29 Jun New Drini Bridge
General description
“Albanian Highway Concession” ltd in order to respect concessionary contract signed with MIE, where one of the most important point was to build a new bridge over Drini river, near Kukes city has started work with implementation design and after that with construction of new bridge over Drini river. This bridge as part of capital investment of concessionary contract will fulfil requirements for highway category of Milot-Morine highway, because the existing bridge over Drini river was built to fulfil only requirements for interurban road.
1.2 Design principle
The new bridge over the Drini river consist in a through arch bridge with a main span of 270 m. In this kind of bridge is a bridge with biggest span that will be constructed in Europe. The arch structure is steel and the deck structure is a composite section with main steel longitudinal beams and concrete deck slab. The choice of such type of structure was driven by the following principles:
Use piles only at the riverside and not in the midst of the river, maximizing the freeboard and reducing concrete elements that could be subjected to erosion at foundations.
Architectural integration with the landscape of Kukes.
With respect to other typical structural schemes for bridges, the arch bridge offers the following advantages:
o Minimize the number of piers required (a common composite girder bridge would not be cost-effective on a span of 270 m and would require more piers inside the river).
o The arch is an architectural shape that harmonically fit in the surrounding context avoiding tall structures that would affect the landscape in a more prominent way such as the antenna of a cable stayed bridge.
The construction process of the arch bridge will use the available areas for pre-assembly and will then be transported with a barge to its final position (see also the report of the construction phases): this process will minimize the erection time and interference with the nearby infrastructures. This will also allow to maximize the prefabrication of components ensuring high quality standards.
The whole bridge will be fabricated with weathering steel which will be self protected from corrosion without any additional painting. This choice of material aims to:
Reduce the maintenance costs and ensure durability;
A better insertion of the bridge in the landscape thanks to the colour and texture of weathering steel.
1.3 Description of the structure
The new bridge over the Drini river, near Kukes, is composed of three consecutive spans of length respectively 20+270+20 m.
The main bridge structure of total width equal to 23.213 m, is comprehensive of the road track of width 19.690 m and two side platforms of width 1.762 m.
The external longitudinal spans, are continuous with respect to the main girders, and have a length of 20 m; they are made up of a composed section with steel beams connected to a reinforced concrete slab.
The composition of the bridge allows to define the central span as a suspension inferior-deck arch bridge, where the deck fulfil the function of arch’s tie beam. The base length assumed for the arch calculation is equal to 270.0m.
The deck’s structural elements of the central span is a composite section with two H-section’s welded beam connected through welded studs to a concrete deck with a total thickness of 25 cm.
The transverse elements (diaphragms) consist in a truss composed of bolted angles. The suspension hangers of the bridge deck are connected at the ends of the diaphragms.
The arch structural elements is a box section with three different dimensional shape: the central part of the arch has a rectangular box section of width 5.0 m, height 2.9 m, variable thickness from 25 mm to 40 mm; the external portions of the arch are divided in a steel fork system, whose dimensions are B*H=4.0*2.9m and variable thickness from 20 mm to 40 mm.
The composite deck is endowed with a pre-stressing system that is composed by two cables (37 wires each) per girder. The compression provided by the cables ensure that the concrete slab is uncracked and will fully provide its additional stiffness and resistant area to the composite tie-deck.
The action introduced in the system by the cable, allow to reduce the stress in the principal longitudinal beams and maintain the slab in a compression state for all the design combinations: this specific procedure allow to reduce the slab stresses due to the variable action avoiding transversal crack in the deck.
In addition, the concrete slab is endowed with its own pre-stressing system. The slab will be made of prefabricated elements. After the installation of the slab elements on the deck, the slab will be prestressed with post-tensioning cables uniformly distributed inside the slab: in this operation, only the slab will be stressed and no stresses will be added to the steel deck. Next, concrete will be poured to solidarize the slab elements end the deck all together.