The proper functionality of any state or country relies heavily on the infrastructure of that particular region. Infrastructure facilitates transportation and ensures a mode of connectivity. Bridges in particular have the responsibility of maintaining their structural integrity while keeping the architecture of things in check.

Bridges need to be designed so that they are safe as well as workable during their lifetime. For that, a detailed analysis is done before construction. Various loads are estimated, soil conditions are tested, appropriate materials are selected—all with the aim of optimizing the strength of the structure. But when the necessary calculations aren’t done, or the design is questionable, the consequences can be dire. The following three cases highlight that very fact.

The Quebec Bridge

Quebec Bridge and its Chord A9L. (Image courtesy of Semantic Scholar.)

The Quebec Bridge, a cantilever structure made to be suspended over the St. Lawrence River, was a design by Theodore Cooper—an engineer with many earlier reputable designs. While working on this project, his objective was to take a comparatively less expensive route, and in doing so he set up the structure for disaster.

Cooper’s design placed the piers in such a manner that the bridge spans were extended from the original 490 m span to 550 m, thereby introducing very high stresses in the structure. On top of that, he left Peter Szlapka—a less experienced engineer—in charge of the construction site management. The limitations soon started to manifest in the physical form of the bridge, as engineer Norman McLure pointed out. Upon his inspection, he found out that the lower compression chords at the south end were compromised, and on his follow-ups the deflection had grown further. Though McLure’s concerns reached Cooper, nothing was done to address the problem.

The bridge, which collapsed in August 1907, was doomed when the first chord—namely Chord A9L—had a buckling failure. As a result, the Chord A9R also gave in and the collapse followed soon afterwards, killing 75 workers and injuring 11. This, however, was not the end of the story.

Construction for another bridge was initiated in 1913 at the same location. This time around, three engineers were to contribute to the work: H.E. Vautelet, a former Canadian Pacific Railways’ engineer who was to head the project, Ralph Modjeski and Maurice Fitzmaurice (who was especially credited on his work for the Forth Bridge). As the previous accident had been a result of chord malfunction, special attention was paid towards the chords and a stronger alternative was used. A K-truss design was also added.

Everything went according to plan at first. The two cantilevers were placed opposite each other, and just the middle section was left to be hoisted into position. The span—which weighed 5,100 tons—was to be lifted two feet at a time using a hydraulic mechanism of lifting hangers (attached on each arm), until it was safely attached to the cantilevers.

However, after five lifts of the southern limb and four lifts of the northern limb, the workers left the project for a break only to come back later to find a detached southwest end. The situation escalated fast, and soon the other ends were ripped as well and the structure collapsed into the river underneath. Dated September 11, 1916, this incident took the lives of 13 workers and injured many others.

Tacoma Narrows Bridge

Tacoma Narrow Bridge’s collapse in 1940. (Image courtesy of Britannica.)

On November 7,1940, the Tacoma Narrows Bridge in Pierce County, Washington collapsed. The disaster was wholly attributed to a flawed structural design.

The initial proposal—pitched by the engineer Clark Eldridge—suggested a suspension bridge made from trusses, each 25 feet high, with a cost of approximately $11 million. However, due to a considerable price margin, an $8 million design by engineer Leon Moisseiff, with 8-foot-long plate girders, was chosen instead.

The structure got its name “Galloping Gertie” during construction, when subtle winds would prompt its up-and-down oscillations. This, of course, raised some concerns. A few options—50-ton concrete chunks, hydraulic buffers for the main span, and anchoring cables—did not pan out as expected. A professor from University of Washington, Frederick Farquharson, was eventually invited for wind-based analysis of the structure. Against the wind speed of 22 m/s, his models displayed some torsional action—which, in his words, could be “the end of the bridge.” This was eventually the case.

On November 7, the recorded winds were raging at 19 m/s and the narrow, flexible bridge—due to its low depth-to-width ratio—was in a torsional motion of about 36 cycles/min. This was torsional flutter, a phenomenon described by the Innovative Bridge Design Handbook as “coupled oscillations in bending and torsion of the deck, fed and amplified by the action of the wind.”

A vortex shedding was also created as a result of the wind hitting the bridge’s deck. The wind separating effect grew as a consequence of the twisting, in turn generating a vortex—a rotating region within the fluid.

Both the torsional cycle and the vortex synced together to trigger a “self-generating” tendency of the bridge, so that it was itself creating the force that caused its torsional motion. The more the bridge twisted, the more it facilitated further twisting, and thus its failure was inevitable.

The visible wave-like motion of the bridge was terror-inducing, but luckily all the people had been evacuated in time. Only a dog ended up losing its life.

Kutai Kertanegara Bridge

Kutai Kertanegara bridge after its collapse in 2011. (Image courtesy of New Civil Engineer.)

The Kutai Kertanegara suspension bridge, which was built over the Mahakam River and connected Samarinda and Tenggarong in Indonesia, was in its entirety a 710-meter-long structure out of which 270 meters of the length was suspended. The project’s construction was initiated in 1995 and completed by 2001. The bridge functioned well enough until November 26, 2011.

According to a report by New Civil Engineer, one of the towers’ foundations was displaced horizontally by 200mm, which resulted in the sagging of the structure; what was originally a 3.7m rise at midspan was reduced to 3m. Maintenance ensued to restore the bridge to its initial state.

A contractor jacked up one end of the bridge deck by 150 mm, while keeping the other end open for traffic. This shifted extra weight on certain hanger connections. The engineers, however, failed to take this effect into consideration while doing their analysis. Due to the additional loading, a shear failure occurred at the bolts, causing one connection to break apart. As a result, the load compounded on the other connections, and provoked the structure to collapse within a mere 20 seconds.

Most design practices incorporate a degree of safety with regard to cable connections, ensuring that if one cable fails, a system would back up the redistributed weight while the necessary replacements are made later. “Indonesia does not have any suspension bridge design codes and engineers used the Japanese equivalent, but it is unclear how accurately these were adhered to,” stated engineer Declan Lynch.

After the collapse, it was discovered upon inspection that the bolts were made of cast iron, which was brittle. Thus, the structure was prone to immediate failure upon the application of a certain amount of force. Normal construction projects make use of ductile components, as they provide a warning before they break, leaving room for maintenance when needed. This was not the case with the Kutai Kertanegara bridge.

Apart from the structural damage, the incident cost 20 individuals their lives, and left 40 others injured, while 19 remained missing.

Though these bridges collapsed, they proved to be good case studies for structural engineers. Under the spotlight, details of the incidents have been analyzed over time to assess where mistakes were made—so that future approaches are different.