Transcript Lecture 14
1 Lecture#14 CE-312 Engineering Geology and Seismology Instructor: Dr Amjad Naseer Department of Civil Engineering N-W.F.P University of Engineering and Technology, Peshawar 2 Outlines of the Presentation • Seismograph and Seismogram • Accelerograph and Accelerogram 3 Seismograph and Accelerograph • The identification and evaluation of ground motion parameters requires access to measurements of strong motions in actual earthquakes. Accurate, quantities measurements of strong ground motion is critical for both seismological and earthquake engineering applications. As stated by the National Research Council Committee on Earthquake Engineering Research (Housner, 1982): “The recording of strong motion ground provides the basic data for earthquake engineering. Without a knowledge of the ground shaking generated by earthquakes, it is not possible to assess hazards rationally or to develop appropriate methods of seismic design”. 4 Seismograph and Accelerograph • The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. 5 Seismograph and Accelerograph • Although written description of earthquakes date back as far as 780 B.C., the first accurate measurements of destructive ground motions were made during the 1933 Long Beach, California earthquake. Measurement of ground motion has only advanced considerably since then, most rapidly in the past 20 years or so. 6 Seismograph and Accelerograph Zhang Heng's Seismoscope In 132 CE, Zhang Heng of China's Han dynasty invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (lit. instrument for measuring the seasonal winds and the movements of the Earth). The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu in 143 CE, the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks; this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragon's mouth. The first ever earthquake recorded by this seismograph was supposedly somewhere in the east. Days later, a rider from the east reported this earthquake. 7 Seismograph and Accelerograph Seismographs are used to measure relatively weak ground motion; the record the produce are called seismograms. Strong ground motions are usually measured by accelerograph and expressed in the form of accelerograms. The simplest type of seismograph can be illustrated by a mass-spring-damper single-degree-of-freedom (SDOF) system. 8 Seismograph and Accelerograph • A rotating drum is connected to the seismograph housing with a stylus attached to a mass. The mass is connected to the housing by a spring and dashpot arranged in parallel, and the housing is connected to the ground. • The relative movement of the mass and the ground will be indicated by the trace made by the stylus on the rotating drum. A typical seismograph station may have three seismographs oriented to record motion in the vertical and two perpendicular horizontal directions. • Seismoscope is a relatively inexpensive ground motion instrument. Seismoscopes are conical pendulums in which a metal stylus attached to a suspended mass inscribes a record of ground motion on a smoked glass plate, producing a two directional record. 9 Seismograph and Accelerograph • A rotating drum is connected to the seismograph housing with a stylus attached to a mass. The mass is connected to the housing by a spring and dashpot arranged in parallel, and the housing is connected to the ground. • The relative movement of the mass and the ground will be indicated by the trace made by the stylus on the rotating drum. A typical seismograph station may have three seismographs oriented to record motion in the vertical and two perpendicular horizontal directions. • Seismoscope is a relatively inexpensive ground motion instrument. Seismoscopes are conical pendulums in which a metal stylus attached to a suspended mass inscribes a record of ground motion on a smoked glass plate, producing a two directional record. 10