AECT360 Lecture 26

8/2/2019 AECT360 Lecture 26

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Lecture 26 - Page 1 of 11

Lecture 26 Seismic Principles

Basic Seismicity:

An earthquake is the sudden oscillatory movement of the ground.

Through the study of seismology, it has been found that earthquakes arecaused by plate tectonics. Basically, the earths crust is made ofplates, similar to an egg shell. These plates are being heated by theliquid-hot magma and are under a tremendous amount of thermal stressand thermal elongation. The boundary between the plates is called afault. Perhaps the most famous fault in the US is the San Andreas Faultlocated in California. Eventually, these plates slide relative to each otherand release energy. This energy is the destructive damage known as anearthquake.

The map below shows the location of the major plates world wide:

Notice that most of the faults (plate boundaries) are located in oceans. An

earthquake occurring in the ocean creates destructive waves known astsunamis.

The epicenter of an earthquake is the point on the earths surfacelocated directly above the hypocenter (or, focus). The exact location ofan earthquake is generally described by the geographic location of theepicenter and the focal depth.


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The graphic below shows a cut-away view of plate tectonics:

During an earthquake, seismographs are used to record ground motionin terms of displacement, velocity and acceleration. Below is a graphicshowing the basic operation of a seismograph:


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Below is a photo of a modern, portable seismometer (an instrument thatmakes seismographs):

Below is a seismograph showing the ground-induced acceleration of theEl Centro, California earthquake of 1940:


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Below is the USGS Earthquake Hazard Map. The numbers represent therelative potential hazard for earthquakes compared to areas with no knownseismicity such as Florida and much of Texas.

Earthquake Magnitude, Intensity & Damage:

Earthquakes are measured in several ways, and there is no exactcorrelation between the methods.

a) Earthquake Magnitude:

The magnitude of an earthquake is measured by the familiar

Richter Scale. The Richter magnitude, M, is calculated from themaximum amplitude of the seismograph trace. A0 is theseismometer reading produced from a standard calibratedearthquake of amplitude = 3.94 x 10-5 inches.

M = log10 )(0A

A

The Richter magnitude is on a logarithmic scale. That means thatan earthquake with a magnitude 4.0 is 10 times as powerful as anearthquake with magnitude 3.0. The Richter scale is between 0 10 with the most powerful earthquakes ever recorded being in the8.7 8.9 range. In theory, a magnitude 10 earthquake would bepowerful enough to completely destroy the earth. However, thenature of the rock strata is such that the rocks will liquefy longbefore the doomsday earthquake could occur.


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The table below shows the approximate correlation betweenRichter magnitudes, peak ground acceleration (PGA) andduration:

Magnitude: Peak Ground Acceleration (%g): Duration (sec):

5.0 0.09 25.5 0.15 66.0 0.22 126.5 0.29 187.0 0.37 247.5 0.45 308.0 0.50 348.5 0.55 37

Below is a graphic of the energy released by earthquakescompared to other nasty things:

b) Earthquake Intensity:

The Modified Mercalli Intensity (MMI) scale is based uponobserved damage, and has no real scientific basis. It ranges from Ito XII, where the PGA ranges from 0.0 g up to 0.90 g for an MMI ofXII.


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Below are some pictures from actual earthquake damage:

Japan - 1964

Japan - 1964


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Anchorage, Alaska 1964

San Francisco 1906

Kobe, Japan 1995


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Earthquake Effects on Structures

When the ground shakes, objects fastened to the ground (i.e., buildingsand structures) shake along with it. If the structure were to stay incontinuous motion under constant velocity along a straight line, it would

experience little additional damage much like sitting in a moving vehicleduring continuous velocity in a straight line. The problem is that theground-induced motion of an earthquake is NOT continuous and hasmultitude of quick starts and stops.

Consider that the simplest structure acts like a Single Degree ofFreedom (SDOF) mass model, which is essentially an upside-downpendulum:

A more complex structure, one in which has multiple floors for example,behaves as a series of masses and is called a Multiple Degree ofFreedom (MDOF) mass model:

Mass

Real single-storystructure

Assumed single degreeof freedom (SDOF)mass model

Spring simulatesthe structuresstiffness

Lumped mass simulates thestructures weight

Real multiple-storystructure

Floor mass

Assumed multipledegree of freedom(MDOF) mass model


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The ground-induced motion acting on a structure creates a Base Shearwhich is in response to the momentum of the mass in motion of thestructure being suddenly stopped. Therefore, the heavier the structure

the larger the base shear.

Direction ofground motion

Story Drift

Structure initially at rest Structure responding toinitial ground motion

Structure responding toreverse ground motion

Base shear (as a% of buildingweight)


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Seismic Retrofit:

In an era of increasing new construction costs, many owners opt to retrofittheir existing buildings to accommodate the threat of a potentiallydevastating seismic event instead of building a brand new structure.

Below are several graphics depicting various retrofit solutions.


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Addition of dampersand/or bracing(shown RED)

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AECT360 Lecture 26