Image main Defining and Designing Protective Structures to Thwart Terrorism

Defining and Designing Protective Structures to Thwart Terrorism

Lima Charlie News hears about some of the latest in Protective Design from international engineering firm Thornton Tomasetti. The firm’s recent projects include providing protective design services for One World Trade Center (Freedom Tower), WTC Towers 2-4, structural and protective design services for the new U.S. Embassy in London, and protective design for ship and submarine survivability among NATO navies.

In the wake of ever-changing tactics of lone-actor terrorists, such as using a vehicle as a weapon to plow down crowds, managers of public venues as well as building owners and government and military enterprises are increasingly scrutinizing how to adapt security measures to mitigate terrorism risks. It’s a delicate balancing act – making sure a venue is welcoming while keeping it safe and protected.

So how is protective building design changing in response?

Whether a football stadium, corporate headquarters, oil refinery, or barracks on a forward operating base, the optimal protective design will take a “holistic” approach, considering as many forms of attack as possible – from backpack bombs, active shooters, and rocket-propelled grenades (RPGs), to biological attacks, cyber subterfuge via eavesdropping, and even attacks by miniature drones. There will always be a way for a terrorist to do some damage. The intent is to outthink them to minimize injuries to people and structures.

We can’t always predict exactly how a terrorist might attack a public venue, but we try to outthink them.”
-Peter DiMaggio, senior principal & Weidlinger Protective Design practice leader, Thornton Tomasetti

Image Oklahoma City bombing
The bombing of the Alfred P. Murrah Federal Building in Oklahoma City in 1995 caused progressive collapse. The Weidlinger Protective Design practice performed a vulnerability assessment of the new Oklahoma City Federal Building in 2015 to address the full range of potential threats, including airborne glass shards and floor slab failures due to upward blast pressures.

Rethinking Protective Design

Everyone’s been there – annoyed at the slow crowds moving out of a stadium after an event, with a limited number of exits available. The concept, of course, has been to maintain controlled, orderly exits and segregate rival fans. In addition, security teams often can’t man every egress point. This security concept of concentrating crowds to control them proved to be a weakness following a concert in Manchester, England this past spring, when a suicide bomber killed 22 and injured scores of others.

This incident among others prompted an urgent rethinking of ingress and egress at stadiums, and protective design in general. Mitigating some risks involves low-tech, easy fixes. For example, where possible, avoid designing buildings with re-entrant corners (nooks and crannies). Not only do they provide hiding places for an aggressor, those spaces can also concentrate blast waves, bouncing them around within the nook, causing them to magnify. Another easy fix is placing air intakes high up on buildings to minimize chances of a biological or chemical element being introduced into the filtration system. It used to be that these intakes were typically placed at ground level for easy maintenance.

More complicated is the physical hardening of structures that need to take into account blasts that can cause progressive collapse, such as when structural elements like columns fail, producing a domino effect that causes an entire structure to give way.

Image Progressive collapse
Progressive Collapse Analysis showing loss of corner building column did not result in disproportionate collapse.

Whether a low-tech or high-tech fix, at issue is the ability to secure both physical and operational elements. Instill too many hindrances that ruin the visitor experience and the terrorists have, to a degree, succeeded in their disruption.

A man walking through bollards near One World Trade Center. Bollards are steel barriers designed to prevent cars from passing through. The One world Trade Center building was designed with multiple restrictions to motor vehicle access, to prevent car bombing.

Balancing Physical Security with Operational Security

The first line of defense for physical security starts at the perimeter of a property line where there’s the risk of a vehicular bomb. Anti-ram barriers such as bollards – vertical steel posts available in a variety of designs – can deter intrusion. In contrast to fences and walls, bollards still offer pedestrians freedom of movement. These barriers recently prevented a more catastrophic outcome in New York’s Times Square, when a car plowed into dozens of people but was put to a stop by bollards.

Image Times Square driver bollards
Car sits atop bollards in New York’s Times Square, after driver drove at pedestrians, May 18, 2017. (AP Photo/Seth Wenig)

The second line of defense is at the pedestrian perimeter of a property where the public enters a venue and where bags might be checked. Logically, these points should never be near structural columns or mission-critical operations like a communications center. Canines trained to pick up the chemical scent of explosives can also be used at the perimeter.

Another aspect of physical security is hardening the facility itself, especially to prevent progressive collapse, as occurred in the Oklahoma City bombing of a federal building in 1995. Glass can be retrofitted with blast-resistant materials. A new building can integrate special design and detailing into its original construction, while an older structure, like a concrete stadium, can have its columns encased with a thin steel jacket, or, alternatively, integrate high-performance concrete retrofits reinforced with steel micro-mesh.

Also extremely important is placing columns at a distance from the controlled perimeter, and incorporating the appropriate strength and ductility (the ability of an element to deform without breaking) into the columns for blast resistance.

Image Structural security simulation
Structural security simulation of a vehicle-borne explosive attack on the new design of a cable stayed bridge using the Thornton Tomasetti developed NL-FLEX structural dynamics software.

While those are some methods for improving physical security, a holistic approach integrates operational security as well, taking a multi-hazard approach to possible risks. At the basic level, this starts with examining where the security guard force is located. Where possible, the command center is located away from a public entrance where suicide bombers might attack.

High-resolution cameras and coordinated lighting are also important, especially for a venue that hosts evening events, as both help extend the visible perimeter. At public assembly venues, extending that visual perimeter as far as possible (including to the parking area) is ideal. There are also air filtration systems that can protect against obtrusive elements, including chemical or biological threats.

Image steel-strengthened columns
Blast simulation of a masonry-encased steel column showing localised damage calculated using the MAZ CFD and NL-FLEX software.

A Holistic Look at Hazards

What about less obvious risks? Will an aggressor try to eavesdrop through walls or windows to steal corporate secrets or threaten the security of an important federal building? A special type of film can be installed between architectural materials used in construction to address this threat.

Where are facility blueprints stored? After all, if easily stolen, they could give a terrorist clues to weak links in a facility’s defenses. What about the location of the mailroom? First choice would be to have it outside the main structure to minimize the impact of a mailed bomb. If that’s not the case, walls may need to be hardened. Following 9/11, for example, several high-profile federal buildings added to their main structure an adjoining one through which all visitors enter. If a visitor were to get through security and set off explosives, the blast’s effect on the main structure would be mitigated.

Image Forensic analysis of the September 11th collapse
Forensic analysis of the September 11th collapse of World Trade Center 7, a high-rise tower that failed under thermal loads caused by unfought office fires. The collapse of WTC 1 showered WTC 7 with hot debris, igniting the fires. The 10th floor fell onto a fire-weakened 9th floor, causing it to collapse, triggering a chain reaction that continued down to the ground.

A holistic approach also goes beyond physical and operational factors in post-event response. Security guards, for example, may be trained in frisking suspicious individuals, but are they trained to minimize casualties in the event of an active shooter? It’s key for security to know the response time of police and have a plan of action for the gap-time.

It may seem an anomaly, but these days engineering firms that specialize in the defense industry and protective building also partner with law enforcement and/or military intelligence officers to offer security services to round out a holistic approach to a facility’s protection.

Military Defenses

Many protective strategies applied to public or corporate buildings have similarities to those utilized at military facilities, such as for barracks and forward-operating bases; though, of course, adjustments are made for risks inherent in combat zones.

Securing the perimeter remains the first line of defense. If an RPG is fired at a barracks, for example, a goal is to have it detonate at a standoff distance from the building in question. If it hits a protective screened wall (pre-detonation screen) positioned several feet from the barracks, that can mitigate the impact to the barracks. Similarly, the barracks’ design may include a protective roofing structure that allows for a similar “stand-off” distance. There are entire areas of research dedicated to such protective measures in hot zones.

Touching on a somewhat cooler area, underwater and on the high seas, naval vessels must also incorporate protective measures. Again, a holistic approach is required and must take into account risks such as torpedoes, electromagnetic pulse weaponry that disables electronic operations, or attack by so-called asymmetric threats such as suicide bombers in small boats. Sophisticated simulations combined with real-scale and full-scale tests allow engineers to design features to protect these floating critical facilities from ballistics, blast and fragmentation, among other risks. The key objective is to make sure vessels can operate in the demanding combat environment and, if necessary, take a hit without serious degradation of capability.

Nato naval exercise (U.S. Navy Image).

All protective measures boil down to trade-offs between physical security and operational capabilities, and in many civilian applications between cost and aesthetics.

Every defensive solution inevitably also brings new challenges. Most recently on the agenda of security and engineering specialists is how to protect against the possibilities of enemy drones, namely small civilian models, not military might. Can a public facility limit air space around it and prohibit drones from being used in certain places and at certain times, such as during a concert at an outdoor arena? Does a corporate headquarters own air rights directly surrounding its perimeter?

In this new world of ever-changing risks, engineers are on the frontier defining and designing defenses to combat enemies.

Peter DiMaggio, Phillip Thompson, for Lima Charlie News


Peter DiMaggio is a senior principal at Thornton Tomasetti and leads its Weidlinger Protective Design practice, which has over 50 years of experience providing physical security analysis, advice and design, including assessing and mitigating vulnerability to multiple hazards.

Phillip Thompson is principal of Thornton Tomasetti Defence Ltd. and serves as director of its U.K. and European operations. It provides expertise in determining the effects of explosions and collisions on facilities, infrastructure, ships and submarines as well as R&D, advanced modeling techniques/simulations, strategic development and project management. Clients include several NATO defense forces, with particular emphasis on naval survivability.

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