22 February 2012
A new lifting bridge is planned for the Basque city of Bilbao. Juan José Arenas de Pablo, Guillermo Capellán Miguel and Miguel Sacristán Montesinos report on its design.
Construction of a new movable bridge in the city of Bilbao in northern Spain is expected to begin next year, with detailed design now completed and the procurement process expected to conclude this year. Basque rail infrastructure administrator Euskal Trenbide Sarea commissioned the design of the bridge from Spanish consultant Arenas & Asociados as the result of a design competition which was held last year, and intends to invite BOT tenders this year for the concession to build the bridge.
Bilbao’s new tram line will link the suburbs of Leioa and Urbinaga and the new lifting bridge, which will have a 161.1m-long main span, will carry it across the estuary of the Nervión River. The new line, the fifth in the system, will create a vital link between the two sides of the estuary, connecting densely populated areas which are currently divided, and also providing access to the university campus.
The new lifting bridge will be built approximately 2km upstream of the historic transporter bridge Puente Colgante. This structure was built in 1893 and is protected as a World Heritage Site. Construction of the new bridge is expected to start within a year under a concession agreement which is due to be negotiated this year. The tram line project is led by ETS, with the engineering consulting services of Fulcrum and Arenas & Asociados.
Transporting the new Leioa-Urbinaga Tram line across the Nervión River involves the construction of a large viaduct 1.39km long, which includes a movable bridge over the navigation channel of the estuary as the main structure. This same structure will also incorporate pedestrian and cycle path access between the two banks of the river, with a total deck width of 12m; a 3m-wide footway on the west, tram lines of 6m wide and the east footway and bike path which is also 3m wide.
One major consideration in the design of the new bridge was that it should not impact on maritime traffic; hence the incorporation of a movable span that can be raised for passage of large vessels. The vertical clearance required for the bridge must be greater than 48m, surpassing that available in the existing transporter bridge downstream.
On the other hand, however, it was considered necessary to minimise the number of openings so that the disruption to trams could be reduced and the smooth flow of regular shipping could be assured. To achieve this, the main deck was designed with a vertical clearance of 21m in the closed position. It is this high-level crossing and the need to cross over various road junctions, which led to the design of such a long viaduct. With the 21m-high navigation clearance in its closed position, the bridge is expected to have to open just 20 to 40 times in a year.
The 161.1m-long span, along with the need for a vertical clearance of up to 48m, came about after a detailed alternative analysis which resulted in a lifting span being chosen as the most suitable for this structure. The reasons are that this type of bridge is best suited to large spans as it operates with the same static scheme in the closed and open position; it is less influenced by wind forces during its operation than other types of movable structures, and it is less dependent for its structural performance on mechanical or moving parts, since in the closed position it operates as a fixed structure, which increases its reliability and reduces maintenance.
The size of the lift span will make it the second longest lift span in the world after the USA’s Arthur Kill Vertical Lift Bridge in New Jersey. Steel was the obvious choice for the movable deck structure in order to reduce the weight of the main span to a minimum. The deck structure is a network arch with cable hangers inclined in two directions to increase efficiency and overall stiffness. The arch assumes a lenticular configuration integrating its formal design with the lifting towers and access viaducts.
Having an appreciable depth in the end support sections is beneficial because it allows for two levels of deck guiding devices, which ensure torsional stability against the wind during operation.
The total length of the 1.39km viaduct is made up of nine independent structures that have different deck types and materials which have been selected to address specific functional, structural and construction reasons. These structures include prestressed concrete box-girder viaducts and composite bridges, as well as the main lifting bridge.
The lifting deck moves vertically into the open position; it is 12m wide and measures 157.1m between the lifting cables anchorages at each end. It is supported in the closed position by the lifting towers, which also enable it be raised into its open position. The lifting equipment consists of upper sheaves, four sets of four cables each 82.5mm diameter that suspend the deck, counterweights which counteract the deck’s weight and two sets of four motor cables each 63.5 mm diameter which extend down to the engine room and are rolled onto the drums driven by the engines which allow operation of the bridge. The lifting system allows the deck to be raised by 27m from its closed position. The lifting towers are mainly concrete and are supported on 17 1.8m-diameter piles. The pile cap has exterior dimensions of 33m by 11.7m and the towers are 80m high; inside is the engine room which hosts all the engines drives and mobile bridge mechanisms. The counterweights are each made of ten cubes of grey iron, which weigh 30t each.
The network arch bowstring structure of the deck has an inclined mesh configuration of the hanger cables. The structure takes a lenticular form with curved edge lower girders independent from the horizontal platform and the total weight of the deck is about 1,550t. The four counterweights are each 300t, making a total weight of 1,200t; this means that there is always an imbalance towards the closed position. As a result it is not necessary to have mechanical locking elements in the support sections, and the structure will always have positive reactions in the supports, even under torsion forces generated by lateral and vertical wind.
The deck itself is designed as a system of edge girders, upper arches, hangers, and arch bracing. The total height of the lifting span in the centre of the span is 19.45m and this reduces to 9.5m at the end sections. The top bracing is formed by a set of closed profiles every 9m and K-shaped cross-bracing which gives lateral stability to the two arches. The arches themselves have diamond-shaped sections of variable depth and are arranged in inclined planes.
The edge girders also have diamond-shaped sections arranged in matching inclined planes. The edge girders are suspended from the arches by 30mm diameter locked-coil cable hangers, forming an inclined mesh in two directions, so hangers cross up to four times in the central area. The hangers are anchored every 4.5m in the edge girders, which coincides with the deck diaphragms. The inner and outer hangers are arranged in separate planes so that there is no need for any special devices for the point where the cables cross.
The deck itself consists of steel diaphragm cross-beams spaced at 4.5m centres, and rolled longitudinal support rails for the tramway, lightweight concrete decking between lanes on the tram deck, and for the footways and bike lanes. Footways and bicycle decking surface will be GRP composite material planks with a non-slip finish. The balustrades are stainless steel handrails which incorporate the functional lighting for the footways and bike lanes. The procedure for construction that is envisioned for the bridge is for the lifting deck to be built off site, then floated into position and lifted up using the bridge’s own lifting system.
Juan José Arenas de Pablo is president, Guillermo Capellán Miguel is technical director and Miguel Sacristán Montesinos is project coordinator at Arenas & Asociados.