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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
CICIND REPORT Vol. 27, No. 1
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.
CICIND REPORT Vol. 27, No. 1
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
CICIND REPORT Vol. 27, No. 1
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
FROM ACCELEROGRAM, q=1.5
FROM ACCELEROGRAM, q=3.0
Fig. 4.3: Contractual versus actual response spectra.
CICIND REPORT Vol. 27, No. 1
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
CICIND REPORT Vol. 27, No. 1
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.