TB-Lecture11-Earthquakes

TB-Lecture11-Earthquakes - EGN-5439 The Design of Tall...

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EGN EGN -5439 The Design of Tall Buildings 5439 The Design of Tall Buildings Lecture #11 Lecture #11 Earthquakes Earthquakes © L. A. Prieto-Portar - 2008
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On 8 October 2005, an earthquake of magnitude 7.6 hit Islamabad, Pakistan, killing 30,000 and seriously injuring another 60,000. Some structures collapsed next to others of the same age that remained intact. This zone was classified as 2b (magnitude 5.5 to 6.5) considered as only moderate. The UBC was applied by private consultants.
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Seismic analysis and design is commonly based on the simplistic model of a structure’s behavior under seismic “static” loads. Important structures however, require that highly competent engineers know how to analyze structures under complex dynamic loads, such as gusting high winds, earthquakes and bomb blasts. Examples of these dynamic loads are predominantly earthquakes. However, there is an increasing interest on the effects from bomb blasts. Other common dynamic loads are the operation of very heavy or unbalanced machinery, mining, construction (such as pile driving, deep dynamic compaction, etc), heavy traffic, wind and wave actions. The study of most dynamic loads show patterns that can be used to simplify their study. Some of these simplifications are shown in the following slides.
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This plot represents the intensity of the load from a low-speed machine versus time upon its foundation.
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The previous real-time plot is typically simplified to this type of sinusoidal idealization.
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This diagram shows the loading upon the soil below a foundation invert due to a vibrating machine. Notice the static load offset.
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A rotating machine that has an unbalanced mass will generate these centrifugal forces.
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Dynamic loads vary in their magnitude, direction or position with time. It is possible for more than one type of variation to coexist. Earthquake loads, for example, vary both in magnitude and direction. Thus, they have three orthogonal directions and their corresponding rotation components: a total of six component forces and moments which each vary in magnitude with time. The figure above could be a wheel load rolling over a bridge deck, and is the instance of a force that varies in location with time. This is a periodic load, and the era of load duration is a cycle of motion. The time taken for each cycle is the period. The inverse of the period is the number of cycles per second, the frequency of the load.
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A simplified loading diagram of the single impact of a steel hammer upon a steel plate.
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Contrast the simple hammer plot on the previous slide to this plot that shows the vertical acceleration of soil particles close to a pile driving hammer when it hits the pile head-cushion interface.
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The blast wave or shock wave that is caused by the detonation of a conventional explosive such as TNT or ANFO (ammonium nitrate/fuel oil) results in the rapid release of a large amount of energy. This is shown by the peak overpressure (pressure above atmospheric pressure) whose front consists of highly compressed air. The rapid
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This note was uploaded on 08/29/2011 for the course CES 4600 taught by Professor Staff during the Fall '08 term at FIU.

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TB-Lecture11-Earthquakes - EGN-5439 The Design of Tall...

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