Tacoma Narrows Bridge (1950)

The 1950 Tacoma Narrows Bridge is a suspension bridge in the U.S. state of Washington that carries the westbound lanes of Washington State Route 16 (known as Primary State Highway 14 until 1964) across the Tacoma Narrows strait, between the city of Tacoma and the Kitsap Peninsula. Opened on October 14, 1950, it was built in the same location as the original Tacoma Narrows Bridge, which collapsed due to a windstorm on November 7, 1940. It is the older of the twin bridges that make up the Tacoma Narrows Bridge crossing of the Tacoma Narrows, and carried both directions of traffic across the strait until 2007. At the time of its construction, the bridge was, like its predecessor, the third-longest suspension bridge in the world in terms of main span length, behind the Golden Gate Bridge and George Washington Bridge; it is now the 46th longest suspension bridge in the world.

Design work on a new Tacoma Narrows Bridge began shortly after the collapse of the original bridge. However, several engineering issues, the demand on steel created by the United States' involvement in World War II, and the state of Washington's inability to find an insurer, all pushed the start of construction to April 1948. The new bridge was designed with a wider deck and taller and wider towers than its predecessor, and addressed the wind issues that led to the original bridge's collapse. It opened to the public on October 14, 1950, and carried both directions of Primary State Highway 14 for over 40 years. Tolls were charged on the bridge until 1965, when the bonds were retired 13 years ahead of schedule. “The price tag for construction was one-third more than the Toll Bridge Authority estimated--$11.2 million. The final construction cost estimate, made just prior to the bond issue, reached $13,738,000.”

By 1990, population growth and development on the Kitsap Peninsula caused vehicular traffic on the bridge to exceed its design capacity. In 1998, voters in several Washington counties approved an advisory measure to create a twin bridge to span the Tacoma Narrows. After a series of protests and court battles, construction began on the second span in 2002. The second span opened in July 2007 to carry eastbound traffic, and the 1950 bridge was reconfigured to carry westbound traffic.

Design

The designs for the 1950 Tacoma Narrows Bridge were drawn up not long after the 1940 collapse of its predecessor. In July 1941, the Washington Toll Bridge Authority appointed Charles E. Andrew (who had been involved in Gertie's design and construction as a consultant) as principal engineer and chairman of the consulting board in charge of designing a new span across the Narrows. Members of the new design board included Dr. Theodore von Kármán, Glenn Woodruff, and the firm of Sverdrup and Parcel of Chicago, Illinois. To lead the design team, Andrew picked Dexter R. Smith as lead designer and principal architect. As early as October 1941, less than a year after Gertie's collapse, the WTBA had adopted a rough design for a new span. The new design closely resembled the original design for the 1940 span drawn up by Clark Eldridge. The cost of construction on the new design was then estimated at $7 million (US$ in present terms).

Since the original bridge became a major asset in the short time it was in service, the U.S. Navy lobbied heavily for a combination highway/railroad span across the Narrows to replace Gertie, and proposed a steel-cantilever-type bridge over a suspension span. However, the extra steel needed to construct such a structure across the Narrows would have added an extra $8.5 million (US$ in present terms) to the construction cost, ruling out any possibility of such a structure ever being built.

Furthermore, the proposed design needed new testing. A purely mathematical solution to designing suspension spans was not possible because little was known about the forces that brought down Gertie. In light of that fact, engineers chose to design the replacement, then subject scale mockups of the design in a specially-built wind tunnel constructed at the University of Washington. According to Charles Andrew, "The only way to attack the problem, was to design a bridge, then build a model of that design and subject it to wind action." The testing was performed by Professor F. B. Farquharson, who had researched Gertie's motions prior to its collapse on November 7, 1940.

From late 1941 onward, Professor Farquharson (as well as von Karman, who did his work at the California Institute of Technology in Pasadena, California) continued to work on the new bridge design. By 1943, he was working in a specially-designed wind tunnel laboratory built on the University of Washington campus in Seattle. The facility was large enough to house a scale model of the completed bridge as long as , plus section models for further testing. After Farquharson confirmed that Gertie collapsed due to its excessive flexibility and the aerodynamic forces as a result of the flexibility in its span, testing was then done on designs drawn by Smith. All of the new designs would feature a deep open stiffening truss instead of a solid plate girder.

Testing on the new bridge design was begun in November 1943 and continued through 1945. The studies included 200 different configurations, to wind forces hitting the span at up to plus-or-minus 45-degrees perpendicular to the deck. Then, testing was performed on a design using open strips of wind grating placed in the roadway, which added even greater stability against torsional movement. A design with bottom lateral bracing on the stiffening truss was also tested to test the resistance against lateral movement. Then, a design was tested with motion damping devices located on the deck at three locations: one at each tower (one at each end of the main span, and one on each side span at the tower), and a set of damping devices at mid-span on each main cable. Each of these steps in the design and testing phase were performed to reduce as much lateral and torsional movement as possible.

After $80,000 (US$ in present terms) was spent in the design and testing of the new span, the design was completed on December 5, 1945. The WTBA finalized and approved revised designs from Dexter's drawings in December in April, 1946, and minor revisions continued on until September. The new span was to have a construction cost of $8.5 million (US$ in present terms).

The final designs of the Tacoma Narrows Bridge, once finalized, were a sharp and drastic contrast from the design by Leon Moisseiff. Instead of a thin plate girder, an open-air stiffening truss with a depth of would form the new road deck. Newer, larger towers that rose higher and wider than Gertie's towers, would support the bridge's main cables, now in diameter versus Gertie's . Newer, larger anchor blocks would support a load that weighed 1.6 times as much as the original bridge. However, some elements of Galloping Gertie were incorporated into the 1950 span. The tower pedestals were enlarged and raised . On the west end stood a long approach viaduct with the same deep girders Gertie's main deck had. This approach viaduct used three support towers, two with thin support beams and one with the structural complexity and design of one of Gertie's main towers, each spaced apart. The viaduct, after a structural examination, was kept and utilized as part of the 1950 bridge's design, with an additional box strut brace added to the tower closest the shoreline (officially known as Tower No. 3 in the design plans), and widening of the upper box strut for the new bridge's deck.

The road deck itself was seen as a major innovation in suspension bridge design. Lanes of traffic on typical suspension bridge roadways are divided by dashed paint lines, a solid strip, or a set of two strips of paint. On the 1950 span's final roadway design, the -wide roadway was split into four lanes of traffic, each lane being wide. Each lane was separated by a deep, open air wind grate. Bordering the outside lanes was a open air wind grate that supported a pipe curb elevated off the roadway. These also formed the separation between the roadway, and a -wide sidewalk on both sides that was fenced in by a railing.

Construction

Constructing the replacement Tacoma Narrows Bridge was delayed for nearly a decade primarily due to the demand on steel created by World War II, and the fact that the state had trouble arranging insurance for the new span. On April 30, 1947, Governor Monrad Wallgren had announced that insurance had finally been arranged. It wasn't until August 1947 that Washington finally requested bids for the construction, and by that time, the price tag for construction went from $8.5 million (US$ in present terms) to $11.2 million (US$ in present terms). On October 15, the state opened bids for the construction, with Bethlehem Pacific Steel Corporation bidding $8,263,904 (US$ in present terms) on construction of the superstructure, and John A. Roebling's Sons Company of San Francisco bidding $2,932,681 (US$ in present terms) for the construction of the suspension cable system.

After several delays involving final financing, the WTBA finally offered a $14 million (US$ in present terms) bond that was to be repaid via tolls on the bridge, as well as Pierce County offering a $1.5 million (US$ in present terms) bond guarantee fund. On March 12, 1948, the state finally completed bond financing. And after steel became more readily available, the puzzle pieces began falling together in rapid fashion. The construction contracts were finally awarded on March 31, and April 1, and by April 9, earth moving began at the remains of Gertie's east cable anchorage.

Anchorages

Since the replacement span was going to be 1.6 times heavier than the bridge it replaced, as well as four lanes instead of two, modification and partial demolition was necessary to begin construction on the newer, and much more massive, cable anchorages at each end of the bridge. The center of the old anchorages were kept as cores, and a newer eyebar system set at apart (versus for the previous bridge) were constructed. They would support a much larger cable load from the original to a much heavier , and consisted of steel eyebars long, with diameter shoes, embedded into new concrete. Each new anchor would weigh and would be embedded deep into compacted soil. Construction of the new cable anchorages was started during the summer of 1948 and continued into 1949.

Tower pedestals

Due to the water depth of the narrows, the Tower Pedestals are each the size of a 20-story office building, with an overall size of , on which rest the towers. They are designed to withstand daily currents and a twice-daily tidal swing. Each tower pedestal used of concrete for construction.

The tower pedestals had creosote timber fenders, which were installed in 1948 to deflect marine debris and traffic, which were removed sometime between 1995 and 2000.

The tower pedestals that supported Gertie's towers were found to be structurally sound and unharmed after the failure of that span, and were reused for the current bridge. The towers of the 1940 span in their short service time experienced corrosion from salt water spray at their bases, so engineers constructed new tops to the pedestals, at the same time the anchorages were being built, to allow for tower construction to begin in near unison. The new tower pedestals (called piers in the designs) were raised to prevent the corrosion issue, and were enlarged where the towers would stand to provide more structural rigidity. The east and west piers were completed in mid-December 1948.

Towers

The original towers for the 1940 Tacoma Narrows Bridge featured four deep, box-shaped struts (two below the roadway and two above), as well as being tapered from being wide at the base to at the top. The new towers of the replacement span were to be the same width both at the bottom and the top, and were to feature a deep X-bracing system with three single X-brace struts below the road deck and a series of three double-X brace struts of varying depths above the roadway (top being , middle being , and the bottom being ). Tower construction began January 1, 1949, and progressed rapidly. Work went ahead of schedule and by April 1949, the steel cable saddles were prepared for lifting. On April 13, 1949, the north cable saddle on the east tower was bolted into place on a set of shims to allow workers to rivet the top plate into place. Then, disaster struck. That morning, an earthquake measuring 7.1 rattled the Puget Sound region. The earthquake caused the towers to sway from vertical, causing the cable saddle to shear its bolts and fall off the tower. The saddle fell below. It missed the pedestal, and plunged straight through a barge, to a final resting spot below the surface of the Narrows, away from the pedestal. The resulting impact sank the barge, taking a compressor and numerous other tools with it. Divers soon located the cable saddle in a deep hole during a slack tide (the only time divers could operate, and slack tides only occurred at night). After a three-day delay, it was raised to the surface, repaired (the impact bent a corner on it), and reinstalled. The earthquake delayed completion of both the east and west towers on the bridge. After the earthquake's aftershocks subsided, work resumed, and by July 17 both towers were announced as being completed.

Cables

Construction of the main cables began by erecting their wide catwalks in July. On July 17, 1949, the first catwalk line was towed via tugboat across the Narrows, then lifted onto the towers. The catwalks, consisting of wire base cables, cyclone-wire fencing, and a center section of wood slats, were erected in sections. On September 15, the catwalks were completed and the spinning gear was in place. Harold Hills, a field engineer for Roebling's Sons Company, became the first man to cross the Narrows via the catwalks. On the following day, Harry Cornelius, an inspector for the WTBA became the second to cross the Narrows on the catwalks.

To begin preparations for spinning the main cables, Roebling's Sons had set up a reeling plant on the Tacoma tidal flats. of galvanized steel wire, the total amount needed for both main cables, began arriving in coils. Shipped in from Trenton, New Jersey

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