1. GENERAL DESCRIPTION
The bridge has a total length of 1488m, with a span
distribution of 45 + 9x60 + 57 + 324 + 57 + 7x60 +
45m. After a comprehensive typological analysis, in
which cable-stayed and three-dimensional truss
alternatives were evaluated, the arch solution was
selected as the best structural approach. The
harmonic balance of span distribution, deck and arch
geometries and pier shapes provide an optimum
structural performance, and configures an
aesthetically remarkable design.
1.1. Deck
The deck is a prestressed concrete box girder with a
maximum height of 4.00 m. The upper slab is
cantilevered on both sides so that the total usable
surface is 14.00 m with variable thickness. The lower
slab is 5.00 m wide and has a thickness of 0.30 m. The
web thickness is 0.50 m, although it thickens where it
meets the upper and lower slabs.
Concrete: with 50 MPa strength is used in the
approach spans and 70 MPa concrete in the spans
over the arch. Very often the actual strength of the
poured concrete reached 100 MPa.
Five prestressing tendons, using between 25 and 37
Ø15.2 strand units are used in each web. The spans
over the arch are complemented with upper and
lower straight tendons.
Due to the length of the viaduct, a detailed study on
where to place the fixed point was undertaken. This
study showed that the arch would be able to
withstand the increase of stresses with the fixed point
placed at its center. This resulted in the typical
configuration of expansion joints in the abutments.
The team cast one span per week (launching, steel
reinforcement works, formwork, pouring, curing and
prestressing).
1.2. Arch
The form of the arch has been designed such that the
bending due to the dead loads was reduced to a
minimum. The arch axis is completely embedded in a
vertical plane.
The cross section consists of a variable rectangular
box section with chamfered corners. The width varies
between 12.00 m at the abutment and 6.00 m at the
crown. The height varies between 4.00 m and 3.50 m
with the web and slabs thickness also varying to
achieve an almost homogeneous distribution of
compressive stresses. Due to the magnitude of the
stresses, the 70 MPa concrete was used.
Because of the location of the arch in the valley
scoured by the river Tagus (Tagus means “cut” in
latin), it was necessary to carry out a study of the
effect of the wind on the structure during the
construction process. Hence, a reduced scale model
was developed and tested in a wind tunnel,
confirming that the structure was not sensitive to
wind induced instability phenomena.
1.3. Piers
The variable terrain elevation causes the piers to have
a very variable height (between 9.60 m and 71.50 m).
A basic hollow box section form is used in all of them,
3.50 m wide but with variable transverse dimension.
Fig. 1: General definition
4/2016