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BRIDGE-ABUTMENT-BACKFILL INTERACTION: BENEFICIAL OR
DETRIMENTAL FOR INTEGRAL ABUTMENT BRIDGES?
Hassan Ibrahim1, Arjun Baladas2 and Stergios A. Mitoulis3
1 BSc, MSc, MIStructE, CEng, PE, P.Eng
Principal Engineer, Parsons, Canada
hassan.yassin@parsons.com
2 GM.ICE, GHM.IStructE
Undergraduate Student, Department of Civil and Environmental
Engineering, FEPS, University of Surrey, Guildford, UK
ab00870@surrey.ac.uk
3 Dipl Eng, MSc, PhD, MASCE, EAEE
Associate Professor, Department of Civil and Environmental
Engineering, FEPS, University of Surrey, Guildford, UK
s.mitoulis@surrey.ac.uk, www.infrastructuResilience.com
Abstract
Integral Abutment Bridges (IAB) are becoming the structural system of choice for many bridge
jurisdictions in North America and Europe, because of their superior durability over conventional
bridges. IABs design out the movement joints and bearings, eliminating the need for costly maintenance
throughout the life of the bridge and improving the durability of the bridge structure. IAB bridges may
appear as simple structural systems, but the interaction of the many structural and non-structural
elements comprising the bridge structure during its lifetime make this type of bridges unique in their
structural analysis and behaviour prediction under the different loads imposed on the bridge during its
service lifetime. Thermal effects and vehicular load effect are well-studied areas in bridge engineering,
and their effects on integral abutment bridges have been the subject of many published literatures, but
little research is available for the effect of earthquake loading on the behaviour of the IAB bridge
systems and its interaction with the backfill soil. This paper aims at answering a critical question in IAB
seismic design, i.e. whether the consideration of the backfill soil and the abutment is beneficial or
detrimental in earthquake-resistant integral abutment bridges.
Keywords: integral abutment bridge, seismic response, design, interaction, SSI, beneficial, detrimental
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COMPDYN 2019 7th ECCOMAS Thematic Conference on
Computational Methods in Structural Dynamics and Earthquake Engineering M. Papadrakakis, M. Fragiadakis (eds.)
Crete, Greece, 24–26 June 2019
Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2019) 3673-3686
ISSN:2623-3347 © 2019 The Authors. Published by Eccomas Proceedia.Peer-review under responsibility of the organizing committee of COMPDYN 2019. doi: 10.7712/120119.7178.19334
1 INTRODUCTION
Conventional bridge systems have experienced maintenance challenges related to bridge deck joints
and bearings. Joints and bearings are expensive to buy, install, maintain, repair and replace and
maintenance activities can cause significant traffic disruptions. These challenges drove the engineering
community into finding innovative solutions to come up with jointless bridge systems. Integral
Abutment Bridges (IAB) have proven themselves as viable solution to these issues, and it has been used
since in Canada, the US and Europe to develop a “get-in, get-out, stay-out” sustainable, low maintenance
bridges. [1]
IABs are unique in terms of their structural analysis and behaviour under diverse actions due to the
interaction between the structural and non-structural, e.g. backfill, elements comprising the bridge
structure through the life of the bridge. Thermal movements of the bridge will result in movements in
the backfill soil, which in-turn affects the soil pressure acting on the abutment walls. The interaction
between the backfill soil and the abutment affects the design of the abutment foundation. The
configuration of the abutment foundation, in turn, affects the stiffness of the whole system, and that
influences the forces attracted to the abutments and piers, which requires many iterations from the
designer to reach an optimal design.
This research paper aimed at answering the question: is the backfill soil of Integral Abutment Bridges
(IABs) a factor that can reduce the seismic response of the bridge or is it a factor that can increase the
seismic response of bridges? For example, if the backfill is a source of damping and acts as an additional
external support to the bridge, then it is safer to consider this as a second line of defence and ignore it
when analysing the bridge. Thus, we can use the backfill soil as a potential retrofitting measure for
existing bridges and enhance its earthquake resistance. On the other hand, if the backfill is mainly a
source of inertia mass, then the effect of this additional mass should be considered in the analysis and
design of the bridge, as this might lead to larger displacements and bending moment in the structural
components of the bridge. This question can be broken down to smaller questions, such as, what is
the damping ratio of IABs [2, 3]; what is the stiffness of the backfill soil; what are the soil properties of
the IAB; what is the geometry and the typology of the bridge and the type of the abutment and how the
above factors may affect the response of the bridge. There are also other factors that may seem irrelevant,
but can still influence IAB responses drastically, for example, the thermal expansion and contraction of
the deck and the consequent movement of the abutment during the bridge service or prior to the seismic
action, which imposes backfill deterioration, i.e. soil flow, compaction and ratcheting. Other factors
might be the deterioration and/or fatigue of the abutment and the foundation of the abutment, for
example, when abutments are supported on piles. There are also other factors that may seem irrelevant
but can still influence IAB responses drastically, for example, the thermal expansion and contraction of
the deck and the consequent movement of the abutment during the bridge service, i.e. before the seismic
action, which imposes backfill deterioration, soil flow, compaction and ratcheting [4]. Other factors
might be the deterioration/fatigue of the abutment or the foundation of the abutment (e.g. the piles if
any). Currently, appears to be no agreement in the engineering community on the above question, and
the following are few variables that can influence the bridge behaviour.
With regard to the length of IABs, they are usually short bridges, and that is what the codes prescribe,
e.g. there are length limitations in each State in the USA. However, practical designs resulted in much
longer bridges. The research by Zhang and Makris [2] suggests that the seismic movement of the bridge
increases by a factor of two as the backfill almost drives the motion of the bridge, which refers to the
case where the bridge is of relatively small length and moves together with the backfill soil. Potentially
this is the source of the q=1 in Eurocode 8-2 [5,6]. However, for a longer bridge length, it is expected
that a different bridge behaviours will be obtained. The key here is to identify the effective length of the
backfill soil as this defines the amount of dissipation that occurs due to the plastification of the backfill
soil, which is beneficial for seismic design, but causes settlements and the need for backfill replacement.
On the other hand, the backfill is not only a source of dissipation and stiffness, but also a source of inertia
3674
mass, which potentially may increase the seismic response of the bridge, yet may as well increase the
period of the bridge.
Figure 1 Schematic diagrams of the ratcheting flow (Mitoulis, 2014)
In respect of the type and height of the abutment, the taller the abutment, the more significant the
interaction with the backfill soil, because a substantial part of the backfill soil is mobilised. However,
this might mean considerable energy dissipation and larger backfill inertia mass. On the other hand, a
short abutment has less interaction with the backfill, yet the abutment is seating on the backfill soil, as
opposed to retaining it, in which case the backfill is the foundation soil for the abutment, i.e. the backfill
soil may induce larger movements to the bridge as it is softer than original foundation soil, thus is
expected to magnify the movements. In the latter case, the abutment movement is strongly dependent
on the stem movement of the backfill soil.
The foundation type can as well affect the behaviour of the bridge, whether the bridge is supported
on shallow foundation or piled foundation. For IAB supported on piles, the flexibility of the piles will
play a role in determining the displacement capacity of the bridge during an earthquake. The interaction
between the soil and the piles is a factor in the overall behaviour of the bridge. This foundation type was
not considered in this research but is suggested for future work on the subject.
With regard to the condition of IAB components (structural and non-structural), concerns the
properties of soil, abutment and bridge components and how they might have been deteriorated during
the bridge service due to several factors, such as thermal movements, environmental conditions,
corrosion, and fatigue. This has not been examined in this research but is suggested for future studies of
the problem [4].
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2 FE ANALYSIS MODEL
For the purpose of this study, the complex soil-structure interaction for Integral Abutment Bridges
(IAB) under seismic load was analysed using the plain strain finite element code PLAXIS 2D (2017).
Multiple models were developed to consider the effects of backfill on the dynamic response in different
geometric scenarios such as length of bridge (spans) and height of abutments.
Dynamic absorbent boundaries were used to stimulate the far field behaviour of the medium, while
at the base the boundary conditions were fully restrained in translation and rotation [7]. The width
adopted for the model is sufficient to minimise the boundary effects, without significant increase in
computational cost verified by preliminary sensitivity analysis.
Grade C30/37 concrete is used for the abutment, piers and foundation. Typically for this grade of
concrete, the Young’s modulus can be taken as E=33GPa (Eurocode 2) but to account for creep,
shrinkage and cracking in the concrete cross section it has been reduced it to 10.5GPa.
Plate elements were used to model the concrete box deck to which axial and bending stiffness are
directly applied. Rotational restraints, to account for high flexural stiffness of the deck, were applied at
the top of pier and abutment cluster elements.
The IAB backfill soil was modelled with 14 layers behind the abutments at 500mm each. To better
model the interaction between the abutment and backfill material, interface elements have been input.
The bearing soil/ foundation consists of undrained very stiff clay which has a depth of 24m. This has
been modelled as 10 layers to have a realistic idealisation of how the soil would be present in reality.
However, for the multilayer model the layers of backfill have been specified differently which can be
seen further on. The interaction between different structural elements and the soil can be detailed and
captured by using the “interface elements” PLAXIS feature. This allows soil structure interaction
idealisation internally within the software for the chosen elements. Interface elements are defined by a
material type as well a virtual thickness. Earthquake induced ground motion in the analysis of earthquake
effects is one of the many possible sources of uncertainty and previous research has shown this to have
the highest effect on variability in the observed structural response [8]. Five dynamic motions were
identified for the study. They were chosen to reach the most unfavourable dynamic response of the
bridge system based on the expected natural period of the structure.
Figure 2 - Analysis Model
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3 PARAMETRIC STUDY
Six different variations of integral abutment bridge (IAB) geometries were chosen to represent the
variation of the bridge length, the abutment height and the type of fill retained by the bridge abutment.
The analysis models represent IABs of a single-span, two-span and three-span bridges, representing
bridges of 34m, 68m and 102m lengths respectively. The three-span structure considered three variations
for the abutment height of 3.5m and 5.5m and 8.0m. The sixth model represent a three-span bridge with
the full abutment height of 8.0m, but with varying shear wave velocity and other soil properties of the
backfill layers (Table 2). Also, models without fill were analysed.
Model Designation Model Description
REF/WF base reference model, 3-span bridge, 8m high abutment
REF/MLF 3-span bridge, 8m high abutment, backfill consists of multiple layers
REF/NF 3-span bridge, 8m high abutment, without backfill soil
REF/3.5m-WF 3-span bridge, 3.5m abutment
REF/5.5m-WF 3-span bridge, 5.5m abutment
1S/WF single-span bridge, 8m high abutment
2S/WF two-span bridge, 8m high abutment
Table 1 - Analysis Models Description
Figure 3 - General Arrangement of Base Model (REF/WF)
Figure 4 – Abutment of REF/3.5m-WF model Figure 5 – Abutment of REF/5.5m-WF
This enabled the study of the soil mass effect on the bridge response to determine whether it
influences the overall bridge system behaviour, or the integral bridge structure is driving the behaviour
of the system. Research showed that one of the many possible sources of uncertainty in the analysis for
earthquake effects is the assumed earthquake-induced ground motions used in the analysis model, and
research has shown this to have the highest effect on the variability observed in the structural response
[8].
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The models are subjected to five earthquake excitation records, which were carefully selected based
on real earthquakes to obtain the bridges responses to these applied motion records, see Figure 6 below.
These records were chosen based on the expected natural period of the structure under investigation, to
reach the most unfavourable dynamic response of the bridge system.
The signals obtained from seismic motion databases available are all recorded at 0.15g, therefore, to
investigate the structural system response in the range of moderate to high strains, these signals were
scaled to 0.3g and 0.6g, with maintaining their frequency content. The selection of 0.3g represents the
upper limit for Performance Zone 2 (AASHTO 2010), and it appears to be a typical range of PGA in
European Earthquake prone areas [7].
Figure 6. The response spectra of the five seismic motions and the elastic RS of Eurocode 8-1 soil A, PGA of 0.3g
Through time history analysis, the outputs from these models were used to determine the effect of
earthquake loading on the bending moment and shear forces of the bridge deck as well as the variations
in Pier drifts due to the earthquake loading on the structure. These models were analysed under
earthquake motions scaled at 0.3g and 0.6g to compare the earthquake effects. The above models were
re-analysed precluding the backfill soil (NF models) to determine the effect of the soil mass, stiffness
and damping on the bridge system behaviour. The results of the models that include the fill and the
models that do not include fill were compared to determine the contribution of the backfill soil on the
overall behaviour of the bridge system. The study looked at bending moments and shear forces at deck-
to-abutment and pier connections, bending moments at the deck mid-span, and compared relative
displacements, i.e. drift values, at the abutments and piers.
3.1 Backfill Soil Properties
The soil properties for the backfill were chosen for a soil above the phreatic line, hence the drained
soil assumption, the properties were assumed for sandy soil. The properties of the backfill soil are
assumed to be constant along the depth of the abutment. This model will be considered as the benchmark
for subsequent analyses.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Sa (
T) g
T (s)
Elastic Response Spectra for Input Motions (scaled at 0.3g)EC-8, A(0.3g)
Kypseli 0.3g
Gebze 0.3g
Duzce 0.3g
Hectormine 0.3g
Umbria 0.3g
Mean Spectrum (0.3g)
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Figure 7 - Backfill Soil Layers Definition (Typical Backfill and Multiple-layer backfill)
The following properties are assumed for the backfill soil used for the different models:
Layer
Designation
Thickness
(m)
Undraine
d Shear
Strength
(kPa)
Shear
Wave
Velocity
(m/s)
Effective
Shear
Modulus
(kPa)
Poisson’
s Ratio
Young’s
Modulus
(kPa)
MC 7.0 0.01 271 138900 0.40 833,600
BF1 1.0 0.01 174 40892 0.40 245,300
BF2 1.0 0.01 206 57034 0.40 342,200
BF3 1.0 0.01 227 69288 0.40 415,700
BF4 1.0 0.01 243 79548 0.40 477,300
BF5 1.0 0.01 256 88541 0.40 531,200
BF6 1.0 0.01 268 96639 0.40 579,800
BF7 1.0 0.01 278 104061 0.40 624,400
Table 2 Backfill Soil Layer Properties (Applicable for REF/MLF)
The base soil is assumed to have average damping of 6%. In PLAXIS 2D this is calculated by the
software by defining Rayleigh Damping Coefficients. To reach the target damping of 6%, the
frequencies used are 1Hz and 3Hz.
4 RESULTS
The time history analysis results of the different motion records were compared to the values from
running the static analysis model, representing the bridge self-weight. The self-weight bending moments
and shear forces at the critical locations shown in Figure 8 were considered as the reference datum for
the comparisons to follow. The ratio of bending moment and shear force resulting from each motion
input scaled to the self-weight effects represent the change in the deck forces due to earthquake effect.
This ratio indicates the amplification or reduction in the bending moment/shear forces due to the seismic
loading above the bending moment or shear forces resulting from the dead loads of the structure. This
is important from the design engineer’s perspective, since the bending moment and shear forces
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ultimately affect the selection of the different structural element sizes, which subsequently affects the
mass and stiffness of the bridge structure.
4.1 Bending Moments and Shear Forces
Results extracted from REF/WF and REF/NF analysis models showed that the backfill soil had a
surcharge effect on the bridge behaviour. An average increase in shear forces at abutment face of 8.1%
in the case of REF/WF compared to 5.6% increase in the shear forces for the case of REF/NF was
noticed. The bending moment at the abutment face increased by an average of 9.2% for REF/WF
compared to 6.9% in REF/NF case. This suggests that the backfill soil increased the earthquake actions
on the structure and worsened the demand on the structural elements. Table 5 below shows the ratio of
outputs from REF/WF to outputs from REF/NF.
Figure 8 - Critical Sections Locations
Msupport-
1
Qsupport-
1
Mmid-
span
Msupport-
2/Left Qsupport-2/Left
Msupport-
2/Right Qsupport-2/Right
Kypseli/VL 7.40% 5.30% 10.00% 7.20% 5.60% 1.40% 2.00%
Gebze/VL 8.30% 7.10% 10.00% 8.00% 7.30% 1.40% 2.00%
Duzce/VL 9.70% 8.70% 12.10% 9.60% 8.30% 4.10% 3.00%
Hector/VL 8.80% 7.40% 11.60% 8.60% 8.10% 2.20% 2.00%
Umbria/VL 11.90% 11.90% 14.40% 11.90% 9.30% 2.80% 3.00%
Average 9.22% 8.08% 11.62% 9.06% 7.72% 2.38% 2.40%
Table 3 - Ratio of Bending and Shear from Dynamic analysis to the Dead load – Model REF/WF (Base Model REF/WF
Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge).
Msupport-
1
Qsupport-
1
Mmid-
span Msupport-2/Left Qsupport-2/Left Msupport-2/Right Qsupport-2/Right
Kypseli/VL 5.83% 4.63% 7.11% 5.64% 4.46% 0.72% 0.93%
Gebze/VL 5.36% 4.36% 6.35% 5.15% 4.16% 0.50% 0.99%
Duzce/VL 8.84% 7.59% 10.68% 8.36% 6.55% 2.73% 2.75%
Hector/VL 6.50% 5.13% 8.52% 6.76% 5.51% 0.46% 0.86%
Umbria/VL 7.83% 6.13% 9.73% 8.23% 6.44% 0.80% 1.31%
Average 6.87% 5.57% 8.48% 6.83% 5.42% 1.04% 1.37%
Table 4 - Ratio of Bending and Shear from Dynamic analysis to the Dead load – Model REF/NF (No Fill Model REF/NF
Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge).
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Support 1 Mid-Span1 Support 2-L Support 2-R
MwFill/MnoFill QwFill/QnoFill MwFill/MnoFill MwFill/MnoFill QwFill/QnoFill MwFill/MnoFill QwFill/QnoFill
Kypseli -0.08% -0.03% 0.00% 0.08% 0.03% 0.00% 0.00%
Gebze 1.44% 0.62% 2.66% 1.58% 1.17% 0.65% 1.31%
Duzce 2.69% 2.55% 3.46% 2.79% 3.02% 0.85% 1.48%
Hector 0.70% 1.04% 1.26% 1.24% 1.65% 1.38% 0.57%
Umbria 2.06% 2.12% 2.84% 1.81% 2.45% 1.74% 1.50%
Average 1.36% 1.26% 2.05% 1.50% 1.66% 0.92% 0.97%
Table 5 – Ratio of M and Q from No Fill Model (three span) to the Reference Model (REF/NF vs. REF/WF) (Base Model
REF/WF Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge)
Figure 9 - Variation of Bending Moment (mid-span) and Shear due to variation of the fill type
The effect of the backfill soil considering the varying properties of soil layers, i.e. the multi-layer fill
(MLF) showed that the backfill properties had a burdensome effect on the bridge response as it increased
the demand on the structural elements of the bridge. The reference model (REF/WF) utilised a constant
set of soil properties for the backfill mass, and the shear velocity was fixed at 271 m/seconds for this
model, as compared to the multi-layer model (REF/MLF) which utilised varying soil properties for each
1m thickness of the backfill soil; the lowest shear velocity was taken at 174 m/s for the top layer and
increases to 278m/s for the lowermost layer, demonstrating that the softer the backfill soil the worse for
the seismic behaviour of the IAB. Table 6 below shows the bending moment and shear from REF/MLF
compared to the self-weight effects. An average increase in shear at abutment of 16%, and a 21%
increase in bending moment at face of abutment was recorded, the mid-span moment showed and
average increase of 28% to the datum self-weight effects.
Msupport-
1 Qsupport-1
Mmid-
span
Msupport-
2/Left
Qsupport-
2/Left
Msupport-
2/Right
Qsupport-
2/Right
Kypseli/VL 15.37% 12.02% 18.35% 14.38% 11.21% 1.92% 3.06%
Gebze/VL 16.03% 13.46% 17.66% 13.55% 11.23% 1.87% 2.51%
Duzce/VL 31.82% 22.40% 42.40% 35.69% 27.26% 6.83% 7.33%
Hector/VL 17.30% 14.34% 21.02% 16.79% 12.09% 3.39% 3.49%
Umbria/VL 22.10% 20.31% 40.87% 22.07% 18.69% 4.65% 5.65%
Average 20.53% 16.51% 28.06% 20.50% 16.10% 3.74% 4.41% Table 6 - Ratio of Bending and Shear from Dynamic analysis to the Self-weight effect - Model REF/MLF (Multi-Layer
Model REF/MLF Flexure (kN.m) and Shear (kN) - results shown per 1m width of bridge)
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Figure 10 Variation of support moment (left) and mid-span moment (right) due to variation of abutment height.
The results showed that the abutment height had a beneficial effect on the bending moments and
shears on the bridge system, i.e. the higher the abutment the better for the earthquake resistance of the
bridge. This is clear form Figure 10 above, which showed a reduction in bending moment as the
abutment height increases. This may be attributed to the backfill acting as an energy dissipation medium.
Figure 11 - The bending moments at abutment and at mid-span for variable bridge lengths (spans varying from 1 to 3
spans) - BM correspond to a strip of 1m width of the 13.5 m wide deck)
Single Span Bridge (1S/WF) Single Span Bridge (1S/NF)
Flexure (kN.m) and Shear (kN) Flexure (kN.m) and Shear (kN)
Msupport-1 Mmid-span Qsupport Msupport-1 Mmid-span Qsupport
Kypseli/VL 4.09% 3.12% 4.68% Kypseli/VL -0.02% 2.41% 4.20%
Gebze/VL 2.38% 2.37% 5.67% Gebze/VL -0.16% 5.50% 9.37%
Duzce/VL 5.70% 5.44% 5.84% Duzce/VL -0.31% 6.51% 9.46%
Hector/VL 1.64% 1.45% 5.15% Hector/VL -0.21% 3.94% 7.01%
Umbria/VL 5.08% 4.93% 6.07% Umbria/VL -0.17% 5.79% 14.47%
Average 3.78% 3.46% 5.48% Average -0.17% 4.83% 8.90%
Table 7 - Single Span Bridge (With Fill vs. No Fill)
Analysis showed that increasing the length of the bridge had worsened the behaviour, since the
bending moments and shear forces increased with increasing the number of spans of the bridge. Figure
11 above shows that increasing the bridge length, i.e. the number of spans, resulted in an increase in the
bending moments and shear force values at the abutment face. FEM results showed that for the single
span bridge, the existence of backfill had a beneficial effect on the bending moments and shears at the
abutment face and the mid-span. On average, there was 9% increase in shear at abutment for the case of
no fill (1S/NF) versus a 5.5% increase in shear when the fill existed. This, however, was not the same
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for bending moment at the abutment face, and the moment at the abutment showed an average increase
of 3.78% for case with fill (WF) versus a negligible reduction in moment from the case of no fill (NF).
These results show that the existence of backfill tends to increase the bending moments and shears
on longer IABs, which should be accounted for in the design of the superstructure elements. On the
contrary, structures that are relatively short showed that the backfill is beneficial to its behaviour under
seismic loading.
4.2 Pier and abutment drifts
The following results represent the recorded displacements at the top of the abutment or pier and the
recorded displacements at the abutment or pier bottom. These results represent the maximum
displacements due to the earthquake loading. The difference between the top displacement and the
bottom displacement, occurring simultaneously, i.e. at the same time instance, represents the differential
movement of the bridge abutment or pier during the earthquake excitation and hence it is a measure of
the expected bending moment for given boundary conditions. Residual displacements, after earthquake
loading, represents the displacements recorded at the last step of the time history analysis, i.e. after the
completion of the seismic motion as per the equation below:
𝐷𝑟𝑖𝑓𝑡 = 𝑈𝑡𝑜𝑝 − 𝑈𝑏𝑜𝑡𝑡𝑜𝑚 𝐻𝑒𝑖𝑔ℎ𝑡⁄ (mm/m) (1)
Figure 12 - Drift Calculation
The drift values were summarised for both models including backfill and models without backfill.
The results show that the backfill has a significant effect on the displacement of the bridge as expected.
Comparing the drift values of models with backfill to those without backfill, show that the backfill
limited the displacements of the bridges significantly.
Comparing the outputs from the 3.5m and 5.5m models to the base model, the results showed that
the backfill acted as a mass driving the motion of the bridge, this behaviour tends to be very obvious
when comparing the base model to the 3.5m and 5.5m abutment height models. The lower mass of
backfill, behind the abutments of the 3.5m and 5.5m models, driving the motion resulted in reduced
displacements as compared to the base model, which had a larger mass of soil retained by the abutment.
This can also be attributed to the change in the bridge stiffness, which in turn affects its response to the
input seismic motion.
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REF/WF REF/NF
Kypseli 1.31 2.67
Gebze 1.53 2.87
Duzce 2.81 6.05
Hector mine 1.98 2.17
Umbria 2.56 3.38
Table 8 - Drift ‰ (mm/m) for 0.3g Input Motion – Bridge with fill (REF/WF) vs No fill (REF/NF)
The following tables show a comparison between the drift values at the abutments of REF/WF and
both REF/1S-WF and REF/2S-WF for input motions scaled at 0.3g and 0.6g. Single span and two span
bridges showed low stiffness compared to the three span bridge, and this resulted in the backfill soil
acting as the driving mass of the system, and therefore the drift values showed an increase in drift values
of both 1S and 2S bridges compared to the base model.
REF/1S-WF REF/2S-WF REF/WF
Kypseli 1.32 1.84 1.31
Gebze 1.57 2.28 1.53
Duzce 3.31 4.53 2.81
Hector mine 1.15 1.72 1.98
Umbria 1.89 2.54 2.56
Table 9 – Drift ‰ (mm/m) for 0.3g Input Motion – With Fill
REF/1S-WF REF/2S-WF REF/WF
Kypseli 2.40 2.90 1.31
Gebze 4.60 5.80 1.53
Duzce 5.00 7.50 2.81
Hector mine 2.20 3.00 1.98
Umbria 4.50 5.90 2.55
Table 10 – Drift ‰ (mm/m) for 0.6g Input Motion - With Fill
The analysis results using 0.6g input motions confirm the behaviour and conclusions drawn from the
0.3g analysis results. The results show that the backfill acted as a mass driving the motion of the bridge,
and this behaviour tends to be obvious when comparing the base model (REF/WF) to the 3.5m and 5.5m
abutment height models. The shorter height of the abutment meant a lower mass of backfill which
reduced the bridge displacements compared to the full height base model.
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REF/3.5m-WF REF/5.5m-WF REF/WF
Kypseli 1.00 1.90 1.30
Gebze 1.00 1.60 1.50
Duzce 1.20 2.60 2.80
Hector mine 1.20 1.70 2.00
Umbria 1.20 2.60 2.60
Table 11 - Drift ‰ (mm/m) for 0.6g Input Motion - With Fill (Abutment Height Effect)
5 CONCLUSIONS AND FURTHER RESEARCH
The present work aimed at establishing the effect of backfill soil on Integral Abutment Bridges (IAB)
seismic response and to answer the question as to whether it is a governing factor in the design of these
types of bridges. The effort comprised the development of finite element models using PLAXIS 2D for
the study of soil-structure interaction effects on a number of variations of integral abutment bridges
under seismic ground motion. The study covered in detail the seismic behaviour of six IAB bridge
configurations that varies in length, abutment height and type of backfill soil.
The study showed that the presence of backfill soil had affected the dynamic behaviour of the bridges
in multiple ways, and it also showed that the interaction between the backfill soil and the bridge structure
is complex enough to warrant soil-structure interaction sophisticated modelling to obtain reliable
responses with regard to the behaviour of the bridge under seismic loading. Although more research is
required on the topic, the present study confirmed that the interaction between the bridge and the backfill
is of importance to be scrutinized by the design engineers and to be considered using sophisticated
analysis models, rather than using simplified design formulas. The results to date do not lead to clear
conclusions, i.e. whether the backfill soil is beneficial or detrimental for the seismic response of IABs,
however it is evident that this is a strongly case-dependent effect and this hence this is an ongoing
research project.
The effects of thermal cycles, creep and shrinkage, and prestressing forces in concrete bridges and
the long-term effects’ evolution over time is neglected in the present research, but such effects on the
behaviour of the bridge is expected to have significant effects on the behaviour of the bridge and should
be considered in any future studies.
Future studies should research into establishing design guidelines or design formulas to be used in
the design office as access to an FE sophisticated software is not always available for design offices, in
addition to the computational cost of running such complicated analysis software.
The study also considered the case of supporting the abutments and piers on shallow foundation. This
system may be standard for European countries, but the more common system in North America is to
use piled foundation. The piles have some flexibility and would allow movement of the abutment and
piers which would affect the natural period of the structure, and this effect should be considered in
further studies.
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