Inadequate structural stability during construction of the Kolkata flyover project could have triggered its fatal collapse, according to engineers.
The failure of the under-construction flyover occurred last month and is reported to have killed at least 26 people and injured many more in the densely populated Girish Park area in India’s West Bengal state.
“[The structure] appears to be a steel-composite bridge with six plate girders supporting a concrete deck slab, all resting on steel crossheads and one or two steel columns,” explained bridge consultant Simon Bourne.
The construction site is located near a busy road and surrounded by bustling neighbourhoods with narrow streets and buildings constructed with little space between, making initial rescue and recovery efforts more challenging.
Prior to the fatal incident, contractor IVRCL had completed 59 sections of the Vivekananda Flyover with a total length of around 900m. The collapsed section is the 60th. Detailed images of the scene appear to show the collapsed concrete deck had been resting on a steel crosshead, over a single supporting central column.
Kolkata flyover collapse
Source: NEWZULU/Tanmoy Bhaduri/NEWZULU/PA Images
The failure looks to have affected two spans of the concrete deck which have been displaced, leaving the supporting steel crosshead and column exposed. The first span of concrete (pictured above), visible in the background, is likely to have been cast at least a week before the incident occurred. This section of the concrete deck looks to have partially collapsed in two halves, with a longitudinal steel girder piercing upwards through space in-between.
The second span of the collapsed deck is just visible in the mid-ground of the image. The concrete slab in this section appears to have completely collapsed and fallen away from the rebar on the left side, and is thought to have been cast only a short time before the incident occurred.
Bourne said initial images of the collapse suggested a fault had occurred with the central crosshead because both of the beams on either side had buckled and parts of the crosshead were vertical, when they should be horizontal. However, he added that the absence of visible bolts in the connection detail meant the crosshead and column were likely to have been welded together, a possible cause of weakness in the structure.
“It looks fully welded and, therefore, I can only assume that the right hand side clean failures are simply weld failures,” Bourne told New Civil Engineer. “On that basis, I would say that these probable weld failures are the key issue – not surprising perhaps, as they probably include some design errors, some detailing errors, some fabrication errors and some construction errors.”
Kolkata flyover collapse
Source: Bhaduri Tanmoy/ABACA/PA Images
WSP PB head of bridges and ground engineering Steve Denton said most failures occur during the construction phase, and can often be attributed to uneven loads which cause more instability than the structure would incur when completed. “Over the years we have examined structure failures around the world to understand the reasons for them. One of the clear things you see is the disproportionate number of failures that occur during construction, despite the construction phase accounting for only a relatively short period in any structure’s life,” said Denton.
“This can be understood because during construction, structures can be subject to loading patterns not experienced in service and they can be in temporary configurations that are inherently less stable than the final structure.”
Atkins’ transport division technical director Chris Hendy agreed that the investigation into the cause of the Kolkata collapse should look at how the construction processes may have affected the stability of the structure. Images of the incident showing the span of concrete deck that appears to have completely failed have led Hendy to question whether the weight of the under-construction slab caused the supporting crosshead to fail.
“[From images of the collapse] you can see one span concreted and the other looks to be in the process of being concreted,” he said. “If I was an investigator, based on what I’ve seen in the pictures, I’d first be looking at whether the trestle crosshead box buckled under the weight of the partially completed deck and the wet concrete.”
While also acknowledging that most bridge collapses occur during the construction phase, Bourne has suggested that the eventual investigation into the cause of the collapse may well uncover underling issues with the permanent aspects of the structure. He said: “The failure is nearly always due to temporary conditions or temporary bits of falsework. However, this case does seem to suggest that the permanent design, detailing, fabrication or construction of that central steel crosshead might be the issue.”
Kolkata flyover collapse
Source: ADG PI - Indian Army
IVRCL said it grieved the loss of life and injury, and would cooperate with the authorities investigating the disaster.
“Such an incident has occurred for the first time in the company’s history,” IVRCL said in a statement. “All necessary process and quality steps have been taken as per standard operating procedure and after obtaining all clearances.
“The design of the flyover has been made by a reputed consultant from Kolkata. The project manager and engineers deployed at this site are highly experienced in constructing large and complex projects.”
Investigations into the cause of the collapse are ongoing.
Learning from disaster
Engineering experts have called for improved construction standards in developing countries following the Kolkata flyover collapse.
The failure of a section of the Vivekananda Flyover scheme occurred last month in India’s busy Burrabazar district.
Bridge consultant Simon Bourne said the level of standards imposed on the design and construction process is often much higher in the developed world than in developing countries. Bourne explained that a series of bridge failures in the 1970s led to the implementation of more stringent regulations and assessments in Europe. “Most of the independent checking we carry out today is as a result of those failures,” he said. “Every bridge in Europe now goes through an independent checking process but we don’t necessarily see the same level of rigour in the developing world.”
WSP PB head of bridges and ground engineering Steve Denton added that the UK, in particular, had learned lessons through the publication of the Bragg Report in 1975 and the subsequent code of practice for falsework design, BS5975, in 1985 which covers both technical and procedural requirements.
While the scale of the collapse in Kolkata would be extremely unlikely to occur in the UK, Denton said it is important that the industry is not complacent and continues to maintain high standards across the construction industry.
“Major structural failures are rare, and extremely rare in the UK,” he said. “This demonstrates that, by and large, we are getting things right. However, because failures are rare, there can be a risk of overconfidence and it is essential that we guard against that.” Denton added that incidents like these highlight the importance of recognising the specific risks during the construction process, and ensuring the right processes, clear accountabilities, effective transfer of information between parties and the right technical expertise is in place.
Gordon Masterton, chairman of independent body the Standing Committee on Structural Safety, has called for a forensic investigation of the causes of the Kolkata collapse.
Masterton, who is also chair of future infrastructure at the University of Edinburgh – and a former president of the ICE – said an independent expert engineering team should be given full access to the site, the construction records and eye witness reports.
“If there are lessons to be learned to benefit future projects in India and elsewhere, the technical results of that expert team’s findings should be made available as soon as they have reached a conclusion,” he added.
“For the benefit of our industry, we need to disseminate any useful new information from collapses like this that may benefit designers and contractors. As with the Tay Bridge disaster and many other examples, we have a noble tradition of enhancing knowledge from disaster. Let’s not miss the opportunity here.”
Additional reporting by Greg Pitcher.