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Effectiveness of Rubber Isolators for the Seismic Retrofitting of a Peruvian Highway Concrete Bridge

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This paper investigates the effectiveness of rubber isolators for retrofitting highway concrete bridges. The Huamani Bridge is analysed as a case study. Located on the central coast of Peru, this bridge was constructed around 1950 with obsolete seismic provisions, using fixed steel bearings. It suffered from severe damage during the 2007 Pisco Earthquake (8.0 Mw). As-built and isolated models were analysed and their results were compared. For the isolated option, the fixed steel bearings are to be replaced with Lead Rubber Bearings (LRB), in order to uncouple the movement of the super and sub-structure, thus increasing the flexibility of the system and decreasing the forces transmitted to the lateral stability elements (piers and transverse beams). Flexible links having the isolators' properties were introduced into the as-built model. The analysis of the isolated model was performed following the AASHTO GSID Simplified Method. It was found that implementing the isolation system increased the structure's overall damping ratio by more than 3 times, as well as its flexibility. This effectively reduced the solicitations in the sub-structure and transverse beams by an average of 75%. It is concluded that the use of rubber isolators is effective for retrofitting highway concrete bridges and that this analytical procedure can be applied to other bridges of similar characteristics in Peru.
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Effectiveness of Rubber Isolators for the Seismic Retrofitting of a
Peruvian Highway Concrete Bridge
Anibal Tafur1*, Thomas Swailes2
1* Department of Civil Engineering, Pontifical Catholic University of Peru, Peru (atafur@pucp.edu.pe)
2 School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, UK
(thomas.swailes@manchester.ac.uk)
Abstract – This paper investigates the effectiveness of
rubber isolators for retrofitting highway concrete
bridges. The Huamani Bridge is analysed as a case
study. Located on the central coast of Peru, this bridge
was constructed around 1950 with obsolete seismic
provisions, using fixed steel bearings. It suffered from
severe damage during the 2007 Pisco Earthquake (8.0
Mw). As-built and isolated models were analysed and
their results were compared.
For the isolated option, the fixed steel bearings are to
be replaced with Lead Rubber Bearings (LRB), in order
to uncouple the movement of the super and sub-
structure, thus increasing the flexibility of the system
and decreasing the forces transmitted to the lateral
stability elements (piers and transverse beams). Flexible
links having the isolators’ properties were introduced
into the as-built model. The analysis of the isolated
model was performed following the AASHTO GSID
Simplified Method.
It was found that implementing the isolation system
increased the structure’s overall damping ratio by more
than 3 times, as well as its flexibility. This effectively
reduced the solicitations in the sub-structure and
transverse beams by an average of 75%. It is concluded
that the use of rubber isolators is effective for
retrofitting highway concrete bridges and that this
analytical procedure can be applied to other bridges of
similar characteristics in Peru.
1. INTRODUCTION
Peru is a country seismically high active due to its
location on the Pacific ‘Ring of Fire’, suffering from
disastrous seismic events in the past years such as the
1970 Ancash Earthquake (7.9 Mw; 70,000 deaths), the
2001 Arequipa Earthquake (8.4 Mw; 145 deaths); and
more recently the 2007 Pisco Earthquake (8.0 Mw,
519 deaths) [1]. These events caused high personal
and material losses.
Nevertheless, the seismic protection technologies
in Peru are not in wide use. In the last years, isolation
systems have started to be implemented in buildings,
but bridges remain overlooked. In January 2016 the
Peruvian Government published the current version of
the Peruvian Seismic Code (“Norma Tecnica Peruana
NTE E030”) making mandatory the implementation of
seismic protection systems for essential buildings
(schools, hospitals, etc.), without further specific
requirements for bridges (NTE E0.30, 2016). The
Peruvian Bridge Design Manual (MTC, 2016) [3]
provides minimal and vague provisions for the
analysis and design of isolated bridges, leaving local
engineers with insufficient information and guidelines
about this type of structures.
Considering that in Peru many bridges built long
time ago with obsolete seismic provisions are still
operational, and given the high seismic activity in the
country (especially on the Peruvian Coast); it is then
observed the necessity for vulnerability assessment
and retrofitting of these bridges. It is crucial to
improve their seismic performance, seeking
compliance with current seismic criteria applied in
other seismically high active and more developed
countries, such as USA and Japan.
According to Siqueira et al. (2014) [4], seismic
isolation has been recently used for the retrofitting of
existing bridges by increasing their flexibility, and
thus, their natural periods of vibration. This effectively
reduces the seismic energy input into the bridge
structure and the solicitations in the lateral stability
system [4]. Furthermore, the implementation of
isolators is generally easier to execute and less
expensive than the retrofitting of the sub-structure
(piers, columns or foundations) [5]. Even though to
the author’s knowledge there is no record of bridges in
Peru retrofitted using this technique, in other parts of
the world this has been successfully implemented.
Some notable examples are the retrofitting of the
Golden Gate Bridge in San Francisco (USA), the Bolu
Viaducts in Turkey; and the Chemin des Dalles Bridge
in Canada [4].
Figure 1 illustrates the principle of this retrofitting
method. In the case of an isolated bridge system (b),
the deformation takes place at the isolators during an
earthquake, preventing the damage of the sub-
structure; whereas in a conventional bridge system (a),
this deformation will occur at the sub-structure, which
is not desirable. The sub-structure elements (columns
or pylons) are the most demanded during an
earthquake, and it is crucial to prevent their damage. It
is clear that the most widely used isolation system for
bridge applications is the Lead Rubber Bearings
(LRB), as stated by Buckle et al. (2006) [6].
Fig. 1: Comparison of a conventional and seismically isolated
bridge [6]
The aim of this paper is to evaluate the efficiency of
rubber isolators for the retrofitting of a multi-span
concrete highway bridge, namely the Huamani Bridge
in Peru, by comparing the responses and solicitations
(displacements and forces) obtained from linear
dynamic analyses of as-built and isolated models.
Thus, the reduction of the seismic forces induced in
the structure due to the implementation of the isolation
system is evaluated.
2. CASE STUDY: THE HUAMANI BRIDGE
The Huamani Bridge (see Figure 2) is a reinforced
concrete continuous bridge that spans over the Pisco
River, in the Ica Department of Peru. It was built
around 1950 and suffered from considerable damage
during the 2007 Pisco Earthquake (8.0 Mw), located at
approximately 60 km from the epicentre. It is located
close to San Clemente Town in the Pisco Province, at
coordinates S13°41’13” W76°09’31”, and it is one of
the most important bridges on the Pan-American
South Highway, the main road network on the
Peruvian Coast.
This bridge is representative of many old concrete
bridges still operational in Peru, which were designed
with none or obsolete seismic provisions. The seismic
design criteria used for this bridge was to apply as
base shear force equivalent to 4-8% of the bridge
weight, with no provisions regarding ductility [7].
Fig. 2: The Huamani Bridge over the Pisco River after the Pisco
EQ (2007) [7]
The bridge consists of 5 spans, 3 typical of 30 metres
each and 2 spans of 23 metres on the sides, giving a
total length of 136 metres (see Figure 3, top). The
super-structure is supported by 3 hunched beams
which are 700 mm wide and 1.80 m high (away from
the supports), and at the supports the beams are 3.5 m
high. The bridge deck is a 300 mm thick reinforced
concrete slab. There are also several diaphragm
(transverse) beams that restrain the structure laterally.
Fig. 3: Layout of the Huamani Bridge (dimensions in mm)
The substructure consists of two abutments and four
interior pier walls measuring 1.10 m by 8.20 m, with
shear keys implemented on their sides as shown in
Figure 3 (bottom), with the objective to control
excessive lateral displacement. These pier walls are
founded on deep foundations (caissons) [7].
Elevation view:
Cross section:
At the intermediates pillars, the super-structure is
supported on steel pot bearings, fixed in both
directions. On the abutments, simple steel rollers were
used, which are free to move in the longitudinal
direction, but restrained in the lateral direction by
means of steel stoppers. According to the survey
reports, the bearings were found in very bad state,
corroded and with rust and dirt covering [7].
Both structural and geotechnical damage was
observed during the surveys at the Huamani Bridge
after the earthquake. The most complete reports on
this matter are the works of Tang & Johansson (2010)
[7], Johansson et al. (2007) [8] and Taucer et al.
(2009) [9], these being the main sources consulted for
this paper. Some examples of critical structural
damage are shown in Figure 4.
Fig. 4: Structural damage observed in the Huamani Bridge after
the 2007 Pisco EQ [7], [8].
Severe cracking was observed in one pier, at the
junction with its shear key. This was due to loss of the
lateral restriction in the bearings which allowed the
superstructure to move freely at this point, pounding
the shear lock and causing severe damage to it (see
Figure 4.a). The shear lock, although damaged,
prevented the bridge superstructure from suffering
excessive displacement and overturning (100 mm
permanent displacement was observed) [7]. Also,
severe cracking was observed in the transverse beams,
which provide lateral stability to the super-structure
(see Figure 4.b).
The steel roller bearings at the abutments were
meant to behave as laterally restrained by means of
steel stoppers welded to their base plates.
Nevertheless, the steel rollers at the southern abutment
suffered severe damage due to excessive displacement,
losing their lateral restraints (see Figure 4.c). On the
other hand, at the northern abutment the damage was
less severe, with the lateral restraint being still present,
but with evidence of permanent displacements (See
Figure 4.d). Severe corrosion was observed in all the
bearings [7].
3. METHODOLOGY
As-built and isolated models of the Huamani Bridge
are analysed and their results are compared. For the
isolated option, the fixed steel bearings present in the
as-built model are to be replaced with flexible links
having the isolators’ properties. The modelling process
is discussed more fully in Section 4 of this paper.
The efficiency of the isolation system is assessed
by comparing the following results from the as-built
and isolated model analyses: super and sub-structure
displacements, base shear forces, forces in key
structural elements, periods and damping ratios.
Furthermore, the piers and transverse beams’
capacities (shear and flexural) are calculated, in order
to illustrate the beneficial effects of the isolation
system on the demand-capacity ratios of these
elements.
The response of an isolated structure is very
complicated to analyse, due to the high-non linearity
induced by the isolators. Non-linear models for the
structure elements have to be combined with hysteretic
curves obtained from laboratory tests of isolators, and
time-history functions for the seismic loads.
Nevertheless, for the purpose of this paper only linear
analyses are carried out, following the AASHTO
GSID simplified method as described in Section 7.1 of
the AASHTO Guide Specifications for Seismic
Isolation Design GSID (2010) [13].
LRB isolators dissipate energy due to internal
friction inside the rubber material (also called viscous
damping), and deformation in the lead core. Figure 5
(top) shows a typical section and the displacement—
force diagram of an LRB isolator, where its hysteretic
curves and non-linear behaviour can be appreciated.
a) [7]
c) [7]
d) [8]
b) [8]
The cyclic behaviour of these isolators can be
approximately represented by a simplified bi-linear
model as shown in Figure 5 (bottom).
Fig. 5: Top: Section of an LRB and its forcedisplacement
diagram [10], [11]. Bottom: Bilinear simplified model of a rubber
isolator [12]
Where:
K1 = Isolator initial stiffness
K2 = Isolator post-yielding stiffness
Kisol = Isolator effective stiffness
Qd = Isolator characteristic strength
Fy = Isolator yield force
Fisol = Isolator maximum force
dy = Isolator yield displacement
disol = Isolator maximum displacement
And K2 may be estimated as follows [12]:
r
T
GA
K
2
× (1.15 to 1.20) (3.1)
Where Tr is the total thickness of rubber (addition of
layers), G is the rubber shear modulus and A is the
section area. K1 is in the range of 15 to 30 times K2.
Note that the effective stiffness Kisol is dependent on
the isolator maximum displacement disol; therefore, it is
not possible to know its exact value beforehand. The
isolator effective stiffness Kisol is useful when only
linear methods are considered. The value of Kisol is
input into the software SAP2000 to define the
properties of the link elements used to represent the
isolators in the model.
The methodology presented is this paper to determine
the isolation system properties and isolated structure
response consists of three stages:
First, the Simplified Method presented in AASHTO
GSID is applied. For the first iteration a
displacement is assumed and a preliminary set of
characteristic isolator resistance properties is
defined. The displacement response of the isolated
structure is calculated taking into account the
reduction by damping. Then, convergence is checked
comparing the obtained displacement with the initial
assumed value.
In the next stage, a Multimodal Spectral Analysis is
carried out, similarly to what is done with the as-
built structure, but with a modified response
spectrum (reduced by damping). It is used the same
as-built models, but replacing the rigid links with
flexible links having the representative isolator
properties determined in the previous stage. The
objective is to obtain similar response to that
estimated by the GSID Simplified Method. After
reaching this second convergence, displacement
response and forces are obtained for the sub-
structure and isolators.
The final stage is the Design of the Isolators in order
to secure their viability, using the forces and
displacements obtained from the spectral dynamic
analysis. The requirements and limit checks provided
by the AASHTO GSID are followed. The output of
the design includes sizes and material properties,
which should be checked against the existing bridge
dimensions.
The AASHTO simplified method reduces the isolated
structure to an equivalent single degree-of-freedom
model with equivalent properties, in order to estimate
its response (displacement of the super-structure) [12].
The super-structure displacement d is estimated as
follows:
(3.2)
Where:
SD1 = Spectral acceleration coefficient
Teff = Effective period of the isolated structure
BL = Damping reduction factor given by Eq. 3.7
SD1 is defined as 0.62 from the seismic hazard study.
Since the values of Teff and BL are unknown
beforehand, for the first iteration Teff may be taken as
1.0 seconds and BL may be taken as 1.0 (5% of
damping). To estimate the system
totald
Q,
and
total
K,2
required for the first iteration (addition of all the
isolators), it is recommended by Buckle et al. (2011)
[12] that
totald
Q,
should be at least 5% of the bridge
effective weight. Since for this bridge the piers are
very stiff (no vibration), it is assumed that only the
mass of the superstructure contributes to the vibration,
therefore the effective weight
eff
W
is taken as the
super-structure weight. For
total
K,2
it is recommended
to take 10% of
dWeff /
. For this case, since the mass
of the existing structure is very large (relatively
compared with bridges designed considering isolation
from the beginning), 15% of
dWeff /
is taken. The
assumed
totald
Q,
and
total
K,2
are distributed according
to the percentage of the dead load acting at each
support (abutments and piers).
Fig. 6: Calculation of the combined effective stiffness at the
supports [12]
It is then necessary to calculate the combined effective
lateral stiffness at each support
eff
K
, accounting for
the stiffness of the sub-structure and the isolators
combined. As shown in Figure 6. The stiffness
distribution factor
is defined as follows:
dsub
d
QdK QdK
2
subeff KK
1
(3.3)
The displacements of the sub-structure
sub
d
and
isolators
isol
d
at each support (abutment and piers),
are calculated as follows:
1d
disol
isolsub ddd
(3.4)
The effective period of the isolated structure Teff is
calculated using the effective weight
eff
W
and the
summation of the effective stiffness’ at each support
eff
K
, as follows:
eff
eff
eff Kg
W
T
2
(3.5)
The damping ratio of the isolated structure
and the
damping reduction factor BL are calculated using the
following expressions:
 
 
2
2
subisoleff
yisold
ddK
ddQ
(3.6)
3.0
3.0
70.105.0
3.0
L
B
Eq.7.1-3 AASHTO GSID (3.7)
Using these values of Teff and BL, the expected super-
structure displacement is calculated using Eq. 3.2.
After a series of iterations, convergence should be
found between the initial assumed displacement and
the calculated after the iteration. Once convergence is
reached, the effective stiffness of the isolators Kisol are
obtained using the following expression, in order to be
used for the multimodal spectral analysis in the next
stage.
2
K
d
Q
K
isol
d
isol
(3.8)
In order to carry out the dynamic analysis of the
isolated model, the response spectrum is modified to
take into account the higher damping present in the
fundamental modes introduced by the isolators. The
acceleration values are divided by the damping factor
BL, at periods above 0.8Teff. Figure 10 in next section
shows the reduced spectrum resulting from the
Huamani Bridge analysis, and is used for both
directions of analysis.
K2
In order to define the properties of an isolation system
used for retrofitting, a controlling parameter has to be
defined. For existing bridges supported on columns,
the shear force in these elements is likely to control
the design. For the Huamani Bridge, the existing piers
are of massive dimensions; therefore the shear force in
these will hardly control the design. The size of the
isolators, which will have to properly fit between the
existing piers and beams, is likely to control the
design. Other parameter to take into account is the
maximum displacement of the super-structure, which
will have to be controlled in order to avoid excessive
pounding on the existing abutments [12]. Figure 7
shows a flowchart summarising the methodology
described in this section.
Fig. 7: Flowchart summarising the methodology for the analysis of
the isolated bridge
4. BRIDGE MODEL SIMULATION
Figure 8 shows a perspective view of the model
defined using finite elements and analysed using the
software SAP2000. The seismic solicitations applied
to the as-built model are represented by Response
Spectrum and Time History linear cases. The
Response Spectrum (RS) is defined according to the
AASHTO specifications and using seismic hazard
information from a similar project in Peru [16]. The
Time History (TH) cases were defined using 10
signals, including the accelerogram of the 2007 Pisco
EQ, properly scaled to PGA 0.45g, which is the
characteristic value of the peak ground acceleration
specified for the bridge location in the Peruvian
Seismic Code (Seismic Zone 4). The most
unfavourable of these cases will be considered to
obtain the design resistance demanded on the
structural elements of the as-built case (envelope).
Due to the limitations of the AASHTO Simplified
Method, for the isolated model only the Response
Spectrum case is carried out, considering the reduction
by damping.
Fig. 8: Model perspective view as seen in the software interface
For the purpose of this paper, the piers were assumed
as fixed at their base for both models. According to the
reports consulted, the foundations are caissons
embedded 8 metres into the soil [7], [8]. The material
properties input into the software were: for the
concrete f’c = 30 MPa, Young’s Modulus E = 24.6x109
MPa, specific weight γ = 25 kN/m3 and 5% damping
ratio. For the reinforcement steel it was considered
ASTM A615 Gr60 with fy = 420 MPa.
Regarding the bearings at the supports, to simulate
the fixed bearings of the as-built bridge, they were
idealised using rigid links. For the isolated model, the
rigid links where replaced with flexible links having
the isolators’ characteristic properties, in this case, the
isolators’ effective stiffness Kisol. The element meshing
was defined in the software by establishing a
maximum characteristic dimension of 300 mm. The
Assume displacement, characteristic strength Qd
and post-yielding stiffness K2
Find the effective period and the equivalent
dampingratioof the isolatedstructure usingthe
GSID Simplified Method
Find the theoretical displacement, reduced by
damping. Is it the same as the assumed?
Use the assumed displacement, Qdand K2to
find the isolators' effective stiffness Kisol to be
used in the model.
Modify the 5% damped response spectrum used
for the as-built model, reducing it by the damping
factor BL.
Run a dynamic linear analysis of the isolated
model. Are the displacements and periods
obtained similar to those calculated with the
simplified method?
Obtain the isolators' design forces and
displacements from the dynamic analysis results.
Design the isolators. If suitable dimensions cannot
be reached repeat the process from the beginning.
NO
YES
NO
YES
software has an option to automatically decrease the
size of the mesh in areas where higher discretization is
required, such as corners and curved parts.
The AASHTO Seismic Code, Section 3.10.4
Seismic Hazard Characterization’ [14], presents the
procedure to define the response spectrum for 5%
damping (typical concrete bridges). This spectrum is
defined as elastic, i.e. without considering ductility
reduction factors for design. Figure 10 shows the
spectra defined for the bridge analyses, for both as-
built and isolated models. Note the reduced response
due to damping for the isolated model (solid line).
The accelerograms used for the Time-History
Cases were recorded by many seismic monitoring
stations in Peru. These stations are part of the large
seismic observation network managed by the CISMID
(Peruvian-Japanese Centre of Seismic Investigation
and Disaster Mitigation), and their records are
available on their website (www.cismid-uni.org). It is
notable the case of the 2007 Pisco EQ accelerogram,
which affected the Huamani Bridge. The closest
accelerometer to the bridge is located at the ICA002
station (63 km away) and its records are shown in
Figure 9.
Fig. 9: Accelerograms recorded at the ICA002 station during the
2007 Pisco Earthquake (CISMID)
Fig. 10: AASHTO Response Spectra (as-built and isolated cases)
defined for the Huamani Bridge analysis
5. RESULTS
After the iterative process of the isolated bridge
analysis, convergence was found for the
following values:
d
=241mm,
totald
Q,
=1251 kN,
total
K,2
=15573 kN/m,
eff
T
= 2.203 s,
= 0.16 and
BL = 1.42. Table 1 summarizes the required
properties found for each isolator after the last
iteration, considering a ratio K2/K1 = 0.1 and dy =
0.1dt. These values were obtained diving by 3 the
properties found at each support.
Table 1: Required properties of the isolators obtained with the
GSID simplified method
Location
K1
(kN/m)
K2
(kN/m)
Qd
(kN)
dy
(mm)
Fy
(kN)
Kisol
(kN/m)
Abutments
2701
270
22
8.93
24
360
Piers
11627
1163
93
8.93
104
1551
Figure 11 shows a comparison of the results from the
modal analyses of the as-built and isolated models,
where important increment in the flexibility due to the
implementation of the isolation system is appreciated.
The periods increased 15 times for the longitudinal
and 10 times for the transverse direction. This caused
a very important increment in the super-structure
displacement as well, 60 times in the longitudinal and
14 times in the transverse direction. Since the as-built
model was found to be very stiff (small
displacements), theses increments are as expected.
The overall structure damping ratio increased from
the initial 5% estimated for the existing concrete
structure, to 16% for the isolated structure (more than
-400
-300
-200
-100
0
100
200
300
400
025 50 75 100 125 150 175 200 225
Acceleration (cm/s2)
Time (seconds)
North-South direction
-300
-200
-100
0
100
200
300
025 50 75 100 125 150 175 200 225
Acceleration (cm/s2)
Time (seconds)
Eat-West direction
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.00 1.00 2.00 3.00 4.00
Elastic Seismic Coefficient Csm (g)
Period Tm (seconds)
Reduced response
1.0g
PGA = 333.66 cm/s2 (0.34g)
PGA = -272.82 cm/s2 (0.28g)
0.50g
0.8Teff
(1.76s)
3 times). This is a very important feature that allows
the structure response to be reduced. The forces in the
piers were reduced by 80% in average. The forces in
the transverse beams were reduced by an average of
75%.
Fig. 11: Comparison of the analyses modal results for the as-built
and isolated models
To evaluate the significance of the seismic force
reduction in the structural elements, demand-capacity
ratios of the piers and transverse beams are calculated
(shear, flexural, and flexural-compressive capacities),
following the guidelines provided by the ACI-318
Code (ACI, 2008) [15]. Two Ultimate Limit State
(ULS) loading scenarios are considered: for the as-
built bridge the envelope of the Response Spectrum
and Time-History cases, and for the isolated bridge
the Response Spectrum reduced by damping.
Table 2 shows that the shear demand-capacity
ratios in the piers decreased significantly for all cases.
In the case of the transverse direction of Pier 2, the
reduction is crucial since the as-built demand is 1.16
(larger than 1), indicating that strengthening of the pier
would be needed. The implementation of the isolation
causes this ratio to decrease to only 0.20.
Tables 3 and 4 show the shear and moment
demand-capacity ratios for the transverse beams. It
can be appreciated the important reductions for the
isolated case. The isolation system causes the ratios to
decrease from values above one in both piers
(strengthening needed), to very low values, in the
order of 0.30 to 0.40.
Table 2: Comparison of pier’s shear demands and capacity
Direction
Element
ϕVn
(kN)
AsB
(kN)
Isol
(kN)
Ratio
AsB
ϕVn
Ratio
Isol
ϕVn
Longitudinal
X-X
Pier 1
5731
5003
1113
0.87
0.19
Pier 2
5731
3545
1113
0.62
0.19
Transverse
Y-Y
Pier 1
5446
3579
1110
0.66
0.20
Pier 2
5446
6320
1109
1.16
0.20
Notes: AsB = As-Built demands (Envelope of RS and TH cases)
Isol = Isolated Structure demands
Table 3: Shear demand capacity ratios for the transverse beams
Case
Location of
Beam
Shear
Demand
(D) (kN)
Shear
Capacity
(C) (kN)
Ratio
D/C
As-Built
@ Pier 1
403
360
1.12
@ Pier 2
708
360
1.97
Isolated
@ Pier 1
120
360
0.33
@ Pier 2
120
360
0.33
Table 4: Moment demand capacity ratios for the transverse beams
Case
Location of
Beam
Moment
Demand
(D) (kN-m)
Moment
Capacity
(C) (kN-m)
Ratio
D/C
As-Built
@ Pier 1
569
414
1.37
@ Pier 2
999
414
2.41
Isolated
@ Pier 1
172
414
0.42
@ Pier 2
172
414
0.42
Regarding the flexural-compressive capacity of the
piers, the most critical combination is ‘0.9 D ± S’.
Figure 12 shows the positions of these combinations
within the Piers’ Design Interaction Diagrams. It is
observed that for Pier 1 (top) the as-built design forces
combinations are very close to the design critical
curve. This indicates that the element is highly
demanded in the as-built case, possibly resulting in
necessity for strengthening due to bending.
Furthermore, it is seen that the isolation system
helps to significantly decrease this demands (4 times
in average). A similar reduction in the demands is seen
for Pier 2 (bottom), although the as-built forces are not
located on the critical curve. For this case, the
reduction due to the isolation implementation is
Pier 1
Isolated Y-Y direction
Max disp. = 240 mm
Period = 2.213 s
Mode 1
(94% participation)
Isolated X-X direction: Max disp. = 240 mm, Period = 2.206 s
(94% participation) Mode 2
Pier 2
As-built X-X direction: Max disp. = 3.91 mm, Period = 0.148 s
(49% participation), Mode 6
As-built Y-Y direction
Max disp. = 16.90 mm
Period = 0.222 s
Mode 1
(75% participation)
Pier 2
Pier 1
efficient but not critical. In the transverse direction of
the piers the demands are located far away from the
critical curves for both the as-built and the isolated
cases, due to the massive bending resistance in this
direction.
Note: As-built combinations
Isolated combinations
Fig. 12: Interaction diagrams and seismic demands for the piers in
the longitudinal direction
6. CONCLUSION
This paper investigated the effectiveness of rubber
isolators as a retrofitting measure for highway
concrete bridges. The object of the study was the
Huamani Bridge, a continuous concrete highway
bridge built around 1950 with obsolete seismic
provisions. The methodology consisted of comparing
key parameters from the as-built and the isolated
bridge models, leading to the following conclusions.
It was found that the implementation of rubber
isolators would substantially increase the modal
parameters of the bridge. It was observed that the
natural periods increased more than 10 times (from
0.15 s and 0.22 s to 2.21 s). The effective damping
ratio increased from 5% (initially estimated) to 16%
(obtained with the AASHTO Simplified Method).
The increment in the flexibility and damping ratio
of the structure caused the seismic force solicitations
to be reduced by an average of 80% for the piers and
75% for the transverse beams. It is concluded that the
implementation of rubber isolators is very efficient to
decrease the demands in the lateral stability elements.
Calculating the capacities of piers and transverse
beams it is observed that these would be exceeded by
the seismic demands of the design earthquake (1000-
yr return period). Furthermore, it was observed that
the isolation system would help to reduce substantially
the demand-capacity ratios (e.g. down to 0.20 for piers
and 0.33 for beams), in such way that no strengthening
of these elements would be needed; thus confirming
the efficacy and worth of the isolation system
implementation.
ACKNOWLEDGEMENTS
The authors acknowledge the economic support of the
Peruvian Ministry of Education that funded this
investigation. The authors would also like to thank
Julio Arias for providing his Master’s thesis, from
which essential information was obtained about the
seismic hazard parameters for this bridge.
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ResearchGate has not been able to resolve any citations for this publication.
Article
Seismic isolation can be used as a practical method to mitigate earthquake hazards for designing new highway bridges or retrofitting existing ones. To realize a reliable and effective seismic isolation design, several important and often interacting factors should be considered, including the ground motion characteristics, structural configurations and properties, mechanical properties of isolation devices and soil-structure interaction etc. This paper adopts the performance-based evaluation approach to investigate the effectiveness and optimum design parameters of isolation devices so as to minimize the overall damaging potential of seismically-isolated bridges. Fragility functions, which define the probability exceeding a performance state at a given set of earthquake intensities, are derived using nonlinear time history analyses of typical highway bridges (conventionally designed or base-isolated) subject to a suite of 250 earthquake motions. The nonlinear models for bridge columns and isolation devices are incorporated and various combinations of isolation parameters, e.g. elastic stiffness, characteristic strength and post-yielding stiffness, representing common types of isolation devices are evaluated. Both Probabilistic Seismic Demand Analysis (PSDA) and Incremental Dynamic Analysis (IDA) methods are used and compared in generating the fragility functions. Damage criteria for both piers and isolation devices are established to relate the component response quantities to global damage states of bridges. The study shows that the mechanical properties of isolation devices have a significant effect on the damage probability of isolated bridges. By evaluating the earthquake intensity required to achieve specified damage states of base-isolated bridges, the optimum combinations of mechanical parameters of isolation devices are identified as a function of structural properties and damage states. The findings can serve as a practical guide for isolation device designs where the uncertainties with ground motions and variability of structural properties are effectively incorporated under the fragility function framework.
Seismic response of multiple span steel bridges in central and southeastern United States. II: Retrofitted
  • G H Siqueira
  • D H Tavares
  • P Paultre
Siqueira, G. H., Tavares, D. H., & Paultre, P. (2014). Seismic fragility of a highway bridge in Quebec retrofitted with natural rubber isolators. Revista IBRACON de Estruturas e Materiais, 7(4), 534-547. (2004). Seismic response of multiple span steel bridges in central and southeastern United States. II: Retrofitted. Journal of Bridge Engineering, 9(5), 473-479.
Seismic isolation of highway bridges (No. MCEER-06-SP07)
  • I G Buckle
  • M C Constantinou
  • M Diceli
  • H Ghasemi
Buckle, I. G., Constantinou, M. C., Diceli, M., & Ghasemi, H. (2006). Seismic isolation of highway bridges (No. MCEER-06-SP07).
lifeline performance
  • A K Tang
  • J Johansson
Tang, A.K. & Johansson, J. (2010). Pisco, Peru, earthquake of August 15, 2007: lifeline performance. American Society of Civil Engineers (ASCE).
A Reconnaissance Report on The Pisco
  • J Johansson
  • P Mayorca
  • T Torres
  • E Leon
Johansson, J., Mayorca, P., Torres, T., & Leon, E. (2007). A Reconnaissance Report on The Pisco. Peru Earthquake of August, 15, 2007.
LRFD Seismic Analysis and Design of Bridges Reference Manual. US Department of Transportation. Federal Highway Administration
  • Fhwa
FHWA (2014). LRFD Seismic Analysis and Design of Bridges Reference Manual. US Department of Transportation. Federal Highway Administration, Washington.
Seismic isolation design examples of highway bridges
  • I G Buckle
  • M Al-Ani
  • E Monzon
Buckle, I.G., Al-Ani, M. & Monzon, E. (2011). Seismic isolation design examples of highway bridges. NCHRP Project, pp.20-7.
Guide Specification for Seismic Isolation Design
AASHTO (American Association of State Highway and Transportation Officials) (2010) "Guide Specification for Seismic Isolation Design" Third Edition, AASHTO, Washington, D.C.