Earthquake Engineering Introduction PDF

Earthquake Engineering ROLANDO A. BITAGUN JR. PREFACE  Earthquakes were the cause of more than 1.5 million deaths worldwide during the 20th Century. During the beginning of the 21st Century the number of deaths was about half a million. This is an unacceptable finding, because earth...

Earthquake Engineering ROLANDO A. BITAGUN JR. PREFACE  Earthquakes were the cause of more than 1.5 million deaths worldwide during the 20th Century. During the beginning of the 21st Century the number of deaths was about half a million. This is an unacceptable finding, because earthquakes can no longer be regarded as natural disasters, since the main cause of this huge number of casualties is the inadequate seismic resistance of the building stock, lifelines and industry, which could be avoided. Earthquakes do not kill people, but the building collapse can do it. It is an unbelievable situation that, after a century of research works, each strong earthquake brings new surprises and creates the situation that new lessons have to be learnt.  The basic concepts of today’s Earthquake Engineering were born almost 70 years ago, when the knowledge about the seismic actions and structural response were rather poor. Many initial concepts were changed due to the progress in research works, but additional improvements still remain to be concretized for reaching a satisfactory level of seismic design.  The challenge for a proper seismic design is to solve the balance between earthquake demand and structural capacity. The earthquake demand corresponds to the effects of earthquake on the structure and depends on the ground motion modelling. Structural capacity is the structural ability to resist these effects without failure. Aspects to be considered to improve the understanding of Earthquakes  Improve earthquake monitoring - seismic hazard identification and risk assessment are critical components of earthquake mitigation strategy. Under this goal, a monitoring system, based on the regional networks of instrumented stations, on the use of satellite-based observations(GPS monitoring stations) and associated data centers, has been developed. The most useful data for seismic design are obtained from the seismic stations, by means of recorded accelerations, velocities or displacements.  Improve understanding of earthquake occurrence - in the last decades, Seismology has made significant progresses in understanding the basic physics of earthquakes. Together with these progresses, modern technologies such as Global Positioning System (GPS) allow seismologists to forecast the overall long-term seismic activity. Yet, earthquakes continue to be a major threat to our society, as we have witnessed during the seismic events of the last century and the beginning of the new one. A major difficulty is due to the fact that an earthquake involves a large number of elementary processes, so that, even if we understand the physics governing each elementary process, the complex interaction between them makes accurate forecasts of earthquakes very difficult.  Improve fundamental knowledge of earthquake effects - Among the most important contributions to reducing earthquake losses, there are both the improving of understanding and the modelling earthquake effects, including the source properties, the wave propagation from the source to site and the local conditions characterizing the site. This task implies the development of methods to generate synthetic seismograms for the expected future earthquakes, incorporating improved understanding of the rupture process and information about the fault type and the properties of the surrounding earth’s crust. At the same time, the identification of the parameters of ground motions causing soil liquefaction, land sliding and damage of structures (such as peak acceleration, ground velocity and displacement, shaking duration, spectral content, etc.).  Improve the seismic design of structures - A new facet of Earthquake Engineering research concerning the seismic structural response is based on the reliance of integrated experimentation, theory, databases and model-based computer simulations. Under this objective the priorities refer to improving the understanding of behavior and collapse mechanisms of various classes of structures under different earthquake types, in order to establish new methodologies for performance-based earthquake engineering. These new methodologies must consider different design philosophies for structures situated in low to moderate and strong seismic areas. The objective implies also developing new materials, new technologies and new structural systems for earthquake resistant structures.  Start development of next generation performance-based codes - The goal of these activities assures the ability to reduce seismic vulnerability of structural systems, learning from the lessons given by the last strong earthquakes and from the remarkable knowledge development during the last decades in the frame of Seismology. The main considered task is to transfer the accumulated results from the academic research works to the design practice, filling the existing gap between these to activities, disseminating upgrade guidelines and new codes, cooperating with professional associations, promoting education for practicing professionals. TOPICS INVOLVED IN SEISMIC DESIGN  Engineering Seismology- developed to solve the problems of the Earthquake hazard, is a branch of Seismology, having the purpose to use the seismological knowledge for the seismic design of buildings, by proposing the seismic actions function of the source and site characteristics.  Earthquake Engineering - with the task to solve the problems of construction vulnerability, is a branch of more general field, the Structural Engineering Science, having the purpose to develop specific methodologies for analyzing the effects of seismic actions on constructions, very different from that used in case of other actions like dead, live, wind, snow, etc., loads.  Seismic Design - collects the data given by Engineering Seismology referring to seismic loads and using the methodologies proposed by the Earthquake Engineering and performs a complex examination of structures, including numerical analysis, structural conformation, solutions for details, and eventually an engineering overview on the designed structure. The main scope of Seismic Design is to obtain the economical victory over a strong earthquake by reducing structural damage controlled by the designer. Earthquake Engineering  Earthquake Engineering is a 20th Century development. In 1963, the International Association for Earthquake Engineering (IAEE) was founded, aiming at organizing World Conferences on Earthquake Engineering (WCEE). Since the IAEE foundation, these Conferences have been held every four years. The last ones, presenting very important progresses in seismic concepts were held in Madrid in 1992, Acapulco in 1996, Auckland in 2000, Vancouver in 2004, and Beijing 2008. These World Conferences represent a wide arena giving the opportunity to scientists, engineers, industrial professionals and government officials to present their scientific and engineering works, to exchange ideas and knowledge for the mitigation of seismic risk. They also provide a common platform for delegates for all over the world to initiate new cooperation. Earthquake  It is a sudden, rapid shaking of the earth caused by the breaking and shifting of rocks beneath the earth surface.  It is the result of sudden release of energy in the earth’s crust that creates seismic waves.  It is the shaking of the earth due to the movement of the Earth’s crust Magnitude vs. Intensity  Intensity: The severity of earthquake shaking is assessed using a descriptive scale – the Modified Mercalli Intensity Scale.  Magnitude: Earthquake size is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake. When an earthquake occurs, its magnitude can be given a single numerical value on the Richter Magnitude Scale. However the intensity is variable over the area affected by the earthquake, with high intensities near the epicenter and lower values further away. These are allocated a value depending on the effects of the shaking according to the Modified Mercalli Intensity Scale. Ritcher Magnitude Scale Modified Mercalli Intensity Scale Main Types of Seismic Waves  Body Wave – is a wave that is generated by the release of energy at the focus that is the epicenter and moves in all directions travelling internally through the body of Earth. v P-Wave v S-Wave  Surface Wave – is a wave that moves along the surface of the Earth where buildings and people are. v Love Wave v Rayleigh Wave Body Wave Example Surface Wave Example Primary Wave Sample Secondary Wave Sample Love Wave Sample Rayleigh Wave Sample Earth’s Internal Structure Sources of Earthquake  Plate Tectonic Movement  Volcanic Eruption  Human Intervention: Explosions  Meteor Crash Plate Tectonic Movement  The earth’s crust is broken into separate pieces called tectonic plates. Recall that the crust is the solid, rocky, outer shell of the planet. It is composed of two distinctly different types of material: the less-dense continental crust and the more-dense oceanic crust. Both types of crust rest atop solid, upper mantle material. The upper mantle, in turn, floats on a denser layer of lower mantle that is much like thick molten tar.  Each tectonic plate is free-floating and can move independently. Earthquakes and volcanoes are the direct result of the movement of tectonic plates at fault lines. The term fault is used to describe the boundary between tectonic plates. Most of the earthquakes and volcanoes around the Pacific ocean basin—a pattern known as the “ring of fire”—are due to the movement of tectonic plates in this region. Other observable results of short-term plate movement include the gradual widening of the Great Rift lakes in eastern Africa and the rising of the Himalayan Mountain range. Tectonic Plates in the Philippines The plate tectonics in the Philippines is complex and includes plate boundaries that are changing rapidly. Several micro-plates are getting squeezed between two convergent plate margins. Stratigraphic evidence indicates cessation and reactivation of subduction at some trenches. The currently active volcanoes in the Philippines define two north-south trending arcs. The scale and type of volcanism varies from monogenetic cinder cone fields to large stratovolcanoes and calderas. Composition of volcanic rocks range from tholeiitic basalt to andesite to shoshonite.  Black triangles = active subduction zones with "teeth" on the over- riding plate  White triangles = inactive subduction zones with "teeth" on the over- riding plate, arrows = transform or major strike-slip faults,  Red triangles = volcanoes active in the last 10,000 years  Elastic Rebound Theory First proposed by M.F. Reid in 1906, attributes the occurrence of tectonic earthquakes to the gradual accumulation of strain in a given zone and the subsequent gradual increase in the amount of elastic forces stored. Where the plates collide, they may be locked in place, that is, these may be prevented from moving because of the frictional resistance along the plate boundaries. This causes building up of stresses along the plate edges until sudden slippage due to elastic rebound or fracture of the rock occurs, resulting in sudden release of strain energy that may cause the upper crust of the earth to fracture along a certain direction and form a fault. This is the origin of an earthquake. The gradual accumulation and subsequent release of stress and strain is described as elastic rebound. The elastic rebound theory postulates that the source of an earthquake is the sudden displacement of the ground on both sides of the fault, which is a result of the rupturing of the crustal rock.   Interplate earthquakes are earthquakes that occur along the boundaries of the tectonic plates  Intraplate earthquakes are earthquakes that occur within the plates themselves, away from the plate boundaries Motion Patterns of Plates  Collision: when two continental plates are shoved together  Subduction: when one plate plunges beneath another  Spreading: when two plates are pushed apart  Transform faulting: when two plates slide past each other Earthquake Measurement A seismograph is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismogram. A seismometer is the internal part of seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with seismograph. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes except for the mass on the spring, which has inertia and remains in the same place. As the seismograph shakes under the mass, the recording device on the mass records the relative motion between itself and the rest of the instrument, thus recording the ground motion. Reading a Seismogram Characteristics of Ground Motion The motion of the ground can be described in terms of displacement, velocity, or acceleration. The variation of ground acceleration with time, recorded at a point on the ground during an earthquake, is called an accelerogram. The ground velocity and displacement can be obtained by direct integration of an accelerogram. Typical ground motion records are called time histories—the acceleration, velocity and displacement time histories. From an engineering viewpoint, the amplitude, the frequency, and the duration of motion are the three important characteristics of the ground motion parameters. For structural engineering purposes, acceleration gives the best measure of an earthquake’s intensity. Volcanic Eruption  A volcanic eruption occurs when hot materials from the Earth's interior are thrown out of a volcano. Lava, rocks, dust, and gas compounds are some of the ejected materials.  Eruptions can come from side branches or from the top of the volcano. Some eruptions are terrible explosions that throw out huge amounts of rock and volcanic ash and kill many people. Some are quiet outflows of hot lava. Several more complex types of volcanic eruptions have been described by volcanologists. These are often named after famous volcanoes where that type of eruption has been seen. Some volcanoes may show only one type of eruption during a period of activity, while others may show a range of types in a series.  Volcanic eruptions can cause earthquakes, fast floods, mud slides, and rock falls. Lava can travel very far and burn, bury, or damage anything in its path, including people, houses, and trees. The large amount of dust and ash can cause roofs to fall, makes it hard to breathe, and is normally very smelly. The ground around the volcano is not secure and can cause big earthquakes.  The main good effect that volcanoes have on the environment is to give vitamins to the soil around them. Volcanic ash contains minerals that help plants grow, and if the ash is very soft, it will quickly get mixed into the soil. Nuclear Explosion  A nuclear explosion is an explosion that occurs as a result of the rapid release of energy from a high-speed nuclear reaction. The driving reaction may be nuclear fission or nuclear fusion or a multi-stage cascading combination of the two, though to date all fusion-based weapons have used a fission device to initiate fusion, and a pure fusion weapon remains a hypothetical device.  Atmospheric nuclear explosions are associated with mushroom clouds, although mushroom clouds can occur with large chemical explosions. It is possible to have an air-burst nuclear explosion without those clouds. Nuclear explosions produce radiation and radioactive debris.  The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives, but the energy produced by a nuclear explosive is millions of times more per gram and the temperatures reached are in the tens of megakelvin. Nuclear weapons are quite different from conventional weapons because of the huge amount of explosive energy they can put out and the different kinds of effects they make, like high temperatures and nuclear radiation.  The devastating impact of the explosion does not stop after the initial blast, as with conventional explosives. A cloud of nuclear radiation travels from the epicenter of the explosion, causing an impact to life forms even after the heat waves have ceased.  Any nuclear explosion (or nuclear war) would have wide-ranging, long- term, catastrophic effects. Radioactive contamination would cause genetic mutations and cancer across many generations Seismic Zoning The problem of designing economical earthquake-resistant structures rests heavily on the determination of reliable quantitative estimates of expected earthquake intensities in particular regions. However, it is not possible to predict with any certainty when and where earthquakes will occur, how strong they will be, and what characteristics the ground motions will have. Therefore, an engineer must estimate the ground shaking judiciously. A simple method is to use a seismic zone map, wherein the area is subdivided into regions, each associated with a known or assigned seismic probability or risk, to serve as a useful basis for the implementation of code provisions on earthquake-resistant design. The Philippines archipelago is divided into two seismic zones only. Zone 2 covers the provinces of Palawan (except Busuanga), Sulu and Tawi-tawi while the rest of the country is under Zone 4  Definition of Terms Stiffness – the ability of a structure to resist deformation Degree of Freedom (DOF) – the number of independent variables that define the possible positions or motions of a mechanical system in space. Frequency – the rate at which something occurs or is repeated over a particular period of time. Amplitude – the maximum extent of a vibration or oscillation, measured from the position of equilibrium. “Earthquakes do not kill people, but the buildings do" THE END