When explosions occur there is a sudden release of energy in the form of heat, light and rapid expansion of the gases produced. The pressure wave created by this expansion travels in the form of a highly compressed band of air. Substances that detonate create shock waves and are known as high explosives. Substances that burn, but do not detonate, are known as low explosives and they will produce a less intense pressure. Here the rise in pressure is more gradual than the almost instantaneous rise produced by high explosives. The burning of a sufficiently large quantity of low explosive begins to mimic the effects of a high explosive.
The high explosive shock wave, of very high pressure but short duration, is very destructive, especially to humans. The shock wave travels at supersonic speed, reflecting off any surface in its path many times before decaying. When enclosed inside a room or confined in a street between tall buildings, these reflections will set up interference patterns, whose peak pressures are momentarily many times greater than the source pressure wave. There is also a high explosive effect known as brisance, that can cause shattering of steel, masonry or concrete close in to the seat of the explosion. As a gross oversimplification, high explosives close to a building tend to break things, whilst sufficiently large charges further away tend to push things over. For any given charge weight the pressure declines extremely rapidly with distance from the explosion. Thus, achieving the greatest separation between the possible sites of an explosion and the building itself is an important design objective.
For both high and low explosives, the positive pressure wavefront outwards from the explosive is followed by a negative pressure inwards, albeit less intense. Thus a body can experience a high overpressure pulse outwards, followed by a lower suction pressure back towards the seat of the explosion. This can be especially damaging to cladding and windows over a wide area. Glass windows can be pushed inwards by the positive pressure wave and then pulled outwards by the pressure reversal, just as the glass is rebounding after the initial inward push, causing it to fall outwards into the street.
High explosives are usually commercial materials, such as those used in quarries, whilst an improvised explosive can be made by mixing nitrogenous farm fertilizer with other substances such as sugar or diesel oil. High explosives need a detonator to initiate the charge, whilst improvised explosives need a detonator and a small amount of high explosive as a booster for initiation. In between lies a third class of explosives, known as thermo-baric. Here the energy is released by detonating a mix of vaporised fuel and air. Although this has the potential to be the most destructive explosion of all for a given weight, it is technically challenging to disperse the fuel as a mist before detonation and a two-stage mechanism is required to achieve this. However, once created, a fuel-air mix produces a powerful explosion of long duration and is especially damaging inside buildings and confined spaces.
In passing, mention should be made of earthquakes. Although the energy applied to buildings by ground movements during earthquakes can be gigantic, it is transferred over a much longer period and generally causes all but very stocky buildings to sway rather than shatter. Some measures designed to protect buildings against earthquakes can enhance resilience against explosions but they are not sufficient alone.
Types of Building Structure
Older buildings and some low-rise modern structures support their roof and upper floors by raising the walls brick upon brick. These are termed compressive structures, since the weight of the upper structure is resisted by the compressive strength of the bricks. The weight of the building pre-stresses the bricks which, in conjunction with the buttressing provided by internal walls, gives adequate resistance to sway against a strong wind. Taller, newer buildings usually have a strong skeleton made of steel or reinforced concrete, with non-structural panels being used to close the space between the members. Here members, able to resist both tension and compression, support the building and, crucially, are tied together to form a frame.
When a sufficiently large blast wave hits a compressive brick-upon-brick building, the walls are blown sideways, support for the upper floors is removed and the building partially or wholly collapses. These buildings are very vulnerable to terrorist attack because, once damage occurs, the remaining brittle brickwork may be unable to bridge any gaps formed resulting in widespread collapse. If an explosion occurs inside the building the internal walls may be removed, leaving the structure weakened against sway forces. In particular, over time, many older shops have been opened up to provide a larger display space, but in doing so the internal walls that once provided sway resistance have been removed.
The effect of an explosion on a framed building is completely different. The frame members have strength in tension and can resist a sideways force much better, whilst anyway presenting a smaller surface area to the blast. Here the pressure wave may blow away the infill panels between members, but the frame itself is more likely to survive. In extremis, if a frame member is in distress or destroyed completely, the building loads can be automatically redistributed amongst the remaining members, bridging the new gap from above, below and alongside. Under current UK Building Regulations, multi-storey buildings are designed to resist widespread collapse following local damage.
But there are some caveats to the otherwise good resistance of a framed building to a terrorist attack. The first concerns the way the frame is tied together. If the frame joints are inadequate, the building will now act as a 'house of cards'. Individual parts - the cards - may remain intact, but the frame will disintegrate allowing a partial or complete collapse. A particularly harrowing terrorist attack occurred in a crowded pub in Northern Ireland, the Dropping Well near Ballykelly. A small bomb was placed by one of the uprights in a large public space. That upright was damaged, but remained standing. Because the concrete slab roof was only lightly connected to the walls, the explosion pushed the walls outwards and the roof slabs collapsed down onto the customers below. The collapsing concrete crushed and killed people, whilst people close to the explosion and by the column that remained standing were badly injured but survived as the column continued to support the roof around them. Where the frame members are tied together inadequately a framed building can be as lethal as a compressive building.
A second caveat is an attack that creates a fire. Steel loses its strength rapidly at temperatures above 400°C. Members bend and joints are weakened, leading to the failure of a frame that could otherwise resist a large explosion or kinetic energy attack at a lower temperature. This was well demonstrated by the two terrorist attacks on the World Trade Centre in New York. In the 1993 bomb attack, the North Tower survived a very large explosion in the underground car park despite severe local damage. But on 11 September 2001 both the North and South Towers were destroyed when burning aviation fuel from two deliberately crashed aircraft took away the strength of the frames after little over an hour. In this case, the energy released by the fuel overwhelmed fire protection designed for a more normal fire, which was probably damaged by the impact of the aircraft as well. Prior to '9/11' however no fire-protected steel framed building had ever totally collapsed. What made the difference at the WTC was how a combination of fire and impact damage triggered the kind of progressive collapse described next.
A third caveat is if an explosion or kinetic energy attack occurs high up a building. Whilst the majority of the frame might resist the attack, the members and floors close to the attack may be seriously overloaded. For example, an explosion may force the floor above upwards in a direction that it was not designed to resist. If the explosion breaks this floor slab and detaches it from the frame it can fall downwards. The kinetic energy of the falling slab might then break the weakened floor below. The increasing momentum of these two falling floors could then be sufficient to break an otherwise unscathed floor below, producing a catastrophic situation where all the floors collapse progressively downwards, leaving the outer frame standing but the infilling floors destroyed.
A similar effect can occur when the frame itself is overloaded. This occurred to the frame of the World Trade Center, where the kinetic energy of the collapsed upper floors falling and impacting on the undamaged frame below the aircraft impact area was sufficient to overload it. The result was the collapse of the otherwise unscathed structure lower down. It also explains why the building collapsed vertically downwards on itself, rather than falling over.
In summary, buildings will remain one of the most attractive targets for terrorist attack and explosives the favourite means of achieving it. The response of buildings to attack varies dramatically according to their method of construction. A building that is framed, with robust individual members, strong but ductile connections to tie the members together and good fire protection will provide the greatest resistance to such attacks.
Christopher Elliott is a consultant for Arup Security Consulting
St Mary's Axe, City of London April 1992
The resilience against collapse of a framed building is well demonstrated here. Despite massive overloading, following a very large explosion, an important proportion of the building frame remains standing. A compressive structure would have been reduced to rubble.
Even so, this building would have performed very much better if the connections between the individual members had been stronger, as is normal in more recent construction. The building was designed before the partial collapse in 1968 of Ronan Point, as the result of an internal gas explosion, and the changes in the Regulations that followed.