The Damage, Cause Of Takoma Bridge Failure

View of damaged cables and towers looking west, February 1943 WSDOT

A bridge inspector checks the damaged cable WSDOT

The Damage

View from below the deck at buckled steel beams WSDOT

Main cables: During the collapse, the main suspension cables were thrown violently side to side, twisted, and tossed 100 feet into the air. They slipped from their positions in the cable saddles atop each tower. And, they fell hard on the approach spans. On the north cable at mid-span, where the cable band loosened, it broke more than 350 wires. Other wires were severely stressed and distorted. The main cables were a total loss, but salvage was undertaken. Their only value was as scrap metal.

Suspender cables: The violent collapse broke many suspender cables. Some were lost, some severely damaged, M3-5 Sagging east side span GHM, and some undamaged. Their Bashford 2795 only value now was as scrap metal. Towers: The main towers (West Tower, #4; and East Tower, #5), including the bracing struts, were twisted and bent. Stress beyond the elastic limit of the metal resulted in buckling and permanent distortion. Their only value now was as scrap metal.

Deck-Floor System: Not surprisingly, the concrete and steel of the center span that now lay on the bottom of the Narrows was deemed a total loss. The remainder of the broken concrete on the side spans needed removal. The floor system had sections that were bent and overstressed. Their only value now was as scrap metal. Side Spans: The loss of the center section, followed by the dropping of the side spans, caused substantial damage. The events stressed and distorted the plate girders and floor beams. Some buckled beyond repair. Piers: Both the West Pier (#4) and the East Pier (#5) sustained no damage. The collapse of the center span caused partial sheering of rivets that attached the towers to the tops of the piers. Anchorages: The anchorages for the main cables were undamaged. For building a replacement bridge, removal of part of the concrete would be necessary in order to spin the new main cables. Exactly where were Galloping Gertie's remains later that day? Weird Fact

First Investigations-Partial Answers to "Why" The collapse of the 1940 Tacoma Narrows Bridge stunned everyone, especially engineers. How could the most "modern" suspension bridge, with the most advanced design, suffer catastrophic failure in a relatively light wind? The State of Washington, the insurance companies, and the United States government appointed boards of experts to investigate the collapse of the Narrows Bridge. The Federal Works Administration (FWA) appointed a 3-member panel of top-ranking engineers: Othmar Amman, Dr. Theodore Von Karmen, and Glen B. Woodruff. Their report was the Administrator of the FWA, John Carmody and became known as the "Carmody Board" report. In March 1941 the Carmody Board announced its findings. "Random action of turbulent wind" in general, said the report, caused the bridge to fail. This ambiguous explanation was the beginning of attempts to understand the complex phenomenon of wind-induced motion in suspension bridges. Three key points stood out: (1) The principal cause of the 1940 Narrows Bridge's failure was its "excessive flexibility;" (2) the solid plate girder and deck acted like an aerofoil, creating "drag" and "lift;" (3) aerodynamic forces were little understood, and engineers needed to test suspension bridge designs using models in a wind tunnel. "The fundamental weakness" of the Tacoma Narrows Bridge, said a summary article published in Engineering News Record, was its "great flexibility, vertically and in torsion." Several factors contributed to the excessive flexibility: The deck was too light. The deck was too shallow at 8 feet (a 1/350 ratio with the center span). The side spans were too long, compared with the length of the center span. The cables were anchored at too great a distance from the

side spans. The width of the deck was extremely narrow compared with its center span length, an unprecedented ratio of 1 to 72. The pivotal event in the bridge's collapse, said the Board, was the change from vertical waves to the destructive twisting, torsional motion. This event was associated with the slippage of the cable band on the north cable at mid-span. Normally, the main cables are of equal length where the mid-span cable band attaches them to the deck. When the band slipped, the north cable became separated into two segments of unequal length. The imbalance translated quickly to the thin, flexible plate girders, which twisted easily. Once the unbalanced motion began, progressive failure followed. The investigation Board's most significant finding was simple and obvious: the engineering community must study and better understand aerodynamics in designing long suspension bridges. Meanwhile, Professor F. B. Farquharson continued wind tunnel tests. He concluded that the "cumulative effected of undampened rhythmic forces" had produced "intense resonant oscillation." In other words, the bridge's lightness, combined with an accumulation of wind pressure on the 8-foot solid plate girder and deck, caused the bridge to fail. Leon Moisseiff, who was contacted immediately after the failure, said he was "completely at a loss to explain the collapse." Moisseiff visited the ruined bridge one week later, touring under the watchful eye of Clark Eldridge. Moisseiff's design, while pushing beyond the boundaries of engineering practice, fully met the requirements of accepted theory at the time.

"Blind Spot"-- Design Lessons of Gertie's Failure At the time the 1940 Narrows Bridge failed, the small community of suspension bridge engineers believed that lighter and narrower bridges were theoretically and functionally sound. In general, leading suspension bridge designers like David Steinman, Othmar Amman, and Leon Moisseiff determined the direction of the profession. Very few people were designing these huge civil works projects. The great bridges were extremely expensive. They presented immensely complicated problems of engineering and construction. The work was sharply limited by government regulation, various social concerns, and constant public scrutiny. A handful of talented engineers became pre-eminent. But, they had what has been called a "blind spot." That "blind spot" was the root of the problem. According to bridge historian David P. Billington, at that time among suspension bridge engineers, "there seemed to be almost no recognition that wind created vertical movement at all." The best suspension bridge designers in the 1930s believed that earlier failures had occurred because of heavy traffic loading and poor workmanship. Wind was not particularly important. Engineers viewed stiffening trusses as important for preventing sideways movement (lateral, or horizontal deflection) of the cables and the roadway. Such motion resulted from traffic loads and temperature changes, but had almost nothing to do with the wind. This trend ran in virtual ignorance of the lessons of earlier times. Early suspension bridge failures resulted from light spans with very flexible decks that were vulnerable to wind (aerodynamic) forces. In the late 19th century engineers moved toward very stiff and heavy suspension bridges. John Roebling consciously designed the 1883 Brooklyn Bridge so that it would be stable against the stresses of wind. In the early 20th century, however, says David P. Billington, Roebling's "historical perspective seemed to have been replaced by a visual preference unrelated to structural engineering."

Just four months after Galloping Gertie failed, a professor of civil engineering at Columbia University, J. K. Finch, published an article in Engineering News-Record that summarized over a century of suspension bridge failures. In the article, titled "Wind Failures of Suspension Bridges or Evolution and Decay of the Stiffening Truss," Finch reminded engineers of some important history, as he reviewed the record of spans that had suffered from aerodynamic instability. Finch declared, "These long-forgotten difficulties with early suspension bridges, clearly show that while to modern engineers, the gyrations of the Tacoma bridge constituted something entirely new and strange, they were not new--they had simply been forgotten." An entire generation of suspension bridge designer-engineers forgot the lessons of the 19th century. The last major suspension bridge failure had happened five decades earlier, when the Niagara-Clifton Bridge fell in 1889. And, in the 1930s, aerodynamic forces were not well understood at all. "The entire profession shares in the responsibility," said David Steinman, the highly regarded suspension bridge designer. As experience with leading-edge suspension bridge designs gave engineers new knowledge, they had failed to relate it to aerodynamics and the dynamic effects of wind forces.

End of an Era The collapse of Galloping Gertie on November 7, 1940 revealed the limitations of the "deflection theory." Now, engineers no longer believed that suspension bridges needed to be stiffened only against the stress of moving vehicles and the "minor" effect of wind. The failure of the Tacoma Narrows Bridge effectively ended Moisseiff's career. More importantly, it abruptly ended an entire generation of bridge engineering theory and practice, and the trend in designing increasingly flexible, light, and slender suspension spans. Othmar Amman said of the collapse of the 1940 Narrows Bridge, "Regrettable as the Tacoma Narrows Bridge failure and other recent experiences are, they have given us invaluable information and have brought us closer to the safe and economical design of suspension bridges against wind action."

Aerial view of 1950 Narrows Bridge WSDOT

Suspension Bridge Design Since 1940

"Mere size and proportion are not the outstanding merit of a bridge; a bridge should be handed down to posterity as a truly monumental structure which will cast credit on the aesthetic sense of present generations." ---- Othmar H. Amman, 1954 The end of the 1950s witnessed the construction of two of the greatest suspension bridges in the world, built by two of the 20th century's greatest bridge engineers. The Mackinac Strait Bridge, which opened in November 1957 in Michigan, was the crowning achievement of David B. Steinman. In New York the Verazzano-Narrows Bridge, designed by Othmar Amman, was 10 years in the making and finally opened in November 1964. Both of these monumental spans directly benefited from the legacies of the failed 1940 and the successful 1950 Tacoma Narrows Bridges. Over the course of the last 60 years since Galloping Gertie failed, bridge engineers have created suspension bridges that are aerodynamically streamlined, or stiffened against torsional motion, or both. Now, wind tunnel testing for aerodynamic effects on bridges is commonplace. In fact, the United States government requires that all bridges built with federal funds must first have their preliminary design subjected to wind tunnel analysis using a 3-dimensional model. Failure of the 1940 Tacoma Narrows Bridge revealed for the first time limitations of the Deflection Theory. Since the Tacoma disaster, aerodynamic stability analysis has come to supplement the theory, but not replace it. The Deflection Theory remains an integral part of suspension bridge engineering. Today, the theory's principles serve as a model for the complex analytical methods (such as "Finite Element" computer programs) used by structural engineers to calculate stresses in the suspension cable system. Since the 1990s, advances in computer graphics technology and high-speed processing have enabled such calculations to be performed on desktop computers. Today, engineers recognize the importance of a thorough aerodynamic analysis of the structures they design. Advanced modeling software programs assist the complex calculations.

Why Did Galloping Gertie Collapse? For over six decades, engineers have studied the collapse of the 1940 Tacoma Narrows Bridge. The experts disagree, at least on some aspects of the explanation. A definitive description that meets unanimous agreement has not been reached. The exact cause of the bridge's failure remains a mystery. Why is it important to know the exact cause of the 1940 bridge's collapse? Engineers need to know how a new suspension bridge design will react to natural forces. The more complete their understanding, the better their problem solving, and thus, the stronger and safer their bridge. The fact that engineers still argue about the precise cause of the Galloping Gertie's collapse is testimony to the extraordinary complexity of natural phenomena. Today, the 1940 Tacoma Narrows Bridge's failure continues to advance the "scientific method." The primary explanation of Galloping Gertie's failure is described as "torsional flutter." It will help to break this complicated series of events into several stages. Here is a summary of the key points in the explanation. 1. In general, the 1940 Narrows Bridge had relatively little resistance to torsional (twisting) forces. That was because it had such a large depth-to-width ratio, 1 to 72. Gertie's long, narrow, and shallow stiffening girder made the structure extremely flexible.

2. On the morning of November 7, 1940 shortly after 10 a.m., a critical event occurred. The cable band at mid-span on the north cable slipped. This allowed the cable to separate into two unequal segments. That contributed to the change from vertical (up-and-down) to torsional (twisting) movement of the bridge deck. 3. Also contributing to the torsional motion of the bridge deck was "vortex shedding." In brief, vortex shedding occurred in the Narrows Bridge as follows: (1) Wind separated as it struck the side of Galloping Gertie's deck, the 8-foot solid plate girder. A small amount twisting occurred in the bridge deck, because even steel is elastic and changes form under high stress. (2) The twisting bridge deck caused the wind flow separation to increase. This formed a vortex, or swirling wind force, which further lifted and twisted the deck. (3) The deck structure resisted this lifting and twisting. It had a natural tendency to return to its previous position. As it returned, its speed and direction matched the lifting force. In other words, it moved " in phase" with the vortex. Then, the wind reinforced that motion. This produced a "lock-on" event. 4. But, the external force of the wind alone was not sufficient to cause the severe twisting that led the Narrows Bridge to fail. 5. Now the deck movement went into "torsional flutter." "Torsional flutter" is a complex mechanism. "Flutter" is a self-induced harmonic vibration pattern. This instability can grow to very large vibrations.

Tacoma Narrows Failure Mechanism - original sketch contributed by Allan Larsen When the bridge movement changed from vertical to torsional oscillation, the structure absorbed more wind energy. The bridge deck's twisting motion began to control the wind vortex so the two were synchronized. The structure's twisting movements became self-generating. In other words, the forces acting on the bridge were no longer caused by wind. The bridge deck's own motion produced the forces. Engineers call this "self-excited" motion. It was critical that the two types of instability, vortex shedding and torsional flutter, both occurred at relatively low wind speeds. Usually, vortex shedding occurs at relatively low wind

speeds, like 25 to 35 mph, and torsional flutter at high wind speeds, like 100 mph. Because of Gertie's design, and relatively weak resistance to torsional forces, from the vortex shedding instability the bridge went right into "torsional flutter." Now the bridge was beyond its natural ability to "damp out" the motion. Once the twisting movements began, they controlled the vortex forces. The torsional motion began small and built upon its own self-induced energy. In other words, Galloping Gertie's twisting induced more twisting, then greater and greater twisting. This increased beyond the bridge structure's strength to resist. Failure resulted.

What if . . . ? Sometimes it is fun and worthwhile to ask the question, "What if . . . ?," about important historical events. Here's one with an answer that may surprise you. What if Clark Eldridge's original design for the 1940 Tacoma Narrows Bridge had been built, instead of Leon Moisseiff's? Would it have blown down on November 7, 1940 like Galloping Gertie?

Eldridge's design; elevation detail, May 23, 1938 WSA, WSDOT records

Answer: The bridge would still be there. That's the opinion of leading bridge engineers who have carefully studied Eldridge's design, with its 25-foot deep stiffening truss. "I believe without a doubt," said one senior structural engineer, "that the bridge would have been aerodynamically stable for the wind speeds that destroyed Galloping Gertie."

Used For Long Spans Suspension bridges are amazing, wonderful structures. They appear almost fragile, viewed from a distance. Yet, they are very strong and nowadays are the world's longest bridge type. They are used for the longest crossings. Typical lengths for spans of this type range from 2,000 feet to 7,000 feet. The longest suspension bridge in the world is Akashi Kaikyo Bridge in Japan. Its center span measures an astounding 6,527 feet, and the bridge's total length is 12,828 feet.

Akashi Kaikyo

12,828'

Golden Gate

8,981'

Tacoma Narrows

Brooklyn

5,979'

3,460'

They All Move! Because they are relatively light and flexible, suspension bridges are all susceptible to wind. They vibrate and move, both vertically (up and down) and laterally (sideways). The challenge for the bridge engineer is to keep this motion within safe limits. The most vulnerable part of a suspension bridge is the suspended roadway, or deck. Do you have gephyrophobia? Weird Fact #15 Engineers designed the 1940 Narrows Bridge to withstand winds up to 120 mph, but it collapsed in a wind of only 42 mph. Why?

The Parts

Illustration of basic suspension bridge part

A suspension bridge does just what the name implies. The deck, or roadway, is suspended by large cables that are secured at each end and pass over the tops of high towers. The basic parts of a typical suspension bridge fall into two categories, "superstructure" and "substructure." The superstructure (super = above) is composed of a deck (or roadway, or girder), two towers, and the main suspension cables. The substructure (sub = below) is composed of the piers (caissons, or tower foundations) in the middle of the span that support the towers, and the anchorages (anchors) for the cables at each end of the bridge. The deck, or girder, is where we drive or walk. It is continuous and may be a truss, or a box girder, or a plate girder. Large anchors, or anchorages, at both ends of the bridge act as counter weights that hold the ends of the main cables. The anchorages are normally either a mass of concrete or solid rock. In the anchorage, the cables splay into separate strands to distribute the tension load evenly and safely. The main cables stretch from one anchor over the tops of the tower and attach to the opposite anchorage. The cables are compacted strands of parallel wires carried back and forth across the water. At the anchorage, each cable strand wraps around a strand shoe. Each strand shoe connects to an eye-bar. The eye-bars are firmly cemented in the anchorage. At the top of the towers each cable passes over a cable saddle. At the cable saddle the cable transfers the load from the cables to the tower. The main cables are attached to the deck by suspender cables. These are sometimes also called "suspender ropes" and "hanger cables." In a suspension bridge, the main cables suspend the deck (girder, roadway). Most of the bridge's weight (and any vehicles on the bridge) is suspended from the cables. The cables are held up only by the towers, which

Illustration of tower and cable saddle

means that the towers support a tremendous weight (load).

The steel cables are both strong and flexible. This makes long span suspension bridges susceptible to wind forces. These days, engineers take special measures to assure stability ("aerodynamic" stability") to minimize vibration and swaying in a suspension bridge under heavy winds. The 1940 Tacoma Narrows Bridge is the world's most famous example of aerodynamic instability in a suspension bridge

Salvage of the 1940 Narrows Bridge After the center span fell into the Tacoma Narrows, the towers, main cables, side spans, and anchorages remained. The approach spans sustained no damage. The process of dismantling and salvaging the ruined bridge proved as intricate and dangerous as its construction. In March 1941 estimates for salvage operations looked gloomy. Experts calculated the scrap metal value of the towers, deck floor system, and other steel parts at $206,000. It would cost three times that amount, an estimated $636,000, to dismantle the bridge. Dismantling of the towers began on September 8, 1941. With the start of World War II in December that year, hopes rose that a steel shortage would bring a better price, and perhaps a profit. That hopeful vision faded. By the end of March 1943 workers had removed more than 4,000 tons of steel from the towers and 3,000 tons from the cables and deck. The salvaged metal was sold for scrap during World War II. In the end, salvage operations lost money. The Toll Bridge Authority paid nearly $646,661 for the effort. In return for 7,000 tons of scrap steel the State received a meager $295,726. The net cost for the operation was $350,933. Interestingly, the process took about 29 months, roughly the same amount of time that it would take to build the replacement Narrows Bridge at the end of the decade.

Engineering Challenges For Gertie's Replacement Engineers faced two major challenges in uilding the second (current) Narrows Bridge. First, they had to better explain what happened to the 1940 bridge, and to design one that would not meet the same fate. Second, what remnants of the old bridge, especially the piers, could be used? In July 1941 Charles E. Andrew, consulting engineer for the Washington State Toll Bridge Authority (WSTBA), appointed Dexter R. Smith as chief design engineer to plan the new structure. By October the state had a new design. The plans roughly resembled Clark Eldridge's original design. The proposed replacement bridge with a deep, open deck truss would cost about $7 million.

Tower face drawing, Current Narrows Bridge WSA, WSDOT records

The proposed design for the new Narrows Bridge needed testing. Issues of aerodynamic stability in the design of suspension bridges had never before been investigated. Testing the bridge design fell to F. B. Farquharson, professor of engineering at the University of Washington. Dexter Smith and the State's bridge design team consulted extensively with Professor Farquharson and his research group at the University. Their work represented the leading edge. They were pioneering the field of bridge aerodynamics. For the four years of World War II, and occasionally afterwards to 1947, Farquharson studied the 1940 span and the new proposed Narrows Bridge in a specially built, 100-foot long Structural Research Laboratory that housed a wind tunnel and scale models of the bridges.

1950 Narrows Bridge, elevation sketch WSDOT

First, Farquharson confirmed that the 1940 Narrows Bridge had collapsed because of its excessive flexibility and susceptibility to aerodynamic forces. Galloping Gertie became the first suspension bridge ever studied using both visual observation and wind tunnel testing. These investigations laid the foundation for continued research. If a dynamic scale model of a proposed bridge design could pass Farquharson's wind tunnel testing, then Smith and his bridge engineers could build the real thing with confidence at the Tacoma Narrows. They envisioned a new Narrows Bridge designed to offer the least wind resistance. The solution would be to use deep, open stiffening trusses with trussed floor beams. The truss members would be shallow, to avoid creating any large, solid surfaces like the ones associated with the failure of the 1940 Narrows Bridge.

Warren truss of the 1950 Narrows Bridge, sketch WSDOT

View of 1950 Narrows Bridge's 33-foot deep Warren truss WSDOT

Farquharson built a 1:50 full scale model and sectional models of Smith's design. The tests included subjecting the model to wind forces striking the bridge at angles up to plus-and-minus 45 degrees perpendicular to the deck. This wide range of wind angles helped give the new bridge design even greater stability. (Today, design engineers typically use a narrower range of plus-to-minus 5 degrees.) The tests proved that the proposed bridge would be far more stable than Galloping Gertie. Farquharson decided to test the model equipped with open wind grates to permit freer airflow and minimize the wind's effects. It worked. The model now showed virtually no torsional movement. Smith and Farquharson decided to take additional steps. They wanted to eliminate as much vertical and twisting motion in the model as possible. Mechanical devices would enhance their design's natural damping ability. First, they added a double lateral bracing system to the stiffening truss to increase torsional stiffness. Second, they added hydraulic shock absorbers at

three strategic points in the structure: at mid-span, between the main span and side span, and at each tower.

Tower & deck cross-section showing damping mechanism, Current Narrows Bridge WSA, WSDOT records

The existing piers posed the second engineering challenge. Would they support the proposed four-lane bridge, which was 60 percent heavier than the old superstructure? The piers had been stressed by collapse of the 1940 bridge, as well as by 17 earthquakes that struck the area between 1939 and 1946. Two of those reached 5 on the Richter scale. Engineers discovered, to their relief, that the piers would prove to be solid foundations for the heavier new span. The tests from 1941 to 1947 cost over $88,000 when completed. By comparison at the time, a pound of coffee cost 29 cents, and you could buy a new car for about $1,500, or rent a 3-room duplex for $50 a month. But, the extensive investigations gave the State's engineers confidence in their new design. The proposed bridge would stand safe and solid in winds up to 127 mph for a 3-second gust.

Current Narrows Bridge, view from beach level, September 26, 1950 Tacoma Public Library copyright information

An "Epoch-Making" New Span Rises Thanks to the failure of its predecessor, Galloping Gertie, the current Narrows Bridge affected the course of suspension bridge engineering and design. The years of research into aerodynamics, and the new mathematical knowledge of vibrations and wave phenomena ushered in a new era of more stable suspension spans.

The next generation of large suspension bridges featured deep and rigid stiffening trusses. Completion of the 1950 Narrows Bridge was soon followed in the United States by the Delaware Memorial Bridge in 1951, the Chesapeake Bay Bridge (Preston Lane Jr. Memorial Bridge) in 1952, and David Steinman's great Mackinac Strait Bridge, built from 1954 to 1957. "Epoch-making," is how one writer describes the current Narrows Bridge. It represented a remarkable achievement. Its design, engineering, functions, and stability were unprecedented. Its aesthetic appeal marked a milestone as well. The visually stunning "Sturdy Gertie" helped shift popular perceptions about "beautiful" suspension bridges. Setting tower base plate, 1948

Construction began on April 8, 1948. The contract for steel WSDOT fabrication and erection went to Bethlehem Pacific Coast Steel. John A. Roebling Sons Company won the cable spinning contract. Some 29 months later, the bridge opened to the public on October 14, 1950. Once again, the Tacoma Narrows became home to the third longest suspension bridge in the world. View WSDOT VIDEO of Installing Catwalks Overall, the 1950 Narrows Bridge has a suspended structure of 5,000 feet--a center span of 2,800 feet and two side spans of 1,100 feet (the same as its predecessor, since the same piers were used). The motion damping devices tested in Farquharson's wind tunnel all appeared in the finished bridge. The new four-lane bridge had several features that made it 58 times more rigid than the 1940 Narrows Bridge. They immediately earned the span a distinctive place in bridge engineering history. In 1950 the Tacoma Narrows Bridge was the most technically advanced long span suspension bridge in the world. Innovations & Special Features: •

The deck system's prominent 33-foot deep steel Warren stiffening trusses: These gave the bridge a depth-to-center span ratio of 1 to 85, the deepest stiffening system on a major suspension span since the 1909 Manhattan Bridge.

•

Double (top and bottom) lateral bracing of the stiffening trusses: This feature, combined with the 33-foot deep stiffening truss, gave the bridge exceptional torsional rigidity.

•

Wind grates: Three slots of open steel grating 33 inches wide separating all four traffic lanes, and a strip 19 inches wide along each curb.

•

Hydraulic shock absorbers at three strategic points in the structure: (1) at mid-span, at the main cable center tie, between the main suspension cables and the top of the stiffening truss; 6 devices per cable (a "first" for a long suspension bridge); (2) between the top chords of the main span and side span stiffening trusses; and (3) at each tower, where it joins the bottom of the deck truss.

•

Ends of the west and east side spans were anchored securely to the ground.

•

Cable sag ratio of 1:12. This required the towers to be higher than the 1940 bridge, which had a sag ratio of 1:10.

Fire, Ice, and Earthquakes Engineering challenges were not the only difficulties faced by the builders of the 1950 Narrows Bridge. Some of the reasons that construction took 29 months had to do with several challenges from "Mother Nature." On April 13, 1949 an earthquake measuring 7.1 on the Richter scale shook the Puget Sound region. The trembler caused no damage to the Narrows Bridge's piers or towers, then under construction. Weird Fact #16 The severe winter of 1949-50 slowed construction. Engineers already had concerns about starting cable spinning in the coldest season. That winter proved their worries valid. For six weeks rain and snow, driven by high winds, lashed the area. Once, ice an inch thick had to be removed from steel so men could do their jobs. Especially rough was work on the cable spinning. At times, workers had to use blowtorches to thaw out cable strands for adjustment and banding. By February 1950 the cable spinning contractor, John A. Roebling Sons Co., was fighting a 2-month delay. Who were the engineers and bridge workers who endured these hardships and successfully met the challenges? You'll find the two main engineers who launched the 1950 Narrows Bridge, Charles Andrew and Dexter R. Smith. Take a look at a couple of the bridge workers, Earl White and Joe Gotchy.

Components of the Current Narrows Bridge Piers The piers designed by Clark Eldridge for Galloping Gertie stood ready to handle the new span's towers. The twin steel legs of each tower were 60 feet apart, center-to-center. The original pedestals were too narrow for the new towers. They were also too close to the water. In Gertie's short life, the tower legs showed signs of salt water corrosion. The new pedestals were the same width, but taller by 18 feet to keep the tower legs safe from salt spray. This pushed the height of the towers on the new span to 58 feet higher than those on the 1940 Narrows Bridge. The additional weight of the superstructure (1.6 times the load carried by the original piers) actually was better, more evenly distributed by the greater width between tower legs. The new design increased the dead load pressure on the piers by only 6 percent. Towers Towers with catwalks, 1949 Tower erection used a "crawler crane," which advanced WSDOT upward as workers completed each tower leg. The legs of each tower are vertical. They are made of hollow steel cells, stacked on top of one another. The cell sections are four columns arranged to form a hollow core in the center. Each section is 32 feet long and weighs 27 tons. Viewed in cross-section, the four cells in each leg form a cross shape. To stabilize the towers during construction, temporary "outriggers" were added until the cables and deck trusses could be completed. When the top

cross bracing was completed, Chicago booms hoisted the 28ton cast-steel cable saddles and placed them at the top of each leg. The cable saddles were secured by 36 bolts. Anchorages The new bridge had a much bigger cable load, increased from the original 28 million pounds to 36 million pounds. This required modification of the anchorages for the 1940 bridge. The old anchorages, spaced 39 feet apart, were retrofitted for the new span's 60-foot spacing between cables. The original structures became the cores of the new, heavier and wider 54,000-ton anchor blocks. The anchorages included 62-foot long eye-bars, fitted with 26-inch diameter shoes, embedded into the new concrete. Cables Once the towers and cable saddles were in place, spinning began for the 20-1/4 inch diameter main suspension cables. Spaced 60 feet apart, each cable contained 19 strands of 458 No.6 gauge wires. The strands looped around the eye-bar shoes to form a continuous cable from anchorage to anchorage. Deck The 33-foot deep Warren stiffening truss system was assembled at the bridge site from shop-fabricated components. Four rolling derricks (2 per tower) moved each way from the tower. Two riveting crews and traveler operators worked from the tower piers to the center of the main span, while two other crews worked from the piers to the shore. First, workmen placed the top and bottom chords and their diagonal bracing. Next, the floor beams were placed between the chords. Then, deck stringers were laid lengthwise on top of the beams. Finally, crews pinned the members in place, and the riveting gangs finished the process.

Anchorage construction, 1948 WSDOT

Anchorage view with cable and eye-bars, 1949 Earl White

Deck cross-section, Current Narrows Bridge WSA, WSDOT records

The deck measures 46 feet 9 inches (curb-to-curb) for the 4 traffic lanes, plus two sidewalks 2 feet 9 inches wide. Steel reinforcing rods were placed in the roadway. Then, workmen placed the deck slab, a light-weight concrete 5-3/4 inches thick with a 5/8-inch asphalt riding surface (used to lessen the load on the piers). The open steel wind grates were installed between driving lanes and at the curbs. Suspender Cables and Cable Bands As each section of the deck was assembled, crews hung the suspender cables. Each set of two cables, looped over the main cable appears to be four wires. These 1-3/8 inch diameter wire cables were attached to the deck at 32-foot intervals by zinc "jewels." Special Features & Motion-Damping Devices Finally, workmen installed the mechanical motion-damping devices. Sand blasting and painting The current Narrows Bridge is painted "Narrows Green." Sub-contractor for the bridge's first coat of green was H. P. Fisher & Sons Company of Seattle for painting the suspender cables, cable bands, and various steel parts.

"Span Stats" – Statistical Profile of the 1950 Narrows Bridge LENGTH: Current Narrows Bridge Total structure length Suspension bridge section Center Span Shore Suspension Spans (2), each East Approach and Anchorage West Approach and Anchorage

5,979 feet 5,000 feet 2,800 feet 1,100 feet 365 feet 614 feet

ANCHORAGES: Current Narrows Bridge Weight of each anchorage 66,000 tons (shore anchors) Concrete in each anchorage

25,000 cu. yds.

West Anchorage (concrete 164 feet long anchor block and gallery) East Anchorage (concrete 185 feet long anchor block and gallery), plus approach, administration buildings and toll house West Anchorage, construction & cost East Anchorage, construction & cost

Woodworth & Co. $406,000 (est) Woodworth & Co. $386,000 (est)

PIERS: Current Narrows Bridge West Pier, total height 215 feet West Pier, depth of water 120 feet West Pier, penetration at bottom 55 feet East Pier, total height 265 feet East Pier, depth of water 135 feet East Pier, penetration at bottom 90 feet

Deck construction, view from water level, spring 1950 Earl White

Area

118 feet, 11 inches

by 65 feet, 11 inches

CABLES: Current Narrows Bridge Diameter of Main Suspension 20.25 inches Cable Weight of Main Suspension Cable 5,441 tons (each) Weight Sustained by Cables 18,160 tons Number of No. 6 Wires in Each 8,705 Cable 104,094,390 feet, = 19,715 Total Length of Wire miles Sag Ratio 1:10

Roadway view of open grating, June 1951 Tacoma Public Library copyright information

TOWERS: Current Narrows Bridge Height above water 500 feet Height above piers 467 feet Height above roadway 280 feet Weight of each tower 2,675 tons Night work to complete the bridge on time, 1950 WSDOT

ROADWAY - DECK: Current Narrows Bridge Center Span height above water 187.5 feet Weight of center span 7,250 lb./ft Traffic lanes 4 Width of roadway 49 feet 10 inches

Width between cables Width of sidewalks(2), each Number of girders and type Depth of girders Suspender cables Thickness of roadway

60 feet 3 feet 10 inches 2 Warren trusses 33 feet 32 foot intervals 6-3/8 inch reinforced concrete

SPAN RATIOS Bridge

Width to Length (of Center Span) 1:72 1:46 1:47

1940 Narrows Bridge Current Narrows Bridge Golden Gate Bridge George Washington 1:33 Bridge Bronx-Whitestone Bridge 1:31

Girder Depth to Length (of Center Span) 1:350 1:112 1:168 1:120 1:209

Vortex-Induced vibratrion: When fixed in a fluid stream. bluff (nonstreamlined) bodies generate detached or separated flow over substantial parts over their surfaces: that is, the flow lines do not follow the contours of the body, but brake away at some points. At low Reynolds number, when the separation first occurs, the flow around the body remains steady. At some critical Reynolds number two thin layers – often termed the free shear layers—from the lee of the body. These unstable layers interact nonlinearly with each other in the body wake to produce a regular periodic array of vortices (concentrations of rotating fluid particles) termed the Strouhal vortices. Such wakes were symmetrically investigated for circular cylinders by Benard. These vortex arrays arrange themselves in two rows, with opposite directions of circulation. each vortex is located opposite the midpoint of the interval between the two vortices in the opposite row (fig. 2). The beauty of this "vortex street" – often termed the Karman vortex street – long attracted attention. The frequency of the shedding vortices over a fixed (restrained) body is often termed the Strouhal frequency (fs) and follows the relation: fs.D/U=S

U is the cross-flow velocity. D is the frontal dimension S is the Strouhal number (nearly constant) appropriate to the body in question. In the original Takoma Narrows bridge D=8ft , S=0.11