edce: civil and environmental engineering civil 706 ......the seismic problem earth surface and...
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -1-
EDCE: Civil and Environmental Engineering CIVIL 706 - Advanced Earthquake Engineering
Introduction
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The seismic problem Earth surface and everything built on it are subjected to
strong vibrations from time to time
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Historical approach
Basel 1356 M~6.5-7 (300 fatalities) Today: • 100-150 Billion CHF damage (~3 times the Swiss Federal Budget) • 6000-22000 fatalities for M=6.9 (Wyss et al., 2007)
Source : Wyss et al. (2007)
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Why ? Plate tectonics Mantle viscous convection driven by heat flows
induce deformations in the crust
Tackley (Nature Geoscience, 2008)
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Why ? Plate tectonics Large-scale: deformations in the crust
accomodated at plate boundaries
Tackley (Nature Geoscience, 2008)
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Why ? Other causes
Strong ground motions can also be caused by: • Volcanoes (cracking in the crust due to magmatic flow) • Cave collapse • Fluids driven by human activities (dam filling, geothermal
flows…) that may trigger earthquakes • Anthropogenic explosions (mines, quarries, nuclear
tests…)
But with less energy released as mechanical waves
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Magnitude/Intensity Please do not mix: • Magnitude linked to the energy released by the
source. Many ways of computing it, all scaled to the Richter magnitude (or Local Magnitude Ml, wrong for large earthquakes).
“Best” magnitude=Moment magnitude • Intensity linked to vibration amplitude in a
place. Fixed number of grades (e.g. EMS98 I-XII), estimated using the observed effects (damage…)
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Why ? Wave propagation
Source : USGS
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Attenuation
Sources : O’Connell and Ake (2005)
• Wave amplitude attenuates with distance (empirical relationships)
• Amplitude depends on: distance, frequency, magnitude, source type and soil conditions.
T=1 s
T=0.1 s
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Site Effects
• Amplification at the interface between a hard and a soft layer • Additionnal damping in soft layers • Wave trapping in geological or topographical entities Often ground motion amplification especially in sedimentary
basins and valley (e.g. Alpine valleys)
Additionnaly, at the ground surface, modification can occur:
Incident wavefield
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Site Effects Mw=6.8 Tottori Earthquake (Japan) - Records
http://www.eps.s.u-tokyo.ac.jp/jp/guidance/solid/furumura.html
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When ? Hazard assessment Earthquake cannot be predicted nowadays and
may never be predicted except in some very particular cases
Earthquake Hazard estimated through the Probabilistic Seismic Hazard Assessment (PSHA)
Parameter of interest: the probability of exceedance of spectral acceleration for one frequency in one place. Source : Swiss Seismological Service
Basel
(at 5 Hz)
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When ? Hazard assessment Poisson process with a constant occurrence rate λ
Probability of having k occurrences during t years: Probability of having at least 1 occurrence during t years: Poisson process is associated to a return period: T A probability of 10% in 50 years corresponds to:
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When ? Hazard assessment PSHA of Mediterranean basin
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When ? Hazard assessment PSHA of Switzerland
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What ? The EQ loading The earthquake loading is dynamic: - Large amplitudes (plastic behaviour) - Cyclic (several cycles) - Amplitude highly frequency-dependent (~complex
frequency content) - Time-varying frequency content - 3D input - Soil-structure interaction (SSI) Generally much simplified in computations
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What ? The EQ loading Acceleration/Displacement ex: Aquilpark station in L’Aquila
(Italy), North component, 2009/04/06 Mw=6.3 earthquake at 5.6 km from epicentre
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What ? The EQ loading Particle motion:
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What ? The EQ loading Amatrice earthquake 24.08.2016:
46
7. Comparison with the Italian seismic code The pseudo-acceleration response spectra associated to the horizontal ground motions recorded by the four
stations with lowest epicentral distance (AMT, NRC, RM33 and SPD) are compared with the elastic spectra
provided by the Italian seismic code (NTC2008) at the corresponding sites for soil class provided in Appedix 1
and four different return periods (TR): 50, 475, 975 and 2475 years. Note that comparison of individual earthquake
recordings with probabilistic hazard is a delicate issue and no direct conclusions can be drawn (see Iervolino,
2013)
References
Bindi, D., F. Pacor, L. Luzi, R. Puglia, M. Massa, G. Ameri, and R. Paolucci (2011). Ground motion prediction equations derived from the Italian strong motion database, Bull. Earthq. Eng. 9, 1899–1920.
Iervolino I. (2013) Probabilities and fallacies: why hazard maps cannot be validated by individual earthquakes. Earthquake Spectra, 29(3): 1125–1136.
Paolucci, R., F. Pacor, R. Puglia, G. Ameri, C. Cauzzi, and M. Massa (2011). Record processing in ITACA, the new Italian strong motion database, in Earthquake Data in Engineering Seismology, Geotech- nical, Geological and Earthquake Engineering Series, S. Akkar, P. Gulkan, and T. Van Eck (Editors), Vol. 14(8), 99–113.
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What ? Induced effects Other effects of earthquakes Due to ground motion: - Liquefaction of the soil - Landslides, rockfalls triggering
Due to fault rupture: - Tsunami - Surface faulting (scarp) - Fluids flow modification (springs…)
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Who ? The role of the researcher
• Provide tools to estimate the earthquake hazard (source, propagation, site effects)
• Assess the earthquake hazard (PSHA)
• Provide earthquake resistant design methods
• Provide assessment methods
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Who ? The role of the engineer
• Evaluate local earthquake hazard (including microzonation)
• Design of new structures (building, bridge, dam, industrial facility…) in earthquake prone regions (even moderate)
• Assessment of existing structures in order to, eventually, propose retrofitting.
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Who ? The role of the community
Creating and updating the design codes including:
• Hazard code maps, design return period,
design response spectra, admitted risk • Appropriate design and assessment
methodologies • Dissemination, information to authorities
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Existing buildings
Main issue: Vulnerability of existing buildings • No earthquake resistant design • Renewal rate around 1%/yr
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New/Existing buildings
New Existing Cost of earthquake resistant measures Negligible
(~1%) Important (~10%)
Choice in conception, construction material, desig n …
Entirely f ree None
Knowledge of the structure (dimensions, material…)
Under control Gaps
Design code Modern Old or even unknown
Behaviour factor q Chosen Estimated, low
→ More advanced models → “Best estimate” analysis methods
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Existing buildings in Switzerland
Example surveyed main cities in Valais:
• Sion:
• Martigny:
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Course Plan
• Simple/rapid seismic evaluation methods
• Static non-linear analysis
• Ambient vibration measurement
• Dynamic non-linear methods
• Displacement-based methods
• Masonry out-of-plane analysis
• Risk-based assessment (SIA 2018)
• Retrofitting
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The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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Stone masonry buildings
Low deformation capacity, bad mortar, bad link between walls (out-of-plane behaviour), much material variability
Source : Courtesy of G. Jacquet
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Masonry buildings - Wooden floors
No diaphragm effect (differential displacements between walls), out-of-plane behaviour of walls
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Brick masonry
X cracks → cyclic loading
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RC - Hooks Problems with 90° hooks - Prefer 135°
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RC - Rebar overlaps Overlap in plastic zones
Plastic zone
Overlap
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RC frames - short columns
Generally associated with partial infill
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RC Frames - Soft story
Also transformed ground floors for commercial in city-centres
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Non-structural elements Façades
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Bridges Bearing length
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The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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Laboratory tests
• Dynamic (shaking table) + real phenomena - which ground motion ?, size of the table, expensive
• Pseudo-dynamic (reaction wall) + still dynamic, low speed (analysis), less expensive, distribution
between test/computation, between laboratories - which ground motion ?, a priori model, damping difficult to model,
low speed (~static?)
• Static-cyclic + easy to perform, hysteresis curve - not dynamic
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Laboratory tests
Static-Cyclic tests (Devaux, 2008)
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The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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In Situ vibration recordings
• Earthquakes
• Ambient vibrations
• Forced vibrations
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The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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Earthquake Engineering methods • Earthquake Engineering = very approximate
methods • 2 significant digits maximum
Refinements should follow the principle of consistent crudeness
Ex: multimodal elastic numerical modelling not appropriate for earthquake engineering problems
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Earthquake Engineering methods • Traditional=forced-based (very crude for inelastic) • Modern=displacement-based
Joe’s
Beer! Food!
Lateral Displacement
Lateral Force
= Base Shear
Joe’s
Beer! Food!
small change in force level
large change in deformation level
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Seismic demand - Response spectrum • Seismic demand is much frequency-dependent
• In any seismic method, the shaking level has to be represented
• Elastic response spectrum gives the maximum response of SDOF systems of various frequencies (periods) (Biot, 1932)
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Seismic demand - Response spectrum
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Response spectrum WARNING DO NOT MIX
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -50-
Response spectrum Displacement and Acceleration
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -51-
Fundamental structural period Key parameter to estimate the seismic demand
that has to be taken into account How to estimate the period ? - Using the period-height relationhips in the building codes - Assuming a simplified beam model using the Rayleigh
coefficient or modal analysis; issue of cracked stiffness - Numerical modelling; issue of cracked stiffness - In situ recordings for low amplitudes; issue of frequency
decrease with increasing amplitude - In situ recordings under strong motion (too late…)
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -52-
Earthquake Engineering methods Non-linear approach justified in case of
assessment of existing buildings
non-linear
static dynamic
elastic
structureaction
Equivalent Force Method
Response Spectrum Meth.
Non-Linear Dynamic
Pushover
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -53-
Equivalent force method Standard use for design Fd = mass . acceleration / behaviour factor
-4
-3
-2
-1
0
1
2
3
4
2.0 3.0 4.0 5.0 6.0 7.0 8.0
Fd
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -54-
Response spectrum method Structure decomposed into SDOF systems
1.00
0.72
0.45
0.22
0.06
1er
-0.90
0.29
1.00
0.96
0.39
2ème
-0.97
0.63
1.00
0.90
-0.47
3ème
0.85
1.00
-0.72
-0.31
-0.21
4ème
0.52
1.00
-0.88
-0.14
-0.91
5ème
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -55-
Response spectrum method Maximum response of several modes
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -56-
Response spectrum method Superposition rule (e.g. Square Root of the Sum
of the Square SRSS)
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -57-
Response spectrum method Important parameter: the modal mass
∑
-0.90
0.29
1.00
0.96
0.39
Proportional to base shear
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -58-
The 5 mainstays of earthquake engineering
• Post-seismic surveys
• Laboratory tests
• In situ vibration recordings
• Models
• Codes
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -59-
Design codes
• Loads depending on the seismic zone, soil (eventually microzonation), importance, period and damping
• Conception guidelines (most important)
• Design computations
• Assessment guidelines
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -60-
Efficiency of design codes Damage in Kobe (1995) as a function of
construction year
87 RC and steel buildings of Chuo district in Kobe
Source: Architectural Institute of Japan, 1995
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CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -61-
Evolution of design codes in CH
< 1956
1970 - 1989
> 1989
1956 - 1970 Lateral force (wind & earthquake)
Increase in seismic actions for a typical building in Basel
• Before 1970: none
• 1970: rough (2-5% of weight)
• 1989: conventional (spectra, hazard zones…)
• 2003: modern (capacity design)
• 20??: Eurocodes