6

                                              Chapter 3
                                              Why Adopt a Seismic Code?

                                                                                         7.1 on the Richter scale and was the
                                                                                         strongest to affect a U.S. city since the
                                                                                         1964Alaskan earthquake.' It shook
                                                                                         the San Francisco Bay Area and killed
                                                                                         sixty-three people. Two-thirds of the
                                                                                         deaths were a result of the Cypress
                                                                                         viaduct collapse. Although the
                                                                                         ground-shaking was intense within
                                                                                         the metropolitan area, few buildings
                                                                                         collapsed. Most of the damage
                                                                                         occurred to unreinforced masonry
                                                                                         buildings built before the adoption of
                                                                                         seismic codes. Nearly all major
                                                                                         reinforced concrete structures built
                                                                                         after World War II survived without
                                                                                         collapse. Even at the quake's epicen-
                                                                                         ter new buildings and buildings
                                                                                         located on firm ground suffered little
                                                                                         damage. Informed observers attribute
                                                                                         the success to the required UBC
                                                                                         seismic codes.2 This example illus-
                                              The term seismic code refers to the
FIGURE 3.1 Most of the butilding damage                                                  trates that code requirements reduced
                                              seismic design requirements in-
in the 1989 Loma Prieta earthquake wvas
                                      to
                                                                                         the damage and loss of life during
olderunreinforcedmasonry buildings            cluded within building codes. In the       this moderate earthquake.
built before the adoption of seismic codes.   past, local governments sometimes
                                                                                           The 1994 Northridge, California,
(Photo: Rob Olshansky)                        viewed the seismic sections of the
                                                                                         earthquake shows similar evidence.
                                              model codes as optional, adopted at
                                                                                         Almost all the buildings in the
                                              local discretion. Now seismic
                                                                                         affected area were built during the
                                              provisions are fully integrated into
                                                                                         past fifty years under one of the UBC
                                              all three model codes. Local govern-
                                                                                         seismic codes. Virtually all buildings,
                                              ments should adopt the latest
                                                                                         even in the areas of strongest shaking,
                                              version of a model code in its
                                                                                         remained standing and allowed for
                                              entirety, including the seismic
                                                                                         safe evacuation of occupants. Regret-
                                              sections, in order to be operating at
                                                                                         tably, one apartment building col-
                                              the current standard. This point is
                                                                                         lapsed on its residents, and two high-
                                              very important and is emphasized
                                                                                         occupancy concrete-frame buildings
                                              throughout this book.
                                                                                         collapsed, fortunately with no
                                                                                                               3
                                                                                         occupants at the time. Still, these
                                              Seismic Codes Are Effective
                                                                                         three buildings were built under an
                                              Experience with recent earthquakes         older version of the UBC code, and
                                              in the United States and throughout        damage and life loss would have
                                              the world shows that seismic codes         been immeasurably greater without
                                              work. Cities with seismic codes            the seismic-resistant construction
                                              suffer much less damage than those         prevalent in the San Fernando Valley.
                                              without such codes.
                                                                                            A Kyoto University study of the
                                                 The Loma Prieta earthquake              1995 earthquake in Kobe, Japan,
                                              clearly illustrates the effectiveness of   Richter magnitude 6.9, found that
                                              seismic codes. Occurring on October        damage to reinforced concrete
                                              17, 1989,this earthquake measured          buildings closely paralleled improve-
WhyAdoptA Seismic
                Code?                                                                                                        7

ments to seismic provisions in the
Japanese building code. More than
55 percent of pre-1970 buildings (old
version of code) were severely
damaged, compared with no post-
1980buildings (newest version of
code). Similarly, steel buildings built
before 1970 sustained severe dam-
age, compared with little damage in
post-1981 buildings. 4 Ohbayashi
Corporation studied buildings it had
constructed in Kobe and found that
58 percent of pre-1971 buildings
were damaged, compared with 28
percent of 1972-80buildings and
only 16 percent of post-1981 build-
ings. 5

   In contrast, a Richter magnitude
6.9 earthquake in Armenia in 1988
destroyed entire communities and          tions are as important to building        FIGURE 3.2 Lessons about underlying
killed 25,000people. This disaster        stability as the epicenter location. In   soil conditions learned in the 1985 Mexico
has been attributed to several                                                      City quake can help areas built on fill,
                                          response to this new information,
factors: design deficiencies; poor                                                  such as the Back Bay area of Boston shown
                                          ICBO in the 1988 and 1991 UBC
quality of construction; and the                                                    above, ninitnize damage. (Photo: Greater
                                          editions has emphasized soil
                                                                                    Boston Convention & Visitors Bureau)
earthquake's intensity exceeding          conditions by increasing the force
                             6
that anticipated by the code. Similar     requirements according to the type
problems exist in much of the             of underlying soil. The National
United States.                            Earthquake Hazard Reduction Programn
                                          (NEHRP)Reconiended Provisions
  Even smaller earthquakes can
                                          (described on page 8) have also
cause extensive damage where
                                          taken into account soil conditions in
buildings are not designed for
                                          the latest edition.
seismic shaking. A Magnitude 5.6
earthquake in 1993 at Scotts Mills,
                                          Today's Seismic Codes Are Based
Oregon, caused significant struc-
                                          on More Than Sixty Years of
tural damage to a number of
                                          Earthquake Experience
unreinforced masonry (brick)
buildings in the area.7 A high school     Seismic codes in use now reflect a
building was significantly damaged        long history of learning from
and vacated, 16 residences and 54         earthquakes and represent the
businesses sustained major damage,        collective knowledge of hundreds of
and the Oregon State Capitol, in          design and construction profession-
Salem, suffered cracking in the           als. The following is a brief account
rotunda. The estimated damage cost        of that history. See Appendix A for a
to public facilities alone was nearly     more detailed account.
$13 million. This earthquake con-
                                            The earliest seismic design
firmed the susceptibility of
                                          provisions in the U.S. were intro-
unreinforced buildings to severe
                                          duced in the appendix to the 1927
damage, even in a minor earth-
                                          Uniforn Building Code, the first
quake.
                                          edition of the UBC. By the 1950s,
  New lessons are learned from            some California municipalities had
every earthquake and incorporated         adopted additional seismic-resistant
into U.S. seismic codes. For example,     design and material specifications.
the 1985 Mexico City earthquake           The 1949 edition of the UBC con-
confirmed that the local soil condi-      tained the first national seismic
                                          hazard map. After the 1971San
                                                                                                                                              Chapter 3
a

                                                                                                                 Fernando earthquake, revisions were
                                        a.. - l.
                                  U1
          Un U   -                                  j
                     i'   .   ;     L
     "
     _
                                                                                                                 made to the 1973 UBC, and new
                                                                                                                 requirements were introduced in the
    Earthquake magnitude is a measure of the absolute size of an earthquake
                                                                                                                 1976 edition. 9
    so that we may compare earthquakes with one another. Generally
    speaking, earthquakes that release more energy                                                                  Early in the 1970s the National
                                                                                                                 Science Foundation (NSF) funded a
    * shake for a longer amount of time,
                                                                                                                 project, under the guidance of the
    * affect a wider area, and                                                                                   National Bureau of Standards (NBS,
                                                                                                                 now the National Institute of Stan-
    * produce more violent shaking near the source.
                                                                                                                 dards and Technology), to evaluate
        Because we cannot measure the energy released by an earthquake,                                          existing earthquake-resistant design
    Charles Richter in 1935 devised a substitute measure-the Richter                                             provisions. This extensive multi-year
    magnitude scale. The scale is based on what a seismograph would                                              project relied on the input of a large
    measure; it has no inherent meaning of its own. The Richter scale is                                         number of seismic design experts
    logarithmic, and each whole number increase in the scale represents                                          and resulted in a 1978 report by the
    approximately a 31.5-fold
                                                                                                                 Applied Technology Council titled
    increase in energy release: that X                                V
                                           22'3                  121;30
                                                        ioros
                                                                                                                 Tentative Provisionsforthe Development
    is, a magnitude 7 earthquake       e        "a', vi
                                                                                                                 of Seismic RegulationsforBuildings
    releases about 31.5 times more       .                              6
                                                                                                                 (ATC 3-06).
                                         ,
    energy than does a magnitude                              6

    6 earthquake. Several different                                                                                 Under a contract with the Federal
    magnitude scales are now in3                                                                                 Emergency Management Agency
    common use, and they all                                      v                                              (FEMA), the Building Seismic Safety
                                                   vce
    share basic characteristics with                                                                             Council (BSSC, formed in 1979 within
    the Richter Scale.                                   i
                                                                                                                 the National Institute for Building
                                                                                                                 Sciences, NIBS) revised ATC 3-06 by
       Shortly after an earthquake
                                                                                                 ~ ~~~~ 37$\0'
                                                                 azn%   ,        C        S
                                                                                                                 a consensus of its members. In 1985
    occurs, the surface wave
    magnitude or body wave magni-                                                                                FEMA released the NEHRP Recom-
    tude is often reported. The                                                                                  mended Provisionsfor the Development
                                                                            7
    scale that most accurately                                                                                   of Seismic Regulationsfor New Build-
                                                                    } I AL                JX~~~~~~V
    represents the energy of an                                                                                  ings, commonly called the NEHRP
    earthquake is the moment                                                                                     Provisions. Although not a code, the
                                                                    0       10       20       30MILES
    magnitude scale. For smaller                                                                                 NEHRP Provisions are designed to
    earthquakes (less than magni-                                                                                provide guidance to the writers of
                                                   FIGURE 3.3 The Loma Prieta, California,
    tude 6), the scales are nearly                                                                               building codes. FEMA and BSSC
                                                   earthquakeof 1989 had a magnitude of
    identical, but only the moment                                                                               continue to update the NEHRP
                                                   7.1, but intensities in the affected area
    magnitude scale can distin-                    rangedfrom MlMI VII to IX. (Source:                           Provisions every three years, with the
    guish differences among very                   USGS Circular1045,1989)                                       latest edition being published in
    large earthquakes.
                                                                                                                 1994. The 1997 edition is due out in
        Earthquake intensity is a measure of the actual shaking experienced at                                   December 1997.
    a location. The United States uses the Modified Mercalli Intensity Scale, a
    twelve-point qualitative scale that describes observable effects of                                          AllThree Model Codes Contain
    earthquakes. For example, Intensity VIII is described, in part, as "dam-                                     State-of-the-Art Seismic
    age slight in specially designed structures; considerable in ordinary                                        Requirements
    substantial buildings with partial collapse; great in poorly built struc-
                                                                                                                 The past two decades have seen great
    tures ... fall of chimneys, factory stacks, columns, monuments, walls.
                                                                                                                 strides in the knowledge of building
    Heavy furniture overturned." Whereas magnitude is an inherent quality
                                                                                                                 responses to earthquakes. Based on
    of an earthquake, intensity generally decreases with greater distance
                                                                                                                 the collective efforts of engineers,
    from the earthquake's center. Intensity is a very useful measure because
                                                                                                                 scientists, and tradespeople, the
    it describes what is most important to society-the degree of damage to
                                                                                                                 NEHRP Provisions contain seismic
    structures built by humans.
                                                                                                                 design provisions that are technically
                                                                                                                 advanced and widely accepted.
                                                                                                                    Since 1992 all three model codes
                                                                                                                 require seismic design standards
                                                                                                                 consistent with the NEHRP
                                                                                                               9
     Why AdoptA Seismic Code?

EQ                                   Modified Mercalli Intensity Scale

         Figure 3.4 Percentage of Buildings Expected in Each Damage State for Various Shaking Intensities:
                    Buildings Designed for Seismic Zone 4 under the I99 I UBC

         Size of Earthquake                                        Standardized Damage States
             (Magnitude)                                  A         B          C           D           E

           6.0-6.5      7.5-8.0                         None      Slight    Moderate    Extensive   Complete

            Distance t o Fault

           30 mi.       50 mi.                          60-90%    10-40%      I-5%        <I%          0
                                                        35-60%    35-45%
                        40 mi.                                               10-30%       <5%
           5 mi.                                                                                      <I%
           I mi.        30 mi.                                                           3- 10%
                                                        25-40%    25-40%     20-40%                   <2%
           -            3 mi.                                     5-25%      40-70%      I0-30%       <5%
                                                        5-25%

     Source: E E N Ad HOCCoiiziiiiftee (see 17ote12).

                                                              4