Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Northridge Earthquake
Seismology and Performance of Bridges
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Plate Tectonics
Southern California
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Local Geography
San Gabriel Mountains
Santa Monica Mountains
San Fernando Valley
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Faults of southern California
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismograph Locations
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Current Seismic Activity
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
The Event
• January 17, 1994, some 10 million people
were awaken by the shaking of a magnitude
MW 6.7 earthquake
• The epicenter was located in the town of
Northridge
• It began as a rupture on a hidden fault at a
depth of about 17.5 kilometers beneath the
San Fernando Valley
• The rupture lasted for 8 seconds, propagating
northwestward along the fault plane at a rate
of about 3 kilometers per second
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Rupture Plane Time Line
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
The Rupture Plane
• The rupture front spread out across the
fault plane, eventually covering an area
of approximately 15 by 20 kilometers.
• The rupture terminated at a depth of
about 5-6 kilometers.
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Rupture Plane Video
• The Green dot is the
hypocenter.
• The Red dots are
the origins of the
aftershocks.
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Slip and Ground Velocity
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Aftershocks
• Over 10,000 aftershocks have been
produced.
• Two had magnitudes over Mw=6.0
– The first being just over a minute after the
mainshock
– Were adjacent regions already close to failure at
time of the earthquake
• Seismologists estimated that aftershocks
would continue for approximately 20 years
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Magnitude of Aftershocks
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Quantity of Aftershocks
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Recorded Ground Motion
• Near-record ground motions were generated by the
Northridge earthquake
• Ground motion was recorded by more than 200
accelerograph stations
• Sites located within 10 km of the earthquake had
recorded ground accelerations between 0.3g and
1.2g
• Sites located 20 to 50 km away had recorded ground
accelerations between 0.1g and 0.6g
• The exception being at Tarzana were ground
accelerations of 1.0g and 1.8g were recorded
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Ground Amplification
• Amplification of the ground motion caused isolated
areas of heavy damage
• Ground motion amplification is caused changes in the
geologic structure
– Two cases of ground amplification
• San Fernando Valley- Deep areas of alluvial sediments that
“jiggle like Jell-o”
• Sherman Oaks/Santa Monica- Bowl-shaped dip in the bedrock
beneath the southern edge of the Santa Monica Mountains
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Closer Look At Tarzana
• 6 km south of the epicenter
• Recorded peak ground acceleration of 1.8g and
sustained large amplitudes near 1g for about 7-8
seconds
• Site topography:
– Sits atop a hill 15 meters high 500 meters long, and 130
meters wide
• Amplification factor of 2 between the top and bottom
• Accelerations between Tarzana and a site 2 km away
had an amplification factor of 7
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Site-Response Factor
• Site-Response Factor- a numerical value that
multiplies the amplitude of a reference wave motion
to match the observed ground motion
• Scientists use this information to develop site-
response maps to help predict future ground motion
• There is a good correlation between high factors and
areas of localized severs damage
• You will find reference to these in the Building Codes!
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
“Tagged”
• “Red-tagged” – Building or structure has
been significantly damaged and is no longer
safe to enter
• “Yellow-tagged” – Building or structure has
sustained enough damage that entry is
limited
• “Green-tagged” – Building or structure is
safe to inhabit
• 66,546 building were inspected
– 4,000 were “Red-tagged”
– 11,300 were “Yellow-tagged”
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
• Ground Motion
Amplification Factors
– Largest Amp. In Soil
Regions (orange and
yellow)
– Smaller Amp. In Rock
Regions (red and green)
• Range = 2-6 Hz
• Reference Value:
– 1.0 on Rock
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Amplification
• Site Response
Calculations Show
– Highest Amplification
Factors Nearest to
Bridge Collapse
• Result of Presence
of Soft Sediments
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
I-10/La Cienga – Venice Undercrossing.
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Bridges and Traffic Management
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
LA’s Love affair with the Automobile
• 27 freeways
• 882 miles of highways
• 6 million registered vehicles
• First place in all categories of traffic
congestion
• 85% of workers commute by personal
automobile
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
The Bridges of Los Angeles County
• 2523 State Owned Bridges
• 716 Need Retrofitting
• 16% had been retrofitted at the time of
the earthquake
• 1500 street bridges owned by County
• 800 street bridges owned by the city
• Approximately 1/3 needed seismic
retrofit
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismic Design in California
• 1940 – The California Sate Highway
Department becomes the first state to
include seismic loading
• 1971 San Fernando earthquake
revolutionizes design criteria and
initiates the concept of seismic retrofit
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismic Design in California
• Retrofitting Goals:
– Prevent Collapse
– Permit structural damage
– Prevent the loss of life
– Provide access for emergency vehicles
– Less expensive modifications than
strengthening to comply with design criteria
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismic Retrofit Program of 1971
• Phase I: Addressed the most critical
bridge deficiency – the loss of girder
support at bearings
• Took 17 years to complete
• Cost $55 million
• Then … Loma Prieta hit
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Pedestal failures
Splayed columns
Lack of confinement steel
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismic Retrofit Program – Phase II
• Phase II: Accelerated the program as a
result of the 1989 Loma Prieta
earthquake
• Addressed remaining bridges including
single column bents, double-deck
viaducts, multi-column bents and other
complex bridges
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Locations of Bridge Damage
• The Northridge
Earthquake
generated in a span
of seconds, a year’s
worth of highway
projects with NO
feasibility studies,
impact statements,
or traffic
management plans!
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Bridge Damage
• About 1200 state highway bridges were
subjected to ground acceleration of .25g
or greater
• 212 bridges reported damage
• Only 4 of 1500 county bridges reported
serious damage
• 62 of 800 spans owned by the city were
damaged
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Major Freeway Bridge Collapses
• SR 14 / I-5 Interchange (2 bridges)
• Gavin Canyon Undercrossing (I-5)
• Mission-Gothic Undercrossing (SR-118)
• La Ciengea-Venice Undercrossing (I-10)
All 5 collapses were reinforced and/or
prestressed concrete bridges completed
between 1964 and 1976
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Faults of southern California
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
SR-14 / I-5 Interchange
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
SR-14/I-5 bridge collapse
• I-5 is the primary
north/south route
through central CA
• Average daily traffic
on I-5 before the
earthquake was
approximately
133,000 vehicles
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
SR-14/ I-5 Interchange
• Curved concrete box- girder bridges
• Under construction in 1971, when
damaged by the San Fernando quake
• Two collapsed ramps, evidence of
pounding between spans at several
hinges
• Failure caused by the variability of pier
stiffness between short and tall columns
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Reconstruction
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
I-5 at Gavin Canyon Undercrossing
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Gavin Canyon Undercrossing
• Concrete Box Girder
• Retrofitted in 1974 with restrainer
cables (Part of Phase I retrofit)
• Failure occurred from restrainer failure
and unseating
• Severe skew contributed to failure,
along with end spans that were much
stiffer than the center span
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
SR-118 (The Simi Valley Freeway)
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
SR-118 / Mission-Gothic Undercrossing
• Prestressed box girder superstructure
• Severe damage to pier columns due to
the large column flares resulting in
increased plastic shear
• Failure influenced by unique bridge
geometry and a wide column spacing
which forced columns to take extra load
and move in their stiff direction
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
I-10/La Cienga – Venice Undercrossing.
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
I-10/La Cienga – Venice Undercrossing.
• 2 – 9 span, cast in place reinforced
concrete box beam bridges
• Total length of 870 feet with skews
ranging from 5 to 41 degrees
• Collapsed as a result of column failure
• Seismic retrofitting was imminent at the
time of the earthquake, and likely would
have protected the structure
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Santa Monica Freeway (I-10)
• Pre-1971 columns •
without retrofit did
not provide
adequate
reinforcement to
confine the
concrete-core. This
results in
“birdcaging”.
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Traffic Impacts
• Four freeways were closed
• Parking structures collapsed
• Bus Routes severed
• Emergency routes severed
• Life-lines cut
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Transit agencies respond
• Most nonhighway transportation
facilities were operational within 2 days
• By early February:
– Detours were in place
– HOV lanes created
– New emergency express bus service
added
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
I-10 “the world’s busiest freeway” Detours
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Mass Transit Extensions
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
The Public Response
• Changes to trip schedule
• Heavier use of arterial routes
• Short term increase in Metrolink
(commuter rail)
• Little to no increase in bus traffic
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Traffic Impact Conclusions
• Those who drove before the earthquake
continued to drive after the earthquake
• Many commuters found new routes that
the continued to use even after freeway
traffic was fully restored
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Seismic Retrofit for Dummies
• Previously
retrofitted
bridges with
steel jackets
performed very
well, with no
reported
failures
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
The Costs
• Estimated retrofit cost of approximately
1000 Caltrans bridges was 1.1 billion
• The Estimated repair cost after the
quake (six major collapses, 157 bridge
repairs) was 1.5 billion
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Villanova University CEE 8414 – Structural Dynamics
Dept. of Civil & Environmental Engineering Northridge Earthquake
Conclusions
• Skewed bridges are more likely to experience
damage or failure
• Bridges designed after 1981 performed very
well
• Retrofitting is not a guarantee, but does help!
• There is still a lot to learn about how to
effectively prevent bridge damage
• It’s still more cost effective to prepare for an
earthquake, than clean up after one
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