Creating security & Earthquakes Earthquake Leads to as well as Results

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Creating security & Earthquakes Earthquake Leads to as well as Results Powered By Docstoc
					  Built to Resist Earthquakes
                                                 The Path to Quality Seismic Design and Construction

      Briefing Paper 1
     Briefing Paper 1
    Building Safety and Earthquakes
    Building Safety and Earthquakes
    Part A: Earthquake Shaking and Building Response
    Part A: Earthquake Shaking and Building Response

     Introduction                                              tion. The objective of Briefing Paper 1 (Parts A
                                                               to D) is to inform the stakeholders and partici-
     This Briefing Paper 1, Building Safety and                pants in the design and construction process,
     Earthquakes, consists of four parts describing            including building inspectors and owners, about
     earthquakes and their effects on buildings.               the basic principles of earthquake-resistant
     Part A provides an overview of how earthquakes            building design.
     occur and the ground shaking motion they
     produce. It also explains why different indi-        Earthquake Causes and Effects
     vidual buildings respond differently to the same
     ground motion. Parts B to D build on that            Most earthquakes are caused by rock movement
     information to explain how earthquake motion         along rupturing faults located in the earth’s
     creates forces acting on a building, to describe     crust. On a global scale, the earth’s crust is
     the structural systems used                                                divided into separate
     to resist earthquakes, and      There are more than 160 known              sections known as plates,
     to define the “load paths”                                                 as shown in Figure 1.
                                     active faults located in California.
     of earthquake forces                                                       Major faults are typically
     within buildings.                                                          located at plate bound-
                                                          aries. In California, many lesser faults occur
     Severely damaging earthquakes have repeatedly        near the boundary of the Pacific and the North
     demonstrated the importance of improving the         American plates, which, in California, is
     quality of both earthquake design and construc-      defined by the San Andreas fault. However,




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                                                                                                         40 °
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                                                                                                         40 °
                                                                                                         40

                                                                                                         60 °
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           180 °
           180           120 °
                          120             60 °
                                          60              0°
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                                                                         60            120 °
                                                                                        120          180 °
                                                                                                      180

                                 Figure 1. Global plates and plate boundaries.




Briefing Paper 1, Part A                                                                                          1
ATC/SEAOC Joint Venture Training Curriculum
                    Am p litud e


                                                                                  T im e




                                                    Pe rio d
                                                 (o ne cycle )

                        Figure 2. Cyclic wave of constant amplitude and period.

other parts of California also contain faults. In    Everything in the path of a seismic wave will be
fact, there are more than 160 known active           shaken. However, the amount of ground motion
faults located in this state. New faults continue    at any given location depends on three primary
to be discovered, usually when an unexpected         factors. One factor is the distance between the
earthquake occurs. Essentially, earthquakes can      site and the source location of the earthquake,
affect any location within California, potentially   known as the focus or hypocenter, which in
causing significant damage and loss of life.         California may range from 2 to 15 miles under-
                                                     ground. The shallower the focus, the stronger
Faults move or “slip” when shear stresses deep       the waves will be when they reach the surface.
underground exceed the ability of the com-           As a general rule, the intensity (severity) of
pressed faulted rock to resist those stresses.       ground shaking diminishes with increasing
Fault slip can move the                                                   distance from the source.
nearest ground surface                                                    Buildings located less than
vertically, laterally, or in
                                   A magnitude 7.0 earthquake             15 kilometers (9.3 miles)
some combination. When             releases 31.5 times more               from certain types of faults
this slip occurs suddenly, it      energy than does a magnitude           are required by the 1997
causes seismic shock waves         6.0 earthquake.                        Uniform Building Code
to travel through the ground,                                             (UBC) to be designed to
similar to the effect seen                                                withstand the stronger
when tossing a pebble onto the surface of still      shaking expected in these near-source zones.
water. These seismic waves cause the ground          Maps produced by the California Division of
shaking that is felt during an earthquake.           Mines and Geology and available from the
                                                     International Conference of Building Officials
Ground motion contains a mix of seismic waves        (ICBO) indicate where these faults are located.
having two primary characteristics as shown in
Figure 2. One is the wave amplitude, which is a      The second factor is the total energy released
measure of the size of the wave. The other is its    from the earthquake, measured by its magni-
period, which is a measurement of the time           tude. Because the magnitude scale is logarith-
interval between the arrival of successive peaks     mic, a magnitude 7.0 earthquake releases 31.5
or valleys, known as one cycle. This concept of      times more energy than does a magnitude 6.0
a time measurement can also be expressed as          earthquake. The ground shaking intensity at a
frequency = 1/period, the number of cycles           given location is greater for the magnitude 7.0
occurring per second.                                earthquake, but not 31.5 times greater. Instead,
                                                     the larger energy release produces shaking that




2                                                                                    Briefing Paper 1, Part A
                                                                 ATC/SEAOC Joint Venture Training Curriculum
                                           D ista n ce fro m
                                              e p ice n te r




                                           F au lt
                                                                                              s
                                                                                         cu
                                                                                    fo
                                                                                m
                               E a rth qu a ke                           fr o                         S o il a t
                               m a gn itu d e                       ce                                th e site
                                                          st   an
                                                     Di



                                    F ocu s o r h yp o ce n te r

                  Figure 3. Common terms and factors affecting shaking intensity at a given site.

    is felt over larger distances because the ruptured                              of the earthquake (amount of energy released),
    fault length is greater. Also, the shaking from a                               and the type of soil or rock at the site. These
    larger-magnitude earthquake often lasts longer,                                 factors are illustrated in Figure 3, which also
    because more time is needed for the longer                                      shows the location of the epicenter (point on
    rupture to release the greater energy.                                          ground surface directly above the hypocenter).
    The last of the three primary factors is the                                    More complex factors, such as the type of
    nature of the soil or rock at the site. Generally,                              faulting action, the direction of propagation of
    sites with deep soft soils or loosely compacted                                 the fault rupture, and the frequency range of the
    fill will be more strongly shaken than sites with                               waves, can increase or decrease the severity
    stiff soils, soft rock, or hard rock. For example,                              (intensity) of the local shaking. Consequently,
    during the 1989 Loma Prieta earthquake, the                                     actual ground motion cannot be precisely
    shaking experienced in the San Francisco                                        predicted. However, based on the recorded
    Marina District, which is underlain by mud                                      motions of past earthquakes obtained from
    nearly 100 feet thick, was from three to four                                   instruments located both inside and outside
    times stronger than the shaking measured only a                                 buildings, it is possible to estimate the probable
    few blocks away on bedrock, near the Golden                                     maximum ground motion given the values for
    Gate Bridge. The building codes for new                                         the three factors. These estimates form the basis
    construction (e.g., the 1997 UBC) and the                                       for seismic design requirements contained in
    NEHRP Guidelines for the Seismic Rehabilita-                                    modern building codes.
    tion of Buildings (FEMA-273 report) use
    adjustment factors to account for the stronger                                  Building Response Characteristics
    shaking at soft soil sites and fill sites.
                                                                                    Different individual buildings shaken by the
    To summarize: the intensity of ground motion                                    same earthquake respond differently. The
    at a specific site, caused by a specific earth-                                 effects of earthquake ground shaking depend on
    quake, depends primarily on three factors: the                                  the specific response characteristics of the type
    distance between the source (also known as                                      of structural system used. One important
    focus or hypocenter) and the site, the magnitude




Briefing Paper 1, Part A                                                                                                                 3
ATC/SEAOC Joint Venture Training Curriculum
                                                                                             Tall, Soft, or Weak
      Split Levels                   Cruciform Plan          L-Shaped Plan                        First Story


                       Figure 4. Examples of buildings with irregular configurations.


building characteristic is the fundamental period         the first story is taller or “softer” than the other
of vibration of the building (measured in                 stories. Irregularly shaped buildings, shown in
seconds). The fundamental period of a building            Figure 4, are subject to special design rules
depends in a complex way on the stiffness of              because otherwise they can suffer greater
the structural system, its mass, and its total            damage than regularly shaped buildings.
height. Seismic waves with periods similar to
that of the building will cause resonance, and            References
amplify the intensity of earthquake forces the
building must resist.                                     ATC, 1997, NEHRP Guidelines for the Seismic
                                                          Rehabilitation of Buildings, prepared by the
Structural systems using concrete or masonry              Applied Technology Council for the Building
shear walls are stiff and result in buildings with        Seismic Safety Council, published by the
short periods, whereas more flexible moment-              Federal Emergency Management Agency,
frame systems have longer periods. In general,            FEMA 273 Report, Washington, DC.
a large portion of the earthquake energy is
contained in short-period waves. Therefore,               ICBO, 1997, Uniform Building Code, Interna-
short-period buildings with stiff structural              tional Conference of Building Officials, Whit-
systems are designed for larger forces than long-         tier, California.
period, flexible, buildings. This concept is also
applicable to the amount of force individual                     About this Briefing Paper Series
structural seismic elements and their compo-
                                                         Briefing papers in this series are concise, easy-to-read summary
nents must resist. Stiff elements must be made           overviews of important issues and topics that facilitate the
stronger because they will attempt to resist             improvement of earthquake-resistant building design and construc-
larger earthquake forces than flexible elements          tion quality.
in the same structural system.                           This briefing paper was prepared by the ATC/SEAOC Joint Venture,
                                                         a partnership of the Applied Technology Council (ATC) and the
Shape or configuration is another important              Structural Engineers Association of California (SEAOC). Funding
                                                         for the series was provided by the California Seismic Safety
characteristic that affects building response.           Commission, Proposition 122 Retrofit Practices Improvement
Earthquake shaking of a simple rectangular               Program.
building results in a fairly uniform distribution
                                                         Copies of Briefing Papers can be downloaded from ATC’s World
of the forces throughout the building. In a more         Wide Web site (www.atcouncil.org), or are available from:
complex T- or L-shaped building, forces
                                                                            ATC/SEAOC Joint Venture
concentrate at the inside corners created by                             c/o Applied Technology Council
those shapes. Similar problems arise when a                             555 Twin Dolphin Drive, Suite 550
building has floor or roof levels of adjacent                            Redwood City, California 94065
portions offset vertically (split levels), or when




4                                                                                       Briefing Paper 1, Part A
                                                                    ATC/SEAOC Joint Venture Training Curriculum

				
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Description: Creating security & Earthquakes. Earthquake Leads to as well as Results. Building Response Characteristics