technologies, static approaches began to give way to dynamic ones.
Dynamic experiments on building and non-building structures may be physical, like shake-table testing, or virtual ones. In both cases, to verify a structure's expected seismic performance, some researchers prefer to deal with so called "real time-histories" though the last cannot be "real" for a hypothetical earthquake specified by either a building code or by some particular research requirements. Therefore, there is a strong incentive to engage an earthquake simulation which is the seismic input that possesses only essential features of a real event.
Sometimes, earthquake simulation is understood as a re-creation of local effects of a strong earth shaking.
Structure simulation
Theoretical or experimental evaluation of anticipated seismic performance mostly requires a structure simulation which is based on the concept of structural likeness or similarity. Similarity is some degree of analogy or resemblance between two or more objects. The notion of similarity rests either on exact or approximate repetitions of patterns in the compared items.
In general, a building model is said to have similarity with the real object if the two share geometric similarity, kinematic similarity and dynamic similarity. The most vivid and effective type of similarity is the kinematic one. Kinematic similarity exists when the paths and velocities of moving particles of a model and its prototype are similar.
The ultimate level of kinematic similarity is kinematic equivalence when, in the case of earthquake engineering, time-histories of each story lateral displacements of the model and its prototype would be the same.
Seismic vibration control
Seismic vibration control is a set of technical means aimed to mitigate seismic impacts in building and non-building structures. All seismic vibration control devices may be classified as passive, active or hybrid where:
passive control devices have no feedback capability between them, structural elements and the ground;
active control devices incorporate real-time recoding instrumentation on the ground integrated with earthquake input processing equipment and actuators within the structure;
hybrid control devices have combined features of active and passive control systems.
When ground seismic waves reach up and start to penetrate a base of a building, their energy flow density, due to reflections, reduces dramatically: usually, up to 90%. However, the remaining portions of the incident waves during a major earthquake still bear a huge devastating potential.
After the seismic waves enter a superstructure, there is a number of ways to control them in order to sooth their damaging effect and improve the building's seismic performance, for instance:
to dissipate the wave energy inside a superstructure with properly engineered dampers;
to disperse the wave energy between a wider range of frequencies;
to absorb the resonant portions of the whole wave frequencies band with the help of so called mass dampers.
Lead Rubber Bearing
Lead Rubber Bearing or LRB is a type of base isolation employing a heavy damping.
Heavy damping mechanism incorporated in vibration control technologies and, particularly, in base isolation devices, is often considered a valuable source of suppressing vibrations thus enhancing a building's seismic performance. However, for the rather pliant systems such as base isolated structures, with a relatively low bearing stiffness but with an high damping, the so-called "damping force" may turn out the main pushing force at a strong earthquake. The video shows a Lead Rubber Bearing being tested at the UCSD Caltrans-SRMD facility. The bearing is made of rubber with a lead core. It was a uniaxial test in which the bearing was also under a full structure load.
Building elevation control
Building elevation control is a valuable source of vibration control of seismic loading. Thus, pyramid-shaped skyscrapers continue to attract attention of architects and engineers because such structures promise a better stability against earthquakes and winds.
Besides, the elevation configuration can prevent buildings' resonant amplifications due to the fact that a properly configured building disperses the shear wave energy between a wide range of frequencies.
Earthquake or wind quieting ability of the elevation configuration is provided by a specific pattern of multiple reflections and transmissions of vertically propagating shear waves, which are generated by breakdowns into homogeneity of story layers, and a taper. Any abrupt changes of the propagating waves velocity result in a considerable dispersion of the wave energy between a wide ranges of frequencies thus preventing the resonant displacement amplifications in the building.
Tapered profile of a building is not a compulsory feature of this method of structural control. A similar resonance preventing effect can be also obtained by a proper tapering of other characteristics of a building structure, namely, its mass and stiffness. As a result, the building elevation configuration techniques permit an architectural design that may be both attractive and functional (see, e.g., Pyramid).
Seismic design requirements
Seismic design requirements depend on the type of the structure, locality of the project and its authorities which stipulate applicable seismic design codes and criteria. For instance, California Department of Transportation's requirements called The Seismic Design Criteria (SDC) and aimed at the design of new bridges in California incorporate an innovative seismic performance based approach.
The most significant feature in the SDC design philosophy is a shift from a force-based assessment of seismic demand to a displacement-based assessment of demand and capacity. Thus, the newly adopted displacement approach is based on comparing the elastic displacement demand to the inelastic displacement capacity of the primary structural components while ensuring a minimum level of inelastic capacity at all potential plastic hinge locations.
In addition to the designed structure itself, seismic design requirements may include a ground stabilization underneath the structure: sometimes, heavily shaken ground breaks up which leads to collapse of the structure sitting upon it.he following topics should be of primary concerns: liquefaction; dynamic lateral earth pressures on retaining walls; seismic slope stability; earthquake-induced settlement.
Earthquake construction
Earthquake construction means implementation of seismic design to enable building and non-building structures to live through the anticipated earthquake exposure up to the expectations and in compliance with the applicable building codes.
Design and construction are intimately related. To achieve a good workmanship, detailing of the members and their connections should be, possibly, simple. As any construction in general, earthquake construction is a process that consists of the building, retrofitting or assembling of infrastructure given the construction materials available.
The destabilizing action of an earthquake on constructions may be direct (seismic motion of the ground) or indirect (earthquake-induced landslides, soil liquefaction and waves of tsunami).
A structure might have all the appearances of stability, yet offer nothing but danger when an earthquake occurs. The crucial fact is that, for safety, earthquake-resistant construction techniques are as important as quality control and using correct materials. Earthquake contractor should be registered in the state of the project location, bonded and insured.
To minimize possible losses, construction process should be organized with keeping in mind that earthquake may strike any time prior to the end of construction.
Each construction project requires a qualified team of professionals who understand the basic features of seismic performance of different structures as well as construction management.
Earthquake engineering is the study of the behavior of buildings and structures subject to seismic loading. It is a subset of both structural and civil engineering. Eminent authority on seismic risk mitigation, Caltech professor George W. Housner is widely considered as the 'father' of the modern field of earthquake engineering. Stanford University professor John Blume’s contributions to the dynamics of structures have earned him the title of the 'father' of earthquake engineering too.
The main objectives of earthquake engineering are:
Understand the interaction between buildings or civil infrastructure and the ground.
Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure.
Design, construct and maintain structures to perform at earthquake exposure up to the expectations and in compliance with building codes
A properly engineered structure does not necessarily have to be extremely strong or expensive.The most powerful and budgetary tools of earthquake engineering are vibration control technologies and, in particular, base isolation.
Seismic loading
Seismic loading means application of an earthquake-generated agitation to a structure. It happens at contact surfaces of a structure either with the ground [6], or with adjacent structures , or with gravity waves from tsunami. Seismic loading depends, primarily, on:
Anticipated earthquake's parameters at the site
Geotechnical parameters of the site
Structure's parameters
Characteristics of the anticipated gravity waves from tsunami (if applicable).
Ancient builders believed that earthquakes were a result of wroth of Gods and, therefore, could not be resisted by humans. Nowadays, the people's attitude has changed dramatically though the seismic loads, sometimes, exceed ability of a structure to resist them without being broken, partially or completely.
Due to their mutual interaction, seismic loading and seismic performance of a structure are intimately related.
Seismic performance
Earthquake or seismic performance is an execution of a building's or structure's ability to sustain their due functions, such as its safety and serviceability, at and after a particular earthquake exposure. A structure is, normally, considered safe if it does not endanger the lives and wellbeing of those in or around it by partially or completely collapsing. A structure may be considered serviceable if it is able to fulfill its operational functions for which it was designed.
Basic concepts of the earthquake engineering, implemented in the major building codes, assume that a building should survive The Big One (the most powerful anticipated earthquake) though with partial destruction
Seismic performance evaluation
Engineers need to know the quantified level of an actual or anticipated seismic performance associated with the direct damage to an individual building subject to a specified ground shaking.
The best way to do it is to put the structure on a shake-table that simulates the earth shaking and watch what may happen next.[citation needed] Such kinds of experiments were performed still more than a century ago
Another way is to evaluate the earthquake performance analytically.
Seismic performance analysis
Seismic performance analysis or, simply, seismic analysis is a major intellectual tool of earthquake engineering which breaks the complex topic into smaller parts to gain a better understanding of seismic performance of building and non-building structures. The technique as a formal concept is a relatively recent development.
In general, seismic analysis is based on the methods of structural dynamics. For decades, the most prominent instrument of seismic analysis has been the earthquake response spectrum method which, also, contributed to the proposed building code's concept of today.
However, those spectra are good, mostly, for single-degree-of-freedom systems. Numerical step-by-step integration proved to be a more effective method of analysis for multi-degree-of-freedom structural systems with severe non-linearity under a substantially transient process of kinematic excitation.
Research for earthquake engineering
Research for earthquake engineering means both field and analytical investigation or experimentation intended for discovery and scientific explanation of earthquake engineering related facts, revision of conventional concepts in the light of new findings, and practical application of the developed theories.
The National Science Foundation (NSF) is the main United States government agency that supports fundamental research and education in all fields of earthquake engineering. In particular, it focuses on experimental, analytical, and computational research on design and performance enhancement of structural systems. The Earthquake Engineering Research Institute (EERI) is a leader in dissemination of earthquake engineering research related information both in the U.S. and globally.
Research programs
The NSF Hazard Mitigation and Structural Engineering program (HMSE) supports research on new technologies for improving the behavior and response of structural systems subject to earthquake hazards; fundamental research on safety and reliability of constructed systems; innovative developments in analysis and model based simulation of structural behavior and response including soil-structure interaction; design concepts that improve structure performance and flexibility; and application of new control techniques for structural systems.
NSF also supports George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) that advances knowledge discovery and innovation for earthquakes and tsunami loss reduction of the nation's civil infrastructure, and new experimental simulation techniques and instrumentation.
NEES comprises a network of 15 earthquake engineering experimental equipment sites available for experimentation on-site or in the field and through telepresence. NEES equipment sites include shake-tables, geotechnical centrifuges, a tsunami wave basin, unique large-scale testing laboratory facilities, and mobile and permanently installed field equipment.
NEES Cyberinfrastructure Center (NEESit) connects, via Internet2, the equipment sites as well as provides telepresence, a curated central data repository, simulation tools, and collaborative tools for facilitating on-line planning, execution, and post-processing of experiments.
Earthquake simulation
The very first earthquake simulations were performed by statically applying some horizontal inertia forces based on scaled peak ground accelerations to a mathematical model of a building . With the further development of computational technologies, static approaches began to give way to dynamic ones.
Dynamic experiments on building and non-building structures may be physical, like shake-table testing, or virtual ones. In both cases, to verify a structure's expected seismic performance, some researchers prefer to deal with so called "real time-histories" though the last cannot be "real" for a hypothetical earthquake specified by either a building code or by some particular research requirements. Therefore, there is a strong incentive to engage an earthquake simulation which is the seismic input that possesses only essential features of a real event.
Sometimes, earthquake simulation is understood as a re-creation of local effects of a strong earth shaking.
Structure simulation
Theoretical or experimental evaluation of anticipated seismic performance mostly requires a structure simulation which is based on the concept of structural likeness or similarity. Similarity is some degree of analogy or resemblance between two or more objects. The notion of similarity rests either on exact or approximate repetitions of patterns in the compared items.
In general, a building model is said to have similarity with the real object if the two share geometric similarity, kinematic similarity and dynamic similarity. The most vivid and effective type of similarity is the kinematic one. Kinematic similarity exists when the paths and velocities of moving particles of a model and its prototype are similar.
The ultimate level of kinematic similarity is kinematic equivalence when, in the case of earthquake engineering, time-histories of each story lateral displacements of the model and its prototype would be the same.
Seismic vibration control
Seismic vibration control is a set of technical means aimed to mitigate seismic impacts in building and non-building structures. All seismic vibration control devices may be classified as passive, active or hybrid where:
passive control devices have no feedback capability between them, structural elements and the ground;
active control devices incorporate real-time recoding instrumentation on the ground integrated with earthquake input processing equipment and actuators within the structure;
hybrid control devices have combined features of active and passive control systems.
When ground seismic waves reach up and start to penetrate a base of a building, their energy flow density, due to reflections, reduces dramatically: usually, up to 90%. However, the remaining portions of the incident waves during a major earthquake still bear a huge devastating potential.
After the seismic waves enter a superstructure, there is a number of ways to control them in order to sooth their damaging effect and improve the building's seismic performance, for instance:
to dissipate the wave energy inside a superstructure with properly engineered dampers;
to disperse the wave energy between a wider range of frequencies;
to absorb the resonant portions of the whole wave frequencies band with the help of so called mass dampers.
Lead Rubber Bearing
Lead Rubber Bearing or LRB is a type of base isolation employing a heavy damping.
Heavy damping mechanism incorporated in vibration control technologies and, particularly, in base isolation devices, is often considered a valuable source of suppressing vibrations thus enhancing a building's seismic performance. However, for the rather pliant systems such as base isolated structures, with a relatively low bearing stiffness but with an high damping, the so-called "damping force" may turn out the main pushing force at a strong earthquake. The video shows a Lead Rubber Bearing being tested at the UCSD Caltrans-SRMD facility. The bearing is made of rubber with a lead core. It was a uniaxial test in which the bearing was also under a full structure load.
Building elevation control
Building elevation control is a valuable source of vibration control of seismic loading. Thus, pyramid-shaped skyscrapers continue to attract attention of architects and engineers because such structures promise a better stability against earthquakes and winds.
Besides, the elevation configuration can prevent buildings' resonant amplifications due to the fact that a properly configured building disperses the shear wave energy between a wide range of frequencies.
Earthquake or wind quieting ability of the elevation configuration is provided by a specific pattern of multiple reflections and transmissions of vertically propagating shear waves, which are generated by breakdowns into homogeneity of story layers, and a taper. Any abrupt changes of the propagating waves velocity result in a considerable dispersion of the wave energy between a wide ranges of frequencies thus preventing the resonant displacement amplifications in the building.
Tapered profile of a building is not a compulsory feature of this method of structural control. A similar resonance preventing effect can be also obtained by a proper tapering of other characteristics of a building structure, namely, its mass and stiffness. As a result, the building elevation configuration techniques permit an architectural design that may be both attractive and functional (see, e.g., Pyramid).
Seismic design requirements
Seismic design requirements depend on the type of the structure, locality of the project and its authorities which stipulate applicable seismic design codes and criteria. For instance, California Department of Transportation's requirements called The Seismic Design Criteria (SDC) and aimed at the design of new bridges in California incorporate an innovative seismic performance based approach.
The most significant feature in the SDC design philosophy is a shift from a force-based assessment of seismic demand to a displacement-based assessment of demand and capacity. Thus, the newly adopted displacement approach is based on comparing the elastic displacement demand to the inelastic displacement capacity of the primary structural components while ensuring a minimum level of inelastic capacity at all potential plastic hinge locations.
In addition to the designed structure itself, seismic design requirements may include a ground stabilization underneath the structure: sometimes, heavily shaken ground breaks up which leads to collapse of the structure sitting upon it.he following topics should be of primary concerns: liquefaction; dynamic lateral earth pressures on retaining walls; seismic slope stability; earthquake-induced settlement.
Earthquake construction
Earthquake construction means implementation of seismic design to enable building and non-building structures to live through the anticipated earthquake exposure up to the expectations and in compliance with the applicable building codes.
Design and construction are intimately related. To achieve a good workmanship, detailing of the members and their connections should be, possibly, simple. As any construction in general, earthquake construction is a process that consists of the building, retrofitting or assembling of infrastructure given the construction materials available.
The destabilizing action of an earthquake on constructions may be direct (seismic motion of the ground) or indirect (earthquake-induced landslides, soil liquefaction and waves of tsunami).
A structure might have all the appearances of stability, yet offer nothing but danger when an earthquake occurs. The crucial fact is that, for safety, earthquake-resistant construction techniques are as important as quality control and using correct materials. Earthquake contractor should be registered in the state of the project location, bonded and insured.
To minimize possible losses, construction process should be organized with keeping in mind that earthquake may strike any time prior to the end of construction.
Each construction project requires a qualified team of professionals who understand the basic features of seismic performance of different structures as well as construction management.
Limestone and sandstone structures
Limestone is very common in architecture, especially in North America and Europe. Many landmarks across the world, including the pyramids in Egypt, are made of limestone. Many medieval churches and castles in Europe are made of limestone and sandstone masonry. They are the long-lasting materials but their rather heavy weight is not beneficial for adequate seismic performance.
Application of modern technology to seismic retrofitting can enhance the survivability of unreinforced masonry structures. As an example, from 1973 to 1989, the Salt Lake City and County Building in Utah was exhaustively renovated and repaired with an emphasis on preserving historical accuracy in appearance. This was done in concert with a seismic upgrade that placed the weak sandstone structure on base isolation foundation to better protect it from earthquake damage.
Timber frame structures
Timber framing dates back thousands of years, and has been used in many parts of the world during various periods such as ancient Japan, Europe and medieval England in localities where timber was in good supply and building stone and the skills to work it were not.
The use of timber framing in buildings provides their complete skeletal framing which offers some structural benefits as the timber frame, if properly engineered, lends itself to better seismic survivability
Light-frame structures
Light-frame structures usually gain seismic resistance from rigid plywood shear walls and wood structural panel diaphragms. Special provisions for seismic load-resisting systems for all engineered wood structures requires consideration of diaphragm ratios, horizontal and vertical diaphragm shears, and connector/fastener values. In addition, collectors or drag struts to distribute shear along a diaphragm length are required.
BOOKS ON EARTHQUAKE ENGINEERING
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