The redevelopment of National Westminster Bank's former HQ is an excellent example of the stringent demands and complexities faced by firms engaged in modern urban demolition work. Paul Wheeler reports from the City of London
FOR A DEMOLITION contractor, knocking out the vaults of a bank must rank high in the fantasy job stakes. When that bank is the 164-year-old former headquarters of National Westminster, right opposite the Bank of England, you kind of sense that those working on the job must get a buzz.
But John F Hunt's job at 41 Lothbury in the heart of the City of London is more than a novelty. It is also an excellent example of the complexity and demands of modern urban demolition work.
Arguably, demolition is the wrong word to describe the contractor's activities. Primary contractor Wates used the term 'surgical demolition' in the tender documents, which with the implication of precision and skill is a more accurate description.
Whatever the terminology for the work, 41 Lothbury is an extraordinary building and its Grade II listed status explains the need for such care and tenderness within the redevelopment activities.
John F Hunt's work is a partial demolition - stripping out walls and floors of the east and rear flanks of the building, while retaining the original facade and the palatial banking hall.
Along with the retained areas - designated on site as heritage areas - existing finishes such as stone doorways, marble-clad columns, and other details have been carefully removed by stonemasons. Their task includes cataloguing, packing and storing the items ready for use in the site's redevelopment.
John F Hunt, working essentially top-down, is currently just one floor down in the seven-storey structure and already the complexity and delicacy of the operation is apparent. Demolition debris has to be loaded into a skip, which is lowered through an approximately skip-sized opening into the building's cast iron frame.
A centrally mounted tower crane lifts the skip clear of the building and then lowers it down gently to a truck waiting patiently outside the bank's entrance in Lothbury. Typically, the procedure allows the contractor to remove no more than six truckloads of debris a day.
Work is also going on in the basement, where the tower crane operator is popping skips up and down a lift shaft so demolition debris can be removed. Equipment comes in and out of the building in the same way - without the tower crane the site wouldn't work at all.
The basic demolition process is to install dead shoring - essentially lots of acro props - on the floor below the one being worked upon, together with any necessary crash decks to catch debris. This dead shoring provides support for additional loads resulting from plant and temporary stockpiles of debris during the demolition process.
Working methods have been selected to minimise vibration that could damage the heritage parts of the building. For example, diamond saw cuts are used to structurally isolate those parts of the building that are being kept from those that are being taken apart and rebuilt.
As John F Hunt works its way down the building, it will switch from pneumatic tools to concrete crackers - large jaws that crush the concrete - again to minimise vibration. A sophisticated vibration monitoring system will monitor the effectiveness of these measures.
The plan is to remove one floor every two and half weeks, and at full capacity John F Hunt will have 70 workers on site, says contracts manager Glen Clark.
John F Hunt has a range of conventional plant and three remote-controlled Brokk 'robots'. The smallest of these is currently working at the top of the building, while two larger machines in the basement are proving useful in knocking out the thick and heavily reinforced bank vault walls.
In truth, the demolition robots look pretty much like standard construction plant. The robotic aspect is a remote-control operating pack connected to the rig by umbilical leads.
The equipment does not come cheap and is comparatively rare on demolition contracts, says Mr Clark. But its sophistication is typical of the way the demolition industry is going.
The lightweight Brokk 40 'essentially does the job of two operatives with pneumatic breakers, ' says Mr Clark. 'The larger machines produce an exceptional punching force for their size.'
The Brokk 180 is a 1.8-tonne robot with a breakout force equivalent to a 5-tonne machine; similarly, the 5-tonne robot can achieve similar results to a conventional 13-tonner.
Essentially this is achieved because the Brokks are comparatively small and the hydraulic effort is focused on the one task. The machines are also very compact which is a great advantage when working in the bank's basement area.
The 1.8-tonne robot can track comfortably through an 800 mm door opening and takes the underground labyrinth of corridors and vaults in its stride.
John F Hunt's original contract was worth £2.1 million but the final sum will end up higher, partly because of the re-scoped asbestos work (see above) and also because Mabey Support Structures had to modify its temporary support system for the main facade, which slowed down access to the site.
John F Hunt started on site in January and, despite the increased scope of the work and delays, Mr Clark believes it can still achieve the original 35-week programme. The client is Royal Bank of Scotland, the architect is Collins Melvin/Ward Partnership and consulting engineer is Campbell Reith Hill.
Asbestos work increases
SOME provision had been made for asbestos at the bank, but the extent of it wasn't clear until demolition got under way.
The building's heritage value explains why the client carried out a fairly limited pre-contract survey, suggests Mr Clark, and that is the reason for asbestos removal work increasing 10-fold from £30,000 to £300,000.
Once on site, asbestos was found on lagging around pipe risers and, in particular, on underfloor heating pipes.
Removal of asbestos requires specially trained operatives - provided by John F Hunt's asbestos division - working in a controlled environment.
Essentially the area containing the asbestos is enclosed, sealed off and a slight negative air pressure set up in the controlled area so that any airflow is into the controlled zone.
This should ensure that no asbestos dust of fibres migrate from the control area - even if there is a small perforation in the seal.
Experienced asbestos workers then carry out the removal work - which in the case of the under-floor heating involved gunning out 1,500 linear m of floor screed to expose and remove the pipes and lagging.
All material within the asbestos area is double-bagged - first in a clear bag and then in a red one. These are sealed and then stored in a controlled access area before being removed.
The London and Westminster Bank
THE BANKING house for the London and Westminster Bank was built at 41 Lothbury in 1838, to the design of two eminent architects, Professor CR Cockerell and Sir William Tite.
Cockerell produced the facade, while Tite concentrated on designing a distinctive domed interior - both significantly modified in the remodelling of the bank in the 1860s.
The builder was Samuel Grimsdale & Sons, which built the new bank for £15,654. This, together with the purchase of the site's freehold from JB Pearce & Sons for £23,750, less an allowance of £1,400 for old materials sold and £1,000 from the City's Sewer Commissioners, put the total cost of the bank at £37,004.
General reaction to the building was extremely favourable, as the following extract from Timb's Curiosities of London (1855) illustrates:
'The front, of Portland stone, is one plane, or general face, and proves that a splendid building may be erected without columns or pilasters. The attic storey has a cornice and balustrade, which give dignity to the whole facade.
At the extremities are bold piers, surmounted by sitting figures - the City of London at the east end, and the City of Westminster at the west.
The interior is very original: east and west are aisles with balustraded galleries; their sides being divided from the centre by an arcade springing from Doric columns; and the vast hall, surrounded closely with lofty buildings, is mainly lighted by a dome and semicircular Diocletian windows from above.'
Step forward 164 years and redevelopment continues, involving localised demolition and structural adaptation, with installation of part mezzanine level, alterations to provide new circulation cores, stripping out and refurbishment of office floors from first to sixth floors and extensions on floors four to seven.