Module 1 - Introduction to Earthquake Engineering.pdf

PRINCIPLES OF EARTHQUAKE ENGINEERING Introduction to Earthquake Engineering BRIEF BACKGROUND Numerous earthquakes took place before people arrived on earth, as shown by the enormous movement in the location of the continents without any damage. Therefore, we may contend that damage is not caused by...

PRINCIPLES OF EARTHQUAKE ENGINEERING Introduction to Earthquake Engineering BRIEF BACKGROUND Numerous earthquakes took place before people arrived on earth, as shown by the enormous movement in the location of the continents without any damage. Therefore, we may contend that damage is not caused by earthquakes; rather, infrastructure suffers damage as a result of an earthquake. That is, a poor built environment should be held responsible for the catastrophic effects of earthquakes. Therefore, even the largest earthquakes produce little to no damage when infrastructure is built to take earthquake effects into consideration! However, earthquakes are the most devastating of all natural disasters due to their historical repercussions on human civilization. The main cause is that they can happen with little or no warning and can wreak havoc across a wide area. Before we investigate the design idea. We start off by giving a general review of earthquakes, their effects on structures, sources, and types in this module. OBJECTIVE OF THE TOPIC At the end of this topic students will 1. Explain the concept of earthquake 2. Describe earthquakes, their worldwide distribution, what causes them, their likely damage mechanisms. TOPIC OUTLINE Earthquake Seismicity Causes of earthquake Fault Seismic Waves Earthquake Damage Mechanisms Earthquake Source Models Seismic Risk Evaluation Earthquake and Ground Motion Prediction EARTHQUAKE Earthquake is a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth. Based on USGS EARTHQUAKE Earthquakes can cause local soil failure, surface ruptures, structural damage and human deaths EARTHQUAKE The most significant earthquake effects on buildings or their structural components result from the seismic waves that propagate outwards in all directions from the earthquake focus These different types of waves can cause significant ground movements up to several hundred miles from the source EARTHQUAKE The movements depend on the intensity, sequence, duration and the frequency content of the earthquake induced ground motions EARTHQUAKE Characterization Casualties 10,000 people die each year Damage losses Destruction ground shaking SEISMICITY Seismicity is a description of the relationship of time, space, strength, and frequency of earthquake occurrences within a certain region, and its understanding is the foundation of earthquake study. SEISMICITY For design purposes ground motion is described by the history of hypothesized ground acceleration and is commonly expressed in terms of the response spectrum derived from that history When records are unavailable or insufficient, smoothed response spectra are devised for design purposes to characterize the ground motion SEISMICITY In principle, the designers describe the ground motion in terms of two perpendicular horizontal components and a vertical component for the entire base of the structure SEISMICITY SEISMICITY Earthquakes initiate several phenomena or agents, termed seismic hazards which can cause significant damage to the built environment these include fault rupture, vibratory ground motion (i e shaking), inundation (e g tsunami, seiche, dam failure), various kinds of permanent ground failure (e g liquefaction), SEISMICITY SEISMICITY SEISMICITY Earthquake focus or hypocenter is the point from which the waves first emanate. The point on the ground surface directly above the focus is called the earthquake epicenter. SEISMICITY Foci are classified into two namely shallow and deep focus. earthquakes with foci from 70 to 300 kilometers deep are called intermediate focus and those below this depth are termed deep focus. Some intermediate and deep focus earthquakes are located away from the Pacific region, in the Hindu Kush, in Romania, in the Aegean Sea, and under Spain. The shallow-focus earthquakes (< 70 Kms depth) is the deadliest and contribute about three-quarters of the total energy released in earthquakes throughout the world. SEISMICITY Aftershocks are numerous earthquakes, usually smaller that follow most moderate to large shallow earthquakes in the ensuing hours and even in the next several months. Aftershocks are sometimes energetic enough to cause additional damage to already weakened structures. A few earthquakes are preceded by smaller foreshocks from the source area, and it has been suggested that these can be used to predict the main shock. CAUSES OF EARTHQUAKES 1. Tectonic result from motion between a number of large plates comprising the earth’s crust or lithosphere 2. Explosions 3. Volcanic Earthquakes 4. Collapse Earthquakes 5. Large Reservoir Induced Earthquakes CAUSES OF EARTHQUAKES An earthquake is a transient violent movement of the Earth’s surface that follows a release of energy in the Earth’s crust CAUSES OF EARTHQUAKES FAULT These are offsets of geological structure; may range in length from a few meters to many kilometers and are drawn on a geological map as continuous or broken line FAULT FAULT FAULT 1. Movement of faults Slow slip produces no ground shaking Sudden rupture due to earthquake, most famous is the San Andreas fault Shallow focus earthquakes much shorter and shows much less offset. Fault rupture majority of earthquakes does not reach the surface Geological mappings and geophysical work show that faults seen at the surface sometimes extend to depths of tens of kilometers in the Earth’s crust FAULT 2. Inactive faults Most plotted on geological maps are now inactive faults New discovery are also discovered from fresh ground breakage during an earthquake Thus, Delineated by a line of cracks FAULT 3. Active faults Primary interest in seismology and earthquake engineering Rock displacement expected to occur Exists in a well-defined plate edge regions of the earth Sudden fault displacement FAULT 4. Fault displacement almost entirely horizontal San Francisco earthquake along the San Andreas fault Large vertical motion occurrence as shown in the figure FAULT Several fault mechanisms exist depending on how the plates move with respect to one another Housner 1973 The most common mechanisms of earthquake sources are described FAULT Dip slip faults one block moves vertically with respect to the other Strike slip faults the adjacent blocks move horizontally past one another TECTONIC QUAKES Slip produced large earthquakes along faults Transform faults in these regions, plates slide past each other continent to continent collisions collision zones are regions of high present day seismic activity SEISMIC WAVES Primary or P wave – the faster body wave. Its motion is the same as that of a sound wave, in that, as it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock These P waves, just like sound waves, can travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans. SEISMIC WAVES Secondary wave – the slower body wave. As an S wave propagates, it shears the rocks sideways at right angles to the direction of travel. Thus, at the ground surface S waves can produce both vertical and horizontal motions. The S waves cannot propagate in the liquid parts of the Earth, such as the oceans and their amplitude is significantly reduced in liquefied soil. SEISMIC WAVES Surface wave - third general type of earthquake wave. Such waves correspond to ripples of water that travel across a lake. Most of the wave motion is located at the outside surface itself, and as the depth below this surface increases, wave displacements become less and less. Surface waves in earthquakes can be divided into two types. SEISMIC WAVES Love wave - Its motion is essentially the same as that of S waves that have no vertical displacement; it moves the ground side to side in a horizontal plane parallel to the Earth’s surface, but at right angles to the direction of propagation. SEISMIC WAVES Rayleigh wave. Like rolling ocean waves, the pieces of rock disturbed by a Rayleigh wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling. EARTHQUAKE DAMAGE MECHANISMS Ways Earthquakes can Damage Structures 1. by inertial forces generated by severe ground shaking. 2. by earthquake induced fires. 3. by changes in the physical properties of the foundation soils (e.g. consolidation, settling, and liquefaction). 4. by direct fault displacement at the site of a structure. 5. by landslides, or other surficial movements. 6. by seismically induced water waves such as seismic sea waves (tsunamis) or fluid motions in reservoirs and lakes (seiches). 7. by large-scale tectonic changes in ground elevation. EARTHQUAKE SOURCE MODELS The subject of source models is an area of study for seismologists, the results of which are fundamental to our understanding of the nature of ground motion. From amidst the complexities of this major study area several key parameters are evident as being of interest to earthquake engineers, some of which have already been introduced, such as fault length, fault width, fault displacement (or slip), stress drop on a fault, and, of course, earthquake magnitude. SEISMIC RISK EVALUATION Because of the difficulties involved in seismic hazard evaluation, earthquake design criteria in different areas of the world vary, from well codified to inadequate or non-existent. Hence, depending on the location and nature of the project concerned, seismic risk evaluation ranging from none through arbitrary to thoroughgoing may be required SEISMIC RISK EVALUATION Regional seismicity or risk maps recommended by seismic design codes usually do not attempt to reflect geological conditions nor to take into account variations due to soil properties. It is necessary, therefore, for critical construction in populated regions to make special geological-engineering studies for each site, the detail, and level of concern which is used depending on the density of occupancy as well as the proposed structural type. In inhabited areas, more casualties are likely to result from a failed dam or a damaged nuclear reactor, for example, than from a damaged oil pipeline SEISMIC RISK EVALUATION Three factors which must be considered in assessment of seismic risk of a site have been well-defined in recent times Geological Input - Provision of a structural geologic map, Compilation of active faults in the region and the type of displacement (e.g., left-lateral, strike-slip, etc.). SEISMIC RISK EVALUATION Three factors which must be considered in assessment of seismic risk of a site have been well-defined in recent times Seismological Input - Procedures for the estimation of ground shaking parameters for optimum engineering design are still in the early stages and many are untested. SEISMIC RISK EVALUATION Three factors which must be considered in assessment of seismic risk of a site have been well-defined in recent times Soils Engineering Input - When there is geological indication of the presence of structurally poor foundation material (such as in flood plains and filled tidelands), a field report on the surficial strata underlying the site is advisable. EARTHQUAKE AND GROUND MOTION PREDICTION The most important seismological aspect of hazard mitigation is the prediction of the strong ground motion likely at a particular site. Nevertheless, the aspects of earthquake prediction that still receive the most publicity are the prediction of the place, size and time of the earthquake. Of course, prediction of the region where earthquakes are likely to occur has long been achieved by seismicity studies using earthquake observatories. In addition, useful probability estimates of long-term hazard can be inferred from geological measurements of the slip rate of faults, regional strain changes, and so on. SUMMARY Earthquake is a term used to describe a sudden slip on a fault. Earthquakes has Natural and Artificial sources Faults are offsets of geological structure Seismic Waves has 3 types according to Bolt these are P, S and Surface Waves DISCUSSION 1. How do we determine the seismicity of a Metro Manila and select a set of appropriate ground motions that can match the seismicity of the location. 2. What identify a plate in the Philippines and determine the following Earthquake type Slip rate (v) (mm/year) Recurrence time (year) FURTHER READINGS BANGASH, M. Y. H. 2011. Earthquake Resistant Buildings: Dynamic Analyses, Numerical Computations, Codified Methods, Case Studies and Examples, London, UK, Springer. BOLT, B. A. 2008. The nature of earthquake ground motion. In: NAEIM, F. (ed.) The Seismic Design Handbook. US: Springer US BOZORGNIA, Y. & BERTERO, V. 2006. Earthquake engineering: from engineering seismology to performance-based engineering, UK, Taylor & Francis. BOZORGNIA, Y. & V.BERTERO, V. (eds.) 2004. EARTHQUAKE ENGINEERING: From Engineering Seismology to Performance-Based Engineering, Florida CRC Press LLC. ELNASHAI, A. S. & SARNO, L. D. 2015. Fundamentals of earthquake engineering - From Source to Fragilitys, United Kingdom, John Wiley & Sons, Ltd. HOUSNER, G. W. 1984. Historical view of earthquake engineering. Proceedings of the 8th World Conference on Earthquake Engineering, 1, 25–39. MANOHAR, S. & MADHEKAR, S. 2015. Seismic Design of RC Buildings Theory and Practice, India, Springer India OKAMOTO, S. 1973. Introduction to Earthquake Engineering, New York. , Wiley.