Figure 1. The eastern half of La Pepa (Cádiz Bay) Bridge on October 29th, 2014. Photo: Dennis Smith.
Design engineer(s): J. Manterola, A. Martínez, M.A. Gil Ginés, J. Antonio Navarro, S. García, S. Vriado, B. Martínez, L. Blanco, R. González, G. Osborne, Carlos Fernández Casado S.L.
Owner: Ministerio de Fomento (Ministry of Public Works)
Contractor: Joint venture Dragados S.A. - Drace Infraestructuras
Built: 2009 – 2015
Main span: 540 meters (1772 feet)
Total length: 3092 meters (10444 feet)
Structural system: Cable stayed bridge
Social, Economic Background
Cádiz is one of the oldest cities in Europe, and throughout its history it has been important as a port. Cádiz is situated on one side of a natural harbor, and as with many traditional, older ports, the city of Cádiz has been faced with the problem of finding a quicker, less congested way to get to the other side of the bay. A new bay crossing of some sort can obviously achieve this, but the crossing of a highly used port has to meet the extra challenge of allowing large ships to pass through. To solve this problem, Sydney has the Sydney Harbor Bridge and Virginia has the ingenious Chesapeake Bay Bridge Tunnel, but Cádiz has only the José Leon de Carranza Bridge (Figure 2), which is a bascule bridge. The span of a bascule bridge is composed of one or two movable pieces that are raised and lowered by means of counterweights to allow the passage of water or road traffic, respectively. The issue with bascule bridges is that when ships are passing, land vehicles cannot, and vice versa. This causes backed up traffic, increased pollution, and decreased overall productivity and efficiency.
Figure 2. The José Leon de Carranza Bridge. Photo: Dennis Smith.
The authorities in Cádiz realized this problem and decided that Cádiz would need a new crossing in the near future. At the same time that this decision was being made, there was unregulated growth in the price of homes in Spain. This led to over-optimism, speculation, sub-prime loans, and ever-increasing debt-to-income ratios. Shortly after, the housing bubble burst, and the market came crashing down in the Global Financial Crisis. Spain, like many developed nations that had only seen a short economic boom, was in a recession. But a recession was the perfect time to start building a new bridge in Cádiz. The new construction, which was financed by the Spanish Ministry of Public Works and Transport, injected the economy with money from public spending, created jobs, resolved Cádiz’s bridge woes, and improved the overall quality of life on the Bay. The location chosen for the bridge was also important: it delivers travelers right into the center of the city, which is closer to the port and the historic old town. This is beneficial for both trade and tourism and should lead to even more economic benefits.
Structural Performance
La Pepa Bridge (see Figure 1) was designed by a team led by Javier Manterola from the firm Carlos Fernández Casado S.L. As previously mentioned, the biggest concern in crossing a port is allowing for the free passage of ships, and the design team was very cognizant of this issue. The shipping channel in the area is 400 meters wide, but was increased to 540 meters for the purpose of the design. This extra area leaves more room for navigating around the eastern (Cabezuela) Quay. The eastern tower is also situated about 70 meters out from the edge of the Cabezuela Quay to allow for easier maneuvering with the shipping cranes. Elevation and plan views are shown in Figure 3.
Figure 3. Elevation and plan views of La Pepa. Source: Carlos Fernández Casado S.L. [1].
The bridge has a total length of 3,082 m, which makes La Pepa the longest bridge of Spain. The main structure is the cable-stayed bridge that spans the shipping channel and its side spans. It has a total length of 1,180 m. The deck is a trapezoidal composite box girder with a depth of 3 m designed to be light-weight, aerodynamic and slender [1]. The main span is built using the free-cantilever construction method (see Figure 1) with special cranes that lift deck segments with a length of 20 m and an average weight of 300 tons. Upon completion, the deck, which can accommodate two automobile lanes and one train in each direction at an elevation of 69 meters, will be supported by a total of 176 stays. The stays are arranged in the semi-fan style that makes efficient use of the shortest tower height for the greatest span length. The towers have the shape of a diamond and a height of 183 m (east tower) and 186 m (west tower) [2, 3].
Figure 4. The simply supported span, ready to be lifted into place. Photo: Dennis Smith.
The approaches incorporate a variety of structural systems. A unique feature of the western approach is a 150-meter removable span. This span is a simply supported steel box girder with a cross-sectional depth that varies linearly from 3 meters at the ends to 8 meters in the middle in order to efficiently deal with the maximum moment (Figure 4). This removable span was added in response to a request from Navantia, a shipbuilding company with a factory in the bay, for a span with clearance greater than 69 meters. This clearance could not be met in a fixed structure, however the infrequent passage of very large ships allows the removable span to be an efficient solution. The western approach uses the same steel and concrete composite structure as the main span, however, the eastern approach uses pre-stressed concrete, and part of this side uses portal-frame piers instead of single columns to allow for the flow of traffic below and parallel to the bridge.
Symbolic
The bridge is significant aesthetically and symbolically not only in the local region of Cádiz, but also in all of Spain. Cádiz has been a city difficult to access for a long period of time. The diamond (double Y) shape of the towers acts as a modern gateway into the historic city of Cádiz, drawing tourists and traders alike. The introduction of modern design into the old city represents the development and renewal of the city and the promising capabilities of the port. Upon completion, the bridge will be a landmark as the longest bridge (3092 meters) in Spain.
La Pepa is an elegant work of art. The slenderness ratio of the bridge is 1/180, making the bridge a beautiful ribbon rising over the bay. The double Y shaped tower touches the outside edge of the deck, completing the tower without intruding the deck. Although there are 176 cables, they do not impede upon the transparency of the bridge, and one can see through the cables to the other side of the bay. The symmetrical main span creates a sense of regularity and harmony as shown in Figure 5.
The name of the bridge has also a special symbolism as the bridge is named after the first Spanish Constitution. The Constitution was signed in Cádiz in St. Joseph’s day and as “Constitución” is a feminine noun in Spanish, the Constitution was commonly referred to as the female version of Joseph (Josephine), nicknamed “Pepa.”
Figure 5. Rendering showing a bird's eye view showing the typology of the area and the scale of the completed bridge. Source: Carlos Fernández Casado S.L. [1].
Construction Process
The construction process is unique with La Pepa most notably because of its size. A trip to the job site showed just how large-scale the process is. So large, in fact, that the concrete used for the bridge is produced on site from large stockpiles in the site’s own storage area.
While the bridge is under construction, the bay will stay open to commercial and private boat traffic. This is especially difficult because so much of the construction of the bridge requires specialized boats for installation. One of the more impressive boats used for construction is the crane boat that installed the first few sections of deck on each tower. These boats were used before the bridge cranes could be installed on the bridge. After three 20-meter deck segments were attached to the tower (one in the middle and one on each side, for both towers), a crane was attached to each side of each deck, as seen in Figure 6 below.
Figure 6. The cranes on the deck of the nearly complete main span on October 29th, 2014. Photo: Ignacio Payá-Zaforteza
The cranes retrieve the 20-meter deck sections from boats in the water below, and then pull the pieces to meet the existing deck. Once installed, the crane is rolled forward along the deck, ready for the next piece, and the cables are installed and tensioned on the new deck section to help support the added weight. This process continues, the deck being installed on both sides of each tower as a cantilever, until the decks meet in the center of the main span and at the first piers on the approach spans. The cables are then adjusted to the proper tensions to support the finished bridge and the deck is complete.
Conclusion
La Pepa is an amazing structure designed by one of Spain’s top bridge engineers with a number of distinctive features. Its 540-meter span, the longest in Spain, makes effective use of the cable-stayed form, opening the door for more large-scale bridges in Spain. The bridge also involved some unique innovations, like the custom-made cranes for lifting deck segments into place, and the removable, simply supported span for the passage of extra-large ships. In addition, construction began in a time of crisis, but has provided economic stimulus and will revitalize the city center once complete.
References
[1] Carlos Fernández Casado S.L.. "Cádiz Bay Bridge". www.cfcsl.com. Retrieved on April 28th, 2015.
[2] J. Manterola Armisén and A. Martínez Cutillas. “Puente sobre la Bahía de Cádiz”. Ingeniería Civil 167: 1-10, 2012.
[3] J. Manterola, A. Martínez, J.A. Navarro, S. Criado, S. Fuente, M.A. Gil, L. Blanco, G. Osborne and M. Escamilla. "Bridge over the Cádiz bay, Spain”. Proceedings of the 37th Symposium of the International Association for Bridge and Structural Engineering (IABSE), 2014.