1

The 29 May Ölfus Earthquake: Structural response and

observed damage

Jónas Thór Snæbjörnsson

Contents• Review of the damage observed after the event on

May 29, 2008.

• The earthquake action resisted by buildings in the near fault area and Reykjavik is inspected through a study of response data available from instrumented structures.

• The response data is used to examine system parameters, such as natural frequency and structural behaviour.

• An example is given from a study of a reinforced cast-in-place concrete building where earthquake induced acceleration data has been systematically collected over period of 19 years.

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Review of observed damage

• No catastrophic collapse of residential buildings occurred during the event of 29 May 2008.

• About 3500 buildings were damaged to some extend,about 25 were uninhabitable and about 30 will be demolished.

• Those severely damaged are commonly older concrete or masonry buildings with little or no reinforcement.

• Several buildings sustained tilting or cracking related to differential soil-foundation settlements.

• Damage to building interiors was extensive and widespread. • Insured damage will exceed that of the June 2000

earthquakes (5.5 Gkr -> 7.5 Gkr) even though the event magnitude was smaller– The May 2008 epicentral area is more densely populated and the

exposure of the built area therefore greater

Review of observed damage• There was some damage to the roads

– subsidence of the filling and the underlying sediments causing longitudinal fracturing

– subsidence where structures such as at culverts intersect the roadway.

• Two bridges suffered minor damage, however, both were fully operational.

• The hot- and cold water supply systems in the towns of Hveragerdi and Selfoss suffered considerable damage.

• The earthquake caused extensive rock-falls from the surrounding mountains, and secondary faulting at mountain edges.

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Earthquake Acceleration Records

• Ground acceleration => Earthquake characteristics:– Magnitude, – Calibration of historic earthquake catalogues– Attenuation and duration– Ground motion simulation– Earthquake hazard mapping and zoning– Elastic and inelastic response spectra

• Response acceleration => Structural characteristics:– Structural behaviour – 3D effects – rotation – Natural frequency, damping– Verification of structural modelling– Damage (damage indices)

The Icelandic strong motion networkThe Earthquake Engineering Research Centre, University of Iceland

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Structural Earthquake Response• Structures magnify the ground motion

• The magnification of motion mainly depends on :– The dihedral angle and frequency of the

earthquake wave. – The dynamic characteristics of the structure

• natural period, critical damping ration, etc.

• Magnification of 8 times the base acceleration has been recorded at the top elevation of instrumented structures.

The Town Hall at Selfoss< View from Northeast.

Black arrows indicate location and directionality of accelerometers.

S

v Plan view of the ground floor.

Location of uni-axial & tri-axial accelerometers is shown.

The location of retrofitting elements:

(1) RC wall & (2) steel cross-braces installed in spring 2000

5

6

7

0 2 4 6 8 10 12 14 16 18 20

-0.5

0

0.5K PGA NS = 0.5227g

0 2 4 6 8 10 12 14 16 18 20

-0.5

0

0.5

AC

CE

LER

AT

ION

(g)

K PGA AV = 0.35838g

0 2 4 6 8 10 12 14 16 18 20

-0.5

0

0.5

TIME (s)

K PGA UN = 0.27247g

Selfoss Town Hall: Ground accelerationDuration ~5 s.

PGA

8

0 2 4 6 8 10 12 14 16 18 20-1

-0.5

0

0.5

13 PGA NS = 0.77587g

0 2 4 6 8 10 12 14 16 18 20-1

-0.5

0

0.5

1

AC

CE

LER

AT

ION

(g) 3 PGA VA = 0.48941g

0 2 4 6 8 10 12 14 16 18 20-1

-0.5

0

0.5

1

TIME (s)

3 PGA SN = 0.71384g

Peak values on 3ed floor ~1.4 timeslarger than in the basement

West gable

East gable

Centre

Selfoss Town Hall Acceleration response on 3ed floor

Selfoss Town Hall: Response spectra

0.04 0.1 0.2 0.4 1 2 40

2

4

6

8

10

12

AC

CE

LER

AT

ION

(m

/s2 )

UNDAMPED NATURAL PERIOD (s)

N-S componentE-W componentvertical-component

Response spectra :Envelop of a computed peak dynamic response parameter for single degree of freedom elastic systems having a range of periods, for a given ground motion and viscous damping ratio (usually 5%).

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Selfoss Town Hall – May 29, 2008Relative response acceleration

0 2 4 6 8 10 12 14 16

-50

0

50

0 2 4 6 8 10 12 14 16

-50

0

50

AC

CE

LER

ATI

ON

(%

g)

0 2 4 6 8 10 12 14 16

-50

0

50

TIME (s)

West end: N-S axis

East end: N-S axis

Centre: A-V axis

A_peak=72% g

A_peak = 52% g

A_peak = 44% g

Selfoss Town Hall – June 21, 2000Relative response acceleration

0 2 4 6 8 10 12 14 16

-50

0

50

0 2 4 6 8 10 12 14 16

-50

0

50

AC

CE

LER

ATI

ON

(%

g)

0 2 4 6 8 10 12 14 16

-50

0

50

TIME (s)

West end: N-S axis

East end: N-S axis

Centre: A-V axis

A_peak = 28% g

A_peak = 22% g

A_peak = 36% g

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Normalised SpectrumCentre: E-W axis 2008 Centre: E-W axis 2000

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

OR

DE

R

FREQUENCY Hz

L-AXIS

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

OR

DE

R

FREQUENCY Hz

19.018.117.015.614.613.311.910.19.17.96.85.33.22000

19.317.616.915.213.912.911.19.38.27.06.04.22.62008

Natural frequencies (Hz)

Normalised SpectrumWest end: N-S axis 2008 West end: N-S axis 2000

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

OR

DE

R

FREQUENCY Hz0 2 4 6 8 10 12 14 16 18 20

0

20

40

60

80

100

120

140

160

180

200

OR

DE

R

FREQUENCY Hz

17.816.515.314.212.611.19.99.07.95.64.13.22000

17.115.714.813.512.310.89.47.46.44.63.83.12.32008

Natural frequencies (Hz)

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Normalised SpectrumEast end: N-S axis 2008 East end: N-S axis 2000

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

OR

DE

RFREQUENCY (Hz)

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

OR

DE

R

FREQUENCY (Hz)

18.016.615.514.713.311.99.99.07.96.75.34.13.22000

16.715.414.413.412.311.29.46.95.84.63.93.12.32008

Natural frequencies (Hz)

Location of fracture shown by yellow lines

Damage to the Selfoss Church tower on May 29, 2008

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FE-líkön gerð í SAP2000 og ETABS

1: 0.2019s 2: 0.1793s 3: 0.0618s

4: 0.0523s 5: 0.0494s 6: 0.0440s

Einingarlíkön

Sveifluform FE-líkansins úr SAP2000

SAP2000 ETABS

This loose material can affact theground behaviour underneath the bell tower

Figure: Páll Imsland

Geological profile close to the church tower

7.800 years – Extrusive rock(Þjórsárhraunið mikla - hraun)

7.800 years – Scoria(Þjórsárhraunið mikla - gjall og kargi)

7.800 – 10.000 years - Sedimentary bed (möl, sandur og leir)

~1.500.000 years - Rock formations(ísaldarberg, hraun og móberg)

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Final Remarks• The South Iceland earthquakes in 2000 and 2008 showed

similar characteristics, i.e. – high peak acceleration – short duration of motion is short and – rapid attenuation of motion with increasing distance.

• System identification based on recorded response data can serve as a baseline for damage identification as well as calibration for further structural modelling of buildings.

• Visible damage in the buildings studied is slight, but the system parameters, at least in some cases, reflect structural changes.

• The power spectral densities and natural frequencies evaluated based on the response data indicate strongsoil-structure interaction, especially in the Selfoss area.

Final Remarks

• Although considerable property damage occurred in the South Iceland earthquakes in 2000 and 2008, the vast majority of structures seemed to withstand the strong-motion fairly competently and without significant damage.

• This is believed to be mainly due to:– The low-rise, predominantly reinforced concrete or

timber, style of buildings. – The short duration of strong-motion contributed to the

endurance of structures.

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THE END

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