But as more and more big City institutions go over to paperless systems and move their nerve centres out of the City, the bombers' strategy has failed.
Peter Rees, head of planning at the Corporation of London, says: 'The IRA thinks the City has targets in it that are like military targets, but they are not taking out the big guns, they are taking out an organic structure that can be rebuilt.'
Companies that had not already created separate sites for data and communications networks were encouraged to do so after the Baltic Exchange bomb last year. These high-security, purpose-built computer centres or 'black box parks' - built to house information, not people, and managed by remote control - have burgeoned as the demand for companies to protect themselves from terrorist attacks and industrial espionage has grown.
But this new generation of buildings has not featured in architectural magazines. The companies have discouraged all publicity to maintain security.
Reuter, which provides financial data for the City from its high-security offices in Docklands, has consistently played down the architectural merit of its building, despite it having been designed by Sir Richard Rogers. When the architects won a Royal Institute of British Architects national award for the building, Reuter was embarrassed by the success, according sources at the RIBA. From a distance the building looks like any other hi-tech office, but all its windows can be removed and replaced by aluminium cladding when there are few people inside. Sporting facilities and the restaurant for the few staff who work there are submerged in a bunker.
On the eastern edge of the Isle of Dogs is London Telehouse, another high-security building, designed by the architects YRM, whose Japanese owners store confidential information for the City. The nature of the business places priority on computers, requiring few personnel. Windows have been included only at the insistence of the planners.
Despite suggestions that the intensified IRA bombing campaign will lead to a new architectural style as a defence mechanism, these windowless office buildings, bunkers and even blast-proof net curtains - of the sort favoured by Downing Street
to protect against bomb blasts - are unlikely to win approval in the
Mr Rees believes that buildings with windows can be designed with security in mind. The new office development proposed by Sir Norman Foster for the insurance company Bowrings, which will stand opposite the Tower of London, has a large public atrium. Such public spaces without hidden corners provide no obvious places in which to hide bombs. Mr Rees says: 'If the atrium is properly policed (by those who work in it and the public) it poses no danger.' Nevertheless, Canary Wharf and Lloyd's have both been closed to the public in case.
The City is fortunate in not having public car parks underneath its key office buildings, which was how terrorists gained access to the World Trade Center in New York.
The Bowring building, which will be considered by City planners in two months' time, has large areas of glazing that some fear could be too vulnerable a material for areas prone to bomb attacks. Last week the City engineer Colin Snowdon described plate glass as 'a lethal instrument' and asked building owners and architects to use laminated glass, which has a PVB or plastic interlayer. Although laminated glass still breaks (unlike 'toughened' glass, which cracks into granules), it does not shatter like plate.
In the wake of the Bishopsgate bomb Pilkington has received numerous calls from nervous building owners asking about the cost and effectiveness of blast-resistant glass.
Andrew Rabeneck, facilities manager of the US investment bank Salomon Brothers, says: 'Toughened glass would be a step foward, and it would add only 20 to 30 per cent to something that is only 1.6 per cent of the construction costs.'
He, like many architects, believes that glass is a safer material in the event of a bomb than cladding made of brittle material such as natural stone. 'Natural stone easily shatters and falls off in large chunks,' he warns. 'Glass yields and allows parts of the building to release the blast, so there is less danger of damage to the building's structure.'
Michael Stacey of the architects Alan Brookes, who designed a glazing system for East Croydon station, believes that to use stronger and stronger glass would not be the right cultural response to the problem of bomb attacks. 'Glass has fascinated architects for the last 25 years,' says Mr Stacey. 'It would be sad if the confidence with which glass is now used is set back by terrorist attacks.'
Some buildings in London are already designed to be bomb proof. The Queen Elizabeth Conference Centre in Westminster has an underground bunker and steps out from the third floor to protect the upper floors against damage from bombs placed at ground level. The core of the British Telecom tower is believed to be able to withstand a nuclear blast, with corridors linking it to other BT buildings in London.
Many local authorities, including the City Corporation, have underground shelters, but the City firmly believes that the owners of commercial buildings would do better to invest in more security than make their buildings so fortified that they become obvious targets. Colin Snowdon says: 'We wouldn't want the City turned into a castle with a moat around the outside.'
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