4. 40|The Structural Engineer – 18 September 2007
Fig 4. Anti-uplift sliding bearing / Fig 5. Expansion joint detail / Fig 6. Bottom slab tendon anchorage
4 5
6
monolithically connected to the superstructure,and to form a central frame
to resist longitudinal loading and environmental effects including seismic
forces. At the abutments the deck is free to rotate and move longitudinally
via sliding pot bearings. Potential deck uplift during seismic events is
restricted by anti-uplift devices connected to the bearings (Fig 4). A guided
bearing at each abutment restricts the movement of the deck in the trans-
verse direction. Two elastomeric bearings are provided with sufficient gap
between the deck-ends and abutment cheek-walls to act as bumpers during
seismic events with intensities greater than the design earthquake. The
expansion joints allow a total bridge movement of up to 400mm at each end
(Fig 5).
Each superstructure consists of a box girder with a constant width of 7m
and a depth varying from 13.5m at the piers to 5.5m at mid-span. The web
thickness steps down from the piers towards mid-span taking values of
0.60m, 0.50m and 0.45m over a transition length of 0.75m.The top slab has
a thickness of 0.3m increasing to 0.5m at the edges, and the thickness of the
bottom slab varies from 1.3m at the piers to 0.3m at mid-span. The thick-
ness of the side cantilever varies from 0.65m at the root to 0.25m at the tip
(Fig 3).
Each pair of in situ concrete balanced cantilevers is connected to a 3m long
connecting in situ segment at mid-span: with approximately 16m long deck-
ends supported from the ground during construction. The length of the
cantilevers varies from 107m to 110m,each consisting of 26 in situ segments.
In order to limit the weight of the segments, their lengths vary from 3m to
4.5m for the first 13 segments, thereafter remaining constant at 5m. The
cantilever prestressing consists of 102 internal tendons made up of 19,
15.7mm diameter superstrand (19T15) arranged in two rows over the piers
with a tendon length of up to 227m.The tendons run through the upper slab
and are anchored at the segments’ end face within the slab thickening
without the need for blisters. Internal tendons are also placed in the bottom
slab to provide the continuity prestressing which comprise 32, 19T15 in the
main span and 12, 19T15 in the side spans. The continuity tendons are
anchored in 2m long blisters constructed at the edges of the bottom slab
(Fig 6).
Abutment galleries are provided to access the expansion joints and the
interior of the box girders for inspection and maintenance. Maintenance or
damage inspection of the inside of the hollow piers is by means of an access
hole at the pier top and then via ladders placed between permanent platforms
that are located at 5m intervals inside the piers. Inspection of the bearings
can be carried out from a platform situated in front of the abutments. The
provision of jacking points allows the replacement of the bearings.
Parapets consist of standard metal posts and railings in accordance with
the Greek regulations.The main drainage pipes are made of fibre glass and
run on the outer face of each box girder with connections to the deck inlets
via transverse drainage pipes under the deck cantilevers. Motorway service
ducts are located under the outer footpaths and provision of ventilation and
drainage holes are made near the web tops and the bottom slab,respectively.
A 1.0m by 1.5m opening in the bottom slab near abutment A0 is provided
for access in cases of emergency.
The relatively high (12m) west abutments dictate that the backfill spills
through either abutment between its two tapering legs founded on a spread
footing. The east abutments are of the bank seat type with spread footing
foundations located at two different levels due to the steep slope of the rock
face.
Design
The design of Votonosi Bridge was carried out in accordance with the
performance requirements set out in OSMEO3
, the design guidelines devel-
oped by EOAE. The loading and static design was based on German DIN
Standards including ZTVK 964
supplement for bridge design.To consider the
effect of deck plan curvature,a three dimensional computer model was devel-
oped with measures to include soil structure interaction. The time depend-
ent characteristic of the concrete was modelled in the analysis using an
estimated construction programme.
The analysis and design covered all stages of construction and was carried
out using SOFISTIK computer program which automated part of the design
process. A number of 3D finite element models were additionally developed
and analysed to determine the intensity of stress concentration in the rock
sockets,anchorage zones,and hammerheads.Camber analyses were carried
out during the balanced cantilever construction to determine the setting out
geometry of the segments using the time dependent properties of concrete
and allowing for site temperature and humidity at the time of casting.
Votonosi Bridge is situated within Greek seismic zone II with peak ground
acceleration (PGA) of 0.16g, upgraded by an importance factor of 1.35
. The
seismic design of the bridge followed the Greek codes of practice EAK6
and
E39/997
. Its seismic integrity during construction was assessed for a PGA
value of 0.104g.To allow for structural redundancy the‘behaviour factor’ was
assumed as 3.0 for the completed bridge, thus allowing formation of plastic
hinges at the top and bottom of the piers during major seismic events. The
behaviour factor was taken as 1.0 during the construction to prevent struc-
tural damage before commencement of service.
During the construction, combinations of the following load cases were
considered in the design:
• self weight of the form-travellers
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5. 18 September 2007 – The Structural Engineer|41
8
9
7
Fig 7. Construction programme / Fig 8. Mobile crane for construction of rock sockets / Fig 9. Rock socket excavation
• self weight of the newly cast segments
• prestressing forces
• an accidental drop of the newly cast segments
• construction loading
• temperature difference between the top and bottom of the deck
• variation in the ambient temperature
• differential settlement between adjacent substructures
• wind loading with differential pressure
• seismic loading
For the ‘in service’ design, combinations of the following load cases were
considered:
• dead load and superimposed load
• prestressing forces
• SLW30/60 traffic loading in accordance with DIN 1072
• temperature difference between top and bottom of the deck
• variation in the ambient temperature
• differential settlement between adjacent substructures
• wind loading
• seismic loading
The following classes of material were specified in the design:
• concrete (cube strength):
superstructure 45MPa
piers 45MPa
rock sockets (capping zone) 35MPa
rock socket (shaft) 25MPa
abutments 35MPa
approach slab 35MPa
• steel reinforcement BSt500 (St IV)
• prestressing tendons 1550/1750MPa
The design and constructability of the bridge was reviewed by EOAE,and
a category III check was carried out by an independent consultant. The
details of the analysis and the design are given in Ref (8).
Construction
General
The construction was carried out in accordance with TSY9
, a materials and
workmanship specification produced by EOAE based on national and inter-
national specifications. The works started in January 2000 and were
completed in December 2005 with the outline construction program shown
in Fig 7. The bridge is situated in a mountainous region and the adverse
winter weather dictated a stoppage of the works for approximately 2 months
of the year.
The site was served by two tower cranes placed between piers M1 and M2
with up to 100kN lifting capacity and 60m reach. Other major equipment
included a 300kN capacity mobile overhead gantry, two 125kVA generators,
a 100m3
/hr loader, two compressors with 850cfm and 335cfm capacities, six
8m3
capacity concrete mixer trucks, four concrete 101bar capacity pumps
with 47m3
/h output, a 10m3
truck, a 12hp gunite pump, a 8hp grouting
pump, a mini-excavator, and a drilling machine.
Due to the relatively long distance between the site and quarries, aggre-
gates were extracted from the nearby Metsovitikos River, crushed, screened
and washed for concrete production in a 50m3
/hr batching plant situated on
site. The batching plant for the construction of the nearby Megalorema
Viaduct2
,situated within 2km of the site,was also used in cases of emergency.
The details of the construction of the foundations, piers and superstruc-
ture of the bridge are described as follows.
Foundations
Access roads to the foundations were designed and constructed with the
emphasis on minimising damage to the environment.31 trial boreholes were
drilled at the pier and abutment locations.This was followed by levelling the
ground to install the mobile overhead gantry for the construction of the rock
sockets.Due to the level difference between the south and north rock sockets
at M2, an extended platform was built using a concrete enclosure filled with
compacted soil to allow the operation of the gantry over both sockets (Fig 8).
The excavation for the rock sockets was carried out in depth intervals of 3m
with drilling at the perimeter followed by sequential firing of explosive
charges placed in a helical formation. With the available drilling machines
the acceptable tolerance for vertical drilling could not be achieved for depths
of more than 15m. The drilling operation was, therefore, carried out in two
stages: a larger diameter than the design diameter was first formed for the
upper half of the socket,and the drilling machine was subsequently lowered
and placed on the base of the upper half in order to drill for the design diam-
eter lower half (Fig 9).
The excavated face was regularly inspected by a geologist to verify the geot-
echnical parameters assumed in the design, and to optimize the measures
required to support the exposed face.The rock face was stabilised using rock
anchors, steel beams, and corrugated metal sheets, followed by the installa-
tion of mesh reinforcement and shotcreting. Concreting the sockets was
carried out in 5 or 6 lifts with the depth of each lift varying from 3m to 5m
using up to 600m3
of concrete for each lift. Concrete temperature was
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6. 42|The Structural Engineer – 18 September 2007
paper: ahmadi-kashani et al
controlled by means of early morning concerting in the summer months,and
using Portland cement type I.These measures reduced the risk of micro-crack
formation in the concrete due to excessive heat of hydration.
The top 3m of the rock socket consisted of a heavily reinforced ‘capping
zone’. The lower 4 or 5 ‘shaft’ lifts contained mainly vertical reinforcement
placed at the perimeter of the socket.Reinforcement installation and concret-
ing was carried out with the aid of the overhead gantry.A sump was provided
on the top surface of each segment to remove any excess water.The construc-
tion of each rock socket took between 3 and 4 months depending on the
quality and type of rock mass encountered.The maximum settlement of the
sockets after completion of the bridge was recorded as 4mm.
Piers
The piers were constructed using climbing formwork in 2.8m segments.The
reinforcement for the piers mainly comprised 7m prefabricated cages typi-
cally formed with six longitudinal reinforcement bars connected by stirrups.
The formwork and associated working platforms were fixed onto sockets
embedded on the previously cast segment. The formwork panels were held
in the upright position and were fixed in place with through ties.
Construction of a typical pier segment normally took 4 days to complete: 1.5
days to place the reinforcement, 1.5 days to install the formwork and 1 day
for concrete pouring and curing.
To avoid lapping reinforcement bars in the plastic hinge zones at the top
and bottom of the piers, the reinforcement cages in this region were fabri-
cated with14m/16m long rebars covering three segments, and were held
upright by steel framework that included three levels of working platforms
(Fig 10). Additional horizontal reinforcement links were placed in this zone
to confine both the concrete and vertical reinforcement,and to allow the safe
formation of plastic hinges during a major seismic event.In order for the piers
of each superstructure to resist similar seismic forces,the stiffness of the piers
had to be comparable;this meant that the M1 piers had to be extended down
into the ground (Fig 2). Concrete collars, 13m diameter and 10m deep, were
constructed around these piers to allow the inspection and repair of the
plastic hinge zones after a major earthquake (Fig 11).
Pier segments lower than 25m above ground level were concreted using
lorry mounted pumps. For higher levels, concrete was pumped through a
150mm diameter steel pipe fixed to the pier by means of a pump with an 80m
head capacity. Suitable aggregate grading and admixtures were used to
provide the required slump and consistency for the pumped concrete.
Temporary stairs and platforms were installed and used during the pier
construction. The construction of each pier took approximately 4 months to
complete.
The galvanised steel maintenance inspection ladders and platforms were
installed inside the piers prior to the construction of the hammerheads. Due
to the height of the piers the formwork for the hammerhead was supported
from the pier tops.Each hammerhead,13.5m high with two diaphragms,was
constructed in five phases using a proprietary formwork system (Fig 12).
Steel beams fixed to the inside walls of the piers supported the 2.5m thick
bottom slab.The construction of the hammerhead required special formwork
to include the various access openings to the interior of the piers and box
girders. Once the hammerhead was constructed, a temporary lift was
installed for each pair of M1 and M2 piers which together with a high level
truss spanning between the decks allowed access to both decks.The construc-
tion of each hammerhead took approximately 3 months to complete.
Superstructure
The superstructures were constructed using a proprietary form-traveller
weighing 800kN with a capacity to support a segment weight of 2500kN,and
with its centre of gravity up to 2.5m beyond the end of the segment. Each
traveller consisted of two main ‘A’ frames anchored to the deck and carried
a transverse truss from which the formwork for the webs and the upper and
lower slab soffits were suspended using high strength hangers. The move-
ment of each traveller was provided by two hydraulic rams.The top and sides
of the travellers were enclosed with a waterproofing tarpaulin to protect the
operatives from adverse weather. To meet the completion date, four pairs of
travellers were used during the construction (Fig 13).
The 6.6m length of the hammerheads was sufficient to place a pair of trav-
ellers braced to each other (Fig 14). Once the first pair of segments had been
cast symmetrically on the opposite sides of the piers, the bracing was
removed.The construction cycle for the next pair of segments consisted of the
following stages:
• moving the traveller to a new position
• adjusting the formwork levels for the new segments
• installing the reinforcement, prestressing ducts and tendons
• concreting the new segments
• prestressing the new segments
Each pair of segments was cast alternately in two stages:
1) The bottom slab and part of the webs, 2) The remainder of the webs and
the top slab. This sequence helped to reduce the unbalanced cantilever
loading, and resulted in better scheduling for the teams of labour involved.
Concreting each stage took about 3h and required up to 35m3
of concrete.The
depth of the upper section was kept constant resulting in a similar rein-
forcement arrangement. A number of surveys confirmed that the traveller
was sufficiently stiff to prevent separation of the first stage from the last
segment when the second stage was poured and added its weight to the trav-
eller. Casting the upper section at least 2 days after concreting the lower
section also helped in preventing crack formation at the construction joint
by ensuring sufficient bonding between the concrete and reinforcement.
The transportation of the reinforcement beyond the reach of the tower
crane was made possible by means of a light wagon driven by an electric
tractor on rails. A specialist team were responsible for placing the 100mm
internal diameter galvanized prestressing ducts, threading the tendons,
placing spiral reinforcement, stressing the tendons and grouting the ducts.
A grout test on a prestressed beam was successfully carried out prior to the
grouting the ducts. A steel template was used at the segment face to facili-
tate accurate placement of the tendons. The strands were delivered on coil
drums and were unrolled and pushed through the ducts by means of a
strand pushing machine.The minimum clear distance of 100mm between the
prestressing tendons specified in the design allowed for easy pouring and
compaction of the concrete and for placement of the traveller hangers.
Temporary 1.0m by 1.5m openings were provided in the top slab to allow the
equipment for prestressing the bottom slab tendons to be lowered into posi-
tion.
Following the installation of reinforcement and tendons,the internal form-
work for the box girder was placed in position, and preparation was made
for the concreting phase. For casting segments away from the pier, an addi-
tional concrete pump was installed on the hammerhead to facilitate concrete
delivery. The external formwork was backed with 50mm thick polystyrene
panels to insulate the concrete from the ambient temperature ranging from
–15°C to 40°C during the construction period. Installation of fan heaters
inside the box girder and radiant heaters on the deck during the winter
months provided acceptable conditions for concreting.In the summer months,
concreting was carried out early in the morning to avoid micro-cracking due
to excessive heat of hydration. Concrete samples were taken during casting
to determine its properties such as creep, shrinkage and modulus of elastic-
ity, which were then used in the camber analysis.
The average 28 days concrete strength achieved was 65MPa. The
prestressing was generally applied 3 days after concreting when the concrete
had attained a minimum strength of 40MPa. The tendons were tensioned
symmetrically from both ends in sequence about the centreline of the box
girder using a 3500kN capacity prestressing jack. The measured tendon
extensions were within 5% of their theoretical values.
The geometry of the balanced cantilevers was monitored by surveying the
coordinates of three steel plate markers placed at the tip of each segment.
One marker was installed at the centreline of the deck and the other two
located 5.5m either side of the centreline. The survey was carried out early
in the morning to avoid differential temperature effects.Measurements were
made on all the segments after three events: a) casting a pair of new
segments, b) prestressing the new segments and c) moving the travellers to
a new position. The results were compared with the theoretical values, and
after allowing for adjustments to comply with the design,a set of coordinates
were calculated for setting out the formwork for the next segment. The
extension of hangers transferring the weight of a segment to the traveller
was measured as 11mm, and was taken into account when setting out the
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7. 18 September 2007 – The Structural Engineer|43
paper: ahmadi-kashani et al
Fig 10. Pier reinforcement in plastic hinge zone / Fig 11. Collar for inspection of plastic hinge zone / Fig 12. Hammerhead construction
Fig 13. Utilisation of four pairs of form-travellers / Fig 14. Braced form-travellers / Fig 15. Support system for the deck-end at abutment A0
before backfilling / Fig 16. Restraining the tip of balanced cantilever adjacent to abutment A0
10
11
12
14
15
13
16
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8. 44|The Structural Engineer – 18 September 2007
paper: ahmadi-kashani et al
Fig 17. Support system for the deck-end at abutment A3/ Fig 18. Construction of the deck-end at abutment A3
Fig 19. Construction of the mid-span connecting segment/ Fig 20. Construction of central span connecting segment a) Elevation b) Cross-
section/ Fig 21. Construction of the edge beams / Fig 22. Theoretical and as built road levels / Fig 23. Votonosi Bridge nearing completion
17
19
18
21
23 22
20a
20b
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9. 18 September 2007 – The Structural Engineer|45
paper: ahmadi-kashani et al
formwork for the bottom slab.The construction of a new pair of segments was
planned on a 10 day cycle which was generally achieved after the initial
learning curve. The construction of each pair of balanced cantilever decks
took approximately 12 months to complete.After completion of the balanced
cantilever over pier M1,the travellers were removed and the deck levels were
surveyed. Subsequently, the construction of the 16m long deck-end next to
abutment A0 commenced.The formwork for the deck-end was supported on
a proprietary truss system which in turn was supported from the ground at
the two edges (Fig 15). The lateral and vertical movement of the cantilever
tip was prevented during concreting by means of a braced frame and by
anchoring the tip of the cantilever to the ground (Fig 16). The construction
of the deck-end consisted of the following sequence:
• concreting the bottom slab and part of the webs (stage 1 concreting)
• stressing a third of bottom slab tendons
• concreting the remainder of the webs and the top slab (stage 2 concreting)
• curing the concrete
• removal of the temporary truss
• de-tensioning and stressing the bottom slab tendons
Partial stressing of the bottom slab enabled the lower section of the deck-
end to carry the weight of stage 2 concreting and allowed the removal of the
temporary truss.
The 16m long deck-end next to abutmentA3 was subsequently constructed
by means of two pairs of right angle steel frames, pinned at the top and
bottom, with horizontal extension arms anchored to the cantilever tip (Figs
17 and 18). Before construction, the travellers were moved to pier M2 and a
deck level survey was carried out. The formwork was then placed on scaf-
folding supported on the steel frames. The construction of the A3 deck-end
was carried out in a sequence similar to that described for the A0 deck-end.
The connection of the cantilever tip to the deck required a careful concret-
ing procedure,including the use of a counterweight at mid-span and regular
monitoring.To minimise the effect of differential temperature,concreting was
carried out at night.The lower deck section constructed in the stage 1 concret-
ing was designed with sufficient reinforcement to span between the bearings
and the cantilever end, allowing for the removal of scaffolding after comple-
tion of the stage 1 concreting.
Prior to constructing the central connecting segment, a level survey was
carried out and the maximum level difference between the opposite cantilever
tips was found to be 16mm and 20mm for the south and north decks respec-
tively,bothwithintherequiredtolerance.Theexternalformworkfromthetrav-
eller was anchored to the segments each side of the gap to provide the
shuttering for the connecting segment.To correct the level difference between
the cable ducts in the bottom slab and to prevent relative vertical movement
of the cantilever tips,the two pairs of 600mm deep base beams of the travellers
were placed over the gap and anchored down to the deck (Fig 19).
Two horizontal steel I-beams were installed in the connecting segment
gaps bearing onto steel angles cast into the cantilever ends, and the gap
between the beams and the cantilevers tips were grouted.In order to prevent
relative horizontal movement between the cantilevers during concreting a
number of the bottom tendons were partially stressed (Fig 20). The bottom
slab and part of the webs were subsequently concreted and the steel beams
were removed. The remaining part of the webs and the top slab were
concreted followed by de-stressing the partially stressed tendons and subse-
quent stressing all the bottom slab tendons.As before,concreting was carried
out at night.
The construction of the deck-ends and the connecting segments were
carried out in the calm summer days in order to reduce the risk of any vibra-
tion damage during concreting. It is interesting to note that before casting
the connecting segments the cantilever tips deflected upward by 150mm due
to the sun shining on the deck top. It is also noted that, after completion of
the superstructure,the top of piers moved by 120mm laterally due to the plan
curvature of the bridge, as anticipated in the design.
Other activities carried out to complete the construction of the bridge
were as follows:
• backfilling behind the west abutment
• concreting the access holes in the top slab
• constructing parapet edge beams using counter-balanced form-travellers
(Fig 21)
• installing the parapets
• installing the deck waterproofing
• surfacing the deck
• installing the expansion joints
• installing the electricity cables and lighting columns
• installing the drainage system
• installing the lighting inside the box girder and the hollow piers
As shown in Fig 22, the vertical profile of the main span is conservatively
constructedabovethetheoreticalprofileinordertooffsetanyuncertaintiesdue
to the creep and shrinkage effects. The nearly completed Votonosi Bridge is
shown in Fig 23.
Conclusion
Votonosi Bridge, with a main span of 230m is the longest span for balanced
cantilever bridges constructed in Greece to date. Situated in a seismically
active zone and in an area of outstanding natural beauty, the design and
construction presented a new challenge to the client,designer,contractor and
supervisors.The successful and accident-free construction of this technically
demanding bridge is the result of constant diligence in the workplace and
teamwork from all the participants. The works were constructed through a
quality management system developed by EOAE/KBR which is used on all
projects under EOAE management.The experience gained in the design and
construction of Votonosi Bridge is being used in constructing a more chal-
lenging and longer span balanced cantilever Metsovitikos Bridge currently
under construction as part of Egnatia motorway project.
General data
Structural concrete: 29 000m3
;
Steel reinforcement: 5400t;
Post tensioning tendons: 1260t;
Duration of construction: 58 months;
Cost of construction: €20.7M
Credits and acknowledgment
The design and construction ofVotonosi Bridge is the combined effort of many
individuals in the following organisations:
Client: EOAE, Thessaloniki
Structural design: DOMI OE, Athens
Category III checker: VCE, Vienna
Designer of temporary works: Zografidis, Athens
Contractor: Mechaniki A.E, Athens
Project manager: KBR (UK), Leathearhead
Construction manager: Thales-Omek, Paris/Athens
Climbing formwork: DOKA, Germany
Form-travellers: NRS, Norway
The authors would like to thank Mr P Gibbons, KBR project manager, for
reviewing the paper.The opinions expressed by the authors do not necessarily
express those of EOAE, KBR, Thales-Omek and Mechaniki AE.
1. Hindley, G., Gibbons, P., Agius, M., Carr, B., Game, R. and Kashani, K.:
‘Linking past and present’, Civ. Eng., ASCE, 2004
2. Ahmadi-Kashani, K. and Gavaise, E.: ‘Bridges of Egnatia motorway’, Proc. 6th
Int. Conf. Short and Medium Span Bridges, Vancouver, Canada, 2002
3. OSMEO: ‘Guidelines for conducting road works design’, EOAE Internal
Report, 2000
4. ZTVK 96: ‘Zusätzliche Technische Vorschrift für Kunstwerke’, 1996
5. Ahmadi-Kashani, K.: Seismic design of Egnatia motorway bridges, Proc. Inst.
Civ. Eng., J. Bridge Eng., London, 2004
6. EAK: ‘National Greek anti-seismic regulations’, 2000
7. E39/99 – ‘National Greek directives on anti-seismic design of bridge’, 1999
8. Stathopoloulos, S., Kotsanopoulos, P., Stathopoloulos, E., Spyropolous, I.,
Stathopolous, K.: ‘Votonosi Bridge in Greece’, FIB Symp, New Delhi, 2004
9. TSY: ‘Specification for material and workmanship’, EOAE Internal Report,
2000
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