Engineers call for controlled demolition of Ngau Tau Kok building to avoid ‘domino effect’ as deadly fire continues
After three days of intense heat, experts say the industrial building’s steel reinforcement has been weakened and ‘it’s lucky it’s still there’
A group of structural and civil engineers have warned that the structure of the Ngau Tau Kok industrial building has reached a tipping point after being on fire for three days.
It urged quick, multidisciplinary cooperation to demolish the building in a controlled collapse, instead of continuing efforts to save the property inside.
The appeal came as firefighters were still battling the blaze, which started around 11am on Tuesday, and has since killed two firemen and injured 11 others.
Civil engineer Albert Lai Kwong-tak, convenor of the Professional Commons, said the fire department should immediately cease its current offensive strategy, and other government departments should start a new deployment plan.
He said a controlled collapse would minimise damages to the community in the event that the building collapsed on its own.
Experts in his group were doubtful about the judgement made by Director of Buildings Hui Siu-wai after his inspection of the unaffected second floor and the external wall of the building on Thursday.
Hui said the internal structure of the second floor remained solid, and the cracks seen on the rendering on the external walls of the building would not affect its structure.
“He didn’t see the worst parts of the building. The ceiling of the third floor [where fire started] is the weakest. He didn’t have a chance to check the third floor. Once the ceiling collapsed, there might be a domino effect,’ said Lai.
The steel reinforcement inside the concrete had already been weakened after more than 50 hours of intense heat, Lai said, adding: “It’s lucky it’s still there. This is because of the usually larger safety margins for older buildings.”
A controlled demolition would tear down the building in a bid to force it to collapse inward, not outward, he said.
Lai said there was enough expertise in the city to perform a controlled collapse and he urged the Buildings Department and Civil Engineering and Development Department to work with other experts and draw up a contingency plan.
“There will be only two scenarios: the fire was off before it collapsed, or it collapsed first. The latter is becoming more likely now,” he said.
Speaking on a DBC radio programme, structural engineer Professor Paul Pang Tat-choi echoed some of Lai’s views, saying he was worried the steel structure within the cement walls and ceiling on the third floor of the building “could have lost half of its strength” after almost three days of burning.
“It is likely that the third floor’s ceiling would collapse,”Pang warned, and “because the building was built decades ago”, this could result in a domino effect.
Pang is a former chairman of the Hong Kong Institute of Engineers’ structural and fire divisions.
Benny Lai Siu-lun, chairman of the Association of Registered Engineering Consultants, also called for officers to closely monitor the situation for anything unusual, such as whether part of the building was starting to tilt.
“The interior was completely charred, and it was hard for firemen to judge from inside the building,” he said.
Structural engineer Greg Wong Chak-yan, former president of the Institution of Engineers, told the Post that it was difficult for engineers to assess the risk of collapse as they could not enter the third floor.
But he believed that while the third-floor ceiling could collapse and influence other structures such as the floor, columns and walls, it was unlikely that the whole building would come down.
“Steel bars would start to weaken under 400 degrees Celsius ... If it is as high as 600 to 800 degrees, and burned for hours, its strength may drop by about 20 to 30 degrees,” Wong said.
“So firefighters can only be particularly careful when they enter the third floor, and watch out for signs such as a twisted cross-beam or column, a sunken floor, or melted metal furniture,” he added.
When asked if commanders should stop sending firefighters into the site if many of these signs were found, Wong said: “Yes, or they should be especially careful and not to ask firefighters to go there alone.”
Firefighters streamed water into the third and fourth floors of the burning building throughout the night on Thursday.
The blaze stopped for about half an hour at 5am, when several firefighters inspected the third to fifth floors of the building from the aerial ladder of a fire truck with an electric torch.
At 6am, firefighters - in breathing apparatus teams - resumed entering the floors with water jets, hammers and axes, after a suspension allowed crews to take a short rest.
Speaking shortly before 8am on Friday, Fire Services Department acting assistant director Kong Ping-lam said 42 fire vehicles and 180 firefighters had been deployed to battle the fire so far.
“Half of the fire on third floor is now extinguished, and the progress of our work on the fourth floor matched our expectation. There was only smoke on the fifth floor and the sixth and seventh floor were not affected,” Kong said.
He add that the fire services has set up an investigation group to look into the reason of the fire as well as the death of the two firemen.
Chief Executive Leung Chun-ying paid tribute to the firefighters, and offered his condolences as well the government’s full support to the family of senior firefighter Samuel Hui Chi-ki, the second fireman who passed away on Thursday.
At 2:37am, non-official members of Leung’s cabinet, the Executive Council, also said they were deeply saddened and shocked by Hui’s death.
In a statement, they “expressed their deepest condolences to the bereaved families of the two fire officers who passed away”.
“Exco Non-official Members understood that the Fire Services Department has been doing their utmost to control and put out the fire in a professional, safe and practicable manner,” the statement reads.