For visitors to the water and sustainable development themed, International Expo Zaragoza, 2008 in Spain, the journey to the fantastic site housing 5,000 shows, 2,000 experts and 140 pavilions will be made even more exciting, thanks to the completion of the Third Millennium Bridge.
Spanning the Ebro River in the Aragon region in the north east of Spain the 34m euro ‘Third Millennium Bridge’ is the Worlds largest suspended concrete arch bridge. Built by Spanish construction specialist, Dragados, the bridge construction benefited from engineering expertise and the use of a range of equipment, including heavy duty 750kN Megashor shoring, from formwork and falsework specialist RMD Kwikform Ibérica.
Designed to carry six traffic lanes, two bicycle lanes and two pedestrian lanes across a deck width of 43m, the impressive concrete arch of the bridge reaches up to 36m from the deck and weighs some 5,000 Tm. Connected to the 24,000 Tm composite precast and cast in-situ deck, each of the 64 cables supporting the deck from the arch is charged with withstanding tensions of up to 300 Tm.
Having previously completed a number of projects for Dragados, including formwork and falsework for the Basagoiti Viaduct in the Basque Country and a specialist Façade retention for Las Arenas Bullring in Barcelona, RMD Kwikform Ibérica were awarded the contract to design, supply, erect and dismantle the formwork and falsework system for the bridge in Zaragoza.
With ground works and site preparation beginning in September 2005, the first of 1,700 tons of RMD Kwikform Ibérica equipment required to create the structure, was brought onto site in August 2007. Work then commenced on the nine month formwork and falsework programme that would see RMD Kwikform Ibérica hand over the completed bridge an impressive two weeks before schedule.
Being such an important and iconic structure, representing the gateway to the Expo Zaragoza 2008, which commenced on the 14th June 2008 running until 14th September 2008, it was critical that RMD Kwikform Ibérica completed the project within the defined programme time. With a team of engineers dedicated to the design process, work on the formwork and falsework for the main design of the bridge took over ten weeks of engineering time to complete.
Outside the tight programme time and challenge of constructing the Worlds largest suspended concrete arch bridge, it was the load tolerances demanded by the client and location of the bridge that proved to be the most important considerations in the overall design. Alberto Aldama, one of the RMD Kwikform engineers that worked on the project explained: “The design of the main structure had to have the minimum batter or displacement because of the very low load tolerances permitted by the client. This meant we had to look at the equipment we had available and design special components in order to meet the tolerances required.
“We also had to consider how these tolerances would be affected by the considerable wind force that would be placed on the bridge. This wind force was a direct result of where the bridge is located. In fact because the El Bierzo valley is in close proximity to the project, we had to contend with what can only be considered as a naturally made wind tunnel that is capable of generating enormous forces and pressures directly onto the bridge structure itself.
“So not only did we have to ensure that the final construction of the bridge was able to withstand these pressures, we also had to make sure our formwork and falsework design would not only hold up against the wind, but would protect the bridge throughout the construction phases. Most importantly we had to make sure that all of the individuals working on the site were kept safe and that any heavy crane work was carried out when wind strengths were safe to operate”.
With the RMD Kwikform Ibérica engineering team focused on developing a low tolerance high strength system, the design process was further challenged by the need to put the cables into place before concreting of the keystone of the arch was completed and the falsework removed. In order to achieve this additional requirement, the falsework system design had to be altered to accommodate the partial removal of equipment in set areas, without affecting the structural integrity of the bridge and falsework system. This partial removal meant that access to the cables within a reduced space was possible with the design allowing access for the machinery required to make sure the cables could to be worked through, without affecting the main structure. Once this process, which involved cable placement every six metres, was completed the equipment was then able to be dismantled.
With all of the issues and challenges taken into consideration at the design process, the main formwork and falsework structure itself was based around exploiting the strength of RMD Kwikform Ibérica’s Megashor, heavy duty shoring system. With a supporting base built further out from the left bank of the Ebro River used to help support the construction of the main deck, the deck and support piers were built for the whole of the bridge prior to the arrival of RMD Kwikform Ibérica equipment.
With the design calling for two main supporting ‘legs’ curving up to meet in an upturned V shaped support for the main arch, at each end of the bridge, the first shoring and formwork challenge was to concrete these parts of the structure. Using a combination of Megashor, steel beams and Kwikstage shoring, the formwork and falsework support for the four ‘legs’ was erected, with the legs poured in four main curved sections climbing up towards the arch. At the same time Megashor towers were erected on the main deck as well as the standard steel beams for the gantries at each end.
Alberto: “Once we reached erection stage the project was all about achieving maximum productivity. This is why we simultaneously erected the ‘leg’ support and commenced pouring, whilst also erecting the Megashor towers for the main arch. In total there were twelve separate Megashor tower arrangements constructed at a total of six different heights. Due to the positioning of the deck support piers, each tower was placed at six metre intervals in order for the formwork structure to have enough support to be fixed into position. This was due to the fragility of the hollow composite precast and cast in-situ deck, which meant that the arch had to be was cast in-situ in 12 metre sections.
“In order to ensure that we were able to meet our tight tolerances and deal with the loading of each leg, the design required some of the Megashor towers to be double legged. To achieve this we designed and manufactured some special Megashor plate component parts. These parts were used to join the pairs of feet of the Megashor towers to RMD Kwikform Ibérica’s steel 300 beams, and also the brace plates for two feet.”
“Whilst the ‘legs’ were being cast, Kwikstage shoring was mounted on top of the Megashor towers, to support the main formwork used to cast the arch. This formwork was constructed using a combination of steel Superslim Soldiers and T200 beams, to which high-grade plywood was attached to support the concrete and enable the final finish to meet the customer’s expectations.”
With tolerances and stresses varying across the arch structure, special reinforced steel beams, anchor parts and wooded supports were manufactured to support the main formwork and falsework system. Alberto: “The area of the connection between the ‘legs’ and the arch required extra support so we designed these specialist parts and used the anchors to join the Superslim belts or purlins as they are commonly known, to the concrete in each concreting phase. This was particularly important as because we were using self-compacting concrete we had to decrease the pressures caused by this material in order to meet the tolerances required.”
With the overall design based around Megashor towers and gantries on top of which were placed standard steel beams, the loads produced by the concrete were able to be transmitted solely onto the diaphragms of the decking. By allowing the placement of Kwikstage shoring onto the towers, RMD Kwikform Ibérica were able to give shape to the arch perfectly using the flexibility of the Kwikstage shoring to obtain different heights quickly and effectively.
In order to transmit the enormous pressures that were produced due to the inclination and the fluid state of the concrete, on top of the shoring RMD Kwikform Ibérica placed longitudinal purlins, which in turn had formwork placed on top to form the complicated shape of the arch. Alberto: “The formwork had to be able to support a lot of pressure since the concrete used was white self compacting concrete. There was also an added complication in the fact that we could not use chamfers because all the edges of the arch had to be perfect, including the connection between arch and “legs” with an infinity of surfaces of different shapes.
We also had to take into account the fact that steel slabs had to be embedded lengthwise and transversally into the concrete every 6m for the posterior attachment of the cables all of which factors were incorporated into the overall design.
The final challenge for the RMD Kwikform Ibérica was dismantling the equipment safely and effectively. Alberto: “In order to remove equipment we designed and erected a variety of support frames, which enabled us to remove the formwork, shoring and Megashor towers without interfering with the suspension cables once they had been placed every 6 metres. It is this forward thinking through the planning stages that allowed us to deliver the project two weeks before deadline, spanning the third millennium bridge in time for Spain’s largest ever Expo to take to the water.”