Group 4
TACOMA NARROWS BRIDGE FAILURE
Conor Foy, Kieran Gaston
Killian Shields, Ronan Foley
Group 4
Introduction
The first Tacoma Narrows Bridge opened to traffic on July 1, 1940 Leon Moissieff. Spanning
over a mile it was the third largest suspension bridge in the world and soon to become one of
engineering’s most fabled disasters, revealing the errors in structural design that ultimately
led to its dramatic collapse.
Even during the construction phase unusual things happened to the bridge. Gentle breezes
caused vertical oscillation of the roadbed while stronger breezes often had no effect. A
variety of band-aids were installed to combat this prior to opening and the University of
Washington committed to further study the problem.
The bridge was nicknamed Galloping Gertie because of its undulating center span, and
tourists drove miles to experience it. Only four months after the bridges inauguration on
November 7, 1940, these undulations became so violent that with each twist the motion grew
stronger, causing one of the suspension cables to break. The load became too much for the
others cables, which caused them to break also, and made the bridge collapse.
The Design
The Tacoma Bridge was a suspended bridge of total length 1810 metres. Its middle span was
853 metres in length. It was designed to support two road lanes. The bridge deck was only 12
metres in width, and had a height of 2.45 metres. The bridge was revolutionary in its design
and historic in its collapse. Its failure marked the end of a trend in bridge engineering where a
maximum of lightness, grace and flexibility were the goal. At that time the construction
industry valued structural grace and slenderness to achieve an artistic appearance.
Although unique the bridge still contained all the fundamentals of a suspension bridge. It
used huge steel cables to support the slender deck. The main cables were innovatively
connected to the two piers constructed on the east and west sides of the river allowing them
to distribute the compressive load down through the piers while the cables themselves,
secured deep within the anchorages on either side of the river and remained under constant
tension. The connections at the top of the towers were rigid which wasn’t like most
suspension bridges at the time which had rolling saddles which allowed the cables to roll
over.
Stiffening trusses were the primary tool in counteracting vertical and torsional motions
caused mostly by wind loading at the time. They were usually placed on the underside of a
bridge deck but for the Tacoma Narrows, solid steel plate I beams were used instead. They
measured only 8ft deep which is extremely shallow for a bridge of this size. The beams were
solid and unlike trusses didn’t allow any wind to pass through. Even after the addition of the
concrete roadway the bridge remained the lightest and most flexible suspension bridge of its
kind at the time.
Lead up to failure:
As this bridge used solid I-beams to support the road, the wind had to pass above and below
the deck. This was one of the critical design faults. Engineers found that the solid girders actually
blocked the wind and caused the slender bridge to twist, fig1.
Shortly after its construction the bridge began sway and buckle dangerously in windy
conditions. These undulating vibrations were longitudinal, which are shown below in fig2.
This means that the road was alternatively raised and depressed in certain locations. However
designers and engineers deemed the mass of the deck heavy enough to provide stability so it
wouldn’t collapse.
Fig1
Failure
The bridges destruction was due to a combination of structural and aerodynamic stability
issues (mainly Anti dampening). The designer failed to account for the aerodynamic forces
exerted on the structure. Some buildings react violently as their natural frequency aligns with
the disturbance’s frequency (seen after earthquakes, when some buildings remain standing
beside others which have completely collapsed). This however, was not the case for the
Tacoma narrows bridge, because the natural frequency of the isolated structure was not near
the frequency of the destructive mode. It has been demonstrated that the ultimate failure of
the bridge was in fact related to the aerodynamically induced condition of self-excitation or
‘negative damping’. The ‘aerolastic’ phenomenon involved was an interactive one in which
developed wind forces were strongly linked to structural motion and shows that forced
resonance and self-excitation are fundamentally different phenomena. In this case the
alternating forces that starts the motion is created and controlled by the motion itself. The
wind force which acted on the large area of the ‘H’ shaped girders caused aerodynamic lift as
pressure below the span is greater than that above the span. A moment is induced causing the
twisting motion.
Twisting occurred which instead of being longitudinal, as was the common design problem at
the time, was in this case torsional. The left side would go down and the right side would rise
and then alternate with the right side falling and the left rising while the centreline of the
bridge would remain steady in a fixed position. This is known as ‘second torsional mode’
Anti Dampening.
Fig2
The bridge’s angle of attack rose (like an aerodynamic fin) therefore giving more lift,
twisting the deck even more until it aerodynamically stalls. The elastic energy stored in the
deck returned the deck in the opposite direction. It now gets downwards lift and it continues
to be pushed downwards until the deck stalls and the process is repeated.
With each cycle the amplitude increased because the wind added more energy than the work
done in flexing the deck could dissipate. The ever increasing amplitude caused an overload to
some of the suspender cables which critically weakened the bridge. Once a few of the cables
broke, the weight of the deck could not be supported and it fell.
Preventing Similar Failure:
 Understand forces of nature in particular wind.
 Understand dynamics of the structure that is being designed
 Suspension bridges should have sufficient stiffening supports.
 Consider location and geography
 Sufficient testing with models with “in situ” conditions.
 Determine and design for possible “freak” conditions.
Conclusion:
The collapse of the Tacoma Narrows Bridge can be put down to a lack of understanding of
the effects of Wind loading. Even a great Engineer like Leon Moissieff was unaware of the
danger of aerodynamic oscillations in suspension bridges. No precautions were taken for this
when designing the bridge. However the collapse serves as a harsh reminder to modern
engineers of the capabilities of external and natural forces upon a design.
New Design
The new design called for a wider bridge that
was heavier and sturdier than the original.
 The girders were open trusses that
offered less wind resistance than the
solid girders of the first bridge.
 The bending stiffness of the trusses
was increased providing more twisting
resistance.
 The stiffening trusses were of a box
design for torsional stiffness.
 Wind tunnel analysis was used for the
first time while designing the new
bridge. It gave the engineers a better
understanding of aerodynamics and wind loading. This has become an essential when
designing modern day suspension bridges.