Accidental loads, pre-existing building defects could be behind fatal Tanjong Pagar structure collapse: Experts

SINGAPORE: Failure to follow the approved demolition process, the presence of accidental loads or pre-existing building defects are among the possible reasons why a structure collapsed at a Tanjong Pagar work site on Thursday (Jun 15), engineering experts told CNA.

A 20-year-old migrant worker from India died after he was pinned under a collapsed concrete structure. It took rescuers nearly four hours to free him from the rubble.

Engineering experts said contractors should abide by proper demolition processes that include not overloading the structure with debris or heavy equipment.

Mr Chong Kee Sen, emeritus president of the Institute of Engineers Singapore (IES), explained how walls can collapse after being dislodged from their original support.

This could be due to “many possibilities” such as accidental loads - which occur as a direct result of an accident or exceptional circumstances - or the process of demolition, he noted.

“Failures are usually due to a confluence of situations and events. There are procedures set out as to how a building should be demolished safely,” said Mr Chong, who is also the director of Engineers 9000, a civil and structural consulting engineering company.

Authorities are looking into what caused the collapse of the reinforced concrete wall that measured about 10m and 3.8m high.

They have ordered all work to stop at the site along Bernam Street off Anson Road, where Fuji Xerox Towers was being demolished to make way for a mixed-use development.

Mr David Ng, honorary secretary of IES, said that the building could have had pre-existing defects that went undiscovered.

“These defects may have caused the structure to ... not be able to take even the pre-designated loading, so the structure will therefore collapse during the demolition stage,” he explained.

The project engineer has to physically inspect the structure for any pre-existing defects to take into consideration when planning the demolition sequence, Mr Ng added,

This involves clearly demarcating what should be demolished first so that the remaining structure is still supported at all times when certain elements have been removed, he said. Floor slabs are usually the first to go, followed by the beam and then the column or wall, due to how they support each other.

“Once this proof plan (has been) given to the contractor to execute on-site, the contractor has to follow the sequence strictly. You can’t change it at all,” Mr Ng pointed out.

Other possible causes include the contractor failing to follow the approved demolition sequence, the structure being overloaded by heavy machinery used to demolish the building, or the building being accidentally hit by heavy machinery, Mr Ng said.

According to the authorities, the deceased worker’s employer is Aik Sun Demolition and Engineering, while the occupier of the work site is Woh Hup.

The Ministry of Manpower and the Building and Construction Authority said on Thursday that a professional engineer has assessed that the remaining building is stable with no structural concerns.

Addressing this point, Mr Chong said that the collapsed wall at the Tanjong Pagar work site would not have structurally affected the remaining building structure because it was “locally connected to the building structure or independently supported”.

SAFETY CONSIDERATIONS

Mr Chong listed some safety measures to be considered before demolition:

• Assess the structural system of the building
• Develop a method of statement for a safe demolition
• Do not overload the structure with debris or equipment during demolition
• Conduct a risk assessment, pre-demolition audit and method statement. Assess how the load of the structure will be transferred to the ground during different stages of the demolition, identify critical elements of the structure, and identify the need for temporary supports

As for whether there remain any safety risks to the public, Mr Chong said there appears to be no residual risk from the incident.

Typically, these risks are mitigated by adhering to work procedures and risk assessment to ensure safety, he added.

The site where Fuji Xerox Towers once stood is slated to be redeveloped as a mixed-use development called Newport Plaza.

The developer, City Developments Limited, said in a statement late on Thursday night that it is deeply saddened by the incident and is cooperating fully with the authorities on the investigation. 

A Woh Hup spokesperson said that the company has expressed its condolences to the worker's family and "do our best to support them in any way we can".