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Seismic Design of Buried Structures

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Title: Seismic Design of Buried Structures


1
Seismic Design of Buried Structures
Doug JenkinsInteractive Design Services
2
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

3
Collapse of buried structure in Japan
Spec
  • Possible earthquake related failure south of
    Sydney
  • Consequences of failure

4
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.

5
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.

6
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

7
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.

8
Classification
9
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

10
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

11
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.

12
Typical Finite Element Mesh
13
Mesh Detail
14
Fundamental period, Static Design Force
Coefficient
15
Static analysis, 3m Cover
16
Dynamic Analysis, 3m Cover
17
Maximum Moments, 3m Cover
18
Static analysis, 15m Cover
19
Dynamic Analysis, 15m Cover
20
Maximum Moments, 15m Cover
21
Axial Load - Seismic Increment or reduction, 15m
Cover
22
Animations
  • First mode shape - 3m and 15 m cover, horizontal
    and vertical
  • Arch Fill
  • Fill only

23
Moment-Curvature
24
Push-over analysis, 15m Cover
25
Animations
  • Push-over analysis
  • 15 m fill
  • 3 m fill

26
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.

27
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.

28
Further Information
  • www.interactiveds.com.au
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