Tall buildings in Hong Kong: are they safe?
Share this article
In the light of last year's tragic events of 11 September, Bulletin held a roundtable discussion on the issue of tall building safety. Our panel of experts considered the implications of the terrorist attacks on building safety and security in Hong Kong.
With its large population in a relatively small area and its mountainous terrain,
Dr Albert So kicked off the forum with a short review of the terrorist attacks on
The importance of redundancy
"We must first look at the force of the impact on the WTC," Professor S Kitipornchai said. The two Boeing 767 planes that hit the towers each carried full tanks of fuel, around 91,000 litres. The planes' take-off weight was 200 tons, and they cruised at a height of 10,000 metres at 850 kilometres per hour. The amount of energy to get a plane to 10,000 metres in 10 minutes is equivalent to the output of a small commercial power plant. Assuming that the planes were travelling at cruise speed and that at the point of impact all the energy was dissipated in one second, then the impact force was in the order of 400 tons.
In spite of that, both buildings withstood the impact, which was the equivalent of an earthquake of five to six on the Richter Scale. The buildings were designed to withstand an earthquake of that magnitude, as well as tornado-strength winds of up to 340 kilometres per hour. What brought them down, Professor Kitipornchai explained, was the fuel--if all the fuel detonated at once, this would be equivalent to 2.5 million sticks of dynamite. "It was a gigantic explosion that blew away significant amounts of fire protection material and exposed the steel structure to raging fires."
Why the towers stood for so long, when many other buildings would have collapsed immediately, is because there was a high degree of redundancy within the structure, he said. (Redundancy refers to buildings with a number of load paths, which will not collapse if one or two columns are destroyed.) Even with part of the structure destroyed, the load was redistributed until the fire damage was such that the affected floors collapsed upon themselves. "If we assume that the floor at level 70 were to collapse with the weight of all the remaining floors above it onto the 69th floor, the impact force is around 30 times the weight of the floor itself--a lot more than static gravity force. No structure could withstand that force, which is why the towers just pancaked within about nine seconds from the time they started to collapse," Professor Kitipornchai explained.
The question is, he asked, whether buildings could be better designed against such an attack. "I would say not. It's very difficult to imagine any building, other than a nuclear power plant, that could withstand this sort of force." Any commercial building so designed would be very expensive and very ugly.
So what is the solution? Improve building safety in terms of escape routes? Improve fire protection? Build in a greater degree of redundancy?
Escaping fire hazard
With that, the panel considered the first sub-topic: Limitations in existing buildings relating to disaster escape.
This was an exceptional case, however, Mr Stephen Lee said. Normal fires would be controlled locally and in
On the topic of existing buildings with a low degree of redundancy, Professor Kitipornchai reiterated that the WTC had performed quite well by standing for between 45 minutes and two hours after the attacks because its tube within a tube structure had a high degree of redundancy. The degree of redundancy in a building is proportional to its cost, he added.
Mr Wong wondered whether concrete is better than steel in terms of redundancy. Mr Lee said that these days the construction industry in Hong Kong uses a concrete core wall inside with surrounding steel columns, which helps prevent progressive failure, even in tall buildings. Professor Kitipornchai agreed that reinforced concrete would have performed better than steel in the WTC, especially as the fireproofing round the steel would have been blown away by the impact. But even if the buildings had been constructed differently, nobody could predict what effect being struck by a plane would have. "You have to remember that terrorists attacked the WTC before in the basement but the building remained standing. This time they did the maximum damage at the right place in both buildings and caused them to collapse."
Professor Frederic Williams argued that there is a basic fallacy in thinking that tall buildings don't collapse. "Buildings are built to last a long time and we can't predict the political future anything like that far ahead. Any city in the world could actually become the target for this sort of attack." He believes we should consider external escape routes from tall buildings, so that people have a means of getting round a fire.
New structural designs
Moving on to the second topic: Existing technologies or technologies under research to tackle these problems, the panel considered new structural designs to resist heat and explosion, and heat resistant building materials.
It would be very difficult and unviable to design the kind of building that could resist this form of attack, Professor Kitipornchai pointed out. The alternative is to design buildings with a high degree of redundancy and make them as fire resistant as possible. "Certainly we can look at materials more heat resistant than steel but I'm not sure that they'd have the strength of steel or be able to resist an explosion."
Mr Lee said that there is research underway on a high resistant anti-fire film that could be applied to main structural elements but not to secondary elements such as internal beams. Regardless of local collapse, the building would remain stable, giving people the chance to escape. "But we need to have a balance," he pointed out. "Although it is technically possible to increase fire protection time to four hours, fire protecting steel is very expensive--up to 50% of the cost of steel--and we have to consider that."
The panel agreed that it would be possible to lengthen the time before a building collapsed but that to do so would either vastly increase the cost or involve designing a building with massive concrete columns.
Escape lifts proposal
Dr So then asked the panel to comment on his proposal for escape lifts, a project he and Dr Lo have been researching for some years.Assuming a building 60 to 70 storeys high is allocated one refuge floor for every 15 stories, if there were four groups of lifts enclosed within a safe chamber, the round-trip time for the lifts to take people to the ground floor would be only one or two minutes. "The lifts would operate under computer vision based intelligent control, and would be high-capacity, waterproof, spark-proof, explosion-proof and chemical-proof," he explained. "They would be powered by electricity and the generators would be independent and protected--two, in case one breaks down."
Professor Michael Hung suggested designing a lift based on gravity, using counterweights, to avoid having to use electricity. Professor Williams agreed, pointing out that
The problem of cost
Professor Williams believes that one of the things engineers have to do is to make people conscious of risk. "There has to be an element of risk--you cannot reduce it to zero in buildings," he said. In
Although Hong Kong may have no choice but to continue erecting tall buildings, for Professor Hung the concern is not so much attack by terrorists as the possibility of building collapse because of our steep slopes, use of reclaimed land and building fatigue from battering during typhoons. Recent revelations of irregularities, such as short pilings, in building construction, have also caused alarm.
Although infrared, followed up by human inspection, is currently used to detect building faults, neither technique is reliable, Professor Hung said. He is currently researching a new technique based on detecting stress, which he hopes will be CityU's contribution to building safety in
In conclusion, the participants agreed that although
Around the table:
Other participants included:
(* In his personal capacity as an engineer, his views are not necessarily those of the department )