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Despite concerns over possible terrorist attacks on nuclear power stations, water supplies or even entire cities, individual buildings remain the likeliest targets. Investigators on the trail of the perpetrators of last month's atrocities are reported to have uncovered plans for a helicopter assault on the US embassy in Paris, a light-aircraft attack on the European Court in Strasbourg, and a ram-raid on the Sears Tower in Chicago by a bomb-laden truck.

"Everybody is now looking at the risk of a bomb attacks," says Steve Tanno, facade director at structural engineer Buro Happold. "On jobs we did a year or two ago, clients are now coming back to us to talk about blast protection."

Mark Whitby, director at structural engineer Whitby Bird & Partners, says he expects clients of new office buildings to review security of areas vulnerable to bombs, such as service lifts and underground car parks. "They will need to think about securing vulnerable areas," he says. "If I was Swiss Re right now, I'd be asking how secure my goods lifts are."

Public projects are under renewed scrutiny, too. The Cabinet Office is understood to have ordered a review of the design of all public buildings. "Security is being reviewed all the time, and now more than ever," says John Gibbons, chief architect at the Scottish Executive and adviser to the Scottish parliament project. "I expect it will get worse. It will make building design much more difficult and much more expensive."

Even before recent events, the Scottish parliament was set to be one of the most terrorist-proof buildings in the country. Following the bombing of the Murrah Federal 好色先生TV in Oklahoma City in 1995 鈥� where a truck containing a bomb was parked just five metres from the building's facade 鈥� security consultants appointed by the government ordered all facades within 25 m of an access point to be reinforced. In a few weeks' time, the project team is to take pre-cast concrete wall panels to a firing range in Northumberland, to quite literally test them to destruction with high explosives.

Other defensive measures ordered at the parliament include decorative ponds that double as a deep moats to prevent E E off-road vehicles driving up to the building, and hydraulic vehicle barriers hidden in pavements.

Alan Mack of Bovis Lend Lease, the parliament's project manager, expects further design changes to be ordered in the light of recent events. "Some of the security areas might need to be reinforced or designed differently," he says. "I think they will also stiffen up screening equipment, which will affect the number of people entering the building at any one time." Mack adds that the effects of previously unthinkable events, such as mortar bomb attacks from nearby hills, were now being seriously considered.

好色先生TVs within 500 m of potential targets already have to undergo stringent safety reviews. The London Eye, for example, across the river Thames from the Houses of Parliament, was assessed by police crime-prevention design advisers to ensure passengers would be safe if parliament was attacked. Capsules have shatterproof glass, and passengers can be evacuated within 20 minutes on ropes dropped through trap doors.

The new threat is suicide bombers who are unconcerned about life, including their own

John Haddon, Arup Security

"I don't think the Eye could have been structurally designed to be more secure," says PC Roger Lyddon, crime-prevention design adviser, who was consulted on the wheel. "It is supposed to be used by people, and that means there could always be an element of surprise 鈥� something that hasn't been anticipated. This is every building's weakness." Even so, to prevent the wheel being a target itself, old-fashioned body searches have recently been introduced.

Counter-terrorism has been a growth industry in this country since the IRA bombing of St Mary Axe, Bishopsgate, in the City of London in 1992. This event, and 1996's IRA attacks in London Docklands and Manchester city centre focused the minds of commercial clients, who began demanding buildings that would minimise the risk and effects of an attack.

As a result, the UK's anti-terrorist measures are some way ahead of the USA's. Security experts also point out that, following the collapse of the Ronan Point tower block in 1968, when a gas explosion on the 18th floor triggered the collapse of the floors below, UK building codes now call for greater robustness to prevent progressive collapse than those in the USA. Collapse of parts of the building not directly affected by impact occurred at both the Oklahoma federal building and the World Trade Centre, adding significantly to the number of casualties.

But the attacks in the USA have raised concerns over new types of threat. "Perhaps the biggest thing is the change in the type of terrorist," says Arup's Haddon. "In this country, almost all terrorism has been by Irish republicans, who tend to give warnings and who tend to go for economic targets rather than killing people. The new threat is suicide bombers who are unconcerned about life, including their own."

Haddon believes that taking suicide bombers into account means stepping up existing security measures 鈥� particularly controlling access to buildings 鈥� rather than fundamentally rethinking design guidance. The advice from government counter-terrorist experts is that measures developed over the past decade are still valid.

It鈥檚 not about fortifying our buildings 鈥� there are too many scenarios; you鈥檇 just end up with a bunker

Gordon Masterton, Institution of Civil Engineers

Almost everyone in the industry agrees that the most important issue is to prevent it happening in the first place. "First and foremost, you have to minimise the likelihood of occurrence," says Gordon Masterton, chairman of the Institution of Civil Engineers' structural and building board. "It's not about fortifying our buildings 鈥� there are too many scenarios; you'd just end up with a bunker."

Besides, terrorist-proofing a building to the same degree as the Scottish parliament can triple construction costs, according to Andrew Chan, senior structural engineer at Buro Happold, who has worked on a several bomb-proof buildings. "For many clients, I don't think it is economical to protect a whole building," he says. Instead, the client will be asked to assess the risk of various types of attack and then protect those areas of the building that may be vulnerable. "For example, to protect against a parcel bomb, you protect just one room, the post room, or you put the post room outside the main building in a strong room."

Once they have identified the risk areas, safety engineers consider how to eliminate the threat. This is achieved through security systems such as CCTV, visitor screening and careful road layout. For the rest of the building, the priority, Chan says, is to ensure the structure is robust enough to prevent collapse while people escape.

The scale of last month's atrocities in the USA have led to fears that terrorists may plan even more audacious attacks. The greatest fear is of chemical or biological weapons. At least one City firm is understood to have issued protective suits and masks to its employees in case of such attacks. But security experts admit they can as yet do little to help firms worried about attacks by terrorists armed with these weapons. There has been little research into the subject, and ways of securing air-conditioning systems and other services against such an attack have not been found.

"There is probably not a huge amount to be done to physically modify buildings, though if there are low-level air intakes they should be changed," says Patrick Moore, development director at risk management consultancy BSC Consulting. "But there should be greater control of access to plant areas, which should be classified, and any work done in them under permit. These areas should be kept spotlessly clear of rubbish, so that any little package containing nasties will show up immediately."

The main difficulty, though, lies in knowing when such an attack has been launched. "How do you detect the agent?" asks Arup's Haddon. "And, once you've detected it, how do you close down systems to prevent its spread throughout the building?" Government experts point out, however, that such attacks are highly unlikely and say public concern is now the biggest issue.

In the immediate future, restoring the confidence of building users will be as important to the construction industry as providing safer buildings. Evacuation procedures may have to be completely rethought in the wake of the World Trade Centre disaster. People will understandably be unwilling to wait inside a damaged building while firemen carry out a phased, floor-by-floor evacuation. Similarly, current police advice favouring inward evacuation, in which people are directed to safe areas inside buildings instead of away from the building (a method designed to protect people from flying glass if a bomb goes off) may become unacceptable.