Seismic Design of Buried Structures

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Seismic Design of Buried Structures
Doug JenkinsInteractive Design Services
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Why Bother?

• "Results show that the bending moments in the
  arch from horizontal earthquake loading can be
  significant in relation to the gravity load
  actions. These moments are also very sensitive to
  the backfill and surrounding soil stiffness
  properties and rather less sensitive to the
  foundation soils beneath the arch.
• John Wood

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Collapse of buried structure in Japan
Spec

• Possible earthquake related failure south of
  Sydney
• Consequences of failure

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Seismic design provisions in AS5100

• Specific rules are given for the categorisation
  of bridges.
• A formula is given for the fundamental period of
  bridge structures (for use in category BEDC-1
  designs only).
• Specific structural response factors are given
  for bridges of different types.
• Structural detailing requirements relevant to
  bridges are given.

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Problems with application to buried structures

• The requirements for more detailed analysis
  methods are related to bridge span, and may not
  be relevant to buried structures.
• Vertical earthquake effects may be important for
  buried structures, but only horizontal effects
  are required to be considered for almost all
  buried structures.
• The formula for the fundamental period is not
  applicable to buried structures.
• For static analysis the earthquake design force
  is not applicable to buried structures.
• The appropriate response modification factor is
  not clear.

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Buried arch design study

• Geraldton Southern Transport Corridor
• 14.5 m span x8 m high
• 3m and 15 m cover
• Acceleration 0.10g
• Site factor 1.5
• Bridge Type II

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Classification

• Type III - Bridges and associated structures that
  are essential to post-earthquake recovery, as
  determined by the relevant authority.
• Type II - Bridges that are designed to carry
  large volumes of traffic or bridges over other
  roadways, railways or buildings.
• Type I - Bridges not of Type II or Type III.

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Classification
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Classification

• BEDC-1
• lt 20 m span, no earthquake analysis
• gt 20 m span, static analysis, horizontal forces
• BEDC-2
• Static or dynamic analysis
• gt 35 m span, horizontal and vertical
• BEDC-3
• Static analysis only if a single dominant mode
• Consider horizontal and vertical loads
• BEDC-4
• Dynamic analysis horizontal and vertical

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Buried arch design study

• For each fill height (3m and 15 m)
• Natural frequency analysis
• Deflection under unit horizontal acceleration
• Pseudo-static analysis
• Response Spectrum analysis
• Push-over analysis

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Buried arch design study

• For each of the analyses the following material
  stiffness properties were used
• Typical soil and uncracked concrete.
• Soil stiffness reduced by half and uncracked
  concrete.
• Soil stiffness reduced by half, and concrete
  moment-curvature relationship (static analyses),
  or cracked stiffness (response spectrum analyses)
• A total of 26 separate analyses were carried out.

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Typical Finite Element Mesh
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Mesh Detail
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Fundamental period, Static Design Force
Coefficient
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Static analysis, 3m Cover
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Dynamic Analysis, 3m Cover
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Maximum Moments, 3m Cover
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Static analysis, 15m Cover
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Dynamic Analysis, 15m Cover
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Maximum Moments, 15m Cover
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Axial Load - Seismic Increment or reduction, 15m
Cover
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Animations

• First mode shape - 3m and 15 m cover, horizontal
  and vertical
• Arch Fill
• Fill only

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Moment-Curvature
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Push-over analysis, 15m Cover
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Animations

• Push-over analysis
• 15 m fill
• 3 m fill

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Conclusions

• Buried arch structures with low to moderate axial
  loads have a large reserve ductility.
• Where the failure mode is concrete compression
  failure under moderate earthquake loading is a
  possibility.
• Bending moments found in the dynamic analyses
  were up to 50 higher than the static analysis
  results.

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Recommendations

• Structure classification be related to fill
  height, rather than span.
• Response modification factor to be related to the
  capacity reduction factor.
• BEDC-1 and BEDC-2 design for vertical and
  horizontal earthquake loads, using static or
  dynamic analysis.
• BEDC-3 and BEDC-4 design for vertical and
  horizontal earthquake loads, using dynamic
  analysis.
• Determine the fundamental period from computer
  analysis or formula for BEDC-1 and 2 structures.
• T 0.055 ?0.5 seconds
• ? deflection in millimetres at ground surface
  above the arch under 1g horizontal load.

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Further Information

• www.interactiveds.com.au