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Civil Engineering

Beautiful, safe and cheap

New construction techniques and adequate chemical treatment turn wooden bridges into a good option for concrete

CARLITO CALL JUNIOR/ EESC/USPWooden bridges are back. Now they are more efficient, durable and beautiful, capable of competing with concrete bridges on equal footing regarding security, uniting extensions of up to 30 meters, in towns and on roads. Wood has always been the material most widely used for the construction of Brazilian bridges, assisting in the settling of the national territory by shortening the distances to cross rivers and ravines. But over the years they passed through a process of deterioration and have been substituted many because of the use of inadequately treated wood, bowing out to structures of iron and concrete.

A course that has begun to change with new construction techniques and special actions that have been demonstrated by researchers from the Woods and Wooden Structures Laboratory (Lamem) of the School of Engineering at São Carlos (EESC), part of the University of Sao Paulo (USP). The new construction systems join techniques brought from other countries and original raw material from reforestation in Brazil, especially treated to confront the local conditions of humidity and attacks from insects and fungi.

Also, one of the advantages recovered and confirmed by the researchers is the low cost of these constructions. They can be built at a
cost of between R$ 300 and R$ 600 per square meter (m2), whilst concrete bridges run between R$ 1,000 and R$ 1,400. The new wooden bridges support loads identical to similar concrete bridges, and in many cases, are a prettier landscape to the environment.

In order to demonstrate all of these qualities and to expand this type of construction, seven bridges have been built under the supervision of the researchers, in some town in the interior of the states of Sao Paulo, Minas Gerais and Goiás, with the transfer of the technology to engineers from the Highways Department (DER) of the Secretary of Transport of the State of São Paulo and to the municipal engineers by way of training courses.

The first of them in an urban setting was built a year ago in São Carlos over the stream named Monjolinho. The most recent on the coastal escarpment leading down to the Atlantic Ocean named the Caminho do Mar ( Sea Way), the first road that linked São Paulo to the port of Santos.

“We’re showing that wood competes very well with other materials in characteristics of weight, resistance and durability, independent from the size of the spans that the bridge covers or loads that it’s supposed to support”, explains Carlito Calil Júnior, the research coordinator at Lamem. He also highlighted the environmental benefits. “The use of wood has a renewable character, contrary to steel and cement, which also demand large quantities of energy in their production. The cultivation of wood by reforestation also serves to trap and store carbon in the atmosphere, which occurs intensely during the growth of the tree until its adult age.”

Traffic and loads
The new systems allow the construction of bridges with beams or laminas or even a mixture that combines wood with a covering of concrete. All of them could be used in towns, municipal roads, state or federal highways, including those with heavy traffic, without restrictions in terms of load, because they are projected according to the general norms laid down for steel and safety structures by the Brazilian Technical Norms Association (ABNT in the Portuguese acronym) the new systems that of prestress (application of stress) is the most improved, with sheets of wood forming a single platform, covered over with rigid bars of a specially ultra-resistant steel (a technique called dywidag) or by stay cables, a grouping of various cable wires also of high resistance.

The technique consists of making holes in the wood every meter for the insertion of steel bars or steel cables, which are tensioned and have their force controlled, guaranteeing that the wooden platform gains transversal rigidity (in the direction of the length of the bridge).

In order to protect the set, both the bars and the steel cables are protected with a protective sheath and covered with grease so as to avoid any possible corrosion brought about by the contact of the steel with the chemical products used in the prior treatment of the wood. This chemical protection is fundamental for the long life of the bridges. It is carried out using a water soluble solution of copper, chromium and arsenic salts (CCA) and another that uses boron in place of arsenic (CCB), freeing the wood from the action of fungi and insects, and preserving it for a lifespan greater than thirty years, as against five years for untreated wood.

Developed in Canada during the 70’s in order to reinforce wooden laminated bridges already in existence, the protection system began to be used as well in the United States and afterwards in Brazil. Here pinus and eucalyptus woods were used, being treated for local conditions of humidity and temperature. In São Carlos, the studies on wooden bridges using this system began in 1992, six years after the building of the first bridge of this type by the Forest Products Laboratory (FPL) in Madison, the United States, where Calil Júnior accompanied the evaluation tests for structural loads, as well as the variations in humidity and temperature.

After these technical studies and with a thematic project funded by FAPESP that began in 2001, in São Carlos three species of eucalyptus trees (Eucaliptus grandis, E. citriodora and E. saligna) and two pine trees (Pinus taeda and P. elliottii) were evaluated for reforesting and equally treated. “The pinus used in North America is denser and hence more resistant that the Brazilian species. With the eucalyptus the reverse occurred. Here it is much more resistant, although with the system of prestress there is no need for a wood of such high resistance because both the steel and the transverse cable reinforce its solidity”, Calil Júnior explains.

In the prestress system the weight of the load is distributed through all of the extension of the bridge. Nevertheless, for spans of up to 10 meters it is convenient that the bridge be constructed in wooden plates in sawed parts or glued sheets, always forming a single structure. In the case of spans of between 10 to 20 meters, the bridges must be made in a system of plates with spaces every meter made up of laminas of length three times greater than the others in the structure. Then for spans greater than 20 meters, a box system is made use of, the system consisting of two plates overlaid upon two planes, one above and the other below.

These plates must also be linked to each other by sheets of greater width every meter, forming tunnels by the upper and lower parts of the structure. In Brazil, the first two using this system will be built on USP’s campus No. 2 at São Carlos, one with glued laminated wood and the other with parts composed of glued laminated plywood. Another already completed structure in this town is a curved walkway constructed inside USP, linking the Wood Laboratory to the Structures Engineering Department, in which the technology of the laminated wood plates with prestress, and using reforested pinus, was applied.

A concrete cover
Up until this moment in time, the majority of the bridges built, such as that of the Caminho do Mar, are of a mixed type with wood and concrete because these are more easily built and have a lower cost. This system uses cylindrical logs (trunks of treated trees) in the lower part with metallic connections in bars of common steel thrust into the wood and held in place by concrete, which also covers the structure that will afterwards receive a coating of asphalt. The option for the cylindrical trunks occurs due to the make up of the pieces and their cost, twice as cheap as that of sawed wood. “In this case an important point is the development of the twisted fibers during the natural growth of the tree. However, this characteristic, together with the rigidity, is lost when the wood is sawed”, Calil explains.

Another possibility is the construction of bridges with treated cylindrical logs, of high resistance and low cost, and laminated truss-frame structures, linked with screws, or even with pieces of glued laminated wood, which makes use of a type of extremely adherent resin. The advantage of these glued laminates is the possibility of building bridge trusses without limitation in length and with total control of the material, including extensions beyond 30 meters. However, they represent costs considered to be high, around R$ 2,000 per cubic meter (m3 ) as against R$ 300 for treated cylindrical logs.

Calil recognizes that the wooden structures still suffer from prejudice related to the long use of wood without adequate treatment that would guarantee the long lives of the bridges. This factor, which still generates wariness on the part of builders, added to the lack of skilled labor, blocks the dissemination of this type of structure. A turn around in the shortage of professionals specializing in the construction of wooden bridges was initiated with two courses administered in São Carlos by instructors from the National Service to Industry (Senai), who assisted in the formation of personnel with specific qualifications to work with this type of material. “During the lessons, we covered details raging properties of the wood to those concerning structures and construction.”

Advanced legalization
Brazil leads in the technology for developing wooden structures in South America. Besides the USP Laboratory at São Carlos, other groups are also researching the theme at the São Paulo State University (Unesp), the Polytechnic College of USP and at the State University of Campinas (Unicamp), with the goal of drawing up a specific Brazilian norm for the construction of bridges of this type, as an annex to the wooden structures norm, also formulated by the EESC and by the Polytechnic College and adopted by the ABNT in 1997.

The São Carlos researchers have also requested from the DER a survey concerning wooden bridges in the State of Sao Paulo, with the objective of offering technical support for the recovery of those in existence and for new constructions. “The idea is that this would go on to be valid in other states, for example that of Goiás, where two bridges have already been built in the municipality of Catalão, based on the studies developed here and passed on by courses and training manuals to assist the engineers in any municipality of the country”, the researcher states.

“We send off letters to all of the municipalities in the state of São Paulo, and when there is interest on the part of the prefectures, we guide the projects at no cost and supply the due recommendations on materials, constructing and maintenance procedures.” The municipality that opts for wooden bridges is only responsible for the cost of construction, while the Wood Laboratory researchers are charged with applying the technology developed at the university, including the formation of the specialized workforce. “We don’t want to build, but to teach how to build.”

The project
Emergency Program for Wooden Bridges in the State of Sao Paulo (nº 99/12583-1); Modality Thematic Project; Coordinator Carlito Calil Júnior Engineering College of Sao Carlos USP; Investment R$ 516,094.73 and US$ 112,202.55