News High Rise building Construction; Terrorism Risks

High Rise building Construction; Terrorism Risks

Nov 03, 2015
As we have now just passed the 14th anniversary of 9/11 it seemed appropriate to write an article on high rise building risks with a particular reference to the World Trade Center twin tower collapses. The terrorist attack of September 11, 2001 at New York’s World Trade Center towers (WTC) was the first attack on a mega-city in the 21st century. The insurance claim (for the buildings only) is reported to have been settled at USD 4.55 Billion.

The collapse of the towers provoked detailed studies into the vulnerability of high rise buildings to terrorist attacks at multiple scales, from the level of structural components to the collapse of the towers.

Upon reflection, most of us attribute the collapses of the WTC to the impact of the planes with the tower structures. The buildings had however been designed for the horizontal impact of a large commercial aircraft.

Whereas the WTC structural design had considered the impact of a commercial airline it is incredible, if as it seems, that the design did not consider the effect of heat generated by the ignition of the aviation fuel on the building’s steel structural components. This is analogous to designing a structure to withstand the impact of a military shell without considering the effect of the explosive upon detonation.

The old WTC was designed as two ‘tubes’ with structural columns around the periphery. The core however only had columns to support the central elevators and services. The core was designed to resist vertical loads and was not assumed to transfer any lateral loads.


As is evident from the numerous photos and videos, the towers withstood the initial impact of the planes. The mass of the buildings was about 2500 times the mass of the aircraft, and the buildings were designed for a steady wind load of roughly 30 times the weight of the plane.

Following the impact of the planes the structures failed after nearly 2 hours; it was the effect of the heat of the fire on the building’s steel structure (600ºc- 800ºc caused by the ignition of approximately 40m3 of aircraft fuel) that eventually forced the columns to buckle. This in conjunction with the failure of the steel connections of the trusses supporting the floor slabs is considered the initial cause of the collapse (Source: “The Towers Lost and Beyond: A Collection of Essays”, ed. E. Kausel, MIT, Cambridge, MA, 2002). There followed a free fall of a mass of approximately 30 stories and 14 stories onto the 80 and 96, respectively, floor structure below, resulting in the towers crashing into the ground with a velocity close to that of a free fall.


As a result of the devastating effect of 9/11, new structural design concepts include the use of High Performance Cementitious Composites (HP2C). In comparison with ordinary concrete, HP2C materials have enhanced microstructural material properties and an enhanced material ductility obtained by incorporating small-sized steel or organic fibres.

The high compressive strength-to-low mass density of this material makes it an ideal material for skyscrapers, in which weight is always a limiting factor. In addition HP2C concrete has an intrinsic fire resistance property in that the polypropylene fibres in the material, which contribute in service to the ductile tensile behaviour, melt under high temperatures, allowing the escape of expanding vapour thus reducing spalling and exposure of steel reinforcement to heat.

Built-in redundancy in design can significantly reduce vulnerability to structural failure. In structural design redundancy means the provision of multiple added failure mechanisms that prevent the total system from collapse upon failure of single or several of its components. There are two types of redundancy; active and standby. The system components simultaneously contribute to stability; some elements of the redundancy may be inactive but become active when some active redundancy components fail. Therefore, implementation of redundancy in a system will improve its reliability.


Following the 9/11 event, there are many arguments against the use of steel in high rise buildings. However, just as advances have been made in the use of concrete structures, progress has also been made in the design of steel structures and in the measures available to protect the structures against fire.

New high-rise building structures are usually a composite of steel and concrete. The new WTC in New York although, still considered by some as an obvious terrorist target, has enhanced security and life-safety systems, including a robust, reinforced concrete shear-wall core surrounded by a steel moment frame. The building is part of a post–September 11 trend for New York City office towers to move away from all-steel structures toward composite systems.

Whereas terrorist attacks on buildings are always possible, buildings incorporating this type of design concept are less likely to suffer the same disastrous consequences as that of the WTC 9/11 event. In the specialist realm of terrorism insurance cover, a detailed pre risk acceptance analysis, of the structural design concept and proposed safety measures for the building by specialist structural engineering consultants, is surely a must.

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