19 A proposition for a new construction method for cast-in-situ multi-span integral bridges

A proposition for a new construction method for
cast-in-situ multi-span integral bridges
Journal: IABSE-IASS Symposium London 2011
Manuscript ID: London-0702-2011.R1
Theme: Design and Construction
Date Submitted by the
Author:
n/a
Complete List of Authors: MITOULIS, STERGIOS; ARISTOTLE UNIVERSITY OF THESSALONIKI,
CIVIL ENG
TEGOS, IOANNIS; ARISTOTLE UNIVERSITY OF THESSALONIKI,
CIVIL ENGINEERING
Type of Structure: Road < Bridges
Material and Equipment: Concrete, Steel, Prestressing
Other Aspects:
Conceptual Design and Realization, Innovative Structural Systems,
Seismic Design and Response

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Ioannis A. TEGOS
Professor, Civil Engineering
Department,
Aristotle University of
Thessaloniki, Greece
itegos@civil.auth.gr
Ioannis A. Tegos, born in 1948,
received his BSc, and PhD
degrees from the Aristotle
University of Thessaloniki,
Greece. His main area of research
is earthquake resistant bridges,
creep and shrinkage effects, flat
slab buildings and design against
punching effects and
experimental strength of RC
members.
Summary
A proposition for a new construction method of
cast-in-situ multi-span integral bridges
Stergios A. MITOULIS
Dr. Civil Engineer,
Aristotle University of
Thessaloniki, Greece
mitoulis@civil.auth.gr
Stergios A. Mitoulis, born 1979,
received his BSc, MSc and PhD
degree from the Aristotle
University of Thessaloniki,
Greece. His main area of research
is earthquake resistance and
serviceability effects on RC
bridges.
A new construction method for cast-in-situ bridges is examined. The method is proper for bridges
with span lengths between 30 to 50 m. The construction of the bridge includes the following stages:
Two balanced cantilevers are casted on both sides of the elevated piers. Each cantilever has a length
equal to half of the corresponding span length of the bridge and is connected to the adjacent one at
the mid-span. The height of the cross section of the deck is reduced from the support to the midspan.
The prestressing tendons are straight, while the reinforcements within the bottom flange of the
deck consist of ordinary strength steel bars. The applicability of the proposed construction method
has been examined to a real cast-in-situ box-girder bridge. Comparative results are illustrated
showing that the proposed method has advantages concerning not only the constructability, but also
the serviceability performance, the earthquake resistance and the structural cost of the bridge.
Keywords: bridge; new construction method; variable cross section; cast-in-situ; straight tendons.
2,0-2,50m
pier
straight tendons
15x19T15 (St 1500/1770)
2X41Φ25
polygonal
bottom
flange
2X51Φ25
2X51Φ25
~4,50m
corners of the polygon
d=150mm
2X61Φ25
l=35,0-50,0m
d>150mm
2X71Φ25
mid-span
structural joint
1,00
Pier
7,00 7,00
8,50 5,50
5,50 8,50
L=14,0 m
tendons
(a)
(b)
Fig. 1: (a) Longitudinal layout of the ordinary strength steel bars at the bottom flange of the deck and (b)
layout of the straight tendons and the reinforcements of the deck’s top flange at the support (plan view).

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Ioannis A. TEGOS
Professor, Civil Engineering
Department,
Aristotle University of
Thessaloniki, Greece
itegos@civil.auth.gr
Ioannis A. Tegos, born in 1948,
received his BSc, and PhD
degrees from the Aristotle
University of Thessaloniki,
Greece. His main area of research
is earthquake resistant bridges,
creep and shrinkage effects, flat
slab buildings and design against
punching effects and
experimental strength of RC
members.
Summary
A proposition for a new construction method for
cast-in-situ multi-span integral bridges
Stergios A. MITOULIS
Dr. Civil Engineer,
Aristotle University of
Thessaloniki, Greece
mitoulis@civil.auth.gr
Stergios A. Mitoulis, born 1979,
received his BSc, MSc and PhD
degree from the Aristotle
University of Thessaloniki,
Greece. His main area of research
is earthquake resistance and
serviceability effects on RC
bridges.
A new construction method for cast-in-situ bridges is examined. The method is proper for bridges
with span lengths between 30 to 50 m. The construction of the bridge includes the following stages:
Two balanced cantilevers are casted on both sides of the elevated piers. Each cantilever has a length
equal to half of the corresponding span length of the bridge and is connected to the adjacent one at
the mid-span. The height of the cross section of the deck is reduced from the support to the midspan.
The prestressing tendons are straight, while the reinforcements within the bottom flange of the
deck consist of ordinary strength steel bars. The applicability of the proposed construction method
has been examined to a real cast-in-situ box-girder bridge. Comparative results are illustrated
showing that the proposed method has advantages concerning not only the constructability, but also
the serviceability performance, the earthquake resistance and the structural cost of the bridge.
Keywords: bridge; new construction method; straight tendons; variable cross section; cast-in-situ
1. Introduction
Safety, serviceability, cost-effectiveness, aesthetics and particular technical issues are typically the
controlling factors in the selection of the proper bridge type [1] [2] and construction method. The
selection is further complicated by other considerations such as the deflection limit, the life-cycle
cost, including traffic obstruction during construction stages and during maintenance works,
scheduling, feasibility of falsework layout and seismicity at the site [3]. In many cases, a
prestressed concrete or steel bridge is a cost-effective choice. Typically, segmental concrete bridge
construction is utilized, which is the most common method of bridge construction.
Segmental construction method typically introduces: (a) the conventional cast-in-situ bridge
construction, (b) the precast prestressed I-beam deck construction with continuous cast-in-situ slab
decks, (c) the balanced cantilever bridge construction, which either utilises scaffolding or precast
deck segments and (d) the progressive and span by span incrementally launched bridge construction.
Segmental cast-in-situ bridge construction is preferable in case of straight and curved in plan
bridges with relatively small bent heights and when prestressing is applied in the longitudinal
direction of the superstructure, as shown in Fig. 1(a). The formworks are typically supported
directly to the ground or to a well compacted temporary embankment. In most cases, the first span
and a 15 to 20 % of the length of the second span are casted together. The construction of the next
bridge segment follows after the application of the prestressing force, while keeping the immediate

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prestress losses within normal levels. The final loading of the bridge due to the self-weight of the
superstructure is varying with time due to the influence of the creep effect [4] [5] [6], as shown in
Fig. 1(b). The final loading of the deck can be estimated by the use of simple formulas as follows:
( )
M =ΣΜ + Μ -ΣΜ
∞
Α Ε Α
φ∞
⋅ 1+ρ ⋅ φ
∞
(1)
In Equation 1 where M ∞ is the final bending moment of the deck after the development of creep
effects, M E is the bending moment of the total bridge system in the ideal case that the deck is
constructed at once, ΣΜ Α is the sum of the bending moments of the deck at subsequent construction
stages considering elastic response of the deck, φ ∞ is the final value of the creep coefficient [7], and
ρ is the coefficient of the losses due to the prestressing steel relaxation [7] [8]. It is noted that the
calculation of actions, that are the bending moments of the superstructure, the shear and the torsion
actions have to be performed at different times, that are at t=0 (time of loading) and t=∞ when the
creep effects have been fully developed.
3
2
1
M ΣMΑ
(a)
(b)
Fig. 1: (a) The structural joint at the mid-span at segmental cast-in-situ bridge construction and (b) the
influence of segmental construction on the bending moments of the deck.
This paper proposes a new bridge construction method, which has similarities with the balanced
cantilever method. However, the cantilevers do not require the standard connection according to the
balanced cantilever method that is the jacking of the meeting segments. The connection of the
cantilevers is achieved by the use of tendon couplers. The tendons are straight and the scaffolding,
which is used for the deck casting, is removed after the application of the prestressing force. The
applicability of the proposed construction method has been attempted to a cast-in-situ benchmark
bridge actually built along a major motorway that runs across Northern Greece. The paper shows
that the proposed construction method can actually facilitate construction, while providing an
effective control of the serviceability needs of the deck. Further to that, the earthquake resistance of
the resulting bridge is enhanced due to the monolithic pier-deck and abutment-deck connections,
while aesthetics are improved.
2. Description of the proposed construction method
2.1 Structural assumptions
The proposed structural method, which can be utilised in the construction of cast-in-situ bridges, is
based on the following structural assumptions: (a) The deck cross section has a variable height
1
8
MA,1
ME
2
MA,2
3
MA,3

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along the longitudinal direction of the bridge with a symmetrical bottom flange, which is modulated
by a polygonal shape inscribed in a parabolic arch, as shown in Fig. 2. The cross section of the deck
can be either a box girder or a voided slab. (b) The prestressing tendons are straight and continuous
in all the deck spans and they are installed in the top flange of the deck. The appropriate concrete
cover [7] [8] is provided to protect the tendons against corrosion. Within the bottom flange of the
deck only ordinary strength steel is utilised. (c) The construction of the end spans can follow two
different design alternatives: (c 1 ) The first alternative introduces the construction of the end spans
by maintaining the geometry of the intermediate spans for reasons of aesthetics. In that case, the
deck is chosen to be rigidly connected to the abutments, as shown in Fig. 3. Possible use of bearings,
for the support of the deck on the abutments, would lead to the requirement for their replacement
during the service life of the bridge. Replacement of the bearings is feasible, however it is not
always convenient. Thus, the monolithic connection of the abutment web with the deck is proposed
in this design alternative. (c 2 ) The second design alternative introduces the construction of the end
spans with lengths smaller than the ones of the intermediate ones. Half of the length of the end span
has a deck cross section with variable height. This corresponds to the part of the deck which
extends from the end pier towards the abutment. The other part of the span is seated through
bearings to the abutment. It extends from the abutment towards the pier and has a constant cross
section height. The need for the smaller length of the end spans was found to be dictated by the
relatively small height of the deck cross section that is 0,80 m and by the use of ordinary
reinforcements in the bottom fibre of the deck, as stated in (b) above.
It is noted that the use of prestressing within the bottom flange of the deck was not deemed to be a
rational design selection, as the tendons would induce a large vertical load downwards, due to the
variation of the height of the deck cross section. This constraint loading, namely the one induced by
possible negative prestressing, would not be compatible with the rational use of tendons, which are
typically utilised in order to compensate for the vertical loading.
a-a
a
a
h2=0,80m
h1=2.20m
A1
b-b
P1
Fig. 2: The first stage of the proposed construction method.
2.2 Particular design issues
a
a
b
b
c-c
centre of gravity
of the deck
h3=0,80m
Τhe rigid connection of the deck with the abutments was achieved by the construction of a
counterbalance that is a cantilever which extends from the abutment towards the backfill soil, as
shown in Fig. 3. The length of this cantilever is 5,0 m and its cross section height reduces from the
abutment to the backfill. The end cross section of the cantilever is utilised for the anchorage of the
tendons. The tendons are slightly lowered at their anchorages in order to provide the appropriate
cover for their anchoring devices, namely the bearing plates. A structural tie, namely a reinforced
concrete wall with a thickness of 0,30 m, is utilized in order to receive the bending moments of the
counterbalance-cantilever, which are developed due to the vertical loading of the deck. In fact this
wall, namely the structural tie, is under tension, while the subdivided abutment web is under
c
c

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compression. The structural tie has a length equal to the distance between the wing walls, with
which it is in contact but not connected. The reinforcement bars of the structural tie are anchored in
the pile cap of the abutment’s foundation. This pile cap has a relatively small thickness, as the wing
walls and the wall that retains the backfill soil formulate a stiff concrete “box”, which increases the
stiffness of the pile cap. It is noted that, the web of the abutment, in case it supports the deck
through bearings, can be compact. In case the web is integral with the deck, its in-service
constrained movements can be accommodated by subdividing it in walls, as shown in Fig. 3, with
small thicknesses in order to reduce the shear stiffness enough, while maintaining adequate moment
capacity [9].
backfill
opening
retaining
wall
tendons
(Detail)
counterbalance
cantilever
structural
tie
pile cap
tendons
subdivided
abutment
web
450mm
Fig. 3: The abutment of the proposed construction method.
Detail
2,75
R=30m
2,75
R=30m
150mm
The minimum height of the deck cross section is proposed to be not smaller than 0,80 m. After the
curing of the casted cantilevers, the tendons are stressed. The design of the prestressing force is
based on the objective of the method that is to provide a slight pre-cambering of the cantilevers that
is a slight bending deflection upwards. Therefore, at this stage the cantilevers of the deck are set
higher than the final design height of the bridge. After the application of prestressing, the steel
formwork is removed and the construction procedure is repeated for the adjacent spans. The
tendons of the subsequent spans are coupled with the ones of the casted cantilever and the adjacent
cantilever is constructed. A detailed description of the prestressing application and the rebar of the
deck is given in section 3 of the paper.
After the completion of the deck construction and the application of the prestressing force, positive
bending moments, which are caused due to the eccentricity of the straight tendons from the deck’s
centre of gravity, are developed along the deck. These positive bending moments overbalance the
negative ones that are imposed by the self-weight of the deck. Hence, a pre-cambering of the
cantilevers is achieved. The pre-cambering was deemed necessary in order to compensate for the
pre-determined long-term prestress losses due to the creep and shrinkage of the deck and due to the
relaxation of prestressing steel. The rest of the vertical loads of the deck that are the additional
permanent and the variable loading [10] are imposed after the completion of the total bridge system.
Thus, the frame action of the total bridge structure, in which the meeting cantilevers are connected,
receives the additional vertical loading. The final bridge system is then checked against the
resulting bending moments, the shear actions and the torsion effects after considering the redistribution
of actions. In particular, the design of the deck against shear actions is facilitated due to
the beneficial inclination to the horizontal of the compression zone of the deck in the critical section
for shear, namely where the maximum shear stress is acting. Possible deficiency of the deck at the
supports against the bending moments caused by either the ultimate or the serviceability limit states
[8][11] shall be covered by additional reinforcement bars of ordinary strength steel. The additional
reinforcements cover the safety criteria set by codes [8] [11] and the serviceability requirements by
limiting the crack width according to the code provisions [7] [8].

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3. Application of the construction method to a cast-in-situ bridge
3.1 Description of the benchmark bridge
The bridge of Kleidi-Kouloura belongs to Egnatia Motorway that runs across Northern Greece. It is
a cast-in-situ structure with a total of three spans and a total length equal to 135.8 m. Figure 4
illustrates the longitudinal section of the bridge, the abutment and the cross sections of the boxgirder
deck, the pier and its foundation. The deck of the bridge has a constant height of 2.18 m,
while prestressing consists of tendons 20x19T15 (20 tendons of 19 wires with diameter 15mm each)
with a parabolic geometry. The bridge has a seat-type abutment on which the deck is supported
through two sliding bearings, while is rigidly connected to the piers. The clearance between the
deck slab and the backwall is bridged by an expansion joint with a movement capacity of ±100 mm.
A1 P1 P2 A2
KOULOURA
135.80
KLEIDI
45.10
45.60 45.10
approach slab
length: 4.0m
backwall: 3.0m
backfill soil
Abutment
T200 (±100mm)
deck
sliding bearing
3
6.5
2
2
3 6.75
3
2
2.18
9.50 9.50
0.60
Pier
13.50
6.00
2.0m
Deck at the midspan
Fig. 4: The benchmark bridge (Kleidi-Kouloura Bridge of Egnatia Motorway).
3.2 Results
Foundation
2.00
7.50
1.00 22.50
The benchmark bridge was re-analysed and re-designed according to current code provisions
concerning serviceability [8] [11] and earthquake resistance [12]. The re-design took into account
the construction phases of the proposed method and the following predominant design parameters
were revealed: (1) The required number of straight tendons was less than the one needed in case a
classification category A or B was chosen, (table 4.118 in [8] [11]). However, the total number of
tendons ensures that the bridge is classified in category C, when this requirement refers to the
performance of the top fibre of the deck, while the use of ordinary strength reinforcements in the
bottom fibre of the deck leads to the classification category D. It is noted that, the design of the
prestressing force and the resulting number of tendons aims at providing the required pre-cambering
of the cantilevers against the self weight of the bridge deck, whose length was half of the total span
length that is 45,60/2 = 22,80 m. (2) The re-design of the prestressing showed that 15x19T15 (15
tendons of 19 wires with diameter 15mm each) of high strength steel St1500/1770 are adequate to
receive the bending moment of the deck above its support. Additionally, ordinary steel rebar 76Ø16
(76 bars with diameter 16mm each) above the support were utilized, which gradually reduced to
28Ø16 at the bridge mid-span. The tendons and the reinforcements needed in the top flange of the
deck are illustrated in Fig. 5. The ordinary strength steel bars, which are also required by the code
[8], are the ones which allow the safe transition from the uncracked to the cracked deck section and
the avoidance of non-ductile failure modes. The lengths of the steel bars were chosen to be
submultiples, namely half, of the conventionally produced ones by the steel manufactures in order
to avoid material waste. Figures 6 and 7 show in detail the distribution of these reinforcing bars at
the support and at the mid-span. Figure 8 shows the steel rebar of the bottom part of the deck. The
bars are installed in couples that are 2x71Ø25 (71 couples of bars with diameters 25mm each) at the
mid-span, while 2x41Ø25 were found to be required at the bottom flange of the deck at the supports.
The reinforcement splices were required to extend 2,15 m. The lengths of the bars were selected to
be 7,0 m and they were set parallel to the sides of the polygonal shape of the bottom flange. (4) The
thickness and the reinforcement of the structural tie, that is the wall that restrains the vertical
movements of the counterbalance-cantilever at the abutment shown in Fig. 3, were found to be
0,30m and 3xØ16/100 (3 lines of bars with diameter 16mm at a spacing 100 mm) correspondingly.
3.00
2.00

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Y
X
28O16
L=14,0m
couplers
52O16
L=7,0m
76O16
L=14,0m
Pier
tendons 15x19T15 (St 1500/1770)
52O16
L=7,0m
28O16
L=14,0 m
lapping
3,50 m
Fig. 5: The layout of the straight tendons and the ordinary strength steel bars of the deck’s top flange at the
support, (the scale is distorted: 1 unit at X equals 2 units at Y axis).
1,00
Pier
7,00 7,00
8,50 5,50
5,50 8,50
L=14,0 m
tendons
Fig. 6: Detail of the straight tendons and the ordinary strength steel bars of the deck’s top flange at the
support.

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7,00
lapping
14,00 3,52
couplers
7,00
tendons
structural joint
Fig. 7: Detail of the ordinary strength steel bars of the deck’s top flange at the mid-span and coupling of the
tendons.
~4,50m
2,0-2,50m
pier
top flange
7,0m
2X41Φ25
polygonal
bottom
flange
straight tendons
15x19T15 (St 1500/1770)
2,15m
7,0m 7,0m
2X51Φ25
splicing
length
corners of the polygon
d=150mm
2X51Φ25
2X61Φ25
l=35,0-50,0m
Fig. 8: Detail of the ordinary strength steel bars at the bottom flange of the deck.
d>150mm
2X71Φ25
mid-span
structural joint
The study of the applicability of the proposed structural method by utilising bridges with shorter
and longer spans showed that for span lengths up to 35,0 m the deck is not exhibiting cracking,
while for longer spans the crack widths can be kept low, namely lower than the limit of 0,02 mm,
by means of rational reinforcement ratios of ordinary strength steel nars. As far as it concerns the
bottom flange of the deck, it was found that the bending moments of the mid-spans require
reinforcement ratios lower than the maximum allowable ones, which are set by codes [7][8]. This is
attributed to the fact that the bending moments of the bridge are redistributed due to the frame
action of the final bridge system. Hence, the deck’s bending moments are mainly received by the
supports, due to the beneficial variation of the deck cross section height and the aforementioned
frame action.
4. Conclusions
This paper proposes a new bridge construction method, which can be used as a design alternative to
the conventional construction practices. The method has many similarities with the balanced
cantilever method. The prestressing tendons are straight and installed within the top flange of the
deck cross section, while ordinary strength steel is utilized for the reinforcement within the bottom
flange. The deck has a variable cross section height along its longitudinal direction. A benchmark
bridge, actually built along the Egnatia Motorway by the conventional segmental cast-in-situ
method, was utilized as benchmark to identify the applicability of the proposed method. The bridge
was checked according to the current code provisions and the study came up with the following
findings:
The application of the proposed construction method revealed significant structural benefits. The
use of straight tendons for the prestressing of the deck facilitates and accelerates the construction of
the bridge. The tendons are installed within the upper slab of the deck’s cross section, which is
more preferable than using tendons which are installed in the webs of the box girder. It is noted that
the use of tendons in the webs of the box-girder decks is not allowed according to current code
design, at least for bridges constructed by the balanced cantilever method. Furthermore, the
prestressing losses due to friction are significantly reduced when the proposed construction method

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is employed. The dead load of the bridge deck, which typically constitutes the largest portion of the
bridge’s vertical loading, is decreased due to the reduction in the height of the deck cross section.
However, the variation of the deck cross section along the bridge deck obstructs the falsework as
the scaffolding is more demanding in terms of geometry, compared to the conventional segmental
bridge construction.
The bridge aesthetics are significantly improved compared to the conventional segmental bridge
construction. This is due to the refined arch-type view of the bridge constructed by the proposed
method and the reduced deck cross section height.
As far as it concerns the cracking of the deck, the proposed construction method can be utilized for
the construction of bridges with short to medium spans up to 35 m. The check against cracking due
to the short term vertical loading of the deck, namely against the infrequent loading, showed that
the deck does not exhibit cracking. In case of bridges with longer spans up to 50 m the use of partial
prestress shall be used.
The deflections of the deck were significantly reduced due to the objective set during the design of
the prestressing force, which ensured that the cantilevers had a pre-cambering upwards, at least
when the scaffolding was removed.
Possible differential settlements of the piers can be received by the resulting bridge system without
developing high bending loading to the deck, due to flexibility of the arch-type superstructure.
5. References
[1] CALTRANS, Bridge Design Aids Manual, California Department of Transportation,
Sacramento, 1994.
[2] CHEN W.F. and DUAN L., Bridge Engineering Handbook, CRC Press Boca Raton London,
New York Washington, D. C., 1999, Chapter 1.
[3] MITOULIS S.A., TEGOS I.A., K.-C. STYLIANIDIS, “Cost-effectiveness related to the
earthquake resisting system of multi-span bridges”, Engineering Structures, Vol. 32, Issue 9,
2010, pp. 2658-2671.
[4] TROST H., Lastverteilung bei Plattenbalkenbrucken, Werner Verlag, Dusseldorf, West
Germany, 1961.
[5] PIETRASZEK T.T., “Dealing with varying moments of inertia of girders in bridge analysis: 1
Discussion”, Canadian Journal of Civil Engineering, Vol. 16, 1989, pp. 203-204.
[6] KWAK H.-G., and SON J.-K., “Determination of design moments in bridges constructed
with a movable scaffolding system (MSS)”, Computers and Structures, Vol. 84 Issue 31-32,
2006, pp. 2141-2150.
[7] EN 1992-1-1:2004 Eurocode 2: Design of concrete structures, Part 1-1: General rules and
rules for buildings, 2004.
[8] DIN-FACHBERICHT 102, Betonbrücken, DIN Deutsches Institut fuer Normung e.V, 2003.
[9] TEGOU S.D., MITOULIS S.A., TEGOS I.A., “An unconventional earthquake resistant
abutment with transversely directed R/C walls”, Engineering Structures, Vol. 32, Issue 11,
2010, pp. 3801-381.
[10] EN 1991-2:2003 Eurocode 1: Actions on structures - Part 2: Traffic loads on bridges, 2003.
[11] EN 1992-2:2004 Eurocode 2: Design of concrete structures-Part 2: Bridges, 2004.
[12] EN 1998-2:2005 Eurocode 8: Design of structures for earthquake resistance, Part 2: Bridges,
2005.