Safety considerations for the protection of tall buildings from attack – Part I

Safety considerations for the protection of tall buildings from attack - Part I. Safety experts have known since before 11 September 2001 that fires or the collapse of skyscrapers are the worst possible catastrophes. How big is the risk that something similar happens again? Have we prepared ourselves well enough to protect the thousands of people in tall buildings? GIT SECURITY has reopened this discussion in the light of recent events and analyses the risks and preventative measures for its readers.

The attacks in New York and recent fires and accidents have thrown up new questions about the dangers in skyscrapers and risk assessment. The main threats, like fire, storms and earthquakes seem to have been well covered by building regulations and legal stipulations. The World Trade Center towers would have withstood a record hurricane with wind speeds of up to 320 kph but had no protection against a fueled projectile weighing 180 t.

An Allianz Zentrum für Technik (AZT), Munich study found that at least the consequences and casualties of such attacks can be partially minimised by applying new safety concepts. As European terrorists apparently prefer places where a large number of people are concentrated, it was aircraft, airports, stations, trains and large events that were considered to be likely targets to date. But skyscrapers must also be put on the list of endangered objects and be better protected.

Armed, explosive and incendiary attacks can create significant damage with relatively little effort and thereby highlight again the vulnerability of our modern industrial society.

Extreme Heat

Their flexible construction enabled the over 400 m high towers of the WTC to remain standing for one to two hours after the aircraft hit them before collapsing. Fire-resistant cladding of the steel supports in the center of the buildings, intended to protect against a small office fire, only held for a limited time against the one thousand degree temperatures during the fire. Steel loses about 75% of its strength at 600 degrees, so it was only a matter of time before the steel girders would give way.

„The dynamics of this sudden support failure allowed the weight of the floors above to suddenly land on the undamaged floors below that were not built for this load and therefore also gave way“, is how the Münchner Rückversicherung study described the disaster.

Evacuation Took Too Long

Other high buildings elsewhere were evacuated immediately after the attack on the WTC: the Sears Tower in Chicago (443 m) was cleared as the catastrophe in New York unfolded; an explosives seeking dog raised the alarm in the Empire State Building (381 m); there were anonymous threats against numerous skyscrapers in London, the Messeturm in Frankfurt (257 m) and the Petronas Towers in Kuala Lumpur (458 m).

Everywhere, thousands of people were pressing down never-ending steps, still the only way of evacuating people from a burning or otherwise threatened tall building. Although such evacuations are practiced regularly – and were also in the World Trade Center – many of the implemented measures remain simply untested theory.

There is panic when accidents occur, so people follow a herd instinct and oversee some life-saving evacuation route. The slowest person dictates the tempo and those that could go quicker build a queue. Evacuation plans also often contain much too optimistic assumptions, but these phenomena can now be reliably demonstrated by computer simulation. Researchers recommend considering alternatives like special emergency lifts on the outside or nets that stretch out from the facade every few floors to give those fleeing a chance to bypass a totally destroyed level.

Escape and Rescue Routes

The enormously concentrated fire density spreading upward presents a special challenge for the fire brigade and to the mass of people who evacuate from skyscrapers. The only escape route down so far has been via the stairwells; lateral exits or rescue by fire brigade ladders have not been options.

Rescuers also face significantly more challenging conditions: the distances to be covered are significantly longer than with other buildings; material like hoses, breathing apparatus and tools must be transported up high with great effort; the use of extendable ladders is limited to 23 m so escape and rescue is only possible via the stairwells; communication is more difficult and pumping quenching water to great heights from a standard fire tender is not possible.

Fire teams can also only get to the fire via the inside of the building. These problems have prompted the authorities and lawmakers to place increased safety regulations on skyscrapers, so specific fire protection recommendations regulate the design and usage of facades, ceilings, fire walls, the dimensions of isolatable fire sections, doors and the design of stairwells. Great importance is placed on dividing each floor into protected units (such as an office or habitation unit, the escape and rescue routes and special purpose rooms) with two physical escape routes and being less than 35 m from a stairway.

Spread of smoke and fire is minimised by walls, fireproof doors, flaps in the air conditioning and circulation systems. Escape routes are kept free of smoke by dividing the corridors into smoke sections of 20 m and by a system of pressurised stairwells and double-door sections (‚locks‘) on the principle of canal locks.

Automatic Sprinkler Systems

Sprinkler systems can be activated just where the fire has broken out and limit fires to just the point where they started. Nearby areas not yet burning are wettened and their flammability reduced. The water also cools constructional elements of the building making them more resistant to heat.

An exemplary system has been installed in the Twin Towers of the Vienna Business Park where a supply pipe network of over 40 km was installed. In emergency, the pressure drops in the pipe network, the alarm valve opens and over 12,000 sprinklers quench fires on all levels. Special pumps deliver the water from two communicating 320 m3 water tanks at the fifth underground level.

Simultaneously a hydraulic alarm bell and the alarm display on the central fire brigade control panel are activated. The second and concluding part of this article will be published in the next issue of GIT SECURITY + MANAGEMENT. It looks at extending risk assessment, redefining building protection and provides some expert advice.

Heiner Jerofsky,
Dipl. Admin.,
Criminal Advisory Committee (ret‘d)