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TRANSCRIPT
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Summary
The earthquake of 27 February in Chile had a great effect on structures all over the
country and in
particular in the region of Concepcion. The structure treated in this article is one of
the pile
supported wharfs of the fisheries in city of Coronel. The structure consists of a
walkway, which
connects the land with a mooring platform. The walkway is divided into two partsby dilatations.
Unknown is why the wharf was so heavily damaged and what was the main
cause of the damage. Two cases are considered. Either the wharf was designed to
sustain an earthquake with an
acceleration of 0.40g, while the maximum acceleration of the earthquake in
Coronel was
approximately 0.65g, or the damage is a consequence of improper design or
manufacturing.
Damage to the pierThe damage to the pier manifests itself in a number of ways. The platform adjacent part of the
walkway shows large
vertical deformations
accompanied with
plastic hinges in-
between the concrete slab and beams of the superstructure. The land adjacent part
of the walkway shows large horizontal displacements and the tear out of diagonal
piles. The mooring platform had very little damage. The emphasis of the analyses
will be on the platform adjacent part of the walkway.
Threecommon cases are known, which can cause such damage to the pile supported structure:
1. An inertia force at the deck of the structure2.Liquefaction
of the soil
3. Acombination of the
two above
In thecase of
an
inertia
force,
the earthquake load working on the mass of the structure is the cause
of the damage. In the case of liquefaction the loss of the stiffness due to the
increase of effective pile
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length is the cause. Liquefaction is a significant strength loss in loose saturated
granular soils as a
result of the shaking of ground. As saturated soil deposits are sheared rapidly back
and forth by the
shaking motion, the water pressure in the pores starts to rise. The soil loses its
strength and stiffness.
Hereafter a description is given of the model, which has been developed to analyze
the failure mode.
The Lp modelThe walkway is modeled as a rigid floor slab, supported by two beams on piles. All connections
between slab, beam and pile are rigid. The important part of this case is modeling the soil-pile
interaction. The so-called Lp-model is used. In the Lp-model, after some depth in the
soil the position
of the pile is fixed. This point is the position of the maximum moment in the pile.
The penetration
depth into the bearing sand layer, which necessary to reach the fixing point, is
represented by the
length Lp. In order to obtain the fixation the total penetration into the bearing sand
layer should be3Lp. In the case of liquefaction the bearing sand layer is positioned deeper. In case
the bearing layer
is too deep to obtain 3Lp, the support may act as a hinge or
not at all. Lp formula.
1 E s
is defined by the following
Lp= with: = 4 4EI
EI = flexural rigidity of the pile
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Es = the soil elastic modulus
Response spectrum
The earthquake load can be put in the model using the acceleration response spectrum of the
structure. Two spectra are considered: the spectrum of the Chilean code Nch2369
and the actual
spectrum of the 27 February earthquake. When both spectra are plotted in one
graph, one of the
main questions of the research is almost instantly answered. Except for a small
peak in the lower
period-range, the design response spectrum of the code is larger than the response
spectrum of the
27 February earthquake.
The Eigen period of the structure in the stiffest case (no liquefaction and fixed supports)
corresponds with 0.44s.The critical range where the 27 February earthquake
spectrum exceeds the design spectrum is approximately 0.17s - 0.25s. For caseswhere the structure is less stiff, the Eigen period become larger. Thus can be
concluded that, despite of the lower maximum acceleration, the design code is well
fit for this structure.
Results of the Lp calculation
The structure can be modeled using the Lp-model with Finite Element software. In
this analysis the program RAM Advance is used. Using the average of the properties
of the sand soil layers, the length Lp can be calculated to be approximately 2
meters. Thus a 6 meter penetration of the piles is
necessary to obtain fixation. All the piles ought to have a penetration depth of 17meters, but this is not exactly known. Nevertheless without liquefaction of the soil,
the pile penetration is sufficient.
A ground research indicates that the top 11 meter of the sand layers are
susceptible to
liquefaction. This means that when the whole length of 11 meter truly liquefy,
exactly 6 meters of
pile penetration are left to ensure the fixation. However the calculation used to
determine this
length is merely a rough representation of the reality, the 6 meters may not be
sufficient to ensure fixation, and the true penetration depth is unknown.Assuming that fixation of all the piles is ensured, the structure performs pretty
well both in case with and without liquefaction. Two indicators of performance are
use for comparison with the capacity of the structure: the maximum moment in
axial direction in the cross section of the
superstructure and maximum axial displacement.
Failure mechanism
Because the structure performs very well in the model when all the piles are still
fixed, there must be
an additional effect. The assumption is made that several piles loose all bearingcapacity and have no
support, due to liquefaction of the soil. This assumption is supported by the before
mentioned fact
that the bearing soil penetration may not be sufficient. Based on the real
deformations of the
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structure, other schemes are proposed. By one of them, the outer pile rows and one
row of the
diagonal axial piles keep their fixed supports (+ 3 fixed rows), the other piles lose
their support. Only
for this scheme the axial maximum moment in the concrete deck exceeds the cross
section capacity.
The exceeding of cross section capacity occurs at two places, both above the
diagonal piles. Here
plastic hinges are formed. When the deformation of the 3 fixed rows model is
compared with realdeformation of the structure, the similarities are abundant. With probability close to
certainty, it can
be said that this is the failure mechanism, which occurred during the 27 February
earthquake.
Due to the loss of bearing capacity of several piles, the structure was unable to
bear its own
weight. The contribution of the inertia load on the deck is minor, because in this
unstable state, the
Eigen period of the structure is very high. Thus can be concluded that liquefaction of
the soil is the
main cause of the occurred damage.
iv
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1 Report introduction
The earthquake of 27 February in Chile has caused damage all over the country and
in particular in
the region of Concepcion. Engineering experts from universities, companies and
from abroad are
analyzing damaged structures. In this report one of the damaged structures will be
analyzed. The
structure in question is the fishing pier of the city of Coronel. The pier was designed
to sustain an
earthquake with 0.40g acceleration; the acceleration of the earthquake in coronel
was approximately
0.65g. Unknown is why the pier was so heavily damaged and what was the main cause of the
damage. The high maximum accelerations, significantly higher than the design
accelerations, could be the reason, but maybe the structure could not resist the
design acceleration itself. The
government plans to rebuild the pier using old piles, although no research hasbeen done, whether this is the best way.
In this report a damage investigation and structural analyses on the pier are performed. The
objective is to trace the origin of the damage and under which minimum
conditions the pier would have collapsed.
Before the structural analyses and the damage investigation there are introductory
chapters. The
introductory chapters give background information and structural information, such
as dimensions
and design criteria. Chapter 2 gives a area description, the chapter zooms in fromthe country to the
fishing pier. Chapter 3 explains the basics of subduction earthquakes: the
earthquakes that emerge in
Chile. The chapter forms the basis of the up following chapter. That chapter is about
the Earthquake
of 27 February. The total picture of the earthquake will be put out. Later in the
analyses significant
characteristics of the earthquake will be used. In chapter 5 the structural lay-out of
the pier is
discussed. The emphasis is on the part of the structure that will be analyzed in the
structural
analyses.
The damage investigation is described in chapter 6. This chapter consists of two
parts. The first part is a list of all the damages. The damages are summed up
supplemented with pictures and comments. In the second part the possible origins
of the damage are put out. Three standard deformation cases of piers due to
earthquakes will be treated.
The actual structural analyses is treated in the last three chapters. Chapter 7 laysout the principles
and the method of analyses. This mainly concerns the conversion of reality to a
usable computer
model. In Chapter 8 all the necessary input parameters of the model are
determined. This includes
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soil properties, liquefiable soil layers and response spectra. Chapter 9 is the
chapter where its all
about, the structural analyses. First the use of failure indicators is explained. Next
different models
are compared with each other and with the real damage. The results of the design
and the 27
February earthquake are compared with each other. And the results of the analyses of the two
walkway parts are viewed in relation with the real damage. The chapter finishes
with a sketch of the failure mechanism of one of the walkways and some firm
conclusions.
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2 Area description from country to fishing pier
2.1 Description of Chile
Chile is a peculiar county in the southern part of South-America. With a coastline of
4630 km Chile is the longest country in the world, in north-south direction. The
country is bordered by two natural barriers. In the north is the Atacama desert, a
very dry and uninhabitable desert. And in the east over the full length of the country
is the Andes mountain range. These two barriers make that Chile is
dependent on its ports for any large goods to enter.
The country can be divided in three parts. The
northern part of Chile which mainly consists of
the
desert is thinly populated. The middle part of
Chile,
which is the engine of the economy is
relatively dense
populated. Over 80 percent of the people live
in the
area. The area approximately extends from
Coquimbo
to Puerto Mont. The southern part of Chile is
rather
cold and just like the north thinly populated.The total
population of Chile is 17 million people. 5
million live in
the capital Santiago.
The living standard of the people is high in
comparison
with other Latin American countries. The
average
income per capita is $14.300. In 2010 Chilebecame a
member of the OECD, which is basically an
organization of counties with a high income.
Because of the isolated position, 88% of the
international commercial exchange is carried
out via
maritime transport. The main sea ports of
Chile are
San Antonio and Valparaiso. Because of thelong
coastline, the presence rocky coast and many
bays, a
lot of small ports emerged at the coast of Chile
as well.
The bays give good sheltering conditions and
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possibilities for small economic activity such
as fishing.
2.2 Description of Coronel Bay
Coronel Bay is situated 30 km south of
Concepcin.
The bay is a part of the Gulf of Arauco, the
bay south
of the line Isla Santa Maria - Coronel. Coronel
Bay is indicated in the highlighted yellow box.
The bay is well sheltered from waves. The
northwestern wave heights are reduced by
the Coronel Land end, the south-
western waves by Isla Santa Maria, the largeisland.
Figure 2.1: Map of Chile (Wikipedia)
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Figure 2.2: Map of the area around Coronel Bay (Google Earth)
Coronel Bay houses Port Coronel, Cabo Froward and fisheries. Port Coronel is an
important port in
the region for the export of wooden products and industrial products. The Port is
indicated in the red
highlighted part in figure 3. Cabo Froward is a port with two conveyor belts and is
mainly used for the
import of wood chips and coal. It is indicated in the green highlighted part in the
figure. The fishing
area is indicated with the blue highlighted box and will be discussed in the next
paragraph.
Figure 2.3: Partition of Coronel Harbor (Google Earth)
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2.3 Description of the Fishing Wharf
The fisheries are an important part of the coronel industry. The fishery area contains six fishing
companies. Together the companies used to produce 1.3 million ton fish per year.
There are up to
7000 workers active in the industry. Two types of ships are used: big ships with adraught of 2-3
meters and small ships. The six companies all have their own conveyer: two piers
and four flexible
conveyers. For years the Coronel Bay suffers from sedimentation problems inside
the bay. Because of this extensions of the piers had to be realized. Other companies
use flexible conveyer belts, so the
mooring location can be moved.
The largest fishing pier was heavily damaged by the earthquake of 27 February. The
investigation andanalyses of the damage of this pier is the subject of this report. The pier in question
is highlighted
with a yellow box in figure 3. General information and structural details of the pier
will be elaborated
in chapter 5.
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3 Earthquakes in Chile
3.1 The emergence of a subduction earthquake
Earthquakes are the result of movement of land masses on the earth. Due to a
convection process in
the core of the earth, different tectonic plates at the surface of the earth move in
the direction of the
underlying convection current. In this slow process the plates move in different
directions. The
meeting point of two tectonic plates is called a fault line. Plates can move towards
and from each
other and they can shear against each other. The first and the last type of
movement can cause
major earthquakes. Within a sort of movement there is more than one type ofearthquake. From this
point I will focus on the type of earthquake present in Chile, the subduction
earthquake.
In Chile two plates move towards each other, the Nazca plate and the South-America plate. The
Nazca plate is an Ocean plate and moves eastwards. The South-America plate is a
continental plate and moves westwards. At the fault line the Nazca plate sub ducts
the South-America plate and goes in the depth of the earth. This movement has
average relative velocity of 80 mm per year, but the largest part of the movement
is caused by the Nazca plate (see Figure 3.1). Rupture zones extend to a depth of50 km and their lengths could reach over 1000 km. This convergence is responsible
for strong earthquakes, even the strongest earthquakes we know in the world.
In the most southern part the situation is different. Here the Antarctic plate is sub
ducted under
the South-America plate, this happens at a slower rate. In this region the
earthquakes are mediocre.
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Figure 3.1: Tectonic movement of continental plates (Vigney, 2003)
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In the contact region the convergence is not constant, because of the subduction process. The
convergence of two plates is determined with two stable points on the plates, for
example with GPS, with the points on Easter Island and Buenos Aires. The relative
movement in the contact region is
close zero for a long time. This process is the offset of an earthquake. The process is
graphicallyexplained in Figure 3.2.
While the sub-ducting plate sub ducts the overriding plate, the overriding plate
sticks to the subducting plate. In this process the plates deforms elastically. The
elastic deformation continues to accumulate over a time span of years, but at a
certain time the stress between the two plates
become too high. The deformation accumulates up to the ignition of the fault and
the generation of an earthquake. Ultimately the plates are back in their original
positions; the position of the beginning of a seismic cycle. Dependent on the time
between two major earthquakes in one area in a fewminutes the overriding plate can move 5 to 10 meters during an earthquake.
Figure 3.2: The genesis of an earthquake in case of subduction (Terremonto Cauquenes 27 Febrero 2010)
The intensity of an earthquake is different per
region. In Figure 3.2 can be seen that the
earthquake motion for the most part takes
place in the coastal region. The most effected
part of the country is located from the coast
up to 50-100 kilometers inland, approximately
200 kilometers from the trench. Next to
scientific knowledge also earthquake records
confirm the diminishing effect of earthquakes
inland.
The Chilean building norm uses three
distinct earthquake risk zones. Zone 1
mostly in the Andes, with low risk, and
zone 3 close to the coast with high
risk.
Several design criteria depend on the risk area
where a structure is built. Criteria such as the
design acceleration of an earthquake and
modification design load factors. In figure 6
the risk zones for middle Chile are shown. The
majority of the cities are situated in risk area 2
or 3.
Figure 3.3: Risk zones used in the Chileanbuilding norms (NCh0433-1996)
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3.2 The effects on nature and on structures as a result of earthquakes
The effect on land structures
An earthquake is a dynamic load on the land and on the buildings atop of the land. A good
representation is the maximum acceleration in vertical and in horizontal direction. The load is
temporary, but can cause a lot of damage to structures that are not properlybuilt. Not properly in this case, means: not properly to withstand the earthquake.
The structure can still agree with national building codes.
Additional to the risk area, the magnitude of earthquake load on a structure, is dependent on
more aspects. One aspect is local side effects, such as the properties of the subsoil
and the thickness of the subsoil layers. Another aspect is the fault orientation. The
progressive rupture process
determines the directivity of the ground motion. The orientation of a structure is of
importance here. In their stiffest direction, structures develop the largest forces.
The principle of soil liquefactionLiquefaction is a significant strength loss in loose saturated granular soils. It is a
result of the shaking
of ground. As saturated soil deposits are sheared rapidly back and forth by the
shaking motion, the
water pressure in the pores starts to rise. In loose cohesionless soil, the water
pressure rises rapidly
and a level can be reached where the particles float apart and the strength of the
soil is temporarily
lost altogether. A possible effect of liquefaction is the occurrence of water sprouts
and large ground
movements. After liquefaction the conditions of the soil, such as the density, are
permanently
changed. As strength is reduced, deformations can occur due to a driving static
shear stress such as sloping ground, embankment loads and building loads,
amongst others.
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Figure 3.4: The mechanism of liquefaction (PIANC wg34)
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The particle size and relative density of the granular soils play the key role in the occurrence of
liquefaction. As the rivers travel from east to west, the slope and therefore the energy for
transporting particles decrease. The soils close to the Andes are primarily composed
of gravels. As the rivers approach the coast, smaller particle size are present.
Excess pore pressures or water
sprouts arise more readily in finer granular material than in coarser granular
material, because of the permeability of the material with respect to the rate of
floating. For this matter liquefaction is more common in coastal regions, especially
at or near river banks.
Tsunami waves
A possible effect of an earthquake is a tsunami. In Figure 3.2 a representation is included of the
emergence of a tsunami. The sudden horizontal and vertical movement of the land
is taken over by
the water in the form of a wave. Tsunami waves can reach heights up to 5 meter in
coastal areas and
inundation heights have been measured from up to 30 meters. Total villages have
been wiped out in
the past.
Permanent vertical uplift of the coastline
In addition to temporary effects, there is also a more permanent effect, the
displacement of the land. As mentioned earlier the land can move 5 to 10 meters
in horizontal direction. Also a vertical displacement takes place. The vertical
displacement can be noticed with easy practical methods at the coast. In figure 7
you can see vegetation of litho amnion coralline algae. These algae have a red
color and lose their color when they are not in their natural salt environment. The
white color is a marker for the natural coastal uplift. In this case there was an uplift
of 1.1 meter.
Figure 3.5: Uplift of the coastline visual due to vegetation (Barrientos, April 18, 2010)
3.3 History of earthquakes in Chile
No recorded human generation in Chile has escaped the damaging consequences of a large
earthquake. More than ten events with magnitudes greater then magnitudes 8 on
the Richter scale have taken place during the 20 th century alone. Since the settling
of the Spanish in Chile earthquakes have been recorded. Among these, the event of
1960 is the largest earthquake ever measured, with a magnitude of 9.5.
The return period for a magnitude 8 event for Chile considered as a whole is 15
years. The return period for any given region in Chile on itself for a magnitude 8
event is 80 to 130 years. Mega trust earthquakes like the one of 27 February have a
much longer return period. The last large earthquake in Concepcion was in 1835, a
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period of 175 years. Logically large and very large earthquakes are not independent
events. When the a return period of a magnitude 8 earthquake is long past since
the last earthquake, a much larger earthquake may be expected.
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4 The Earthquake of 27 February 2010
4.1 Properties and effects of the earthquake
Before the earthquake, Chilean geotechnical scientist indentified the Concepcion
area as a mature seismic gap, since no large subduction earthquake has occurred
there since 1835. They noted that the convergence of 75mm/year would have
accumulated up to more than 10 meter displacement, and predicted a magnitude
8.5 event should the earthquake happen in the near future. The time of an event is
unpredictable; however the magnitude if an event would happen can be predicted
pretty well. In this case as well, the prediction was good.
On February 27, 2010 the earthquake occurred off the coast of Maule, near
Concepcion. The epic
centre was located at the coordinates S36.027, W72.834 at a depth of 35 km
(see Figure 4.1). The
earthquake had magnitude of Mw 8.8 in Richter scale and a seismic moment of
1.8 1022 Nm. The
main shock occurred at 3.34 am local time and had duration of approximately 2
minutes. The effects of the earthquake were observed from Valparaiso to Temuco.
Apart from the immediate
consequences, the earthquake resulted in a tsunami that affected a significant
portion of Chile coast. The tsunami has wiped out total villages. Inundation in some
villages was 6 meters. At cliffs and near coastal islands inundation of over 10 meters
has been measured.
Figure 4.1: Intensity radius of the 27 February earthquake (Terremonto Cauquenes 27 Febrero 2010)
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Maximal ground accelerations
A network of ground motion station, monitor the ground motion accelerations continuously. The
outputs the stations give are digital acceleration files and response spectra. From these datathe
maximum accelerations can be subtracted. In Table 4.1 Table 4.1a number of
maximum accelerations measured by ground stations are given. Here g is thegravitational acceleration of 10 m/s2
Accelerations are an important quantity, used in the design codes, to determine the
forces exerted by an earthquake. Striking is the high vertical acceleration in
Concepcion. Both in relative a absolute value Concepcion had the highest vertical
acceleration.
Table 4.1: Accelerations measured by several Chilean ground motion stations (Terremonto Cauquenes27F)
Resulting vertical and horizontal displacements
Due to earthquake the large horizontal and vertical displacement of the land
occurred. In Figure 4.2
the deformation of the land is shown. The figure shows maximum uplift at the coast
of 1.5 to 2 meter
and subsidence inland. The horizontal deformations have maximum levels of 5 to 6
meters on the
coast. These figures however are based on seismic data. From GPS measurements isknown,
horizontal deformations of 8.5 meters have occurred at the coast near Concepcion.
For the vertical
deformation, the model produces are more accurate, based on comparisons with
measurements.
These uplift/subsidence are estimated from the upper growth limits of marine
intertidal organisms,
as explained in paragraph 3.2.
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Figure 4.2: Left: the fault slip inverted form seismic data; Right: the coseismic verticaldeformation (color in m) and the horizontal displacement (arrows). {Terremonto Cauquenes27 Febrero 2010}
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Liquefaction due to the 27 February earthquakeLiquefaction was observed to have occurred over a large area around Coronel. The widespread
presence of river sediments and the long duration of the event contributed to the number of
liquefaction observations. Areas of high spatial density soils susceptible to liquefaction, were
observed to have liquefied. In areas of low spatial density there was intermittent liquefaction of
susceptible soils. Observations indicate that the central region east of the epicenterexperienced less
intense ground shaking, which agrees with the lesser amount of damage of
structures in that region.
Significant properties for Coronel Bay
In this paragraph four earthquake properties/effects are discussed. In this research
ultimately only
the data of Coronel are of interest, particularly of the pier. Table 4.2 is a summary of
the
properties/effects of the earthquake in Coronel. First: The maximum acceleration in
Coronel is similar
to the maximum acceleration in Concepcion, 0.65g. Second: The vertical en
horizontal displacements
are considerable, however not as large as the displacements north of Coronel. Third:
Liquefaction
frequently occurred in the area of Coronel. Coronel is on build on not densely
stacked river soil.
Table 4.2: Properties/effects in Coronel of the 27 February earthquake
distance to epic centre 85km southmaximum acceleration 0.65g North - South
displacements ver. +1.3m Hor. 5.5m west
liquefaction frequently occurred
4.2 Aftershocks of the earthquake
After the occurrence of an earthquake,
aftershocks are expected to occur.
Aftershocks occur mainly in the area that hasbeen fractured and at the ends of the rupture
zones. The change of activation of a similar
major earthquake however is negligible.
Figure 2.2 shows all the (after)shocks with a
magnitude greater than 4.8. The number of
earthquakes and the magnitude decrease
with time. Both seem to decrease
exponential.
For a long period after the major event,earthquakes that matter occur. In the
building and analyses of structures
this long
and intense period of aftershocks
should be
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accounted for. For example when a
structure
is built such that damage is expected
after a
major event. The aftershocks could
hinder
the repair for a long time.Figure 4.3: Distribution of aftershocks of the earthquake ()
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5 The Muelle lo Rojas fishing pier
The Muelle lo Rojas fishing pier is a pile supported pier. The pier is 98m long and has
a walkway and a mooring platform at the end. The walkway is 4.0 meters wide; the
mooring platform is 6.0m wide and 14.0m long. The water depth varies from 0meters at the coast to 10 meters at the end of the mooring platform. The average
penetration length of the piles is 30m. The total length of the piles is the penetration
length plus the length in and above the water.
The mooring platform has the most important function of the pier. The platform
functions both as mooring structure and loading/unloading platform. Mooring is
possible on both long sides of the
platform. The mooring defense consists of 3 meter long wooden fenders, connected
to the platform. Also the platform contains stairs on one side that go all the way
down to the waterline. Both
walkway and mooring platform are 4.25m above sea level (N.R.S.). The pier ismainly used by big
fishing ships with maximum sizes of 20 by 10m and a draft of approximately 3m.
Pictures of the pier before the earthquake are shown in Figure 5.1.
Figure 5.1: Photos of the Muelle lo Rojas before the 27 February earthquake
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5.1 The layout of the structure
Anticipating on the damage of the pier and the pier analyses beforehand, only the
dimensions of the walkway will be discussed in this chapter. In the investigation of
the damage will become clear that the mooring platform only suffered minor
damage. Further the mooring platform and the walkway are not connected witheach other. Hence the mooring platform is of less interest for the analyses of the
structure and will not be discussed.
5.1.1 Dimensions of the structure
In Annex 1 the original drawings of the fishing pier can be found. On the first page a
top view and a
side view are shown. In the top view very nicely the distribution of the piles can be
seen. Piles are
placed every 6 meter and at several row on one or on both sides the piles are
placed diagonal. Thediagonal pile are always in pares. There are two different pares used in the
structure; pares oriented
in the axial direction of the walkway and pares oriented in the lateral direction. Two
cross sections
are of interest, a standard cross section (c.s. B-B), and a cross section with lateral
diagonal piles (c.s.
A-A). Further between row 8 and 9 and between the walkway and the platform,
next to row 16, are
expansions joints.
Piles of the walkway
The foundation of the structure consists of hollow steel piles. The piles are filled
with sea sand up to
the pile cape. The upper 45 mm of the pile is filled with concrete, as part of the pile
cap connection.
The profile of the piles is 123/4x61.7: the diameter is 324mm and the wall
thickness is 8mm. Final
the piles have different lengths, dependent on the water depth. In Table 5.1 the
length of the piles
for each row is shown. The column diagonal length contains the length of only the
diagonal piles.
The column vertical length contains the vertical length of both diagonal as
orthogonal piles.
Pile profile: 123/4x61.7 (d=324mm / t=8mm)
Table 5.1: length piles of the walkway
row pile length vertical pile length Seabed Embedded
orthogonal length diagonal depth (m) verticalpiles (m) diagonals (m) piles (m) length (m)
1 33.6 - - -0.00 30.02 34.0 33.0 34.7 -0.42 30.0 / 29.0
3 35.5 - - -0.85 30.0
4 - 33.9 35.0 -1.33 29.0
5 35.4 34.4 35.6 -1.80 30.0 / 29.0
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6 36.0 - - -2.38 30.0
7 36.6 35.6 36.8 -2.96 30.0 / 29.0
8 38.9 - - -5.30 30.0
9 39.3 - - -5.70 30.0
10 41.2 40.2 41.7 -7.61 30.0 / 29.0
11 - 40.8 42.3 -8.16 30.0
12 42.0 - - -8.44 19.0 / 29.0
13 - 41.3 42.8 -8.72 29.0
14 42.6 41.6 43.1 -9.00 30.0 / 29.0
15 42.9 - - -9.25 30.0
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Cross section of the walkway
The upper structure consists of a concrete floor that is supported by two longitudinal concrete
beams. The floor has a thickness of 200mm and is 4000mm wide. The beams have size h x w =
450x350mm, and a centre to centre distance of 2500mm. The beams and piles are
connected with
pile caps of 90x70x70mm. The top of the pile in the centre of the pile cap. The
system consisting of
the three types of elements is poured in one piece. Also the inner part of the steel
hollow pile at the
connection is filled with concrete. Further the walkway is supplemented with railing
and lampposts,
which have no structural purpose. In Figure 5.2 a cross section is shown with only
orthogonal piles.
Figure 5.2: Cross section of the walkway with two orthogonal piles (dimensions in m and cm)
However the majority of the pile rows have a cross section that deviates, with one
diagonal pare of piles in lateral direction, or two diagonal pares in the axial
direction. The piles stand under an angle of 18, with the vertical position. In case of
the lateral pare, the pile on the outside is connected with the beam. The pile on the
inside is only connected with the system through the block. The block has a
different size, in order to make the connection with both diagonal piles able. The
size of the block here is 90x170x80. In case of the axial pares both pile are
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connected to the beam. From a structural point of view, the floor height is part of
the beam and pile cap height.
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Figure 5.3: Cross section of the walkway with one orthogonal and two diagonal piles (dimensions in m andcm)
Expansion joints
In the introduction of this paragraph the presence of two extension joints was
mentioned. Figure 5.4
shows such a joint. The dilatation of the joint is 30mm. There are no materials
connecting the two
ends together in any way. So structural the dilatation separated parts are
completely independent.
Figure 5.4: Detail of a dilatation in the structure (dimensions in cm an mm)
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5.1.2 Layout of floor and beam reinforcement
The concrete floor, beams and pile caps all contain reinforcement steel. The
reinforcement lay-out can be found in on the second page of Annex 1. In this
paragraph I will lay out the reinforcement of the different elements, with help of
drawings. However it should be noted that the drawings in this paragraph dont give
the complete reinforcement lay-out. For the complete set of drawings I refer to thebefore mentioned annexes.
Floor reinforcement
The floor is reinforced with a mesh of orthogonal bars in the top and bottom of the floor. In the
longitudinal direction are 10 bars, c.t.c. 200 or 220mm, see Figure 5.5. There are
5 additional bars
16 at the supports. The length of the additional bars is 3.2m for intermediate
supports and 2.15m
for end supports. In the transversal direction are 10 bars c.t.c. 200mm. Between
two top/bottombars is an additional bar that crosses diagonally from top to bottom between the
beams, see Figure
5.5, the middle bar. This means that above the beams in the top of the floor the
bars have a c.t.c. 100mm, and similar for the bottom of the floor between the
beams.
Figure 5.5: Transversal cross section of the floor reinforcement (dimensions in m and cm)
Beam reinforcement
The beams are reinforced with a reinforcement cage in the transversal direction and bars in the
longitudinal direction. The cage 10, with has a c.t.c. distance of 250mm in the field and a
150mmc.t.c. distance above the support. The lay-out of the longitudinal bars is somewhat
more complicated. I will suffice with the explanation of the two lay-outs of Figure 5.6.
Over the whole length of the
beam there are four corner and two intermediate bars. The corner bars in the
bottom are 18, the four above are 12. An individual bar has a length of 12m, and
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between bars a weld length of
500mm exists. In the field are an extra 3 bars in the bottom of the beam 3 22. At
the support are an extra 6 bars in the top of the beam 6 22. The transfer of forces
of bottom and top additional bars can be seen well in Annex 1, in the longitudinal
beam reinforcement figure.
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Figure 5.6: Transversal cross section of the beam reinforcement: right - at a support; left - in the field (allin mm)
5.1.3 Structural details and layout of the reinforcement of the pile caps
The pile caps are reinforced with an reinforcement cage. The cage contains of 20 vertical bars12 @
11 and 10 horizontal rectangles 12 @10; see Figure 5.8 and
Figure 5.7. In addition the top of the pile has to the surface welded spiral
reinforcement. The pile top also has elongated cuttings, which are connected to the
inside of the pile and reach to the floor. The spiral reinforcement is 10 and thecuttings 4 16. From the reinforcement lay-out of the structural drawings does not
become clear how the reinforcement of the blocks, beams and floor collaborate in
the joined structural parts. Supposedly the reinforcements are braid together,
obeying maximum reinforcement standards for reinforced concrete.
Figure 5.7: Top view of the pile - block connection reinforcement (dimensions in mm and cm)
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Figure 5.8: Longitudinal cross section of the pile - block reinforcement (dimensions in cm and mm)
The connection of the structure with two diagonal piles is off course different. The
principle of the
spiral reinforcement, cuttings and the reinforcement cage however, is the same.
The difference is in
the amount of bars and the extreme measures of the pile cap. The lay-out can be
found in Annex 1.
5.1.4 Properties of the pier materials
In the structure of the walkway three different materials are used: A36 steel for
the piles, FeB500 steel for the reinforcement and H20 concrete for the
superstructure. The properties of these materials presented below, are the
properties that will be used in the analyses.
Steel of the piles
The piles have steel quality ASTM A36, which is an American standard for steel.
This steel quality close to the European standard S235, but is slightly different.
fs = 250 N/mm2 yield stress
ft = 400 N/mm2 ultimate tensile strength
E = 2.1 105 N/mm2 Youngs modulus
= 7800 kg/m density
Reinforcement steel
The reinforcement quality unfortunately is unknown. Because of this I assume the moststandard
reinforcement quality FeB 500. FeB is a reinforcement quality used all over the
world. This quality is used for all bars.
fs = 435 N/mm2 yield stress
E = 2.1 105 N/mm2 Youngs modulus
= 7800 kg/m density
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Concrete upper structure
In the upper structure H20 (Hormign 20) is used. H20 is quite similar to the
European C20/25. The Youngs modulus is calculated using fck. For the density I
assumed the typical density of concrete, 2400 kg/m, a legitimate assumption for
normal concrete. The design compressive and tensile
strength are the characteristic material strength divided by a material factor.
fcd = 20 N/mm2 design compressive strength
fcd = 1.15 N/mm2 design tensile strength
Ec = 21.4 103 N/mm2 Youngs modulus
= 2400 kg/m density
5.2 Conditions and properties of the structure and the earthquake
In order to determine the design response spectrum and the true response
spectrum, several design need to be known. Within the design conditions are threeimportant subjects, the sort of structure, the area conditions, and the risk area. The
sort of structure influences the design response spectrum through factors. The area
conditions contain soil properties. The soil type is also of importance for
determining the response spectrum. The risk area is discussed before in paragraph
3.1. Coronel is situated in risk area III. Earthquake load conditions depend on the
risk area.
Structural design conditions
In dynamic analyses two structural properties are of importance; the correction factor I and the
damping coefficient . The correction factor is a structure type dependent factor.Structures in the Chilean code are subdivided in the three categories, C1 to C3. The
Muelle de Rojas pier is a category C2 structure. For category C2 structures a factor
of I=1.0 is taken into account. Further every
structure has a typical damping coefficient; the Chilean code gives an estimate for
several structures. For welded steel structures a coefficient is given of = 0.02.
Structure factor I = 1.0
Damping coefficient = 0.02
Further Coronel Bay has minor tidal movements. The difference between the tides isapproximately
0.5m. Loads due to currents are negligible. The extreme wave height in the bay is measured Hs=
1.5m with Tp = 13s. However it is not realistic to account for the extreme
values during the earthquake. The flow and wave loads are not taken into
account in this analyses.
Properties of the design earthquake load
With Chilean code a design earthquake can be determined. This earthquake load is
determined by a formula containing several parameters. The parameters are lay
out here.
Earthquake induced vibrations are modeled with a acceleration. In Chile each
earthquake risk
zone has its own acceleration. Coronel lies in risk zone III; for this zone the
design acceleration is
0.40g. The other risk zones have lower design acceleration. At this acceleration
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the structure is supposed to stay fully intact.
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Figure 5.9: maximum acceleration for seismic design in the Chilean code (NCh2369)
In the Chilean code, soil is divided in four types. Type I being a solid and hard, e.g.
gravel. And Type IV
being soft and flexible, e.g. clay. The soil of Coronel bay and specifically the soil near
the pier is a
saturated sandy soil with fine sand and silt; the relative density DR is between 55%
and 75% and the
blow count N between 20 and 40 for the larger part of the soil. The soil can beclassified as an Type III soil. Soil properties Dr and N are determined in Chapter 8.1.
In Table 5.2 two soil dependent
properties can be found. The definition of these properties is not given by the code.
Both entities are properties of the response spectrum.
Table 5.2: Soil dependent properties of the response spectrum in the Chilean code (NCh2369)
Further in the structural analyses the following modifications factors are used in the
analyses; R the
reaction modification factor; and the seismic coefficient Cmax. Factor R is an empirical
value that takes
the type of structure into account. It is selected from a list with all types of building
withcorresponding R-values. Coefficient Cmax determines the maximal spectrum value in
the design
acceleration response spectrum. The coefficient is dependent on the damping
coefficient and the modification factor R. It is selected from a table.
Table 5.3: The table necessary to determine Cmax
in the Chilean code (NCh2369)
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The pier in relation with the earthquake
Next to properties that apply for the area of Concepcion Bay, there is a property
that is specific for the pier; namely the orientation of the pier in relation to the fault
orientation. The horizontal
acceleration of the earthquake concentrates in one direction. The simplest
approach to obtain this direction is drawing a line from the epic centre of theearthquake to the place of interest. Parallel to this line is the direction of
acceleration. This method is physically not correct, but it delivers a good
approximation of the direction.
In case of the pier: the fault orientation was in the longitudinal direction. This
fault direction was, given the orientation of the axial diagonal piles, in the stiffer
direction of the pier.
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6 Damage report of the fishing pier
As a result of the 27th February earthquake the Muelle lo Rojas fishing pier has suffered severe
damage. In Figure 6.1 and Figure 6.2 you can see some extraordinary displacements both
horizontaland vertical. Most of the piles are out of their original position and are skewed.
Because of the large
displacements the concrete is cracked and burst at critical positions. Striking is the
difference
between the damage of the different parts of the structure. As discussed in chapter
5 the structure is
split up in three parts through dilatations. The part of the walkway adjacent to the
land shows large
horizontal displacements in the axial direction and damage at the pile caps. The
part of the walkwayadjacent to the platform shows large vertical deformations and damage at so-called
imposed hinges.
The mooring platform however has very little damage and no significant
deformations. For this
reason the platform will not be reviewed. Further should be noticed that no part of
the structure
displaced in the horizontal in the transversal direction, despite heavy accelerations
in this direction.
The damages of the walkway will be discussed in the remainder of this chapter.
First all the
damages will be summed up in a list of damages. Second the plausible origin of the
different parts is
discussed and compared to each other.
Figure 6.1: Side view of both parts of the walkway
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Figure 6.2: Front view of the walkway, photo taken from the platform
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6.1 The list of damages
The walkway is damaged with seven different sorts of damages. Most of the
damages are related to each other or a direct result of one another. I include
extraordinary displacements under the name damages; it has a large effect on the
serviceability limit state of the walkway. All the damages will be illustrated withphotos and shortly discussed. Below is the list of damages.
1. Uneven extraordinary vertical displacement2. Bursting of the floor and beams3. Yielding of floor reinforcement4. Extraordinary axial horizontal displacement5. Bursting of pile caps6. Tear out of piles7. Fracture of the pile cap reinforcement
1. Uneven extraordinary vertical displacement
The platform adjacent part of the walkway shows large and uneven vertical
displacement. Near the platform the walkway has risen and at the other side, the
walkway has come down. As a result of the displacement two plastic hinges have
been formed. The formation of the hinges is associated with heavy damage to the
floor and the beams. The piles dont seem to resist the movement and displace
accordingly. Because of this, the pile caps are intact in this area.
Figure 6.3: Extraordinary vertical displacement
2. Crushing of the floor and beams
In the plastic hinges a large angular rotation took place. The rotation is approximately 25. The
rotation is far too large for the concrete, hence large cracks appeared over the full
width of the hinge and large chunks of concrete burst of the deck. At some parts the
whole reinforcement cover came off, see Figure 6.4. In the beams the concrete burst
due to crushing. Also here large cracks and spall off of the concrete can be seen,
see Figure 6.3. The continuity of the structure at the hinge is only and fully
preserved by the reinforcement.
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3. Yielding of floor reinforcement
Together with the bursting of the concrete, the floor reinforcement deformed. The
deformation made the rotation possible. In order to deform the steel yielded. The
deformations are plastic and irreversible. Probably the beam reinforcement
yielded as well.
Figure 6.4: Exposure of the reinforcement, due to complete spall off of concrete
4. Extraordinary axial horizontal displacement
The first three damage topics all occurred solely in the platform adjacent walkway.
At the walkway part adjacent to the land other damages occurred. To begin with, in
this part mainly extraordinary horizontal displacements occurred, the vertical
displacement is of lesser magnitude, see Figure 6.1. For the most part the horizontal
displacement is accompanied by a large rotation of the piles. The rotation is similar
to the vertical displacement of the other part of the walkway. Only here the hinges
are formed between the piles caps and the piles.
5. Crushing of connection blocks
An indirect result of the rotation between piles and the pile caps is the cracking and bursting of
concrete. Because of the imposed force by the pile, the internal stress in the concrete becametoo
large. At some places the whole cover of the reinforcement came off. Logically spall
off mainly occurs at the corners of the pile caps, see Figure 6.5.
Figure 6.5: Spall off of the connection block concrete cover
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6. Tear out of piles
However not all piles displaced along with the superstructure, some piles are
totally tore out of the connection blocks, see Figure 6.6. This happened with three
piles. All three piles have the same property, all are the inner pile of a lateral
diagonal pile pare, see Figure 5.3.
In Figure 5.3 you can see, that the inner diagonal pile is not in line with the beam
above. The pile has a free end connected only with the concrete of the elongated
pile cap. This weaker type of
connection is probably the cause that the piles tore out.
7. Fracture of the block reinforcement
Together with the tearing of the piles, the reinforcement of the connection block
deformed heavily
and broke. In this stat the reinforcement does not perform it task in any way. In
Figure 6.6 and Figure
6.7 examples are shown.
Figure 6.6: Tear out of a diagonal inner pile
Figure 6.7: Tear out of another diagonal inner pile
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6.2 Description of three possible origins of the damage to the pier
From the preceding paragraph we can conclude that all damages to the structure are a direct or
indirect result of the large horizontal and vertical displacement. In order to find the
origin of the pier
damage, we should find the origin of the structural displacements. Out of theliterature of piers and
pile supported wharfs, three cases are known to cause large deformations in case of
earthquake
load: deformation due to an inertia force at the deck itself; deformation due to an
horizontal force
from an anchoring construction and deformation due to liquefaction. All three cases
will be
discussed, with emphasis on the probability, that the mechanism in question is the
actual cause.
Deformation due to inertia force at the deck
The most obvious origin of permanent deformation and damage is an inertia force
at the deck. Every
object with foundation and mass gets deformed under earthquake load; the
question of importance
is whether the deformation is elastic of plastic. Figure 6.8 gives a graphical
representation of this
type of deformation to a similar type of structure, namely a pile-supported wharf.
The image
corresponds with the extraordinary axial horizontal displacements (damage type 5)
of the land
adjacent part of the walkway. The piles follow the large horizontal displacement of
the deck and
plastic hinges are formed at the fixation point in the soil. At the pile cap connection
with the
superstructure a hinge is formed as well, accompanied with severe damage.
It is possible that also at the platform adjacent part this type of mechanism is the
cause of the damage. But this type of deformation does not explain the large
vertical displacement of thewalkway. Deformation due to inertia force at the deck will be considered in the
analyses for both
walkway parts.
Figure 6.8: Side view of the deformation of a pile supported wharf due to inertia force on the deck (PIANCwg34)
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Deformation due to horizontal force from the anchoring construction
Due the horizontal force of an anchoring construction, pile supported constructions
can be damaged.
The mooring platform and the land part of the pier can be classified as anchoring
construction of the
walkway. But these construction parts did not cause the deformation for two
reasons. Firstly the
mooring platform and the land part of the pier did not have great deformations.
Secondly thewalkway and the anchoring constructions are not structurally connected. The
deformation should
have been passed on by structure to structure contact. This could explain little
deformation of the
walkway, but given the minor weight of the platform and the land pier-part, this
cannot be the cause
of the large deformations.
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Deformation as a indirect result of liquefaction
Liquefaction has the result that the fixing point of the foundation will be lower in the
soil. The free
length of the piles becomes larger and the piles are possibly not enough stiffanymore to prevent
buckling. When the whole soil liquefies unto the end of the pile, the pile support
even can be
modeled as a hinge or a role. In this case the foundation of the structure is clearly
the weakest part.
The structure or a part of it may displace as a whole, leaving the superstructure
intact. In fact this is
what we see at platform adjacent part of the walkway. Three parts have displaced
as a whole.
Between the parts hinges are formed. The parts itself, regarding the superstructure,
stayed intact.
It is possible that at the land adjacent part of the walkway liquefaction occurred.
However it cannot
be the only source of the deformation and damage at this part. At this part three
piles were torn out
of the connection blocks. The tearing out requires great force, and the result of
liquefaction is mainly
force introduced by displacement.
Figure 6.9: side view of the deformation of a pile supported wharf due to liquefaction (PIANC wg34)
To conclude over the whole walkway probably two mechanisms caused large
deformations and
damage; an inertia force at the deck and liquefaction of the soil. For the land
adjacent part of the
walkway the emphasis is on the inertia force and for the platform adjacent part
the emphasis is on
liquefaction.
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