Modeling Seismic Response for Highway Bridges in the St. Louis Area for Magnitude 6.0 to 6.8 Earthquakes J. David Rogers and Deniz Karadeniz Department of Geological Sciences and Engineering University of Missouri - Rolla

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Modeling Seismic Response for Highway Bridges in
the St. Louis Areafor Magnitude 6.0 to 6.8
EarthquakesJ. David Rogers and Deniz
KaradenizDepartment of Geological Sciences and
EngineeringUniversity of Missouri - Rolla

• ABSTRACT
• Two highway bridges spanning the Missouri
  River flood plain were selected for evaluation of
  seismic site response for the most probable
  earthquakes we can expect to see in the near
  future emanating from the three principal sources
  in the Midwest the New Madrid Seismic Zone
  Wabash Valley Seismic Zone and Southern Illinois
  Seismic Zone. This study focused on the expected
  range of earthquakes magnitudes M 6.0, 6.3, 6.5
  and 6.8. These have a 25 to 40 probability of
  occurring over the next 50 years. Most previous
  efforts have focused on M 7 to 7.6 quakes, such
  as occurred in 1811-12. The 1811-12 quakes only
  have a 7 to 10 probability of occurrence in the
  next 50 years.
• Our results suggest site amplification
  between 550 and 950, depending on earthquake
  magnitude and epicentral distance. Our study
  suggests that the threshold magnitude for
  significant foundation failure and structural
  damage to Missouri River bridges is between M 6.4
  and 6.6. Above these magnitudes widespread
  liquefaction is predicted, which would dampen
  ground acceleration, but would sever foundation
  elements, such as piles or caissons. Site
  amplification could be expected where the site
  period is gt 1.0 second (s). Therefore, M 6.5
  earthquakes at ranges 110260 km could cause
  severe damage to structures 10 to 25 stories high
  founded on channel deposits with gt 30 to 46 m of
  unconsolidated sediment.

Map at left shows the location of bridges
studied, locations of modeled earthquakes, and
the outlines of the three seismic zones in the
Midwestern USA. Black dots represent the major
historic earthquakes that have occurred in the
region. Map at right shows the three recognized
source zones for historic seismicity in the
Midwestern United States.
The geology of the Missouri River floodplain is
typified by bedrock bluffs and channel floors
developed in dense Paleozoic limestone and
dolomite, covered by glacial windblown silt
(loess). The channel flood plain is filled with
young unconsolidated sands and silt. The upper 2
to 4 meters of the flood plain are usually
covered with recent lake sediments and overbank
floodplain silts and clays. The thickness of the
channel fill varies between 30 and 60 m.
Creve Coeur Bridge
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Hermann Bridge
Rock/Surface Peak Accelerations
Surface Acceleration
Rock Acceleration
Earthquake source and wave propagation were
characterized using SMSIM code of Boore (1993),
from the US Geological Survey. Site Response is
characterized using DEEPSOIL v.2.5, from the
Univ. of Illinois.
Response Spectra
The site response at the ground surface is
markedly different from the base rock input
motion and site response increases as the
magnitude increases
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Amplification of Ground Response Site
amplification is the ratio of the response
spectra of a soil site to the response spectra of
the bedrock beneath that site. Earthquake energy
can be amplified because Resonance within the
soil column overlying much stiffer basement rocks
Impedance Ratio between the rigid basement rock
and the unconsolidated soils lying over them
Conservation of energy of the incoming seismic
wave train (e.g. wave energy arriving at a much
higher rate than can be propagated through the
soft soil cover)

• Liquefaction Potential
• Screening Analysis
• Liquefaction (quicksand) can affect
• Surface Accelerations
• Spectral Accelerations

CONCLUSIONS 1. Preliminary results indicate that
the bridges we analyzed would be subjected to
long period motions, which may pose a significant
threat to simply-supported tail spans founded on
friction piles. 2. Large amplifications can be
expected at both bridge sites. Amplification of
the ground motion is in the range of 550 to
950. 3. Similar site amplification was predicted
for earthquakes at distances of 110 to 210 km,
because there is only slight attenuation of
seismic energy in the stiff Paleozoic bedrock. 4.
Widespread liquefaction is predicted at the Creve
Coeur Bridge site for M 6.8 event, but only
localized liquefaction for M 6.3-M 6.7 events. 5.
The screening analysis did not predict any
liquefaction at the Hermann Bridge site. 6. Soil
softening (liquefaction) may cause a decrease in
response spectra values for periods lt 1 sec. 7.
However, soil softening may cause an increase in
response spectra values for periods gt1 sec.