Seismic Design of Buried Structures Doug Jenkins Interactive Design Services

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

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

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.

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.

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

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.

Classification

Classification BEDC-1 BEDC-2 BEDC-3 BEDC-4 < 20 m span, no earthquake analysis > 20 m span, static analysis, horizontal forces BEDC-2 Static or dynamic analysis > 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

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

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.

Typical Finite Element Mesh

Mesh Detail

Fundamental period, Static Design Force Coefficient

Static analysis, 3m Cover

Dynamic Analysis, 3m Cover

Maximum Moments, 3m Cover

Static analysis, 15m Cover

Dynamic Analysis, 15m Cover

Maximum Moments, 15m Cover

Axial Load - Seismic Increment or reduction, 15m Cover

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

Moment-Curvature

Push-over analysis, 15m Cover

Animations Push-over analysis 15 m fill 3 m fill

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.

Recommendations T = 0.055 0.5 seconds 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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