Rise of the Super Towers
R i s e o f t h e S u p e r To w e r s
Rise of the Super Towers Techniques are failing to keep pace with the surge in super-high tower blocks that could require demolition in the next 25 to 35 years. Mark Anthony reports on an industry facing a nearvertical learning curve.
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R i s e o f t h e S u p e r To w e r s Like millions of people around the world, I watched in horror as the tragic events of 9/11 unfolded before my eyes live on TV. Once I had grasped the sheer magnitude of what I was witnessing, I began to wonder about the evacuation and rescue procedures for buildings that had truly earned the skyscraper nickname. As the second tower fell and the death toll rose inexorably towards a previously incomprehensible and unprecedented final tally of close to 3,000 souls, it was clear that rescue methods and technology had failed to keep pace with the vision of architects and the innovation of the engineers.
Sense of Helplessness When an infographic diagram arrived on my desk showing the massive residential buildings that have sprung up in Dubai, Australia, Russia and the US, I was reminded of the sense of helplessness the firefighters of 9/11 must have felt. If experience has taught us anything, it is that today’s vision of construction innovation is tomorrow’s demolition headache. The problem, of course, is that as these super towers have grown ever-taller, the life expectancy of both commercial and residential properties has grown ever-shorter. In London, where space is at a premium, the demolition of 25 year old buildings is far from unusual. Apply that logic to Dubai – home to eight of the 15 tallest residential buildings in the world - and the global demolition industry is looking at a learning curve of literally vertical proportions. Just think about that for a second. The Burj Khalifa in Dubai tops out at 830 metres tall; half a mile in old money. That’s almost 10 times the reach of the highest high reach excavator ever to turn a track in anger (not that we’re proposing high reach as a suitable method).
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Logistical Nightmare The logistics – not to mention the cost – of putting a crane half a mile in the air would be the first challenge. And finding a demolition-savvy operator willing and able to work at such heights would be an additional and sizeable hurdle. And how do you get men and machines to the upper floors and debris down again. Does a half-mile man-lift elevator even exist? If it doesn’t, the “lucky” contractor working on this imaginary project would have to employ super-fit Marathon-running crews, establish base camp halfway up the tower, or accept that half of each working day would be lost just getting men on station at the building’s summit. And, given the potential wind-rip at those heights, just how do you get thousands of tonnes of rubble to ground level without covering the entire city in dust?
If the building was imploded (again, not our preferred method), the exclusion zone would be the best part of a mile and could actually encroach into a neighbouring Arab state. Even if such a thing were contemplated, the resulting pile of debris would likely be larger than the current world record implosion. And seriously, who wants to pick away at 50 storeys of rubble anyway?
You certainly can’t just drop it down a disused lift shaft – Debris falling 400+ metres would hit the ground like a bomb going off and could undermine the structure itself. It is clear that any such reverse construction is going to require the development of new techniques and new methodologies that – to the best of our knowledge - have not yet been invented. So rather than applying our own limited brainpower to solve this dawning conundrum, we called upon the expertise of some of the best minds in the demolition business to let us have their thoughts on the challenge facing future demolition generations:
All of which points towards deconstruction (or reverse construction) as the only feasible demolition method currently available to us. But even here, current technology, experience and expertise falls a long way short.
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R i s e o f t h e S u p e r To w e r s
Bill Moore, Brandenburg: Despe from Italy has already developed a system for high rises (http://tinyurl.com/kawttss). An alternative would be to figure out a way use the building's freight elevators, lowering the overhead pulleys as work progresses. Or convert the freight elevator into an interior material hoist with sufficient capacity to bring mini excavators and skid-steers up to the work areas.
Rick Wilson, Paragon Worldwide/TKC This is a futuristic challenge requiring a futuristic solution. I would like to see the development of top down systems composed of lightweight composite materials that designed to self-adjust for building height and stresses – engineered for quick assembly. This system would allow more floors to be encapsulated and reduce or eliminate the need for additional support equipment to include tower cranes for material movement operations.
Exterior walls could be "folded" in and debris could go down either on the freight elevator/material hoist or, after removing the elevator cars and rails, down the passenger elevator shafts. The reverse of new construction whereby a tower crane is used might also be possible. Instead of new materials being brought up, debris would go down over the side of the building in skid pans or roll-off boxes.
It is clear that these super towers will place specific demands upon the deployment of equipment. I would like to see innovative electrohydraulic lifting systems – again engineered with composite materials - for both interior and exterior applications and equipment mobilisation to the top floors. These could work in conjunction with ultrahigh speed robotic demolition machines and attachments to feed footprint recycling and downsizing systems for material movement. These might feed self-expanding and sealing composite demolition chutes for both internal and external applications with internal dust suppression systems that will continually filter and recycle the water for the misting system applications.
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Mark Coleman, Coleman and Company 40 years ago, the high reach excavator hadn’t even been thought of, so who knows what we will have at our disposal 40 years from now. But, based on current knowledge, I would advocate deconstruction or, more specifically, reverse construction. Also, you cannot assume that any of the temporary works used in the construction will be safe to use for demolition, tower crane bases for example. It is highly likely that design codes will have changed yet again by the time of demolition.
Typically, buildings like to be demolished in reverse order of construction. Pretty much all buildings of this height will have been constructed using a structural steel frame. So the standard text book way to demolish these structures as we know now would be self-climbing erecting tower cranes erected up the side/sides of the building or with in the lift cores. The patent glazing/curtain walling would be removed as it was installed with special lifting gear using suction pads. With the steelwork dismantled and lowered, concrete floors cut out and lifted or broken from the deck with the deck then cut out after concrete removal.
The lifts could be left on during soft strip while the rest of the project gears up for the main demolition. Of course resource levelling would need to calculated to determine how many lifts would be needed for personnel access/egress to upper floors along with redundancy should there be issues with of one or more of the lifts being used. The remaining lifts would then be used for preselected material segregation and removal from site.
But reverse construction may not be as easy as it sounds. These buildings have been designed depending on their height - to deal with horizontal forces (mainly wind) in many different ways. The most common is a mass damper where the sway and stability of the building is counter balanced like a big pendulum. It is, therefore, vitally important to understand how this pendulum works when reducing the height and weight of the tower and causing instability
Craneage would also present lots of issues. Firstly, the crane would need to be secured to the side or internal core of the building gaining lateral stability and ensuring that it doesn’t get blown over in the high winds. Wind at this height is stronger but, more importantly, changes quickly so what may be OK at one level can be different at another level particularly if you lower past an exposed point.
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R i s e o f t h e S u p e r To w e r s We would probably need welfare facilities on multiple levels to save time moving up and down the building. Emergency evacuation would require a large number of high speed external lifts. The disconnection of the sprinkler system would need to be phased so the building is protected throughout the demolition process.
Another key craneage consideration is the weight of crane’s rope which can drastically reduce the SWL capacity. A typical wire rope weighs 6kg/m. On very tall/deep construction (ultra high sky scrapers/mine shafts) the loss in SWL can be quite considerable – a multiple sheaved crane, lifting 200 metres could be around five to ten tonnes. Very long crane drops are also effected by wind.
Jacking down methods are likely to become more common place, and I can certainly see Despe’s Top Way Down system being used in the right environment more and more.
A possible solution would be to lower a shorter distance where another crane takes over and the lift is repeated. An Intermediate lifting platform would be useful for lifting and lowering of material. The safety and welfare consideration on a project of this nature would also be unlike anything we have ever seen.
The key, of course, is designing the structure with demolition in mind; creating structures that can be lowered down into the basement and demolished at that level or even slid out in pre-cut sections that formed part of the design.
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