Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 1 Northridge Earthquake Seismology and Performance of Bridges

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 2 Plate Tectonics Southern California

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 3 Local Geography San Gabriel Mountains Santa Monica Mountains San Fernando Valley

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 4 Faults of southern California

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 5 Seismograph Locations

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 6 Current Seismic Activity

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 7 The Event January 17, 1994, some 10 million people were awaken by the shaking of a magnitude M W 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 8

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 9 Rupture Plane Time Line

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 10 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.

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 11 Rupture Plane Video The Green dot is the hypocenter. The Red dots are the origins of the aftershocks.

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 12 Slip and Ground Velocity

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 13 Aftershocks Over 10,000 aftershocks have been produced. Two had magnitudes over M w =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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 14 Magnitude of Aftershocks

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 15 Quantity of Aftershocks

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 16 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 17 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 18 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 19 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!

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 20 “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”

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 21 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 22 Amplification Site Response Calculations Show –Highest Amplification Factors Nearest to Bridge Collapse Result of Presence of Soft Sediments

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 23 I-10/La Cienga – Venice Undercrossing.

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 24 Bridges and Traffic Management

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 25 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 26 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 27 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 28 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 29 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 30

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 31

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 32 Lack of confinement steel Pedestal failures Splayed columns

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 33

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 34 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 35 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!

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 36 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 37 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 38 Faults of southern California

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 39 SR-14 / I-5 Interchange

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 40

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 41 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 42 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 43 Reconstruction

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 44 I-5 at Gavin Canyon Undercrossing

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 45 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 46

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 47 SR-118 (The Simi Valley Freeway)

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 48 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 49

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 50

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 51 I-10/La Cienga – Venice Undercrossing.

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 52 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 53 Santa Monica Freeway (I-10) Pre-1971 columns without retrofit did not provide adequate reinforcement to confine the concrete-core. This results in “birdcaging”.

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 54 Traffic Impacts Four freeways were closed Parking structures collapsed Bus Routes severed Emergency routes severed Life-lines cut

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 55 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 56 I-10 “the world’s busiest freeway” Detours

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 57 Mass Transit Extensions

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 58 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 59 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 60 Seismic Retrofit for Dummies Previously retrofitted bridges with steel jackets performed very well, with no reported failures

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 61 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

Villanova University Dept. of Civil & Environmental Engineering CEE 8414 – Structural Dynamics Northridge Earthquake 62 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