Suspended paths

Clean aesthetics and technological advantages explain growth in the number of cable-stayed bridges throughout Brazil and the world

In five years’ time, the city of São Paulo was given three cable-stayed bridges: the Octavio Frias de Oliveira Bridge, in the city’s South Zone, the Orestes Quércia Bridge in the North, and a bridge that is part of the Complexo Viário Padre Adelino, a system connecting some main arteries on the city’s East Side. Throughout Brazil, examples of cable-stayed bridges sprout in different regions. One of the forerunners in the country was the cable-stayed bridge for the Engenheiro Jamil Sabino Subway Station on the Pinheiros River, which was opened in 1999, followed by others that sought to combine technical visibility with the functionality of a bridge, such as the JK Bridge in Brasília; the João Isidoro Bridge in Teresina, Piauí; the Construtor João Alves Bridge, in Aracaju, Sergipe; and the Ponte da Integração in the state of Acre. But what factors have to be taken into account when deciding to opt for a cable-stayed bridge?

To Roberto Alves, an engineer with Figueiredo Ferraz Project Engineering which is responsible for several projects in the country, the technical aspect is the priority in making this decision. "The main technical aspect that justifies the adoption cable-staying is without a doubt the extension of the obstacle to be overcome - whether it’s a river, a valley or even an avenue. While a conventional structure can handle spans of up to 50/60 meters, a cable-stayed bridge can overcome spans of over 500 meters. Today there are already cable-stayed bridges in operation with main spans the extend beyond 1,000 m. Spans of this magnitude are typically found in bridges over shipping channels which, in view of the growth in the dimensions of large freighters, demand a very safe horizontal clearance (distance between supporting structures)," the engineer points out.

Along with his colleagues Roberto Romani and Oswaldo Luis Gonçalves, Roberto Alves points out that the main difference and distinction between the two concepts is their structural behavior. "While a conventional structure overcomes the span between supports by allowing the deck beams to flex, in a cable-stayed structure these beams will overcome the span while practically hanging from stays attached to the tops of the masts and they are, therefore, very ‘slim’ due to the proximity of the staying cables," adds Roberto Romani.

Since its decks are thinner, the cable-stayed alternative winds up also being very competitive for urban works, enabling the execution of large lengths without compromising road clearance (the height left below the deck for vehicles passing under it). A conventional structure, which has a greater thickness of deck to overcome the same span, would have to be raised to offset the restriction on height, adds Romani.

In such cases where both alternatives are feasible, they point out, the cable-stayed bridge will always be more expensive, although economics is not always the sole criterion for making the choice, the experts warn. With the technical command of project design and construction of cable-stayed bridges, designers felt confidence in designing them although, in some cases, certain solutions that appear may seem rather ‘forced’, some say. One must also consider that the complexity of the present-day road projects, especially in large cities, justifies the boldness of such solutions.

In terms of maintenance, the technical group tells us that each project has its own particular characteristics, but regardless of the structural system designed, a preventive maintenance program is fundamental and plays a key role in the preservation of the work, prolonging its life and avoiding the high costs that a larger intervention (which could have been avoided) would bring.

"The good news is that the federal government will invest R$ 5.8 billion (US$ 3.31 Bn) by 2018 in a program for the recovery of 2,500 bridges and viaducts along federal highways in Brazil. The Program for Rehabilitation of Special Structures (Proarte) started when it was found that approximately 500 bridges or viaducts are in derelict condition and require immediate intervention. Without a doubt, it is a matter of political will," stresses Roberto Alves

A cable-stayed project, due to its aesthetics, has a much stronger architectural appeal although this should not be the only factor in determining its choice. "The technical and economic aspects should also be considered," they stress. They believe that it is very difficult to expect a conventional bridge to become an icon, whether from an architectural or even engineering standpoint. But they emphasize that construction of projects that use the system of stays, like any other distinctive work, contributes to the emergence of new construction methodologies, to the technological development of the steel, of concretes, stays, anchors and to the emergence of powerful structural engineering software.

To Roberto Romani, the design of a viaduct is always the result of its inclusion in the road project. The more complex the road system tends to be, the greater the complexity of the structure, hence the importance of the participation of an architect in this integration. In the most recent major works such a concern is already clear.

Roberto Alves underscores a few of the projects developed by Figueiredo Ferraz as examples of good architecture in the area of bridges, namely: the Third Bridge of Vitória,  F.Ferraz/Usiminas (1985); Rodovia dos Imigrantes Descending Lanes, F.Ferraz (2002); II Bridge over the Orinoco River, F.Ferraz/Consórcio Brave (2006); besides the Octavio Frias de Oliveira Bridge, Marginal de Pinheiros (SP), Projeto Enescil (2008) and Millau Viaduct in France, “this one, without a shadow of a doubt, the largest of all; a true ‘work of art’, perfect in every way. In my opinion, nothing the likes of it will be made in the next 50 years”, concludes Alves.

Ponte do Saber is the newest bridge of this type 
On February 17 this year, the newest member of the family of cable-stayed bridges in Brazil was opened to traffic: the Ponte do Saber (or Bridge of Knowledge) connecting the Campus of the Federal University of Rio de Janeiro - UFRJ (aka Cidade Universitária or University City), on the Ilha do Fundão, to the city’s ‘Linha Vermelha’ (Red Expressway) in the direction of Rio’s downtown area, North Zone of Rio de Janeiro. Built over the ‘Canal do Fundão’ channel, with a total length of 780 meters and two lanes with 4.5 meters in width each, this bridge has improved the traffic of Rio’s Cidade Universitária. About 1,800 vehicles use the bridge during rush hours, which means a 60% reduction in the number of vehicles that used to circulate at the exit of the Cidade Universitária.

Designed by architect Alexandre Chan, the bridge is based on the initial studies carried out by Prof. Francisco Lopes - professor of the Civil Engineering Program at COPPE (The Alberto Luiz Coimbra Institute for Graduate Studies and Research in Engineering), and by architect Ivan Ferreira Carmo - current mayor of the Cidade Universitária.

The bridge is suspended by fifteen front stays and six back stays.  The metal sustaining cables (stays) are anchored to a single pillar or mast. The structure, with 100 meters in height, sustains the deck over a free span of 180 meters.

The cable-stayed bridge is one of the stages of the Environmental Recovery Program for the Canal do Fundão which is part of the specified duties of the State Government for the 2016 Olympic Games. With R$ 321 million (US$ 183.43 million) in funding from Petrobras, the interventions include the dredging of 3.6 million cubic meters of sediment from the Canal do Fundão, reurbanization of the Residential Village at the UFRJ, strengthening the pillars of the Linha Vermelha and replanting of mangroves, besides the construction of the cable-stayed bridge for which R$ 69 million (US$ 39.43 million) were allocated from the total R$ 321 million in the Program.

A giant on the Rio Negro 
With 3,595 meters in length, the grand and imposing bridge over the Rio Negro, linking the city of Manaus to Iranduba, in the state of Amazonas, is the largest bridge in Brazil over fresh water. Designed to promote the integration of the Metropolitan Region of Manaus, it is the world’s second largest in this segment losing only to Ciudad Bolivar in Venezuela where the bridge that crosses the Orinoco River and is 3,600 meters long.

The bridge was built by the Rio Negro Consortium comprised of the Camargo Corrêa and Construbase construction companies. In its entire structure, with 73 spans, the consortium placed 246 excavated piles and 213 precast beams. The construction of the submerged part of the bridge consumed a volume of concrete equal to that of Rio de Janeiro’s Maracanã Stadium. The same volume of concrete was used in building the part over the river.

The extension of the cable-stayed section in the central span is 400 meters divided into two 200 m sections with a clearance of 55 meters in height, one on each side of the main mast which was designed to hold the stays. This mast, in turn, is 185 meters tall from the surface of the water (equivalent to the height of a 60-storey building). This central section is supported by 104 diamond-shaped stays.

Regardless of the flooding or ebbing of the river, the clearance at the central span was designed to allow the traffic of ocean liners; large ships that, by passing under the bridge, can reach the archipelago of Anavilhanas or any other destination without concern. This clearance height could have been much smaller, significantly reducing the cost of the construction, but by order of the government of Amazonas state, it was maintained to ensure full navigability toward the headwaters of the river. At ebb tide, the clearance increases. If the river ebbs 10 m, for example, the height of the bridge will increase to 65 meters.

A person who stands at the highest point of the main mast will have a very wide view of the local landscape and can discern, on one side, the Anavilhanas Archipelago - an ecological haven 50 km from Manaus and, on the other, the ‘Meeting of the Waters’ between the Rio Negro and Rio Solimões, downstream of the bridge.

The total width of the bridge is 20.70 meters where we have four lanes of traffic - two in each direction - in addition to a walk for pedestrians on both sides of the lanes of traffic. The width is slightly greater in the cable-stayed section - around 22.60 m - due to the placement of the stays.

Points of access to the bridge, both on the Manaus side as well as in Iranduba, were chosen to afford minimal access-ramp slope of no more than 3.01%. This allows the traffic of cars and cargoes of any size.

More than a work of engineering, the bridge over the Rio Negro is recognized as a gateway to the future and a promoter of economic and social development. With it, the 30 neighboring municipalities around the state capital are now able to share in all the benefits that derive from the Zona Franca de Manaus (Free Trade Zone) project, promoting the generation of employment and income for thousands of people. Prior to the construction of the bridge, the area of the Pólo Industrial de Manaus (PIM - Industrial Hub of Manaus) was 10,000 km2. With this expansion, the territory grew to 101,000 km2. The great benefit for companies is that they now have the opportunity to establish themselves in these municipalities and receive the same tax incentives granted to those established in the city of Manaus.