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Ponte Del Mare: Conceptual Design and Realization of a Long Span
Cable-Stayed Footbridge in Pescara, ItalyMario De Miranda, Prof.,
Dr., IUAV University of Architecture, Venice, Italy; Studio De
Miranda Associati, Milan, Italy; Alessandro De Palma, Civil Eng.
Studio De Miranda Associati, Milan, Italy and Alberto Zanchettin,
Dr., IUAV University of Architecture, Venice, Italy.
The structure is formed by a central bridge completely made of
steel and by two approach viaducts made of prestressed concrete.
The total length is 466 m. The main bridge has two sep-arated
curved decks: the first, 4,10 m wide and 172,8 m long is reserved
for cycles, while the second, 3,10 m wide and 147,7 m long, is set
2 m above the first one and is dedicated to pedestri-ans. At both
ends, the decks join to-gether as one, approximately 7 m wide. The
bridge decks are made of a steel spatial reticular truss, covered
with a bored steel sheet.
Preliminary Design
A central pylon, 49 m high, was origi-nally designed to bear a
single cable, suspending both decks. The original bridge then
consisted of two main curved girders suspended to a two span
parabolic cable, borne by a cen-tral tilted mast. The cable,
anchored to the ground by means of two braced abutments, was also
curved in plan to follow the shape of the main girders. A system of
inclined hangers was used to suspend the decks and transfer the
loads to the main cable (Fig. 1).
In order to allow sailing, the minimum height of the bridge with
respect to the river level is 15 m: the necessary height was
obtained, as said, with ac-cess ramps made of prestressed con-crete
girders spanning about 30 m, on concrete columns. The same bored
sheet that covers the decks was used for the viaduct girders.
Great attention has also been paid to the conception of the
light system, which is positioned on the mast and on the cables in
order to permit the
visibility of the entire bridge from a distance.
Tender Stage and Design Development
The Municipality of Pescara called for an international
competition for the realization of the footbridge on the basis of a
preliminary design. The ten-der was won by a joint venture formed
by three Italian companies. Due to the unusual structural system of
the bridge and its complicated building procedure, the contractors
appointed a consultant to develop the final design, improving the
bridge feasibility and optimizing the structural resources.
The original system was studied in order to find potential
critical aspects, particularly concerning the dynamic response of
the structure. The results showed that the bridge conceived in
preliminary design had a high deform-ability when loaded
asymmetrically on one of the two spans. Moreover, the erection
procedure would have pre-sented various difficulties also keeping
in mind that the channel should have been maintained navigable
during the bridge construction.
Another, less evident, problem was a strong coupling of the
aero-elastic response in the two decks, suspended to the same cable
with inclined ten-sion members; this implied that the displacement
of one deck, caused by an imposed load, generated a displace-ment
on the other deck. Consequently, a deck was subjected to the
aerody-namic force directly applied to it and also to the elastic
action coming from the aerodynamic force applied on the
Abstract
A new cable-stayed footbridge has been recently realized at the
harbour of the city of Pescara, in central Italy, to connect the
southern and north-ern coasts of the city seafront, which are
divided by the Pescara River. The bridge was designed in its
original form as a suspended structure with two curved decks joined
at both ends. The mast, positioned between the decks, is designed
to resemble, together with the steel cables, a sail. Considerations
involving the structural response of the structure, in particular
to dynamic loading on the two decks, suggested employing a stiffer
cable-stayed solu-tion. A new configuration with one layer of cable
stays per deck was then proposed to avoid the problems gen-erated
by the previous solution, with both decks suspended to a single
cable. In this way, the structural behaviour is more efficient,
less deformable and the aesthetic quality of the bridge is
valo-rized.
Keywords: structural architecture; footbridges; curved deck;
dynamic response.
Bridge Conception and Description
Pescara is strongly linked to the sea and its harbour plays a
central role in the economic and social environment of the city.
The preliminary design of a new footbridge, at the mouth of the
River Pescara, aimed at stressing this link: the suspension cables
and the antenna were conceived to resemble the shape of the sail
and of the mast of a boat.
Fig. 1: Layout of the bridge in the preliminary design
8,0%
+1,80Spalla nord
+2,00 +2,50
+18,50 m+16,50 m
+1,40 +1,50 +1,60
7,2%
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other one. Furthermore, the two decks being slightly different
and separated, they had unequal modes of vibration.
It was clear that a sectional wind tun-nel test hardly would
have been able to simulate such a complicate interaction, and a
full model, apart from cost and time, would have modelled the
special cross section with poor precision. To overcome the three
above-mentioned factors, a new suspension system was defined
consisting of an earth anchored cable-stayed system (Figs. 24).
The deformability was reduced by one fourth due to the more
direct transfer of loads to the mast; the erection got simpler; the
aerodynamic wind tunnel tests became feasible due to the
decou-pling of the static response of the decks.
Aerodynamic and Dynamic Analysis
The wind tunnel tests were performed at the Department of
Mechanics of the Polytechnic in Milan. Sectional models of the
bridge in scale 1 : 5 were tested and analysed to obtain the
aerodynamic parameters necessary to perform the numerical
simulations of bridge behaviour under wind loading (Fig. 5).
Numerical analysis was made at the same Polytechnic and at the
Depart-ment of Structural Engineering of the University of
Trento.
The sensitivity of the bridge conceived in the preliminary
design towards wind loading comes from its geometric
char-acteristics: high span/height ratio, deep decks and light
structure.
However, these aspects were asked to be retained in order to
avoid alteration
of the architectonical layout of the bridge. The same parameters
(slender-ness and lightness) also imply a strong sensitivity of the
bridge under imposed crowd loading.
Bridge aerodynamic analysis has been further complicated, due to
a series of factors:
The bridge has two decks which are different in form and
dimensions.
Each deck has an asymmetric shape, leading to a different
behaviour when wind comes from opposite sides.
Decks, being one in the wake of the other, infl uence mutually
their response to wind loading.
The effects determined by the bored steel sheet covering the
decks can be evaluated only by testing specimens on a suffi ciently
large scale.
The decks form in plan an arch with an angle of about 90.
Considering that wind direction is mainly constant, this curvature
implies that every section of the deck is hit by wind at
different angles: only one section is hit perpendicularly, while
all the others have a skew angle varying from 90 to 0. This aspect
was actually not modelled in the wind tunnel tests, so the
precautionary option, although not physically coherent, of wind
always perpendicular to the deck has been considered.
The first tests were performed initial-ly on an elastically
supported model, simulating the dynamic conditions of the first
mode of vibration. These tests have shown a good behaviour with
respect to the Von Karman vortices1 and a sufficiently high
aero-elastic stability. A second series of tests was made with the
model supported by dy-namic actuators, imposing on the deck a
dynamic deformation which allows the definition of the dynamic
char-acteristics depending on the move-ment of the deck and its
frequency of vibration, that is the so-called flutter
derivatives.2,3
Fig. 2: Sketch of final layout
Fig. 3: Bridge elevation (Units: m)
35,000 35,000 34,000 34,000 30,000 30,000 30,000 30,000
40,000172,763
0,00
17,416
49,00
2,60 Quota P.C.1,40 Quota P.C.
p = var.Pedestrian bridge axis
Longitudinal view pedestrian bridge
Accessviaduct axis p 6,69%
Longitudinal view cycle bridge
35,000 30,000
6,96%
34,000 30,000 30,000 30,000Access viaduct axis
p 6,96%
Access viaduct axisp 6,96%
30,000 40,00034,000 147,666
0,00
17,376
49,00
2,60 Quota P.C. 1,40 Quota P.C.
Cycle bridge axisp = var.
Access viaduct axis
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5,00
5,000
40,000
30,000
SPS
End
P8
P7
P6
P5
P4
P3
P2
P1
SPN
Start
35,01130,000
34,000
7,57
3,100
147,666
172,763
4,100
R = 80,938
5,000
30,000
30,000
30,000
34,000
7,57
R = 470,734
R = 157,193
R = 157,859
R = 459,187
North
Pescara River
Porto canale
vibration. The frequency of vibration rose to a factor of two in
comparison with the original layout, with parabolic cables.
This increment enhances the aerody-namic stability of the same
factor, as confirmed by the wind tunnel tests, showing the
effectiveness of the opti-mized suspension system.
The Final Structural System
As previously mentioned the final con-figuration of the bridge
preserves the original aspect but changes radically its structural
response.
Two different groups of cable stays were employed, with respect
to the sea, one suspending the external deck for pedestrians and
the other the internal one reserved for cycles. The cables link the
top part of the mast to the inter-nal part of each deck, realizing
an ec-centric support. Two conic surfaces are defined by the cable
stays. A couple of back stays work as bracings guarantee-ing
stability of the mast in the longitu-dinal plane of the bridge. The
trussed girders (Figs. 6 and 7) are integral over the end piers,
made of steel-concrete composite construction. The required
structural damping was obtained by two couples of heavily damped
stay cables linking the pylon base to the decks, at the location of
maximum modal displacements. Vibrations of stay cables are
contrasted by elasto-meric dampers. The mast (Fig. 8), tilted at 10
with respect to the vertical axis, has variable cross section,
being maxi-mum at mid height and minimum at both ends (pinned
structural scheme). It is made of 1,00 m diameter steel hollow
tubes, externally stiffened by T-shaped ribs, forming a tapered
cy-lindrical shape, with good aesthetic characteristics and at the
same time economic enough.
Fig. 4: Plan view of bridge and viaducts
Fig. 5: The sectional model of the pedestrian bridge on wind
tunnel
The tests showed that the deck section is strongly affected by
the reduced velocity, which is the non-dimensional ratio between
wind speed and the speed that an air particle needs to cover the
width of the deck in a period of time equal to the period of
vibration of the structure. The total
damping guaranteeing a sufficient factor of safety of the deck
for the first mode of vibration with respect to the critical speed
was found to be 2,5% of the critical damping. The tests also
confirmed a direct relation between aero-elastic stability and
first mode of
Cross sectionpedestrian bridge Cross sectioncycle bridge
1900
1900
Fig. 6: Typical cross sections of the pedestrian and cycle
girders (Units: mm)
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lifted and positioned on the piers; at the same time joints
connecting different elements were closed;
the inclined mast was then erected in segments and braced by two
inclined pipes, in order to get a stable and stiff structure during
the subsequent phases;
with both the steel girders joined and bearing on temporary
piers, stay cables were tensioned so that, at the end of the stage,
the girders lifted from the temporary supports were suspended by
the stay cables;
the end pins and the provisional struts were removed, once
tensile forces started acting on the provisional strut of the mast
(compression forces in the provisional phase);
the slabs of the two decks were concreted after checking and
adjusting the forces acting on the cables.
Finally, the paving and the finishing were realized.
ConclusionsA long span footbridge, characterized by high
slenderness and an unusual configuration with two separated decks
has been realized at the harbour of the City of Pescara.
Accurate studies of the bridge, carried on during the final
design stage, suggested bringing some variations to the preliminary
design in order to optimize its structural behaviour under wind
loading, mainly due to the high deformability of the structure and
some concerns regarding its aerodynamic behaviour. The original
suspension bridge type solution has been changed to a cable stayed
one, improving the structural response and at the same time keeping
the aesthetic value of the bridge unaltered.
Fig. 7: Connection between the two girders and the anchor
pier
Fig. 9: Realization on provisional supports of the two steel
decks
The erection procedure, which ended in July 2009, consisted of
the following phases (Figs. 911):
the concrete piers of the viaduct were realized, based on
drilled piles of 1200 mm diameter;
the prestressed concrete girders forming the viaducts were built
by means of a span-by-span process;
the side piers, made of steel tubes were then erected, fi lled
with concrete and vertically prestressed;
the steel girders of the cable-stayed bridge were then assembled
at ground level, in elements about 35 m long and completed with the
corrugated steel sheeting;
a set of temporary piers were then built; the girder segments
were
Fig. 8: Pylon view (Units: mm)
+3,10
+49,00
10461Pylon view
The main bridge is then formed by two curved trussed girders
suspended over their entire length of 172 and 147 m. Structural
details are also given in Ref. [4].
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Fig. 10: The completed steel mast and an aerial view of the deck
before concreting
References
[1] Von Karman T. Aerodynamics. Cornell Uni-versity Press:
Itaca, NY, 1956.
[2] Theodorsen T. General theory of aerody-namic instability and
the mechanism of flutter. NACA Report No. 496, 1935.
[3] Scanlan RH, Tomko JJ. Airfoil and bridge deck flutter
derivatives. J. Eng. Mech. Div. ASCE 1971; 97 17171737
[4] de Miranda M. Il Ponte Del Mare a Pescara. Convegno Collegio
Tecnici dellAcciaio, CTA: Padova, 2009.
SEI Data BlockSEI Data Block
Owner:Comune di Pescara
Architectural design: Walter Pichler, Bolzano, Italy
Structural design:Studio De Miranda Associati, Milano, Italy;
Prof. Ing. Mario De Miranda/Ing. Alessandro De Palma
Foundation design:ng. Brun ianco
Supervision/Engineer: ng. Nicola Di Mascio/Ing. Luciano Di
Biase
Static testing:Prof. Enzo Siviero
Static testing assistant:Dr. Civ. Eng. Alberto Zanchettin
Contractors:Mospeca S.r.l./Angelo De Cesaris S.r.l./Solisonda
S.r.l.
Structural steelwork including cables (t): 480
Concrete including piles (m3): 5170
Total bridge cost (EUR million): 6,3
Service date: November 2009
Fig. 11: View of the bridge from the harbour of Pescara
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