• Location: AP-68 Highway between Bilbao and Zaragoza; P.K. 292+120 SEE MAP
• Scope of Works: Detailed design
• Structural type: Steel tied-arch bridge, with network hanger configuration
• Construction process: Steel structure placed using cranes
• Overall length: 71,0 m
• Spans: 71,0 m
• Deck width: 8,0
• Deck maximum height: 7,0 m

In April 2009, TORROJA INGENIERÍA carried out the Principal Inspection of an overpass located in the AP-68 highway (station 292), as part of the implementation of the Bridge Management System of ABERTIS. In the inspection we detected that top part of central pier had moved, and that the girder bearings were out of position.

The following geotechnical campaign (ground electrical tomography around the central pier), detected evolutive karst pits under and around the central pier. The demolition of the old structure was recommended and performed, as an effective foundation underpinning of the pier was not possible. It was confirmed that such ground karstification was a general problem of the area around the bridge.

The design of the new overpass was undertaken taking into account a few assumptions:

The structure must be light and isostatic, without a central support at median. The weight of the structure must be transmitted to the ground over an area as wide as possible. The structure must be able to accommodate decimetric differential settlements of the supports.

Below the approach embankments, under the abutments, a buried reinforcement structure had to be designed in order to reduce ground settlement, made of a geocomposite canvas working in tension under a rigid compressed gravel cement slab.

Regarding the new deck, two parallel vertical arches have been defined, spaced 10 m apart and made of structural steel. They are 71 m long and with a 15 m rise. They are braced together at the crown, with a Ø700 mm circular section.

There is a longitudinal steel tie structure under each arch. Each tie is fixed to both ends of the arch to create a bowstring typology.

The deck slab, 20 cm thick, is made of reinforced concrete. It holds an 8,0 m wide carriageway.

The link between arch and deck ties is made with two groups of crossed suspension cables one going from left to right and the other right to left, made of fully locked high performance steel cable (Ø30 40 mm).

The two conjugated suspension cables, one from each direction, cross in an intermediate point between arch and tie. There is no constraint between the cables at the cross point. This surface made of crossed cables makes for a resistant network system.