ICE and IStructE members and guests in Essex were treated to a fascinating lecture last night on all things Demolition, Deconstruction, and Dismantling. The lecture – given by McGee Group Director Paul Bland and Head of Demolition Nick Taylor – covered all aspects of building demolition and was augmented with examples from their extensive portfolio of work.
The meeting – organised by the Essex Branch of the Institution of Civil Engineers (by me actually) and held jointly with the Institution of Structural Engineers – drew a crowd of nearly 50 attendees to the Lord Ashcroft Building at Anglia Ruskin University in Chelmsford.
Before the meeting started there was a small matter of an award to be made. The Best Technician Student Award celebrates exceptional young civil engineers in the East of England currently working towards an accredited HND/HNC or Foundation Degree in civil engineering.
The 2014 award was given to Norman Chan who was nominated by Anglia Ruskin University for his consistent high performance in his coursework and exams, excellent talent in engineering design and his positive attitude. Norman received a cheque for £250, a certificate, and shield from Regional Chairman Glen Owen.
With the award out the way and Norman back in his seat I introduced McGee as a company and our speakers Paul and Nick and left them to it. Paul started the proceedings by introducing McGee as a company and giving a few details of him and Nick although I rather stole his thunder as I used information from their slides for my into. So it was very quickly into the meat of the talk. The speakers have very kindly provided a PDF of their slides which can be found on the ICE website here.
The lecture was split into two sections. In the first part Nick described the challenges associated with the demolition of buildings regardless of location, but including the particular challenges in congested city centre locations.
It was immediately evident that there are a great many challenges to overcome and that the majority need to be considered and resolved before any works take place on site. Many of these revolve around information about – and details of – the structures to be demolished including their design and load bearing capacity and hence any temporary works that are required. They also include the materials they are made of, how the materials arising can be segregated and removed from site, and the utility services both in and around the structures.
Other challenges revolve around the stakeholder liaison, licences and permits, paperwork and plans and all the other softer – though no less important – pre-construction activities that are needed.
Nick and Paul covered an interesting topic regarding what designers can do now when designing new buildings to make them simpler to demolish. It raised a number of interesting points such as the fact that to date, none of the really tall London buildings for example have been demolished and questioned what methods will be used when this eventually becomes a reality.
Paul recounted that they had recently considered (purely as an exercise for insurance purposes) how The Gerkin building would be demolished if needed. He described how – because of the unique shape and structural form of the building – the entire building would require scaffolding out with a very complex scaffold in order to do significant elements of the work.
Posed with the question what ‘would be the ideal building to demolish be?’ the response was a rectangular steel framed building four storeys high clad in copper and roofed in lead! Unfortunately not many buildings fit this description and so so complex technical demolition solutions will always be required.
It was noted and accepted that with the trend towards off-site manufacture and on-site assembly and design using BIM. This will result in more ‘assembled’ buildings and the future of demolition is going to be less about demolition by brute force and more about dis-assembley.
A few areas where designers could make demolition easier were dscussed. These included robust mechanical connections, healthy floor loadings, wide stairwells, and the avoidance of un-grouted post-tensioning cables and external cantilevers. Unfortunately though such things will either make the structures more costly or less appealing and hence despite the requirements of the CDM regulations they are unlikely to become mainstream anytime soon.
The second half of the lecture moved on to describe the various different techniques that are adopted nowadays for the demolition and deconstruction of buildings – many of which have come about through technology developments that wouldn’t have existed 10 years ago. The main demolition methods were described as:
- High Reach
Paul and Nick then together talked the audience through a number of current and previous schemes to illustrate these different methods.
First up was a current project at One Nine Elms in Battersea with the dismantling top-down of two existing towers to make way for two new taller towers. Supported logistically by tower cranes the concrete framed buildings are being progressively reduced in height with small plant working at high level. As a first on this project McGee are also use a Doka climbing screen system for noise rediction that climbs down rather than up.
Next up was a project long completed at 122 Leadenhall Street – an address now famous for the new building on the site known by its nickname The Cheesegrater. The existing building was one of only 4 buildings in London where the floor plates were actually hung from from the roof via a significant transfer system itself supported by the concrete core.
In this instance a bottom-up method was adopted. Once the the lower podium levels had been sufficiently demolished the remaining floors were demolished off a large temporary platform that was progressively strand-jacked up the core. Once the floor plates were removed the platform was removed and the core demolished by traditional top-down.
One major benefit of this method was freeing up ground level areas for piling and basement works whilst demolition of the building was progressing above. It did give rise to significant public interest to see a half demolished building with the un-demolished half at the top of the building. A similar method was subsequently adopted for the demolition of the taller 20 Fenchurch Street. The site now contains the building known as the Walkie Talkie building at the top of which is the Sky Garden that has just opened to the public.
Next up was an example of surgical demolition of the Clarenden Tower in Christchurch, New Zealand. We tend to think that demolition is a planned exercise to clear a site for development but in this instance it was to clear buildings damaged beyond repair in the earthquake of Februrary 2011. If I recall correctly McGee were brought in to advise on possible demolition methodologies rather than to do the work themselves.
In this instance – as the building was so unstable – all operatives had to be lanyarded to external supports from a crane and only very small light plant used to demolish the building top-down. Diamond saws and core drills were proposed to cut the building elements into sections that could be lifted off by crane. An added complexity was that a prior soft strip exercise was nigh-on impossible and had to be done at the same time as demolition.
An example of high-reach demolition was provided via a case study of the demolition of a redundant building for Vauxhall. This was cited as by far the preferable method of demolition and applicable mostly to sites with lots of space and no near neighbors. Paul commented that one of the leading demolition plant manufacturers is currently developing a high-reach excavator capable of reaching up to 90m high. That is around 30 storeys. THIRTY STOREYS!
The final example was a variation on explosive demolition. Whilst explosive demolition is carried out there are limited circumstances where it is appropriate. Cooling towers is one such example where other means would just not be practicable. Nick outlined how a form of explosive was used at Great Ormond Street Hospital a few years ago. In this instance the explosives were actually classed as fireworks and took the form of expanding gas rather than explosive materials.
The technique was used to remove massive concrete foundations to the building that were only a few metres from in-use operating theatres and wards. A specially designed shrouded rock drill bored holes into the foundations. Matting and gravel filled bags were then placed on top before detonating the expanding gas which very effectively fractured the concrete.
And with that this most interesting lecture was over. Branch Chairman David Goodliff managed questions from the audience and summed up before thanking the speakers and attendees.
The next Essex lecture is on 12th February and is an introduction to Fracking by Caudrilla Resources Ltd. Should prove interesting!
The Institution of Civil Engineers (ICE) is a leading source of professional expertise in transport, water supply and treatment, flood management, waste and energy. Established in 1818, it has over 80,000 members throughout the world including over 60,000 in the UK. ICE’s vision is to place civil engineering the heart of society, delivering sustainable development through knowledge, skills and professional expertise. For more information on the Institution of Civil Engineers visit www.ice.org.uk
The Institution of Structural Engineers (IStructE) is the world’s largest membership organisation dedicated to the art and science of structural engineering. The Institution of Structural Engineers was founded in 1908. For more information on the Institution of Civil Engineers visit www.istructe.org
McGee Group are a family owned construction and civil engineering company based in Wembley, Middlesex. They were formed in 1959 and provide a range of construction services including environmental works, demolition, ground works and concrete frame construction. For more information on the McGee Group visit www.mcgee.co.uk