EARTHQUAKEENGINEERING

WHAT IS AN EARTHQUAKE?

An earthquake is a shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other.

STRUCTURE OF THE EARTH

The interior structure of the Earth is layered in spherical shells, like an onion. These layers can be defined by either their chemical or their rheological properties. Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravitational and magnetic fields of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.

HISTORY OF THE EARTH

• The history of Earth concerns the development of the planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.• Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.

HISTORY OF THE EARTH

• The first life forms appeared between 3.8 and 3.5 billion years ago. The earliest evidences for life on Earth are graphite found to be biogenic in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48-billion-year-oldsandstone discovered in Western Australia. Photosynthetic life appeared around 2 billion years ago, enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose. During the Cambrian period it experienced a rapid diversification into most major phyla. More than 99 percent of all species, amounting to over five billion species,[that ever lived on Earth are estimated to be extinct.[10][11] Estimates on the number of Earth's current species range from 10 million to 14 million,[12] of which about 1.2 million have been documented and over 86 percent have not yet been described.

• Geological change has been constantly occurring on Earth since the time of its formation and biological change since the first appearance of life. Species continuously evolve, taking on new forms, splitting into daughter species, or going extinct in response to an ever-changing planet. The process of plate tectonics has played a major role in the shaping of Earth's oceans and continents, as well as the life they harbor. The biosphere, in turn, has had a significant effect on the atmosphere and other abiotic conditions on the planet, such as the formation of the ozone layer, the proliferation of oxygen, and the creation of soil.

EARTHQUAKE MECHANISM

• The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution.

PROPAGATION OF SEISMIC WAVES

• The full elastic seismic wavefield that propagates through an isotropic Earth consists of a P-wave component and two shear (SV and SH) wave components. Marine air guns and vertical onshore sources produce reflected wavefields that are dominated by P and SV modes. Much of the SV energy in these wavefields is created by P-to-SV-mode conversions when the downgoing P wavefield arrives at stratal interfaces at nonnormal angles of incidence. Horizontal-dipole sources can create strong SH modes in onshore programs. No effective seismic horizontal-dipole sources exist for marine applications.

EARTHQUAKE PHENOMENA

• The phenomena of earthquakes differ greatly in accordance with the number, duration, and intensity of the shocks, and with the distance of the place of observation from that of the origin of the disturbance. One of the greatest of modern earthquakes is that of northern India of 1897, which is well summed up in the official report.

EARTHQUAKE MEASUREMENTS

• The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.

SEISMICITY- GLOBAL & LOCAL

EARTHQUAKE VIBRATIONS

• Our servo-hydraulic shake table can also be used for simulating vibration in vehicles. We carry out vibration testing not only of transported goods, but also of equipment that is secured to or in vehicle. The equipment is suitable for testing large, heavy items at low and moderate frequencies, while smaller and lighter items can be tested at high frequencies on SP's electromagnetic vibration rigs

FREE & FORCE VIBRATIONS FOR SINGLE DEGREE OF FREEDOM SYSTEM

• This document describes free and forced dynamic responses of single degree of freedom (SDOF) systems. The prototype single degree of freedom system is a spring-mass-damper system in which the spring has no damping or mass, the mass has no stiffness or damping, the damper has no stiffness or mass. Furthermore, the mass is allowed to move in only one direction. The horizontal vibrations of a single-story building can be conveniently modeled as a single degree of freedom system. Part 1 of this document describes some useful trigonometric identities. Part 2 shows how damped SDOF systems vibrate freely after being released from an initial displacement with some initial velocity. Part 3 covers the resposne of damped SDOF systems to persistent sinusoidal forcing.

FREE & FORCE VIBRATIONS FOR SINGLE DEGREE OF FREEDOM SYSTEM

• Consider the structural system shown in Figure 1, where: • f(t) = external excitation force • x(t) = displacement of the center of

mass of the moving object • m = mass of the moving object, fI = d

dt(mx˙(t)) = mx¨(t) • c = linear viscous damping coefficient,

fD = cx˙(t) k = linear elastic stiffness coefficient, fS = kx(t)

STRONG MOTION VIBRATION RECORDS

• The strong motion records were provided by the U.S. Geological Survey (USGS) National Strong-Motion Program (NSMP). At least one strong-motion record is available and has been processed for each building, and as many as sixteen seismic records are available for some buildings. Ambient vibration records were collected using velocimeters (velocity transducers). In order to test the efficiency of the ambient vibration for defining the dynamic parameters of structure, a set of three permanently instrumented buildings has been monitored using ambient vibration.

EARTHQUAKE SPECTRUM AND DESIGN SPECTRUM

• Earthquake Spectra, the professional journal of the Earthquake Engineering Research Institute (EERI), is published quarterly in both printed and online editions in February, May, August, and November. The printed edition is sent to EERI members as part of their membership benefits. The online edition is available free to EERI members who register for the online access. 

EARTHQUAKE SPECTRUM AND DESIGN SPECTRUM

Strength reduction factors (SRFs) continue to play a key role in obtaining inelastic spectra from the elastic design spectra for the ductility-based earthquake resistant design. This study proposes a new model to estimate the SRF spectrum in terms of a pseudo-spectral acceleration spectrum and ductility demand ratio with the help of two coefficients. The proposed model is illustrated for an elastoplastic oscillator in case of five recorded accelerograms and three ductility ratios. The model is used to carry out a parametric study for the explicit dependence of SRF spectrum on strong motion duration, earthquake magnitude, site conditions, and epicentral distance. It is shown that there is no clear and significant dependence of SRF spectrum on strong motion duration, while the dependence on earthquake magnitude, site conditions, and epicentral distance conforms to the trends reported by earlier investigations. In particular, it is confirmed that the dependence of SRF spectrum on earthquake magnitude cannot be ignored.

GROUND MOTION- EFFECT OF GROUND CONDITIONS

• The overall objective of this research is to improve the understanding of the damaging ground motions produced in earthquakes in order to develop better methods for seismic hazard assessment and mitigation in urban areas. Past earthquakes have shown that the amplification of motions due to surface-to-bedrock geology, 3D crustal structure, and topography have a major influence on seismic damage and loss in urban areas. Also of significant importance are the details of the rupture process on the fault, and the way a built structure is engineered.

GROUND MOTION- EFFECT OF GROUND CONDITIONS

• Two important local geologic factors that affect the level of shaking experienced in earthquakes are (1) the softness of the surface rocks and (2) the thickness of surface sediments. This image of the Los Angeles region combines this information to predict the total amplification expected in future earthquakes from local geologic conditions or site effects.

• As the waves propagate they are affected by the earth structure, such as changes in elastic properties resulting in effects such as constructive and destructive interference and basin amplification. Near the ground surface, strong shaking can result in nonlinear soil behavior or raise pore fluid pressure causing liquefaction. Likewise, the geometry of a man-made structure, the construction materials, the type of ground, and its anchorage in the ground affect its vulnerability to damage during the shaking. This research aims to understand each of these processes and to work with the seismic engineering community to bring the best estimates of strong ground shaking to engineering practice.

GROUND MOTION- EFFECT OF GROUND CONDITIONS

• Two important local geologic factors that affect the level of shaking experienced in earthquakes are (1) the softness of the surface rocks and (2) the thickness of surface sediments. This image of the Los Angeles region combines this information to predict the total amplification expected in future earthquakes from local geologic conditions or site effects.

EARTHQUAKE DAMAGES TO VARIOUS CIVIL ENGINEERING STRUCTURES

• Civil engineering structures were subject to enormous damage, considered to be worse than that incurred during the Great Kanto Earthquake, and this included the collapse of overhead Shinkansen rails, the collapse of overhead rails belonging to the Hanshin Hankyu Electric Railways, and the collapse and overturning of the main supports of the expressway. 

• This exceedingly shocking form of damage occurred to the sc overhead support--Mos, considered modern when built, between Ashiya Post Office and Uozaki. Many of the other overhead supports which received damage were constructed between 1965 and 1975, and most of them were designed and installed in accordance with the design policies of the day. The removal of these damage supports was carried out at a frantic pace in order to secure emergency transportation routs and enable recovery work to continue. 

• Damage also occurred in underground structures made of the same material in the metropolis. Underground structures were always considered safe from the effects of earthquakes until now, but subsidence also occurred to the roads which inters ect the Kobe Expressway. This subsidence caused the central pillar on the underground platform and the station ceiling of Daikai Station to collapse. It is thought that the main reason for the collapse of the underground station's central pillar was the fact that the epicenter of the quake was nearby and the fact that the rate of lateral movement was greater than originally provided for in the design, which indicates that greater care should be taken on cause surveys in the future. 

EARTHQUAKE DESIGN PROCEDURES

• Modern earthquake design has its genesis in the1920’s and1930’s. At that time earthquake design typically involved the application of 10% of the building weight as a lateral force on the structure, applied uniformly up the height of the building. Indeed it was not until the 1960’sthatstronggroundmotionaccelerographsbecame more generally available. These instruments record the ground motion generated by earthquakes. When used in conjunction with strong motion recording devices which were able to be installed at different levels within buildings themselves, it became possible to measure and understand the dynamic response of buildings when they were subjected to real earthquake induced ground motion. By using actual earthquake motion records as input to the, then, recently developed inelastic integrated time history analysis packages, it became apparent that many buildings designed to earlier codes had inadequate strength to withstand design level earthquakes without experiencing significant damage. However, observations of the in-service behaviour of buildings showed that this lack of strength did not necessarily result in building failure or even severe damage when they were subjected to severe earthquake attack. Provided the strength could be maintained without excessive degradation as inelastic deformations developed, buildings generally survived and could often be economically repaired. Conversely, buildings which experienced significant strength loss frequently became unstable and often collapsed.

• With this knowledge the design emphasis moved to ensuring that the retention of post-elastic strength was the primary parameter which enabled buildings to survive. It became apparent that some post-elastic response mechanisms were preferable to others. Preferred mechanisms could be easily detailed to accommodate the large inelastic deformations expected. Other mechanisms were highly susceptible to rapid degradation with 2 collapse a likely result. Those mechanisms needed to be suppressed, an aim which could again be accomplished by appropriate detailing. The key to successful modern earthquake engineering design lies therefore in the detailing of the structural elements so that desirable post-elastic mechanisms are identified and promoted while the formation of undesirable response modes are precluded. Desirable mechanisms are those which are sufficiently strong to resist normal imposed actions without damage, yet are capable of accommodating substantial inelastic deformation without significant loss of strength or load carrying capacity. Such mechanisms have been found to generally involve the flexural response of reinforced concrete or steel structural elements or the flexural steel dowel response of timber connectors. Undesirable post-elastic response mechanisms within specific structural elements have brittle characteristics and include shear failure within reinforced concrete, reinforcing bar bond failures, the loss of axial load carrying capacity or buckling of compression members such as columns, and the tensile failure of brittle components such as timber or under-reinforced concrete. Undesirable global response mechanisms include the development of a soft-storey within a building (where inelastic deformation demands are likely to be concentrated and therefore make high demands on the resistance ability of the elements within that zone), or buildings where the structural form or geometry is highly irregular, which puts them outside the simplifications made within the engineering models used for design.

DESIGN CODES

• A design code is a document that sets rules for the design of a new development in the United Kingdom. It is a tool that can be used in the design and planning process, but goes further and is more regulatory than other forms of guidance commonly used in the English planning system over recent decades. It can be thought of as a process and document – and therefore a mechanism – which operationalises design guidelines or standards which have been established through a masterplan process. The masterplan or design framework is the vision. It should be accompanied by a design rationale that explains the objectives, with the design code providing instructions to the appropriate degree or precision of the more detailed design work.

• In this way a design code may be a tool which helps ensure that the aspirations for quality and quantity for housing developments, particularly for large-scale projects, sought by the Government and other agencies are actually realised in the final schemes. It has the potential to deliver the consistency in quality exposed as lacking by CABE’s Housing Audit(2004).

• The following codes and standards have been identified as applicable, in whole or in part, to civil engineering design and construction of power plants.

• • American Association of State Highway and Transportation Officials (AASHTO)— Standards and Specifications

• • American Concrete Institute (ACI) - Standards and Recommended Practices

• • American Institute of Steel Construction (AISC) - Standards and Specifications

• • American National Standards Institute (ANSI) - Standards

• • American Society of Testing and Materials (ASTM) - Standards, Specifications, and Recommended Practices

• • American Water Works Association (AWWA) - Standards and Specifications •

• American Welding Society (AWS) - Codes and Standards

• • Asphalt Institute (AI) - Asphalt Handbook

• • State of California Department of Transportation (CALTRANS) Standard Specification • California Energy Commission - Recommended Seismic Design Criteria for Non-Nuclear Generating Facilities in California, 1989

• • Concrete Reinforcing Steel Institute (CRSI) – Standards

• Factory Mutual (FM) - Standards • National Fire Protection Association (NFPA) - Standards • California Building Standards Code (CBC) 2001 • Steel Structures Painting Council (SSPC) - Standards and Specifications