Disproportionate collapse has always been a risk for large buildings, but the events are rare.Read more
22-09-2021 | Posted by Joaquín Martí
At the time of writing this post, it is precisely 20 years since the New York WTC was destroyed by the deliberate crashing of fuel-laden, commercial airliners against the two towers. Everybody in the planes, many occupants of the towers, and a number of people that happened to be nearby were killed in the attack. And, unfortunately, this is not an isolated case, it is simply a particularly dramatic example of the possible effects of terrorist action.
Prevention starts by trying to avoid having people in the terrorist state of mind and continues by trying to deny them the means and the opportunity to achieve their purposes. But if all this fails, our structures and facilities must be up to the task.
Of course, not all structures can be protected, nor would it be reasonable to attempt it. Certain types of structures are obvious candidates for consideration, such as embassies, government buildings, airports, iconic structures, major bridges, and the like. Besides, physical security considerations are now being applied to select municipal facilities, cultural venues, hospitals, stadiums, and other targets that previously had no precedence to necessitate protection against attack.
Concentrating in the case of bridges, as in many other structures, the general principles of deter, detect, delay, defend, and respond provide a layered response to the threat. These principles take specific characteristics in the case of bridges because of their accessibility to the public and to vehicles. The problem also depends on whether we are dealing with an existing bridge, or the exercise is being conducted for a new project.
For a bridge that provides an essential communication link, a paramount consideration is the avoidance of progressive collapse. If a cable is severed, if a fuel truck is set on fire, or if a Vehicle-Borne Improvised Explosive Device (VBIED) is detonated anywhere, the bridge structure must be capable of redistributing the loads and maintaining its overall integrity.
Several factors complicate the calculations required by a terrorist detonation on a bridge. At short times, the near field effects, with enormous pressures generated and material failures being triggered, must be carefully determined; the range of pressures exerted and the material behaviours activated are indicative of the complexity of the problem. At later times, it is necessary to ascertain that the overall structure, in spite of the local destruction caused by the explosion, is able to redistribute the applied loads and retains global stability.
Principia has the experience of conducting this type of calculations in relation with several major bridges, in some cases for already existing bridges and in others in support of a new bridge project. And apart from bridges, we have also performed similar calculations for other structures, such as airport terminals, government buildings, offshore platforms, petrochemical installations, military facilities, corporate offices, etc.
It is indeed a technically challenging discipline, requiring both competence and experience, but one where the potential consequences are well worth the investment.