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structural Mitigation
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6/19/13
1
Structural mitigation for seismically induced permanent ground displacement Craig D. Comartin, SE
Topics
• Structural behavior under large seismically induced ground displacements
• Illustrative design considerations and examples
• Development of application guidelines and building code provisions
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Surface fault rupture
reverse strike-slip normal.
Vertical and horizontal displacements beneath structures are possible due to surface fault rupture
Block sliding and graben displacements
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Horizontal displacement Tension will lead to collapse at large displacements
Vertical displacement (small)
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Vertical displacement (large)
Plastic moment MP , and tension in beam
Horizontal displacement
Rigid link
DH
Vcap Vcap
Tension in link, T = Vcap= Capacity of foundation soils in friction and passive resistence
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Surface rupture beneath concrete frame
Plan
Vertical displacement (large)
Rigid link Rigid link
Vcap
Vcap
Tension in link, T = Vcap= capacity of foundation soils in friction and passive resistence
DV
DH
Plastic moment MP , but no tension in beam
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Frame compliance to fault displacement
Design strategy for large displacements
• Provide base level structural capacity in tension and shear exceeding the horizontal capacity of surrounding soils in friction and passive pressure.
• Provide capability to maintain vertical load carrying capacity for large vertical displacements.
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Bowles Hall
Hayward Fault location at Bowles
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Estimated displacements due to surface fault rupture
For a 475-year event, the ratio of displacements of approximately 7:1, horizontal to vertical. For this probability, a 14 inch horizontal displacement is expected and thus 2 inches vertical.
Areas of potential large displacements
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Structural embedment
Retrofit strategy
• Remove soil from building above the foundation level.
• Provide a retaining wall with clearance to face of building.
• Underpin library and exercise room to provide new mat with level contact with supporting soils.
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Anchorage Courthouse
Anchorage Courthouse
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Site location plan
Earthquakes in Alaska
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Performance objectives
100 yr.
500 yr.
1000 yr.
5000 yr.
Ground shaking
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00
Sa (g)
Period (seconds)
1000 yr - 7% Damping
100 yr - 3% Damping
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Displacement hazard
Ground displacement probability
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Conventional frame
Stiffened box foundation
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Cantilevered bay
Cantilever failure
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Cantilever success
Flexible bay
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Frame compliance to fault displacement
Foundation analysis
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Mat foundation
Eccentrically braced steel framing
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California Memorial Stadium
Strawberry Canyon,looking south, 1915
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Fault creep
curb offsets major cracks in culvert observed in 1948,
1954, 1965, and 1999
curb offsets
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Offset curbs
Pavement cracks
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Stadium columns
Trenches and borings
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Trench at south side – Main trace
Existing stadium structure: nomenclature
East Bowl: Concrete Slab on Ground
South Fault Rupture Zone
North Fault Rupture Zone
West Bowl: Elevated Concrete Frame
West Berm: Concrete Slab
on Grade
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5’ Gap All AroundFault Rupture Block
Student AthleteHigh PerformanceCenter (S.A.H.P.C.)
New Shotcrete and Castin Place Concrete Shear
(N) Precast Concrete andSteel Bowl Structure
Existing SeatingSlab on Beam
Sliding CoverPlate
Plastic Shee tBelow Mat Footing
(N) Steel FloorFraming
“Stone Columns”to Densify Original Fill:Unconnected to Footing
FAULT RUPTURE B LOCK
PHASE 2 PHASE 1
Concrete MatSlab Footing
Performance-based design objectives
• Continued operation 100yr
• Immediate occupancy 250yr
• Life safety 500yr
• Collapse prevention 1000yr
Performance level Event/shaking intensity
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Performance-based design process for large foundation displacements
• Determine performance objectives. • Estimate displacements for hazard level(s). • Determine structural limit states for each
hazard level. • Verify capability to meet desired performance
level(s)
The way forward
• Develop guidelines for probabilistic assessment of large foundation displacements.
• Develop guidelines for structural application.
• Develop peer review requirements for both.
• Begin code incorporation process.
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Conclusions
• Structural behavior can be conceptualized, quantified, and understood.
• Well-reasoned mitigative actions should be allowed.
• Guidelines and standards are needed. • Provisions, constraints, and compliance
should be regulated in the building code.