Failure Knowledge Database / 100 Selected Cases

Collapse of Tacoma Narrows Bridge
November 7, 1940 in Tacoma, Washington, USAMasayuki Nakao (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)
The Tacoma Narrows Bridge i n the stat e of Washington was com pleted and opened t o traffic on July 1,
1940. The bridge was the first of i ts type to employ plate girders to support the roadbed. Shortly after i ts
construction, the bridge w as disc overed to sw ay and bu ckle da ngerously in windy co nditions. O n
November 7, just four months after the opening, it collapsed due to wind-induced oscillations (19 m/sec ofwind velocity). No human life was lost in the collapse of the bridge.
1. Event
The Tacoma Narrows Bridge at Puget Sound in the state of Washington was completed and opened totraffic o n Ju ly 1, 19 40. O n N ovember 7, j ust fo ur m onths after the opening, it c ollapsed d ue t owind-induced oscillations. No human life was lost in the accident.
2. Course
The Tacoma Narrows Bridge w as completed and opened to traffic on July 1, 1940. It stre tched like asteel ribbon across the Tacoma Narrows in Pu get Sound near the city of Tacoma, Washington (Figure1). It was the third longest suspension bridge of its time with a center span of 853 meters, and had sleek
appearance with i ts l ength in comparison to i ts w idth and thickness of 11.9-meter w ide road bed
providing two traffic lanes and sidewalks (Figure 2). It was a narrow bridge for its long length.

Figure 1. Location of Tacoma Narrows Bridge [1]1

Failure Knowledge Database / 100 Selected Cases

Figure 2. Structure of Tacoma Narrows Bridge [1]Shortly aft er its c onstruction, t he bridge was disc overed t o sw ay an d b uckle dangerously al ong its
length in win dy co nditions. En gineers wereconducting wi nd tunnel ex periments on

airflowcharacteristics around the bridge structure.
When th e b ridge began heav ing vi olently o n Nov ember 7 , the a uthorities notified Professor F . B.
Farquharson of the University of Washington who had been conducting wind tunnel experiments with amodel of the bridge. The professor and his research team recorded the bridge with a camera.

Figure 3. Movements of Tacoma Narrows Bridge [1]While the wind was not extraordinary, the bridge was undulating noticeably, and the center stay wasvibrating torsionally in 9 se gments with a freque ncy 3 6 cy cles/min. The am plitude of t he t orsional
vibration quickly built up in an hour when the north center stay broke and the motion changed from arhythmic rising a nd falling to a two-wav e twisting motion. The bridge twisted violently in two parts
with frequ ency 14 v ib/min, i n w hich the m idpoint of the br idge re mained m otionless w hile t he tw ohalves of the bridge tw isted in opposite d irections (Fig ure 3). T his c atastrophic tw isting m otion wasprobably start ed by the fa ilure of cable band o n the north end, w hich w as c onnected t o t he center
diagonal ties. The twisting motion caused high stresses throughout the bridge, which lead to the failureof the suspenders and collapse of the main span (located near the vehicle in Photo 1).
The weight of the spans sagging into the river pulled the towers towards them, bridge began cracking,and the entire bridge crashed down (Photo 2).

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Failure Knowledge Database / 100 Selected Cases

Photo 1. Twisting Motion of Tacoma Narrows Bridge [1]Photo 2. Breakdown of Tacoma Narrows Bridge [1]3. Cause3

Failure Knowledge Database / 100 Selected Cases
Self-excited o scillation induced by la teral w ind w as respons ible f or the collapse. The cause w as
“unknown” rather than “ignorance.” The self-destruction of the bridge, in fact, took place when windtunnel ex periments w ere underw ay. The Tacoma N arrows Bridg e w as one of th e suspe nsion br idges
designed a pplying t he “ deflection theory”, w hich h ad been form ulated i n A ustria for concre te archbridges. Leon Mo isseiff, a brid ge e ngineer a nd m athematician, applied the t heory for susp ension
bridges, ca lculated th e s tresses a nd c oncluded that she ar a nd be nding l oads are partly c arried i n t hecables, rather than relying on stiffening trusses. The bridge relied on the dead load (the weight of thedeck, m ain c ables an d s uspender ca bles) for its rigi dity with l ittle inherent str uctural d amping. This
theory allowed reducing the amount of stiffening material and the construction costs. It seemed an ideal
design a pproach for long-span sus pension bridges. H owever, the designer e xtended th e s lender spa nconcept too far , an d the n ovel d esign caused t he bridge to b e e xcessively fle xible. T he girder on
Tacoma-Narrow bridge then had flat H shapes as P hoto 3 shows. The new bridge was redesigned andrebuilt with open trusses, stiffening struts and openings in the roadbed to let wind through, allowing
less twisting than the previous design (Photo 4).

Photo 3. The former Tacoma Bridge (its bridge girder has a flat H form) [1]
The investigation of the cause of the failure and wind tunnel testing of 3-d scale model concluded that:(1) The exceptional flexibility and small resistance against twisting of the bridge allowed it to pickup
the oscillation quickly.
(2) The shape was aerodynamically unstable. The H-shaped girders allowed the air flow to easilyseparate at the edges, and the vortex generation happened to match the oscillation of the girders.
The wind-generated vortices moved the girders that then generated new vortices. The designers
were unaware of this mechanism of wind excited vibration.
4. Immediate Action
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Failure Knowledge Database / 100 Selected Cases
The Federal Works Agency appointed three engineers to investigate the failure: Theodore von Kármán
(a hydrodynamic expert well-known for hits Von Kármán vortex), Othmar B. Ammann (the consultingengineer for t he George Washington Bridge), and Glenn B. Woodruff (the c onsulting engineer for theGolden Gate Bridge). They issued their report in a little over four months after the collapse occurred.
This repo rt exonerated th e b ridge designers an d en gineers s aying t hat “ the Tacoma Narrows Bridge
was well desi gned and built to resis t safely all sta tic forces, including wind, usually considered in thedesign of sim ilar stru ctures. … It w as not rea lized t hat t he aer odynamic forces w hich had pr ovendisastrous in the past to much lighter and shorter flexible suspension bridges would affect a structure ofsuch m agnitude as the Tacoma Narrows Bridge”. The report then r ecommend further resear ch andtesting to develop the methods used to calculate aerodynamic forces acting on suspension bridges.
5. Countermeasure
The new bridge was redesigned and rebuilt in 1960 with open trusses, stiffening struts and openings inthe roadbed to let wind through, allowing less twisting than the previous design (Photo 4).

Photo 4. New Tacoma Narrows Bridge (with Open Trusses) [1]6. Summary
The T acoma Narrows Bridge c ollapsed due to w ind-induced osc illations. Profes sor Farquh arson’srecordings of th e c ollapse an d w ind tunnel t esting conducted s ubsequently helped c larifying t hemechanism of vibration and the importance of rigid girders. The method of dynamic analysis from thelessons gave guidelines for designing suspension bridges thereafter.
7. Knowledge(1) Self-excited oscillation must be taken in consideration when engineering a s tructure. Otherwise, abridge can fall. Self-excited oscillation is typically observed as “chatter” that occurs between themachine tool and the workpiece. This chatter can break tools and make machine tools dance during
5Failure Knowledge Database / 100 Selected Cases
the process.
(2) The lessons learned from failures w ill help advancing knowledge and technologies. Good records
can turn even the w orst dis aster i nto val uable ass ets t o te chnologies. The history of fail ures is a
priceless record of human experiences.
8. Background
Leon Moisseiff who designed the Tacoma Narrows Bridge was one of the world-renowned suspension
bridge en gineers at t hat time. He im plemented t he deflection t heory in h is design t o justify thesubstantial re duction i n str engthening m aterials, b elieving t hat the dead w eight of the bridge would
suppress the vibrations caused by wind and traffic. He believed that the suspended structure would act
as a counterweight and restore the bridge to equilibrium, if a bridge were designed flexible enough tobend and sway with the winds. The longer, lighter and narrower bridge design enabled to reduce theamount of st eel ne eded t o build s uspension bri dges. L ess steel greatly reduce d t he cost of a br idge,
which was appreciated during the Great Depression.
References[1] Yotaro Hatamura (Editor), Jissai-no Sekkei (Practical Design) Research Foundation (1996) Zoku-Zoku
Jissai-no Sekkei (Practical Design III), The Nikkan Kogyo Shimbun, LTD.

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