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Northridge Earthquake Seismology and Performance of Bridges. Plate Tectonics. Southern California. Local Geography. San Gabriel Mountains. Santa Monica Mountains. San Fernando Valley. Faults of southern California. Seismograph Locations. Current Seismic Activity. The Event.
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Plate Tectonics Southern California
Local Geography San Gabriel Mountains Santa Monica Mountains San Fernando Valley
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
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.
Rupture Plane Video • The Green dot is the hypocenter. • The Red dots are the origins of the aftershocks.
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
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
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
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
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!
“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”
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
Site Response Calculations Show Highest Amplification Factors Nearest to Bridge Collapse Result of Presence of Soft Sediments Amplification
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
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
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
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
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
Splayed columns Pedestal failures Lack of confinement steel
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
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!
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
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
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
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
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
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