EDCE: Civil and Environmental Engineering CIVIL 706 ......The seismic problem Earth surface and...

CIVIL 706 - Introduction EDCE-EPFL-ENAC-SGC 2016 -1-

EDCE: Civil and Environmental Engineering CIVIL 706 - Advanced Earthquake Engineering

Introduction


The seismic problem Earth surface and everything built on it are subjected to

strong vibrations from time to time


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)


Why ? Plate tectonics Mantle viscous convection driven by heat flows

induce deformations in the crust

Tackley (Nature Geoscience, 2008)


Why ? Plate tectonics Large-scale: deformations in the crust

accomodated at plate boundaries

Tackley (Nature Geoscience, 2008)


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


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…)


Why ? Wave propagation

Source : USGS


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


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


Site Effects Mw=6.8 Tottori Earthquake (Japan) - Records

http://www.eps.s.u-tokyo.ac.jp/jp/guidance/solid/furumura.html


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)


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:


When ? Hazard assessment PSHA of Mediterranean basin


When ? Hazard assessment PSHA of Switzerland


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


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


What ? The EQ loading Particle motion:


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.


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…)


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


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.


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


Existing buildings

Main issue: Vulnerability of existing buildings •  No earthquake resistant design •  Renewal rate around 1%/yr


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


Existing buildings in Switzerland

Example surveyed main cities in Valais:

•  Sion:

•  Martigny:


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


The 5 mainstays of earthquake engineering

•  Post-seismic surveys

•  Laboratory tests

•  In situ vibration recordings

• Models

• Codes






• Models

• Codes


Stone masonry buildings

Low deformation capacity, bad mortar, bad link between walls (out-of-plane behaviour), much material variability

Source : Courtesy of G. Jacquet


Masonry buildings - Wooden floors

No diaphragm effect (differential displacements between walls), out-of-plane behaviour of walls


Brick masonry

X cracks → cyclic loading


RC - Hooks Problems with 90° hooks - Prefer 135°


RC - Rebar overlaps Overlap in plastic zones

Plastic zone

Overlap


RC frames - short columns

Generally associated with partial infill


RC Frames - Soft story

Also transformed ground floors for commercial in city-centres


Non-structural elements Façades


Bridges Bearing length






• Models

• Codes


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


Laboratory tests

Static-Cyclic tests (Devaux, 2008)






• Models

• Codes


In Situ vibration recordings

•  Earthquakes

• Ambient vibrations

•  Forced vibrations






• Models

• Codes


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


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


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)


Seismic demand - Response spectrum


Response spectrum WARNING DO NOT MIX


Response spectrum Displacement and Acceleration


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…)


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


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


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


Response spectrum method Maximum response of several modes


Response spectrum method Superposition rule (e.g. Square Root of the Sum

of the Square SRSS)


Response spectrum method Important parameter: the modal mass

∑

-0.90

0.29

1.00

0.96

0.39

Proportional to base shear






• Models

• Codes


Design codes

•  Loads depending on the seismic zone, soil (eventually microzonation), importance, period and damping

• Conception guidelines (most important)

• Design computations

•  Assessment guidelines


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


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

EDCE: Civil and Environmental Engineering CIVIL 706 ......The seismic problem Earth surface and...

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