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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.