Die revidierten Erdbebenbemessungs- kriterien der Internationalen

Die revidierten
Erdbebenbemessungskriterien der
Internationalen
Talsperrenkommission
Dr. Martin Wieland
Chairman, ICOLD Committee on Seismic Aspects of Dam Design
Pöyry Energy AG, Zürich
Erdbebenbemessung von Talsperren
Dams were among the first structures to be
designed systematically against earthquakes
Concrete dams: Method by Westergaard in the 1930s for
Hoover dam; found worldwide acceptance and was used until
the late 1970s.
• Method accounts for inertial effects of dam and hydrodynamic
pressure and used seismic coefficient of typically 0.1.
Embankment dams: First dynamic analysis by Mononobe
et al. in 1936
• Pseudo-static slope stability analysis and use of seismic
coefficient of 0.10 to 015.
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International Commission on Large Dams (ICOLD)
Committee on Seismic Aspects of Dam Design since 1968
Bulletin 52 (1986): Earthquake analysis procedures for
dams (Zienkiewicz, Clough, Seed) (linear analysis)
Bulletin 62 (1988/2008): Inspection of dams following
earthquakes
Bulletin 72 (1989), Bulletin 148 (2010): Selecting seismic
parameters for large dams
Bulletin 112 (1998): Neotectonics and dams (active faults
in dam foundation)
Bulletin 120 (2001): Design features of dams to effectively
resist seismic ground motion
Bulletin 123 (2002): Earthquake design and evaluation of
structures appurtenant to dams
Bulletin 137 (2011) Reservoirs and seismicity (reservoirtriggered seismicity)
Erdbebengefährdung ist Mehrfachgefährdung
(man kann das nicht oft genug sagen!)
• Ground shaking: most earthquake
regulations are concerned with this hazard
only!
• Fault movements in dam foundation or discontinuities in dam
foundation near major faults which can be activated causing
structural distortions;
• Fault movement in reservoir causing water waves in the
reservoir or loss of freeboard;
• Mass movements (rockfalls): hazard
relevant in mountainous regions which has
been underestimated or ignored!
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Sefid Rud buttress dam, 105 m high
Buildings at dam site, Sefid Rud dam
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Switchyard on fill
Derailed transformer
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Transmission tower failure due to rockfall,
Sefid Rud dam
Rockfalls, Sefid Rud dam
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Erdbebenbemessungskriterien
Dam and safety-relevant elements (spillway,
bottom outlet):
Operating basis earthquake, OBE (145 years)
(negotiable with owner)
Safety evaluation earthquake, SEE (ca. 10,000 years)
(non-negotiable)
Appurtenant structures (powerhouse,
switchyard):
Design basis earthquake, DBE (ca. 475 years)
Temporary structures (coffer dams, river
diversion) and critical construction stages:
Construction level earthquake, CE (> 50 years)
Bemessungserdbeben für grosse
Stauanlagen
• Maximum Credible Earthquake (MCE)
• Maximum Design Earthquake (MDE)
• Safety Evaluation Earthquake (SEE)
• Design Basis Earthquake (DBE)
• Operating Basis Earthquake (OBE)
• Construction Earthquake (CE)
If reservoir-triggered seismicity (RTS) is possible the
DBE and OBE ground motion parameters should
cover those from the RTS scenarios.
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Design Earthquake
Title
Element / Component
CE
Diversion Facilities
- Civil
- Geotechnical
Intake/outlet structures
OBE/
SEE
X
Tunnel, tunnel liner
X
Rock slopes
X
Underground facilities
X
Cofferdams
X
- Electrical/Mechanical
Gate equipment
X
Dam: Dam Body
Dam body
- Individual Blocks
DBE
X
OBE
Crest bridge
X
Crest spillway cantilevers
X
Bottom Outlet cantilevers
X
Foundation/Abutments
Abutment wedges
X
X
Bottom Outlet
Main gates, Valves
X
X
Guard gate
X
Dam: Electrical/Mechanical
Operating equipment
X
Essential parts
X
X
X
Verhaltenskriterien für Talsperren und
Sicherheitsorgane
(i) Dam body:
OBE: fully functional, minor nonstructural damage
accepted
SEE: reservoir can be stored safely, structural damage
(cracks, deformations) accepted, stability of dam must be
ensured
(ii) Safety-relevant elements (spillway, bottom
outlet):
OBE: fully functional
SEE: functional so that reservoir can be
operated/controlled safely and moderate (200 year return
period) flood can be released after the earthquake
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Sicherheitserdbeben
A. Deterministische Gefährdungsanalyse
• Kritische Erdbebenszenarien aufgrund der Seismotektonik
(keine Wiederkehrperiode)
• 84% Fraktilwerte der Erdbebenparameter
B. Probabilistische Gefährdungsanalyse
• Erdbebenbewegungen mit Wiederkehrperiode von 10’000
Jahren für grosse Stauanlagen
• 50% Fraktilwerte der Erdbebenparameter
Erdbebenparameter des Sicherheitsbebens sind die
Maximalwerte aus A und B
Erdbebenerschütterungen
Earthquakes affect all components of a
dam project at the same time:
dam
foundation
safety devices
pressure system
underground works
appurtenant structures
hydro-mechanical equipment
electro-mechanical equipment etc.
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Erdbebenparameter für Stauanlagen
• Peak Ground Acceleration
• Acceleration Response Spectra (5% damping)
• Acceleration time histories (structures ‘responding’
to inertial forces)
• Displacement time histories (underground structures
‘not responding’ to inertial forces (imposed
deformations)) and wave propagation velocities
• Spatial variation of ground motion for special (long)
structures
Beschleunigungszeitverläufe werden für
Erdbebensicherheitsnachweise benötigt
Acceleration time histories of the three components
of the design earthquakes are the most
comprehensive characterization of the earthquake
ground motions required by the dam engineer.
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Ingenieur-Eigenschaften der
Beschleunigungszeitverläufe
• Duration of strong ground shaking to account for aftershocks
(it is not really important that the characteristics – frequency
content – of the aftershocks are different from those of the
main shock)
• Directivity effects (at least three spectrum matched
earthquakes to be analysed without amplification of ground
motion (as used e.g. in some nuclear recommendations)
• Spectrum matching of recorded accelerograms, use of long
records especially for dams, which are vulnerable due to long
duration effects (displacements and pore pressure build up)
Ingenieur-Eigenschaften der
Beschleunigungsteutverläufe
• Artificial accelerograms are acceptable, energy input is rather
irrelevant in case of elastic structures when dominant
eigenfrequencies are in the range of the response spectrum,
energy content will have an effect on nonlinear behaviour
structures
• Accelerograms of earthquake components must be
stochastically independent
• Accelerograms used for seismic safety check are not real
accelerograms but are models of the ground shaking, this is
standard practice in civil design; it is important that the ground
motion model used and the structural models give
conservative results
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Folgerungen
• The seismic hazard is a multi-hazard for most
large dam projects. Ground shaking is the main
hazard considered in all earthquake guidelines for
dams. The other seismic hazards are addressed
less rigorously than the ground shaking or may
even have been ignored.
• Dams are not inherently safe against earthquakes.
• The updated ICOLD guideline on ‘Selecting seismic
parameters for large dams’ covers most structures
and elements of large dams.
• As most dams built prior to 1989 when ICOLD has
published its seismic design criteria of dams, have
not been checked for the SEE ground motion, the
earthquake safety of these dams is not known.
• Moreover, due to changes in the seismic design
criteria and the design concepts it may be
necessary to perform several seismic safety checks
during the long economical life of a large dam.
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• The dam designer needs acceleration time
histories for the dynamic analysis; these time
histories are modified ones to suit the designer’s
requirements and are not natural ones. This may
be misunderstood by seismologists doing the sitespecific seismic hazard analyses.
• Reservoir-triggered seismicity is not a separate
load case in dam design but may be relevant for
appurtenant structures and buildings and
infrastructure in dam area.
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