Gabinete Técnico da

Agrupamento Complementar de Empresas integrada no

  Apresento de seguida um artigo do Engenheiro Luso-Americano José Jardim

OCTOBER 1998    CIVIL ENGINEERING

BRIDGE WITHIN    A      BRIDGE

The new white suspenders are    located 11.5 m from each existing   red suspender.

Engineers redefined the concept of retrofit by adding a dual-track railway deck to a 30-year-old suspension bridge in Portugal. 

The new Tagus Bridge design    features a structural steel     railway deck and evacuation    walkways that run the length of    the bridge. Photos by José    Jardim.

A multinational consortium built a bridge within a bridge in Lisbon, Portugal, adding a second cable system, a railway deck and an additional automobile lane to the existing Tagus River suspension bridge without interrupting traffic flow. The Tagus River Bridge, scheduled to open to rail traffic in early 1999, became the first suspension bridge ever to undergo such a comprehensive retrofit.  Historical problem Spanning the wide estuary of Portugal's longest river has been a long-standing challenge to engineers. In 1876 Portuguese engineer Miguel Pais conducted the first formal study on the feasibility of spanning the estuary. But it wasn't until 1953 that the Portuguese government finally appointed a commission and then a special department, the Gabinete da Ponte sobre o Tejo, under the direction of engineer Jose do Canto Moniz, to complete the project. Decades later Moniz's nephew, Luis do Canto Moniz, would serve as chief engineer for the retrofit project.  In 1960, American Bridge, part of the United States Steel Corp., based in Pittsburgh, won the original bridge project in a worldwide design/build competition and retained Steinman Boynton Gronquist & London, New York, to prepare the preliminary and final designs and supervise construction of the suspension bridge.

The famed bridge designer D.B. Steinman took part in the preliminary design of the bridge but died in 1960. Managing partner Ray M. Boynton then took charge of the design team, proposing a four-lane roadway bridge that could be retrofitted to form a combined suspension/cable-stayed bridge for highway and railroad loading. At the time, the bridge was the longest in the world designed for both highway and railway loading.

In 1966, after four years of construction, the $32 million Tagus River Bridge was completed. With a main span of 1,013 m and measuring 2,278 m from the south to the north anchorage, the bridge was the longest in Europe and included the world's longest continuous truss. The caisson foundation at the south main tower, embedded in rock 79 m below water, is the world's deepest bridge foundation.

The new bridge stimulated economic development of the previously undeveloped and relatively inaccessible Setúbal peninsula, home to Portugal's largest steel mill. Tourist centers, capitalizing on miles of sun-drenched beaches and lush forests of umbrella pines, grew rapidly. Almada, on the south bank, developed into a residential area for Lisbon workers.

As development continued south of the river, and the population and congestion of Lisbon grew, traffic counts soared beyond planners' expectations. By 1990, the increased traffic required reconfiguring the roadway to carry five lanes of traffic on the original deck. By 1993, traffic counts averaged 120,000 vehicles per day, including a significant number of heavily loaded freight trucks. Ferries continued to link Lisbon and the southern rail system, serving as an alternative to the increasingly congested bridge.
 

Two plans were developed to alleviate the congestion. A second bridge built on the east side of Lisbon--the Vasco da Gama cable-stayed bridge--was completed just in time for Expo 98, the last world's fair of the millennium. Meanwhile, on the other side of town, work finally began on the Tagus River Bridge retrofit.  In 1992, the Portuguese roadway authority, Junta Autónoma de Estradas (JAE), appointed Steinman, the original design firm and now the major bridge unit of the Parsons Transportation Group, Pasadena, Calif., to design the retrofit.  A reorganization resulted in the transfer of responsibility for the bridge from JAE to the newly created bridge authority, Gabinete de Gestao das Obras de Installacao do Caminho de Ferro Na Ponte Sobre o Tejo (GECAF). In 1997 GECAF became part of the Rede Ferroviária Nacional, the Portuguese national railroad network.  The retrofit included widening the roadway deck to six lanes, installing the railroad deck between and along the lower chords of the stiffening trusses, reinforcing the existing structure, and repairing, rehabilitating, and repainting the overall structure. The railway deck would carry not only commuter trains but also heavier, long-distance freight trains.

Materials for construction of the    railway system were hoisted    from the contractor's yard while   roadway traffic continued flow-   ing above. Photo by John   Clenance.

The Tagus River Bridge is a national icon and a dominant feature of Lisbon's skyline. The Seven Hills form its backdrop, and the Christ the King statue gazes down on the bridge from 40 m above the roadway. To preserve the scenic and historical nature of the bridge, retrofit designers recommended retaining the classic suspension bridge form.

However, during the years between the original design and the retrofit, the loading criteria had changed considerably. To carry the new dead and live loads, the retrofit design had to strengthen the existing bridge with a second main cable system, in which two new cables would be supported at new anchorages and at extensions to the existing towers and cable bents. The new cables were sized to carry all of the new dead and live loads. In the new design, the old and new cables jointly carry the combined highway and railway loads. The bottom lateral system of the existing stiffening trusses has been replaced by a new lower deck that supports the railroad tracks.

The bridge's response to wind and seismic activity, and the human response to bridge vibrations caused by moving trains, were the subject of elaborate studies in the final design phase. Steinman performed a seismic investigation of the modified bridge, including redetermining the seismic activity in Lisbon. Like its sister bridge, the Golden Gate in San Francisco, the Tagus River Bridge is located in an area with a long history of earthquakes. The firm also conducted site response and liquefaction analyses, evaluated foundation stability and studied soil-structure interaction and multisupport excitation.

Steinman's retrofit design also went through rigorous wind tunnel testing to determine the effect of the railway and the new traffic lanes on the bridge's stability. Rowan Williams Davies & Irwin Inc., Guelph, Ontario, performed the wind tunnel testing, which included a sectional model test and a taut tube (partial aeroelastic) model. The taut tube technique provides information on the response of the bridge deck under full-scale turbulence. The design wind speed criterion established for the bridge was 60 m/s, but the tests revealed no instabilities at maximum wind speeds up to 72 m/s.

In January 1996, Consorcio Tejo won the $220 million construction contract to add the railway deck. The consortium consisted of dsd Dillinger Stahlbau, Saarlouis, Germany, the leader of the joint venture and the company in charge of structural steel works; American Bridge, responsible for all the cable-related works; and Wayss and Freytag, Frankfurt, Germany, which was associated with two Lisbon-based firms: Teixeira Duarte, responsible for pile foundations, civil works and installation of the railroad track work, and Sociedade Lisbonese de Metalização, responsible for painting all of the existing and new structural steel.
 

The multisupport controlled tension system was    used to air-spin bridge wire. Photos by José    Jardim.

Since an average of 140,000 vehicles per day pass over the bridge, the consortium had to carry out the complex retrofit with almost no traffic interruption. Steinman's project manager, Jose Celis, notes that contractors added the two additional main cables and all the new structural steel work for the second railroad deck while widening the road deck and repainting the entire structure, all while the bridge remained fully operational.  At the start of the project, DSD reinforced some of the truss members and cable bents to carry the new live load, and reinforced the full lengths of the diagonals with steel plates that required up to 800 bolts each. Workers reinforced truss upper chords with one or two plates and strengthened cable bents with wide plates along their full height.

The two new cables align 3.7 m directly over the existing cables at the cable bents and towers. New saddles, built over the existing saddles, are mounted on steel frames, and new cable anchorages are located adjacent to the existing anchorages. To reach the new anchorage locations, the new cables splay outward at the cable bents, a design made possible by the addition of a cable tie located 6 m above the roadway that resists the horizontal pull of the new, diverging cables. The tie consists of four galvanized structural strands 762 mm in diameter inside a steel box. Each strand is capable of carrying the full load.

The existing suspenders are spaced 23 m apart at every second truss panel point; the new suspenders are located at the truss panel points halfway between the existing suspenders. Each new compacted suspension cable, measuring 354 mm in diameter, comprises 19 strands, each in turn comprising 216 parallel bridge wires. American Bridge spun 2,800 metric tons of bridge wire in just over three months using an advanced multisupport, controlled tension system (see Civil Engineering, October 1997). Bridge wires were spun in the 70% to 80% range of the free-hanging catenary wire tension.

To meet the tight construction schedule, installation of structural steel for the new railroad deck and the widening of the roadway deck progressed in a predetermined sequence during cable spinning. During this time, the original suspension cables carried all of the new dead load, construction loads and highway loads--resulting in a noticeable yet predicted deflection of the stiffening truss that received extensive coverage in the local news media. In response, GECAF called a news conference to explain that engineers were regularly monitoring bridge deflections and that the 4 m truss deflection matched their calculations.

The original cables and stiffening trusses were relieved of the additional dead load by incrementally jacking groups of 16 suspenders simultaneously until the new cables bore all of the load. This delicate operation lasted seven weeks, from late September to early November 1997, and by its completion the stiffening trusses had recovered their original profile.

After most of the dead load had been transferred, the cables were coated with a waterproofing paste and wrapped with soft annealed galvanized wire 3.5 mm in diameter at a tension of 1,300 N. For durability, the cables received a three-coat application of acrylic paint.

The original design specified that the stiffening trusses be continuous from anchorage to anchorage to prevent abrupt gradient changes caused by future railroad loading. The retrofit railway deck, which carries two tracks, makes use of the space between the stiffening trusses. The structural steel system of the railway deck replaces the original bottom lateral system and bottom struts. The continuous track stringers work with the stiffening truss bottom chords, eliminating the need for reinforcement.

The original bottom struts were replaced by box-shaped floor beams, connected to the bottom chords every 11.5 m or at each panel point. These beams transfer the load of the railway deck to new, stronger transverse diagonals and rest in new suspender-truss connector brackets.

The railway stringers, wide-flange sections 1 m deep, are bolted to the floor beams so their continuity is preserved. To avoid interrupting this continuity, the new bottom laterals are connected to the floor beams adjacent to the stringers, resulting in a modified K-bracing system.

Installation of the railroad deck structural steel system, which added around 36 metric tons of new dead load for each of the 199 bays--the spaces between the panel points--took about nine months (April to December 1997). During installation, the contractor used six temporary movable platforms, suspended from the bottom chords of the stiffening trusses and running the full length of the bridge. These giant platforms also served as temporary bracing systems for the bridge during replacement of the bottom lateral system.

Key to the success of the project was the fact that the contractor used four overhead gantry cranes suspended from structural steel wide-flange rails attached to the bottom of the roadway deck floor beam trusses. The gantries, which traveled the full length of the bridge, transported all structural steel materials for construction of the railroad deck from the contractor's yards to the point of installation. Because the cranes hoisted materials up from the underside of the bridge, it was not necessary to close traffic lanes during the operation.
 

Back to October 1998 CE Magazine Table of Contents

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