Although the structure of each bridge is a classic arch, the unusual construction method made it possible to build the bridge with a reduced budget in nearly half the time it would have taken using traditional methods.

The bridges are part of the new Cantábrico-La Meseta motorway, which is a new radial connection between Madrid and the coast. The two bridges are owned by the Spanish Department of Public Works; the main contractor for the work is Sacyr and the structure has been subcontracted to Estructuras y Montajes de Prefabricados de Puentes. The structural design has been developed by Pondio Ingenieros, working for subcontractor Estructuras to optimise the construction process.

What sets these bridges apart from others of similar characteristics is the way in which they are being built. This process was developed around three basic criteria. The contractor wanted to keep the cost of auxiliary elements as low as possible, minimise the construction time, and optimise the amount of materials used.

It is obvious that building the arch and the deck at the same time, rather than in succession, will reduce the amount of auxiliary equipment required and will also save time. However, this requires the use of cantilevered formwork and a launching gantry at the same time - the former for the arch and the latter for the deck construction.

In order to solve this, Pondio Ingenieros considered eliminating at least one of these two launching elements by using precast materials for both the arch and the deck. The advantage of this solution is that both structural elements can be built simultaneously by using a single customised launching gantry.

The only special construction equipment used to raise the bridge is a specially designed cantilever falsework with trolleys. This system erects both the precast elements of the arch and the precast girders of the superstructure. Both types of elements are picked up on the rear part of the falsework by the two trolleys.

However this process complicates the calculations that are needed for the structure. If precast segments are used for the arch, tensile stresses for the combined forces of bending and compression in every section of the arch must be avoided. This happens throughout the whole line of the arch with the exception of the springing and the crown. In the springing, the hogging moments are supported by the upper reinforcement of the cast-in place slab. In the crown the sagging moments are supported by four Gewi bars joining the bottom flange of the arch.

When using precast beams for the deck, a centred prestressing that can transmit the tensile forces to the abutment must be used. In order to reduce material costs and save work at a later stage, the deck has been designed in such a way that this prestressing is the same as the final prestressing for the whole structure.

One final consideration was to minimise the weight of the structure as a whole during the works, so that the prestressing needed during construction was compatible with the permanent prestressing. For this reason the cantilever wings of the deck are not concreted until the arch has been completed. This also substantially reduces the forces that anchor the abutments to the rock, to 1760t per abutment. These forces are transmitted using 16 soil anchors each 110t capacity per abutment.

The viaduct consists of two separate arch bridges, each of them with a width of 11.7m. It has 13 spans which range from 13m to 18.6m and the total length of the bridge is 240m. The total length of the arch is 141m and its rise is 32m with the profile of the arch following a second order parabola. The arch is formed of a rectangular hollow cross-section which is 5.5m wide and whose height varies from 2.6m at the base to 1.8m at the top of the arch. The method of construction involves the simultaneous advance of arch, piers and superstructure. In order to achieve this, the advancing construction is supported by a triangulated system of tensile bars.

Each semi-arch is formed of 30 segments each 2.5m long except for the root segment at the springing, and a 3m-long central segment for the crown cantilever adjustment. Placing and levelling the root segment is one of the most difficult tasks, requiring accurate placement in order to guarantee that the arch is aligned correctly.

The cross-section for each segment has a U-shape and is connected to the previous one by means of a prestressed bar at the top of the webs. On the front of each segment there are two shear keys per web and the segments are glued using epoxy resin.

Each semi-arch has four 18.6m-long spans, which means that each span consists of approximately nine segments.

The construction process consists of assembling three segments and one provisional stay which consists of two 36mm diameter prestressed bars. The bottom ribs between segments are then concreted, in order to control the deviation force in the bottom and upper slab of the section. This process takes four days and progresses the work by 7.5m.

This cycle must be repeated twice before a pier is reached. Then the diaphragm of the pier must be concreted and the main provisional stays of the triangulated truss must be put in place. Each stay is made out of two steel profiles HEB-450.

Then the columns are cast in place using 4m-long climbing formwork. The duration of this stage depends on the height of the columns. Before launching the deck beams, the line of the arch is calculated topographically, and the stress of the stay is adjusted by using a jack placed on the deck of the bridge. This is another advantage of the method, since it involves only one stop for topographic control per span.

The deck beam is a precast box girder with a height of 1.1m and a width of 4m at the bottom. It carries 18 prestressed cables of 15mm diameter that can support the self-weight of the beam and the slab between webs.

The thickness of the top slab is 350mm between webs and 200mm at the edges.

As mentioned above, during the construction of the arch, only the section of the slab between the webs of the beam needs concreting.

The total height of the slab is 1.45m which gives a slenderness ratio of 1/12.8. This ratio allows for use of a centred prestressing that can cope with the bending moments caused by the service loads. This centred prestressing forms the top tensile chord of the triangulated structure during construction.

Given the thinness of the slab, the idea of using centred prestressing followed by parabolic layout prestressing was rejected. In any case, although at first glance the slab does not look slender, this has not been translated into an excessive use of concrete. In fact the total area is only 5.4m2 which means a total use of 0.47m3 per square metre.

Two types of cables have been used for the prestressing: top cables embedded in the upper slab of the deck, with a straight layout and joined by couplers on each joint of the beam, and bottom cables, with an extra-dosed prestressing on a polyethylene sheet overlapped on every joint of the beam.

The beams are connected by a cast-in-place mortar joint 100mm wide; this is located at 1/10 of the length of the span so that it does not interfere with erection of the segments of the arch.

The next task is to concrete the deck cantilevers, which is carried out using a 12m-long formwork which has a four day turnaround process.

All the systems used during the construction - the customised launching gantry, the jacking systems for stays and the formwork for the different structural elements were developed by Estructuras y Montaje de Prefabricados. Checking and design of all of the elements was carried out by Pondio Ingenieros, which was also responsible for all of the structural calculations for the bridge and for supervision during the stages of stress control in the stays and introduction of axial force in the

crown.

The system described above is very innovative in the way it jointly uses precast segments for the arch and precast elements for the deck. The construction of the arches is proving to be a success with extremely short delivery times.

Construction of the bridge started in November 2003, with abutments, soil anchors, the access road and so on. The first arch was finished in May 2004, and the second is currently under construction with completion programmed for the end of this year.

Michael Muller works for Pondio Ingenieros

Printer friendly version

Email this article to a friend