Epic structural failures

By - Indrajeet Chauhan

What are structures?

The structure of a building (or other object) is the part which is responsible
for maintaining the shape of the building under the influence of the
forces, loads and other environmental factors to which it is subjected. It
is important that the structure as a whole (or any part of it) does not fall
down, break or deform to an unacceptable degree when subjected to such
forces or loads.

Hoan Bridge

A little background information.

The Daniel Webster Hoan Memorial Bridge, part of the I-794 Lake Freeway in downtown Milwaukee, was constructed from 1970 through 1972 (though it opened to traffic in 1977) and spans the entryway to Milwaukee's inner harbor and the mouth of the Menomonee and Kinnnickinnic Rivers. It is a landmark structure seen from many southside streets and freeways and in 1975 won awards from the American Institute of Steel Construction (the "Long Span Bridge Award") and from the American Society of Civil Engineers.

What happened?

The Hoan Bridge was temporarily closed on December 13, 2000, after two of the three support beams of the lakefront span failed, causing the north-bound lanes to buckle and sag by several feet and leaving the span in a near collapsed state. No motorists were injured when the bridge failed. The affected span did not collapse, though, as the third and final support beam kept it from failing, although one source notes "the final girder sustained considerable stress damage until the bridge span fell two weeks later."


the bridge failure began in a joint on the center girder of the northbound section. The stresses of extreme cold and traffic contributed to that failure. Research results from the failed Hoan Bridge span are now being communicated by FHWA to state DOTs so they can check similar bridges to prevent failures. A two-day workshop that shared this information was held in September 2001 with more than 100 transportation officials from all over the United States. Through its partnership with FHWA, WisDOT reconstructed the missing span and reopened the bridge to traffic in October 2001.


On December 28, 2000, engineers used explosives to remove the damaged section. The damaged span was restricted to one lane in each direction for eight months while it was reconstructed, and the remainder of the bridge underwent extensive rehabilitation and retrofitting. Two lanes in each direction were reintroduced on October 10, 2001, and the bridge was fully reopened the following month.According to the Milwaukee Journal-Sentinel, at the time of its failure, the six lanes of the bridge had carried an average of only 36,590 cars per day. A total of $16 million was spent to demolish and replace the damaged section and retrofit the remainder of the bridge.Experts believe that improperly designed welds between the lower lateral bracing and floorbeams along with a period of extreme cold and snow led to the partial collapse of the Hoan Bridge.

Tacoma Narrows Bridge

A little background Info.

The Tacoma Narrows Bridge is a pair of twin suspension bridges that span the Tacoma narrows strait of Puget sound in Pierce County,Washington. The bridges connects the city of Tacoma with the Kitsap peninsula and carry State route 16 (known as Primary state highway 14 until 1964) over the strait. Historically, the name "Tacoma Narrows Bridge" has applied to the original bridge nicknamed "Galloping Gertie" which opened in July 1940 but collapsed due to aeroelastic flutter four months later, as well as the replacement of the original bridge which opened in 1950 and still stands today as the westbound lanes of the present-day twin bridge complex.


(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 failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 miles per hour (64 km/h). This is a so-called torsional vibration mode (which is different from the transversal or longitudinal vibration mode), whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line of the road remaining still. Specifically, it was the "second" torsional mode, in which the midpoint of the bridge remained motionless while the two halves of the bridge twisted in opposite directions. Two men proved this point by walking along the center line, unaffected by the flapping of the roadway rising and falling to each side. This vibration was caused by aeroelastic fluttering

Fluttering is a physical phenomenon in which several degrees of freedom of a structure become coupled in an unstable oscillation driven by the wind. This movement inserts energy to the bridge during each cycle so that it neutralizes the natural damping of the structure; the composed system (bridge-fluid) therefore behaves as if it had an effective negative damping, leading to an exponentially growing response. In other words, the oscillations increase in amplitude with each cycle because the wind pumps in more energy than the flexing of the structure can dissipate, and finally drives the bridge toward failure due to excessive deflection and stress. The wind speed that causes the beginning of the fluttering phenomenon (when the effective damping becomes zero) is known as the flutter velocity. Fluttering occurs even in low-velocity winds with steady flow. Hence, bridge design must ensure that flutter velocity will be higher than the maximum mean wind speed present at the site.


The wind-induced collapse occurred on November 7, 1940, at 11:00 a.m. (Pacific time), because of a physical phenomenon known as aeroelastic flutter. No human life was lost in the collapse of the bridge. Tubby, a black male cocker spaniel, was the only fatality of the Tacoma Narrows Bridge disaster; he was lost along with Coatsworth's car.


Because of materials and labor shortages as a result of the involvement of the United States in World War II, it took 10 years before a replacement bridge was opened to traffic. This replacement bridge is 5,979 feet (1,822 m) long—40 feet (12 m) longer than Galloping Gertie. The replacement bridge also has more lanes than the original bridge, which only had two traffic lanes, plus shoulders on both sides.Half a century later, the rebuilt bridge that was completed in 1950 was exceeding its traffic capacity, and a second, parallel suspension bridge was constructed to carry eastbound traffic. The suspension bridge that was completed in 1950 was reconfigured to solely carry westbound traffic. The new parallel bridge opened to traffic in July 2007.

The Tacoma Narrows bridge failure has given us invaluable information...It has shown that every new structure that projects into new fields of magnitude involves new problems for the solution of which neither theory nor practical experience furnish an adequate guide. It is then that we must rely largely on judgement and if, as a result, errors, or failures occur, we must accept them as a price for human progress.

The End.