Main Suspension Cable Failure Likely Cause of Morbi Bridge Disaster

Here’s what we know so far about the deadly bridge failure in India.

The morning after. Morbi pedestrian deck collapsed after suspension cable gave way.bridge

The morning after. Morbi pedestrian deck collapsed after suspension cable gave way.

A 143-year-old pedestrian suspension bridge over the Machchu River in Morbi in the state of Gujarat, India, collapsed at 6:32 p.m. on October 30—just 4 days after it had been re-opened
with great fanfare culminating 6 months of renovation. An estimated 250 people were on the bridge at the time
it collapsed, according to one source. It appears as if a main suspension cable
snapped, du
mping screaming men, women and children into the river. From figures released today, 135 died, 54 of them children, most under 10 years old, the youngest a 2-year-old boy.

Some victims are thought to have hit the rocks only 10 feet under the water on the riverbed near the banks of the river, according to the commander of the National Disaster Response Force (NDRF) on the scene. The river is about 30 feet deep in the middle.

Basic structural components of a suspension bridge. (Picture courtesy of study.com)

Basic structural components of a suspension bridge. (Picture courtesy of study.com)

Videos posted on social media show individuals crowded on the bridge, some attempting to rock the bridge as if it was a swing. This activity is immediately seized on
by commenters as the cause of the bridge failure. It appears that making the bridge swing is so popular an activity that the bridge is colloquially called “jhoota pool,” or the swinging bridge. The private company that sold tickets to let people stand on the bridge has been condemned in the court of social media for letting too many people on. 

But as structural engineers point out in our online forum, Eng-Tips, a bridge designed by today’s standards should be able to withstand a much bigger static and dynamic load than what the Morbi bridge experienced that fateful day. The real answer may not be the deck plates
below that swung down like trap doors, or men pulling on vertical cables, or rocking the bridge, but the aging, rusty main suspension cables
above on which the vertical cables hang. These cables may have been untouched in the bridge’s renovation.

Where is Morbi?

Morbi is on the Western coast of India, 1100 km southwest of Delhi, with a population of 210,000 (2011 census). It is best known for its production of ceramic goods, such as tiles and bathroom products. The area accounts for 80 percent of the ceramic products produced in the country, according to Sky News. The industry employs many migrant workers and, no doubt, several were on the bridge when it collapsed.

What Do We Know About the Bridge?

The 230 m (755 ft) long Morbi pedestrian bridge, AKA, the swinging bridge, was built in 1879 during the Victorian era of British occupation. The bridge is described as “a nostalgic reminder” of a bygone era and as a “artistic and technological marvel of the period” on the state’s tourism site.

Much of the infrastructure built by the British has been poorly maintained by an
India independent since 1947. In addition, reports of corruption and bribes swirl around infrastructure, building and maintenance contracts in India. Incidents of mass deaths are
horribly frequent. In 2019, a footbridge in Mumbai’s (previously known as Bombay) Chhatrapati Shivaji Railway Terminus building collapsed, killing six. In 2011, more than 30 died when a bridge near Darjeeling collapsed.

What Do We Know About the Bridge Renovations?

The bent aluminum(?) plates on the Morbi bridge had been replaced. Picture from Eng-Tips.com.

Pre-renovation. Distorted aluminum deck plates on the Morbi bridge. Picture from Eng-Tips.com.

The bridge has been closed for renovation for six months but was reopened to
the public a week before the collapse, according to Reuters. The bridge
initially had wooden planks for the deck which were replaced with what appears
to be aluminum plates, which were probably replaced in the recent renovation.
Not much detail has emerged on what the renovations did to the bridge and more
attention is being paid to what was not done. The main cable that broke may have been untouched.

The local police officer investigating the disaster, Deputy Superintendent of Police P. A. Zala, is quoted in India Express as saying that a main cable on the bridge was “rusted” and “had the cable been repaired, this incident would not have happened.”

The bridge was reopened without a “fitness certificate.”

Who Celebrates on a Bridge?

Apparently, celebrating on the bridge is a common practice in Morbi. Some 400 people had bought tickets to be on the bridge in celebration of Diwali and Chahath Puja (the locally celebrated New Year), reported Reuters.

Such celebrations bring crowds into the streets all over India—and in Morbi, onto the bridge. Videos posted on social media show people packed onto the bridge.
Most of them appear to be still, with no intention of crossing to the other
side. Some men are shown moving in rhythm trying to make the bridge swing, while others seem to be pulling on the cables. No music is heard on the video, so rhythmic dancing, the cause
of the 1981 collapse of a skywalk in the Kansas City Hyatt Regency Hotel, can be ruled out. Video shows the hangers (vertical cables) on one side of the bridge going slack all at once, indicating the failure of a main cable (one of the two continuous cables that arches between the bridge’s anchorage and the towers). Losing one of the main cables made the deck drop like one long continuous trapdoor,
dumping people into the river.

How Much of a Load Should the Bridge Take?

Surprising as it may seem, bridges have the highest load per square foot when they are packed with people. If the pedestrians are walking, there is a distance between them, but if they stop walking, people will
get packed together, increasing the load per square foot to the maximum. San Francisco’s Golden Gate Bridge strained and flattened during its 50-year celebration when it was briefly closed to traffic and pedestrians packed its 1.7-mile length. Of course, a pedestrian bridge with no vehicular traffic whatsoever should be expected to withstand a deck packed with people.

Local politician Jayrajsinh Jadeja blamed Oreva, the private company in
charge of the bridge, for selling tickets without restrictions and said that overcrowding led to the bridge collapse.

With a person every 2 square feet, a 49-inch-wide deck could have held a total of 500 people, should they have been arranged in a neat grid. The best estimate of the actual crowd
on the bridge was half of that.

A structural engineer quoted on Eng-Tips.com estimates that the bridge could have supported the weight of over 3,000 people, even if the average person on the bridge weighed 300 pounds and still have had a 1.5 factor of safety. The cable normally used for the bridge (AFAIK wired cable) has a safety factor of 6 or 7 and would not fail unless “unless the cross section has suffered serious corrosion.”

The Cable in Question

  An India Times reporter surveys the broken main cable of the Morbi bridge.

An India Times reporter surveys the broken main cable of the Morbi bridge.
Close-up view of the damaged cable. “This seems like a rust exterior with many of the cable elements broken with what remains looking very like a stretch to beyond yield. The lower strand has actually unwound, so if you wound it back, I think it would be pretty much level with all the other broken ends. I suspect this is either at the top of the tower or one of the first long vertical hangers. It looks like about 2/3 of the wire core had already snapped before the events of the reopening took its toll,” said petroleum engineer LittleInch on Eng-Tips.com.

Close-up view of the damaged cable. “This seems like a rust exterior with many of the cable elements broken with what remains looking very like a stretch to beyond yield. The lower strand has actually unwound, so if you wound it back, I think it would be pretty much level with all the other broken ends. I suspect this is either at the top of the tower or one of the first long vertical hangers. It looks like about 2/3 of the wire core had already snapped before the events of the reopening took its toll,” said LittleInch,
an engineer on Eng-Tips.com.

An online forensic study (no substitute for on-the-scene forensics, of
course) points to a main suspension cable break as the cause of the disaster.

What’s Happening to the Company in Charge of Renovating the Bridge?

Police have arrested nine people connected with Oreva, the private
company that has a 15-year exclusive contract for operating the bridge. Oreva
was responsible for maintaining the bridge, which included selling tickets
to those who wanted to celebrate on the bridge. They may be charged with “culpable homicide not amounting to murder,” said senior police officer Ashok Kumar Yadav, quoted in BBC News. “Of these nine, two work as managers, while two work as ticket booking clerks [all employed by Oreva] at the bridge site.” The others who were arrested include people who may have been contracted by Oreva, such as two people contracted to repair the structure, as well as security personnel at the bridge.

The choice of Oreva, once the “world’s largest maker of wall clocks” before switching to lighting products and electric bikes, is
being questioned. Why would the local government would hand off responsibility for bridge building, maintenance and safety to a company that appears to have no qualifications for it? It’s likely that Oreva contracted construction crews that did have bridge experience,
but that has not yet been confirmed. No one from Oreva has commented on the disaster as of the time of writing.

The managing director of Oreva was recorded at a press conference at the re-inauguration of the Morbi bridge on October 26
saying “the bridge won’t need any major repair work for at lest 8-10 years now[i].”

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[i] Posted on  Twitter https://twitter.com/himansshhi/status/1587086806113480705