collapse of the tacoma narrows bridge

                                     Collapse of the Tacoma Narrows Bridge

            “Galloping Gerite, “ the Tacoma Narrows Bridge’s nickname because of it’s rolling, surging actions collapsed four months after opening to traffic on July 1, l940,

at approximately 11:00 a.m. It huffed and puffed and a forty-two mile per our wind storm took down the 5,939 foot long bridge linking Tacoma Narrows, in Puget Sound near Tacoma, Washington to Gig Harbor/the Olympic Peninsula.  It was the third longest “suspension span,” in the world.  Motorists sensed the 2,800 foot bridge claiming, “they felt as though they were traveling on a giant roller coaster.”

            An eye witness account was provided by the editor of a Tacoma newspaper, Leonard Coatsworth; …”the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car … I jammed on the brakes and got out, only to be thrown onto my face against the curb.  On my hands and knees most of the time, I crawled five hundred hundred yards  or more to the towers…I risked rising to my feel and running a few yards at a time …Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows” (ketchum.org).

            The bridge was constructed with two main piers towers ,and cables.  It had anchorages, weighing 44,000 tons, and a concrete floor system, “created to resist the cable pulls of the bridge.”  The primary cables had a breaking strength of 220,000 pounds per square inch. The floor system was comprised of  “two stiffening side girders; a concrete roadway slab was chosen to counteract the extreme lightness of the structure.”

Slight movement began on the bridge during the floor section construction; which caused the bridge deck to rise and fall, “one and one third feet above and below its normal position.”  The toll bridge “cost fifty-five cents pre car, fifteen cents per extra passenger, and fifteen cents for pedestrians,” it quickly became a tourist attraction.

            Engineers had no doubt the structure was completely safe, but in July l940, the University of Washington, directed Professor F.B. Farquharson to film its movement, and

complete a series of experiments to devise methods to limit the movements.”  The team discovered that the motion on the model bridge lessened, “when anchor cables were attached from the center bottom that approaches to the ground,” however on the actual Tacoma Bridge the cables “snapped in the wind” (ketchum.org).

            Suggestions discussed to eliminate the problem were; “drilling holes in the side girders to keep wind from pushing and pulling on the bridge, and installing semicircular deflector shields to streamline the girders.” The torsional movement was severe at the end of the suspension span, which contributed to the swaying.  This is the rotation “about the axis through the center of the bellows “ (ejsus.com).  When torsions become destabilized, as with the bridge, an expansion joint reduces its ability to contain pressure and absorb movement; which is what the wind caused to happen. At the maximum point of the collapse, the elevation of the sidewalk at the right of the bridge was twenty-eight feet higher than the sidewalk at the left.  A few minutes after the first concrete dropped into the water, a six hundred foot section of the center, broke out of the suspension span turned upside down and plunged into the Puget Sound; with that gone there was nothing to “counter balance the weight of the side spans” (Vuik).

            Professor Farquharson and other University of Washington engineers were employed to study methods to “reduce the movement on the bridge.”  Experiments were conducted, but a solution never came to fruition. On the day of the disaster, Professor Farquharson witnessed and documented the event.  Post-collapse rendered many theories concerning why the bridge fell.  An investigative board for the Washington State Toll Bridge Authority concluded that, “ the failure was due to the bridge’s design reacting to the wind in the Narrows.”  Both suggestions were approved, but to everyone’s astonishment it was too late. “Galloping Girdie” collapsed. Fortunately, the only casualty

was a dog trapped in an abandoned car.

            The next day national newspapers and magazines printed full accounts of the Tacoma Narrows Bridge collapse. She became famous as “the most dramatic failure in bridge engineering history.”  The event changed how engineers design suspension bridges, and safer suspension spans leading to the success of today’s Tacoma Narrows Bridge.

            The bridge was insured with twenty-two insurance policies, and  was declared a total loss.  Eighty percent or $5,200,000 of the structures valuation was covered; which helped build the next bridge. Engineers admitted they knew the bridge was unstable; that the fall was due to its “improper shape and not improper strength.”

             Clark Eldridge, chief engineer felt that the “federal money – lending agencies insistence that the bridge be designed by an eastern firm led directly to the bridge’s unsound design.”  He continued, “We had a tried-and-true, conventional bridge design… we were told we couldn’t have the necessary money lenders.” Eldridge stood firm that the “Washington State Highway Department engineers had pointed out that the solid side girders would act like sails unlike the traditional open truss design that allowed wind to pass through.”

             A consulting designer, Leon Moisseiff said the engineers didn’t “know enough about areodynamics.” The Washington State Board Authority’s review concluded the collapse was due to the “general proportions of the bridge, the type of girders, and floor.”  The reported cause was the “slippage of the center cable bond on the north cable due to forces it wasn’t strong enough to resist”

( lib.Washington.edu.2006).

            The Tacoma Narrows Bridge had a specific purpose when originally constructed.

It was an indispensable economic and military portal to the Olympic Peninsula, its completion was deemed, “ a triumph of man’s ingenuity and perseverance.” The funds

used to build the bridge was split; $2,880,000 came from the Public Works Administration, and $3,520,000 was a Reconstruction Finance Corporation loan (to be repaid through tolls)  The Pacific Bridge Company was commissioned to build the Narrows Bridge; which was later called, “the Pearl Harbor of Engineering,” after her

collapse.  Most engineers involved in the project were shocked my its downfall, but

accounts show the bridge began exhibiting, “wave-like motion,” in the final stages of construction.”

            Evidently the movement of the bridge was a concern from its inception. Yes,

there was a need to connect Tacoma with the Olympic peninsula, but to what extent?

No link between these lands were implemented before this time, therefore shouldn’t the engineers, and powers to be, initial do the job correct?  Damages were that of structure,

financial loss, cars, and one dog; but for the traffic the Tacoma saw each day, the potential for immense fatalities was pertinent.

            Salvaging the ruins was the next step, before conceiving plans for a new structure; also with the beginning of World War II  attempts to rebuild were negligent.   Salvage activity continued through l942, sending the “materials to the United States war effort and the profits saved for the construction of a new bridge.”  The National Register of Historic Places, since l992 has the Tacoma Bridge’s sunken remains to protect them from salvages.

            The design of the next Tacoma Narrows Bridge was “unique and had an effect on the construction of other suspension bridges that followed.”  The design integrated distinctive features such as; “ the open steel grid slots, the greater ratio of the depth of stiffening truss to span length, the double lateral system, the hydraulic energy absorbing, and dumping devices.  No other suspension span construction received as much engineering connotation in technology publications or as much nation-wide publicity than this Tacoma Narrows Bridge.

            Areodynamics was the key factor, “the first time a research program investigated the aerodynamic effects of wind acting upon a bridge” (nwrain.net).  Charles Andrews designed  the new structure, in l947, and Professor Farquharson directed the construction of a wind tunnel.  It was financed through a $14,000,000 bond issue, with bridge tolls repaying the loan.

            Construction started in l948, and was completed in October, l950.  The years

between were spent “studying aerodynamics,” to insure a safer structure.  It is the fifth longest suspension bridge in the United States, located between Tacoma and Gig Harbor; forty feet longer that the original bridge. Designed to carry 60,000 card daily, but handles an average of 90,000.

            Over fifty years later, a new Tacoma Narrows Bridge was built parallel and to the south of the l950 span.  It’s 5,400 feet, the “longest suspension bridge built in the United States since the Verrazan-Narrows Bridge opened in New York, in l964.”  It cost over

eight hundred forty-nine million dollars, was under construction for five years, opening to traffic on July 16, 2007.  The towers are made of concrete reinforced with steel, each are

8,500 cubic yards of concrete. Together the towers contain 2.9 million pounds of steel,

and the foundations weight over 85,000 tons each.  The new bridge is cherished by commuters with a significantly improved traffic flow that reduces the three to four hour backups (wsdot.wa.gov).

                                                    Works Cited

ejsus.com. “Torsional Movement. Expansion Joint Systems 2 May 2009

            http://www.ejsus.com

ketchum.org. “A short History of Galloping Gertie.”  History of the Tacoma Narrows

            Bridge. N.d. 2 May 2009. <http://www.ketchum.org/tacomacollapse.html

lib.Washington.edu.  “History of the Tacoma Narrows Bridge.” University of Washington

            24 Jan. 2006 3 May 2009 <http://www.lib.washington.edu/specialcoll/

            Exhibits/tnbpage5.html

nwrain.net. “Today’s Tacoma Narrows Bridge.” n.d. 3 May 2009 <http://www.nwrain.ne

Vuik, K. “The Tacoma Bridge” Tacoma Narrows Bridge Failure.  n.d. 1 May 2009

            <http://ta.twi.tudelft.nl/nw/users/vuik/informatioin/tacoma_eng.html

wsdot.wa.gov. “SR 16 New Tacoma Narrows Bridge.” Washington State Department

            of Transportation 14 July 2007. 3 May 2009. <http://www.wsdot.wa.gov/projects/

            sr16narrowsbridge.

 

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