Before the Morbi bridge collapse, there were incidents in China, Mexico, Colombia, and many others - from America to Vietnam. The common factor in all these tragedies? They were all suspension bridges.
THE HISTORY
They are one of the oldest forms of bridge engineering. Suspension bridges were constructed by many primitive communities, such as the Incas. In the earliest forms, a walkway was built on vines which were used as cables. A stronger variant emerged in India in 4th century CE. First plaited bamboo, then iron chains were used as cables.
However, till the 1800s, suspension bridges suffered major load-bearing issues, and were vulnerable to strong winds. JA Roebling solved this problem by adding web trusses on both sides of the pathway, making it stiffer.
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In the modern era, famous suspension bridges like the Brooklyn Bridge in New York City, and the Golden Gate in San Francisco have become iconic structures.
So how do these bridges actually work?
THE DESIGN
The basic design involves a pathway hung from cables or ropes. The cables are attached to tall towers. These towers support majority of the weight. The load on the bridge is managed by curved cables which are in tension. This load is then transferred to the towers, which in turn dissipate it into the ground. The pathway is suspended in air so it mustn't sway, and deck trusses are used to reduce this swinging.
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Suspension bridges have many advantages over other types, like arch, beam, cantilever, truss, and tied-arch bridges. But there are disadvantages as well.
PROS AND CONS
Suspension bridges involve lower construction costs, as compared to many other designs. However, their weight-bearing ability is limited by the strength of the material of cable used.
Suspension bridges are suitable for connecting longer distances than other types of bridges, but their usability is limited as excessive weight can break the cables. These structures are capable of withstanding earthquakes, but are weak against strong, stormy winds.
Over the years, many suspension bridges have faced disasters. Let's take a look at what happened, and why.
PAST DISASTERS
One of the most well-documented case is of the Tacoma Narrows Bridge in Washington state, USA. On 7 November 1940, the bridge suspenders broke, and it collapsed. However, since it had been closed in time, there were no casualties. The mishap occurred because the bridge couldn't handle wind gusts at 67 km/hour. The roadway did not have web trusses for extra strength, and was thus too flexible in the strong wind.
Almost two decades later, the Peace River Bridge in British Columbia, Canada collapsed. On 16 October 1957, the bridge's suspension cables broke and fell into the river. No one injured as the bridge was closed to traffic. The cause of the cable rupture was a landslide putting pressure on the bridge. A million cubic metres of shale had reportedly hit a part of the structure over 12 hours.
More recently, it was the Chu Va 6 bridge in Vietnam. On 24 February 2014, the bridge collapsed while a funeral procession was crossing it. 8 people died and 39 were injured after falling into a ravine full of sharp rocks. An investigation reportedly found that faulty turnbuckles had been used in the bridge and these broke. There were also technical errors in the design. The builder and two government officials were reportedly jailed for 10 years.
Four years later, tragedy stuck Chirajara, Colombia. On 15 January 2018, a partially-constructed suspension bridge collapsed. At least 10 workers died, and several were injured. Investigators reportedly said that the mishap occurred due to a flaw in the bridge's design, including wrong assumptions about the capacity of support piles.
Last year, disaster hit a tourist attraction in China - the Longjing glass bridge in Zhejian. On 7 May 2021, the glass walkway on the suspension bridge shattered. Videos captured a man hanging by railing for over 30 minutes before finally being rescued. Local authorities blamed extreme weather for the event. High-speed 150 km/hr winds apparently broke the glass panes on the walkway.
Just months before Morbi, the Cuernavaca bridge in Morelos, Mexico saw a tragedy. On 7 June 2022, the suspension bridge collapsed 2 metres above a stream and rocks. At least 14 people were injured, including the local mayor and his wife. Experts said that the collapse could have occurred due to more people being on the bridge than it could handle. Witnesses also said that one person was jumping on the bridge before it fell. This could have caused added stress to the structure.
THE MORBI TRAGEDY
The exact cause of the deadly Morbi bridge collapse is not known yet. Experts have pointed out that multiple things could have gone wrong. These include overcrowding; cables made weak with age; the walkway being converted from wooden to aluminium, thus changing the basic structure; and weak connection points attaching the vertical cables to the deck.