the february 2010 earthquake in chile actual versus ... · pdf filecicind report vol. 27, no....

CICIND REPORT Vol. 27, No. 1

65

Michael Angelides

Michael is a Civil Engineer with

B.Eng. and a M.Eng from McGill

University, Canada. In Montreal

he worked at orthopedic implant

and at satellites design. He re-

turned to Greece in 1989 and

joined AMTE. Since 1993 he is

the managing director of AMTE, a

consulting company specializing

in the design of all kinds of indus-

trial projects.

The February 2010 Earthquake in Chile

Actual versus Designed Response of 2 Concrete Chimneys

1. Introduction

Two reinforced concrete chimneys were designed in 2008 and

constructed in 2009 in Chile. In February 2010 an earthquake

measuring 8.8 on the Richter scale struck the region. The

present paper analyses the behaviour of these chimneys and

compares the designed response to the actual response.

2. Description of the Project

The two chimneys

were designed and

built for the thermal

power plants of

Colbun and Bocamina

in Puerto Coronel near

Coception, Chile

(approximately 500

km south of Santi-

ago). The power plant

owners are Colbun

S.A. and Endesa,

respectively. Both

power plant projects

had been awarded to

Maire Engineering,

which contracted the

design and construc-

tion of the chimneys

to Karrena GmbH,

Germany. The structural design of the chimneys was carried

out by AMTE Consulting Engineers, Greece, on behalf of

Karrena. The location of the chimneys with respect to the

February 2010 earthquake epicentre is shown in Fig. 2.1.

The layout of both chimneys has been based on the “New

Chimney Design” concept (Hoffmeister and De Kreij, 2008),

which essentially eliminates the free space between the liner

and the windshield. This is achieved by using a lining consist-

ing of Pennguard blocks attached directly to the inner surface

of the concrete windshield. The Pennguard blocks provide for

both the thermal insulation and for the acid resistance of the

windshield. Furthermore, the low weight of the Pennguard

blocks (borosilicate blocks) does not appreciably increase the

mass of the structure, which constitutes an advantage for seis-

mic regions (self weight = 1.9 kN/m3×0.054m thickness = 0.10

kN/m2). Since the lining coincides with the inner windshield

surface, the “New Chimney Design” concept results into

smaller overall reinforced concrete diameters, hence into more

flexible structures. This increased flexibility constitutes an

additional advantage for seismic excitations, due to the fact

that most design response spectra specify significant response

reduction at higher natural periods of vibration.

The Colbun chimney is 130 m high and the outer diameter

ranges from 11.00 m at the base to 5.90 m at the top. The top

50 m have a constant diameter. The concrete thickness ranges

from 40 cm at the base to 25 cm at the top. There are three

openings for flue gas

duct entry: Two at

level +16.50 and one

at +46.32. The bot-

tom openings have

dimensions 4.0×3.7 m

and the top opening

h a s d i m e n s i o n s

3.8×8.3 m. The chim-

ney foundation con-

sists of a circular raft

of 26 m external di-

ameter.

The Bocamina chim-

ney is 100 m high and

the outer diameter

ranges from 10.50 m

at bottom to 6.25 m at

top. The top 60 m

have a constant di-

ameter. The concrete thickness varies from 35 cm at the bot-

tom to 25 cm at the top There are two openings for flue gas

duct entry at level +12.50 with dimensions 3.9×8.4. The chim-

ney foundation consists of a circular pilecap over reinforced

concrete piles.

The design for the two chimneys was carried out in 2008 and

the construction for both chimneys was completed in 2009.

The two constructed chimneys are shown in Fig. 2.2.

Location of

chimneys

Fig. 2.1: Location of chimneys with respect to

February 2010 earthquake epicenter. (source: AON Benfield)

66


3. Design considerations

According to the contractual requirements, the chimneys had

to be designed to ACI 307-98 (Standard Practice for the De-

sign and Construction of Reinforced Concrete Chimneys) and

to ACI 318-05 (Building Code Requirements for Structural

Concrete). Earthquake related issues were specified in the

Chilean codes NCh 433 (Earthquake Resistant Design of

Buildings) and NCh 2369 (Earthquake Resistant Design of

Industrial Installations). Additionally, seismic design specifi-

cations had been prepared for these projects by Prof. E. Cruz,

who was also the design verification engineer on behalf of the

Owner.

The project region lies in an area of particular seismicity.

Most of the west coast of Chile coincides with the border be-

tween the Nazca and the South American tectonic plates (see

Fig. 3.2). This border is the source of frequent seismic activity

through subduction interaction as the Nazca plate pushes

against the Chilean coast. An overview of past earthquake

activity reveals that the region gives rise to a major seismic

event of magnitude 8.0 or greater every approximately 15

years: 1906 (Valparaiso, M8.0), 1922 (Vallenar, M8.2), 1943

(Coquimbo, M8.2), 1960 (Valdivia, M9.5), 1985 (Santiago,

M8.0), 1995 (Antofagasta, M8.0). This indicates that a major

earthquake was practically guaranteed to hit the chimneys

within their service life.

In consideration of the above observations, the following prin-

ciples were used for the design of the chimneys: The design

was carried out on the basis of forces determined from a re-

sponse spectrum analysis. The chimneys were detailed for

ductile behaviour by limiting the horizontal bar spacing, by

increasing the vertical bar splicing and by setting limits on

reinforcement ratio with respect to the axial forces at that level.

The ductile detailing rules were adapted from the CICIND

Code. Finally, the design of reinforcement was carried out

with reduced seismic behaviour factors at critical locations,

due to the perceived limited capability of the structure to con-

Fig. 2.2: Colbun and Bocamina chimneys. (Source: J. Wilson)

STACK DESIGN SPECTRA

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.0

0

0.0

5

0.1

3

0.1

8

0.2

5

0.3

3

0.4

0

0.5

0

0.6

0

0.7

0

0.9

0

1.2

0

1.4

0

1.6

0

1.8

0

2.0

0

2.4

0

2.8

0

3.2

0

3.6

0

4.0

0

5.0

0

6.0

0

7.0

0

period [s]

accele

rati

on

[g

]

COLBUN

BOCAMINA

Fig. 3.1: Design response spectra for the chimneys.


67

sume elastoplastic energy without irreversible damage

(Angelides, 2001) and also in order to ensure satisfactory post

earthquake response, with particular aim at resisting large

aftershocks (see Fig. 3.3).

In consideration of the fact that the design response spectra

involved significant reduction of response for high natural

periods, the specifications required a minimum guaranteed

base shear of 0.15 g for Colbun and 0.10 g for Bocamina.

The final geometry of the chimneys was determined through a

series of iterative calculations, in order to arrive at an optimum

configuration that would lead to the minimisation of the total

construction cost. The total cost consists of the concrete vol-

ume, the reinforcement weight, the formwork surface and the

lining surface. By assigning unit prices to the above and by

varying the diameter and thickness over the height, successive

responses and the associated construction costs were calcu-

lated. As the variation in geometry directly affected the stiff-

ness and the dynamic behaviour of the chimneys, the iterative

calculations were carried out through the use of response spec-

tra analyses using beam element models. The procedure fol-

lowed the methodology outlined in Angelides (1995) and re-

sulted in the final geometry and reinforcement selections.

After the definition of the final geometry, the design was car-

ried out by a 3D finite element model and a dynamic response

spectrum analysis.

Fig. 3.2: Global map of tectonic plates with the Chilean Nazca plate boundary highlighted. (Source: G.R. Saragoni)

23992399

223223

7878

20802080

LF9: earthquake along xLagerreaktionenSigma-y,+Stäbe M-z

Max Sigma-y,+: 1.86, Min Sigma-y,+: -2.18 [°]

S p a n n u n g en

y,+ [k N /c m 2 ]

1 .8 6

1 .4 9

1 .1 2

0 .7 6

0 .3 9

0 .0 2

-0 .3 5

-0 .7 1

-1 .0 8

-1 .4 5

-1 .8 1

-2 .1 8

M a x : 1 .8 6M in : -2 .1 8

25102510

215215

215215

1975

1975

LF8: earthquake along yLagerreaktionenSigma-y,+Stäbe M-z

Max Sigma-y,+: 1.45, Min Sigma-y,+: -1.75 [°]

S p a n n u n g en

y,+ [k N /c m 2 ]

1 .4 5

1 .1 6

0 .8 7

0 .5 8

0 .2 9

-0 .0 1

-0 .3 0

-0 .5 9

-0 .8 8

-1 .1 7

-1 .4 6

-1 .7 5

M a x : 1 .4 5M in : -1 .7 5

Portion of chimney

designed with in-

creased 2nd mode

participation

Portion of chimney

designed elastically

Fig. 3.3: Design considerations.

68


4. The February 2010 Earthquake

At 03:34 on Saturday, February 27, 2010 an earthquake meas-

uring 8.8 on the Richter scale struck the west coast of Chile.

The epicentre was located off Maule (105 km NNE of Concep-

tion) at a depth of 35 km (data from USGS). This earthquake

is currently listed by the USGS as the fifth largest globally

since 1900. The direct consequences included 450 deaths and

more than 30 billion US dollars financial losses.

The recorded ground motion lasted for more than 200 sec,

while the strong motion itself lasted more

than 50 sec. Fig. 4.1 depicts the graph of

horizontal accelerations recorded in Con-

ception (in Colegio San Pedro) and Fig.

4.2 depicts the vertical accelerations from

the same recording station. The maxi-

mum horizontal acceleration was 0.594 g

and the maximum vertical acceleration

was 0.571 g. While these recordings were

made in the city of Conception, hence

approximately 20 to 30 km north of the

chimney locations, they are considered

representative of the ground motion that

the chimneys were subjected to.

The response spectra corresponding to the recorded ground

motion at the Colegio San Pedro station have been calculated

and are plotted in Fig. 4.3 for different values of the behaviour

factor (q or R). The contractual design spectra are also in-

cluded in this figure, for comparison. It is directly apparent

from this figure that, for the natural vibration period range of

the chimneys at hand (2.1 sec for the Colbun chimney and 1.7

sec for the Bocamina chimney), the design spectrum is com-

patible with the calculated ground motion spectrum for a be-

haviour factor of 3.0 (such as the value specified in the Code).

-8.00E+02

-6.00E+02

-4.00E+02

-2.00E+02

0.00E+00

2.00E+02

4.00E+02

6.00E+02

0.01

6.50

12.9

9

19.4

8

25.9

7

32.4

6

38.9

5

45.4

4

51.9

3

58.4

2

64.9

1

71.4

0

77.8

9

84.3

8

90.8

7

97.3

6

103.

85

110.

34

116.

83

123.

32

129.

81

136.

30

142.

79

149.

28

155.

77

162.

26

168.

75

175.

24

181.

73

188.

22

194.

71

201.

20

Fig. 4.1: Horizontal acceleration record from Colegio San Pedro, Conception station. Maximum = 0.594 g.

VERTICAL COMPONENT [CM/SEC2]

-6.00E+02

-4.00E+02

-2.00E+02

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

0.01

6.50

12.99

19.48

25.97

32.46

38.95

45.44

51.93

58.42

64.91

71.40

77.89

84.38

90.87

97.36

103.8

5

110.3

4

116.8

3

123.3

2

129.8

1

136.3

0

142.7

9

149.2

8

155.7

7

162.2

6

168.7

5

175.2

4

181.7

3

188.2

2

194.7

1

201.2

0

time [t]

Fig. 4.2: Vertical acceleration record from Colegio San Pedro, Conception station. Maximum = 0.571 g

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0.0

0

0.0

6

0.0

6

0.0

6

0.0

7

0.0

7

0.0

7

0.0

7

0.1

0

0.1

6

0.2

5

0.5

0

1.0

0

2.0

0

3.0

0

4.0

0

COLBUN DESIGN

BOCAMINA DESIGN

FROM ACCELEROGRAM, q=1.0



Fig. 4.3: Contractual versus actual response spectra.


69

5. Actual response

In order to calculate the actual response of the chimneys, time history analyses were carried out on the basis of the accelero-grams recorded at the Colegio San Pedro Station. The results indicated that the developed forces were in the order of the

design spectrum values for a behaviour factor of 3.0. However, significant vertical axial forces were also devel-oping which were critical for the reinforcement stresses. Fig. 5.1 depicts the calculated elastic time history of base moments for the two chimneys, while Fig. 5.2 depicts the calculated elastic time history of axial forces for the Bo-camina chimney.

Fig. 5.3 illustrates the difference between the design mo-ment level and the calculated elastic response for the base of the Colbun chimney. The design moment level (red line) corresponds to a behaviour factor of 3.0. The design spectrum moments (orange line) are the moments calcu-lated from the design spectrum without the scaling up prescribed in the project specifications to guarantee a minimum base shear of 0.15 g. In the same graph on the right part of Fig. 5.3 are also superimposed the provided capacity moments (purple line) on the basis of the rein-forcements designed in consideration of reduced behav-iour factors, as outlined in Section 3.

Fig. 5.1: Time history of base moments (elastic response).

Fig. 5.2: Time history of axial forces at level +20.00

(elastic response). Bocamina chimney.

Fig. 5.3: Time history of base moments (elastic response) versus design moments. Colbun chimney.

70


The development of elastic stresses during an earthquake event

would lead to reinforcement stresses beyond the yield strength

of the material. The actual section capacity provided however

allows the redistribution of stresses in a way that the maximum

moments may be carried at lower reinforcement stresses.

It was also evident from the calculations that the vertical accel-

erations played a significant part in the structural response. In

the case of the Bocamina chimney in particular, the response

may have led to the development of elastic axial forces in the

order of 1.0 g. It appears that the piled foundation may have

contributed to this increased axial response in Bocamina, since

the raft foundation at Colbun probably provided damping

through rocking action, as illustrated in Fig. 5.4.

After the earthquake, both chimneys were inspected and no

structural damage was reported. The Colbun chimney did not

develop any cracking, while in the Bocamina chimney hairline

horizontal cracks developed, an indication of higher stressing

of the vertical bars caused by the increased axial tensions due

to the vertical acceleration. These observations are in line with

the calculated actual response.

5. Conclusions

The chimneys were subjected to significantly high horizontal

ground motion, as well as to very high vertical ground motion.

The provision for reduced behaviour factors, along with duc-

tile reinforcement detailing allowed for a safe response to ex-

treme seismic loadings.

6. Acknowledgements

I am particularly indebted to Prof. Ernesto Cruz in Santiago,

Chile for constructive discussions and guidance throughout the

design process. I would also like to thank Karrena GmbH for

a good cooperation and for an excellent execution of the pro-

ject that contributed to the overall success. I am grateful to

Prof. Nikos Gerolymos at the National Technical University of

Athens for providing the digital acceleration records from the

Chilean earthquake. Finally, I would like to acknowledge the

valuable contribution of Lena Zannaki at AMTE in the design

calculations of both chimneys.

7. References

[1] ACI, “ACI 307-98: Standard Practice for the Design

and Construction of Reinforced Concrete Chimneys”,

1998.

[2] ACI, “ACI 318-05: Building Code Requirements for

Structural Concrete”, 2005.

[3] M. Angelides, “Cost Optimisation Methods in Chim-

ney Design”, CICIND 43rd Meeting, Paris, April 1995.

[4] M. Angelides, “Earthquake Capacity Design Consid-

erations”, CICIND 55th Meeting, Antalya, April 2001.

[5] AON Benfield, “Event Recap Report: 02/27/10 Chile

Report”.

[6] CICIND, “Model Code for Concrete Chimneys”,

2001.

[7] E. Cruz, “Bocamina II New Coal Power Plant Seis-

mic Design Criteria”, 2008.

[8] E. Cruz, “Coronel Thermo-Electric Power Station

Seismic Design Criteria”, 2007.

[9] H. Hoffmeister, A. De Kreij, “Chimney for Wet Stack

Operation”, CICIND Report, Volume 24, Number 2,

July 2008.

[10] Instituto Nacional de Normalixacion, “NCh 433:

Diseño sismico de edificios (Earthquake resistant

design of buildings)”, Santiago, Chile, 1997.

[11] Instituto Nacional de Normalizacion, “NCh 2369:

Diseño sismico de estructuras e instalaciones

industriales (Earthquake resistant design of industrial

installations)”, Santiago, Chile, 2003.

[12] R. Leon, “The February 27, 2010 Chile Earthquake”,

School of Civil and Environmental Engineering,

Georgia Tech, Atlanta, 2010.

[13] G.R. Saragoni and S. Ruiz, “The 2010 Chile, Mw=8.8

Earthquake”, International Atomic Energy Agency,

2010.

[14] J. Wilson, “Performance of Pennguard Lined Tall

Reinforced Concrete Chimney Structures in the 2010

Chilean Earthquake”, Swinburne University of Tech-

nology, Victoria, Australia, 2010.

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