Modern demolition trades drama for discipline, with engineers applying rigorous calculations to ensure safe and predictable structural collapses.
In 1970s Britain, Lancashire steeplejack Fred Dibnah’s chimney-top feats became engineering folklore. Renowned for his expertise in dismantling tall, Victorian-era industrial chimneys, Dibnah’s demolition method involved carefully cutting a wedge-shaped opening, or “mouth”, at the base of the chimney. He would then support the remaining structure above the cut using wooden props.
To bring the chimney down, Dibnah would light a fire beneath it, gradually burning away the wooden supports. Once the props gave way, the chimney would collapse – ideally in a controlled direction.
Wearing no harness, Dibnah stood above crowds of spectators, and was awarded an OBE for his efforts. His exploits are still talked about in industry today, according to Will Neethling TFIEAust CEngT, Engineers Australia Engineering Technologist Fellow and Director of engineering consulting company De-Consult.
“But if you look at those pictures of guys sitting on top of a skyscraper beam in New York, having a sandwich with no harness on, those days are gone,” he said.
What’s replaced them? A highly engineered, controlled approach that treats the act of demolition with the same technical rigour as construction.
What needs controlled demolition?
Any structure that cannot be demolished mechanically, Neethling said.
Modern excavators, armed with attachments that can grasp, shear and crush, can handle ordinary buildings. But once a structure reaches a height or complexity beyond safe reach, it becomes a candidate for controlled removal – typically by inducing a collapse.
“We’re talking anything taller than a few storeys – mining structures, power stations, process plants, petrochemical facilities – anything too big for a digger to push over,” he said.“That’s when you require a controlled demolition plan, a concept experienced contractors come up with followed by detailed analysis by an engineer to verify that it can be done properly.”
The art of pre-weakening
Before any structure can be safely collapsed, it must first be pre-weakened – altering the structure to allow for controlled demolition.
For example, Neethling managed the dismantling of an 88 m, three-legged flare stack at a refinery by first bringing it down to a manageable height, allowing the remaining structure to be cut up and taken away.
“We pulled one leg out and rolled it over on the other two legs,” he said. “To do that safely, we put a steel cable around the leg, and then a digger at a safe distance away – more than the height of the building.”
This process required the stack’s steel to be partially cut, changing the entire structure.
“You need an engineer who is experienced in demolition to do a calculation to say, ‘Yes, we can pre-weaken this by doing a partial cut on certain components, and the structure will still be stable enough for people to be around it in that partially weakened state’,” Neethling said.
Another critical aspect is evaluating the environmental and operational assumptions that underpin a structure’s stability.
“Tall structures are normally designed for winds of certain storm strengths. So you would do this work when the winds are quite light and the structure won’t blow over.”
Calculated risk
Explosives offer an efficient way to bring down buildings. But when things go wrong, the consequences can be catastrophic – as seen in the fatal Royal Canberra Hospital implosion in 1997, which led to the death of 12-year-old Katie Bender and injured nine others.
According to the coroner’s findings, approximately 480–500 kg of explosives were used for the demolition of the hospital. This excessive quantity led to a debris projectile flying beyond the exclusion zone.
Since then, explosives are mostly used in industrial and mining applications outside of urban areas.
Neethling has worked on the demolition of several local power stations using explosives, including Wallerawang Power Station in Lithgow and Munmorah Power Station in NSW, with ongoing work at the Gove Refinery in Arnhem Land.
“It’s cost-effective and there’s less pre-weakening of the structure, therefore less potential for harm to workers during the preparatory stages,” he said. “Linear shaped charges are used to cut through steel members during demolition. You create a few little holes, put the explosives where they need to be, and they do the rest of the work.”
Tight controls are a must, though. “You need to know that the effects of the explosive substance can be controlled and managed.”
To contain the impact, crews wrap charges with protective materials, often layering sandbags around them. “You might get this good bit of sand flying out, but nobody worries about that,” Neethling said.
These days, there are also fewer crowds gathering to watch building demolitions.
“To quote one industry expert, ‘That’s bad juju. No crowds’.”
While it’s hard to avoid community awareness of explosive demolition events, it can be discouraged.
“You need an exclusion zone which is far enough away, normally about 1 km, and you should pick a time when it’s less likely that people will be around,” he said. “At Munmorah, there was a public park nearby, but it was still outside the exclusion zone. There were actually a few people flying drones, which was another problem – their drones could interfere with ours that we use to monitor and film.”
Urban constraints
In urban centres, where space is limited, the strategy changes.
“You cannot have a controlled induced collapse, where you roll the structure over, unless you have the real estate to land it on,” Neethling said. “The Australian Standard for demolition [AS 2601 – 2001] says you need 1.5 times the height of the building as an exclusion zone.”
That means high rises in the Sydney CBD, for instance, are taken down one floor at a time.
“They put small diggers on the floors, break things down, use the lift shaft as a rubble chute, and take everything out the bottom,” he said. “It’s a slow, progressive process.”
And it’s not just what’s above ground that counts; subsurface infrastructure must be considered too.
“If there’s a large tunnel underground with lots of sewer pipes, you would typically go in and remove all the contents,” Neethling said.
But sometimes the approach is to solidify, rather than demolish.
“We just block either end, put holes in the top, and pour in concrete to fill it up – which is a lot easier.”
If the materials underground are inert, on private property and are not going to cause harm or collapse, they could be left in place. “But if there’s a potential for subsidence, you dig them out and fill the trench again.”
Read more: What will Sydney’s next skyscrapers look like?
Reuse and recycling
While recycling is the norm for post-demolition materials, reuse is the ultimate goal.
“In urban environments, there’s a lot of materials that can be reused – roof tiles, timbers, flooring,” Neethling said. “Contractors will take the roof tiles off if they know they have a resale value.”
But this differs in industrial applications. “Equipment is outdated, and to reuse it you’d have to bring it up to current standards and pay for storage, transport, reassembly … it’s expensive,” he said. “Sometimes we export components to developing countries, but in remote areas like the Pilbara, you’re hundreds of kilometres from the nearest port. The economics of reuse are complex.”
