Canadian Consulting Engineer

BUILDING SYSTEMS: Coping with Terrorism

For a long time, we have known that HVAC (heating, ventilating and air-conditioning) systems have the ability to spread chemical and biological agents throughout a building with considerable efficienc...

December 1, 2001   By Paul Marmion, P.Eng., Stantec Consulting

For a long time, we have known that HVAC (heating, ventilating and air-conditioning) systems have the ability to spread chemical and biological agents throughout a building with considerable efficiency. In 1976, the Legionella pneumophila outbreak at the hotel holding a USA Legions Convention was the first wake up call for mechanical engineers, and in recent years there is a growing concern with indoor air quality and the so-called “sick building syndrome.”

As a practising HVAC engineer, I have spent the better part of the last 35 years thinking about how we can improve human comfort and health inside buildings. Trying to get my mind around how an HVAC system can be used to harm people is a difficult concept for me to handle. Following the news on the World Trade Center and threats of future terrorist attacks I quickly realized that architects and engineers are going to have to add potential terrorist attacks to their list of design challenges. We will have to think of worst case scenarios and try to engineer deterrents that will complicate or counter the terrorist scenario. This will require all concerned to “think outside the box.”

The nature of the attacks on the 11th of September and the previous bombing in 1993 demonstrate that terrorists fully understand the complexities of modern building design. The first bombing of the World Trade Center in 1993 not only caused considerable structural damage, but also knocked out the primary and stand-by electrical power systems that served the building’s life safety systems. After the bomb blast, the stairwell pressurization systems that keep smoke out of the exit stairwell failed to work, turning the stairwells into massive chimneys that vented smoke from the basement blast area to the top of the tower. The lack of power also affected the fire fighting systems and emergency lighting, making it almost impossible to evacuate the building effectively. In retrospect, we should not assume this happened by chance, but that the terrorists wanted to cause maximum panic and maximum loss of life. The September 11 attacks used the structural and mechanical limitations of the twin towers to cause the collapse of the buildings.

Technical information on specific HVAC systems installed in numerous significant buildings as well as detailed technical information relating to the spread of biological or chemical agents by HVAC systems is readily available from academic journals and industry technical societies such as ASHRAE. With this information paired with access to the required professional expertise (which we must assume they have), terrorists can relatively easily gain understanding of the limitations and vulnerabilities of the mechanical and electrical systems installed in most of our buildings.

Buildings throughout the world are vulnerable to biological and chemical attacks. Particularly susceptible are indoor sporting facilities, convention centres, religious gathering halls and airports. These types of buildings have large mechanical HVAC systems serving densely occupied areas. They pack tens of thousands of people in a relatively small area and have central ventilation systems. If a terrorist is looking to contaminate a relatively small number of people, on the other hand, low-rise buildings with compartmentalized HVAC systems become more vulnerable to attacks.

Like the aviation industry, the construction industry and those involved in the design and operation of central HVAC systems need to analyze the risk of terrorist attacks and develop ways to make it more difficult to use building systems as a weapon. Safeguards may need to be added to current facilities and built into new ones. Some safeguards that could be considered are:

Secure design information during the design and construction process, especially for vulnerable high occupancy buildings.

where possible, protect the outside air intake from human tampering (e.g. position to make it difficult to access, and secure roof areas where there are intakes).

Secure air-handling mechanical rooms and air plenums.

Limit access to mechanical equipment and design information to known personnel.

Develop and install air filters that can capture and prevent the spread of chemical and biological agents.

Develop and install air quality sensors that can sense the presence of biological and chemical agents.

Include terrorist safeguards in building codes.

Building design teams will have to understand the properties of possible agents that could be used in an attack. This may require the addition of a new design team member with the required expertise, such as microbiologists who understand the bio-weapon threat. The cost of anti-terrorism protection measures will add to the cost of construction and operation of a building. The “post September 11 world” can’t afford to do without this additional cost if we want to match the resources available to terrorists and if possible, keep ahead of their capabilities.

Author’s postscript:

This article was difficult to write because I realize that it could be perceived as being opportunistic and also as providing potential terrorists with technical information they can use against us. However, I know that, as is stated in the article, they have the same access to technical information that I have and probably have access to the required professional expertise to understand the limitations and vulnerabilities of the mechanical and electrical system installed in most of our buildings. Our only defence is to try to make it as difficult as possible for potential terrorist attacks by developing engineered safeguards that can limit or if possible foil them.

Paul Marmion P.Eng is a managing principal in the mechanical department of Stantec Consulting. He is based in Vancouver.


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