Faris Ali, Professor of Structural Engineering at the University of Ulster, details the complexities of producing suitable construction materials in the modern age…
Professor Faris Ali
The need to enhance the protection and performance of structures under extreme situations is rapidly increasing as such incidents are becoming more frequent in the modern world. A blast can take place in a domestic or industrial environment, or it can be caused by a terrorist attack. Moreover, it is more likely that a blast will trigger a fire than a fire triggering a blast.
The tragic September 11th attacks are perhaps the most pertinent examples of blast events accompanied by fire. It is now clear that fire was the main cause of the collapse of the two towers
Professor Faris Ali
The tragic September 11th
attacks are perhaps the most pertinent examples of blast events accompanied by fire. It is now clear that fire was the main cause of the collapse of the two towers. The effects of fire were most evident in the 47-storey third tower that was brought down approximately seven hours after the twin towers. The 1995 Oklahoma bombing is another example of the devastating effect of blast forces. The blast shock wave was triggered by the detonation of 1,814kg of TNT at a 4.5m standoff distance, and caused devastating destruction to the surrounding buildings.
In recent years, there have been notable attempts to address the issue of fire-blast resistance, by developing blast mitigation methods that can also provide some resistance to fire. Some of the methods available use steel (or a core material covered with steel) as the main material of blast walls. Although steel has a high resistance to blast forces, its very heavy weight (7.8ton per m3
) can cause difficulties with handling and installation. Steel also has very poor insulation properties, which means that it can assist the spreading of a fire by thermal conductivity.
Concrete has also been used in blast walls. Although it is approximately three times lighter than steel, it has two major drawbacks. Firstly, it is very brittle and, therefore, prone to fragmentation during blasts. Secondly, it is susceptible to explosive spalling when subjected to fire. A good example is the M1 fire at Watford in April 2011, which resulted in the motorway's closure due to structural damage caused by the explosive spalling of concrete.
A third approach uses purposely developed materials, or sandwich panels, that contain multilayers of various materials. In general, the costs associated with this group tend to be very high and usually involve the use of resin or polyethylene-based ingredients to assist in the absorption of the blast's shock wave. However, such components can be compromised by fire, as they melt at approximately 160°C. This makes them functionless under fire, particularly if the fire takes place before the blast. The cost of any blast protection system can be an important parameter when being widely used to provide safer environments. The challenge is to produce a system that is cost effective, capable of resisting blasts and fires, and that is logistically viable.
Terrorist attack on World Trade Center
After years of intensive research, the Fire-Blast Protection System (FBPS) was developed at the University of Ulster. The FBPS aims to provide protection for people, buildings (industrial/military) and infrastructure facilities (including tunnels) in events of blasts, fires, and a combination of the two.
The system is based on using low-cost concrete panels that provide a superior 'panel thickness to blast resistance' ratio. This allows for a significant reduction in panel thickness, and consequently a reduction in weight and cost. The developed panels have shown very high ductility when compared with other types of concrete panels, allowing the FBPS panels to behave like steel rather than concrete. Tests have shown that the panels have approximately 12 times higher ductility than standard concrete panels, including Ultra-High Performance Fibre Reinforced Concrete (UHPFRC).
This graph shows the superior capacity of the new concrete to absorb an impact's energy, compared to UHPFRC. The FBPS panels have demonstrated high ductility with a 10º rotation at supports without failure, which, when added to their resistance to fragmentation, makes them capable of providing excellent protection against blast forces.
The preliminary tests have shown that 25mm thick panels can withstand an impact equivalent to a blast of 500kg of TNT at a 30m standoff distance, making them compliant with US Department of Defense UFC 3-340-02 blast deflection response criteria. The panels can withstand higher blast pressures by increasing their thickness. The panels also provide fire resistance as they are immune to the explosive spalling that concrete is prone to under extreme heat. The FBPS panels are non-combustible, which makes them ideal for fire situations. Their ability to slow the progress of fire is one advantage of the FBPS panels over existing blast-proof materials. In addition, the panels have very good thermal insulation properties, enabling them to function as fire wall barriers.
The 'panel thickness to blast resistance' ratio provided by the FBPS system allows the panels to be produced in various thicknesses to resist a wide range of blast forces. For example, if the system is used for cladding or protecting existing walls, the panels can be 10-25mm thick. If the system is to be used to erect a new building, the panels can be manufactured in larger thicknesses such as 50-100mm. The relatively small thickness of the panels makes them lightweight and logistically efficient. Their ability to resist fire and blast forces, their low cost, their light weight and narrowness, make FBPS panels unique.
The panels are also ideal for use in other construction situations, including reinforcing and rehabilitating damaged structural elements. The panels can be fitted on building facades, internal walls and ceilings that are considered to be vulnerable to fire or explosion. In cases of cladding or protecting existing walls, the system can be provided onsite by fitting it into a frame.
Experimental research at Ulster has been supported by a numerical finite element study. This panel model allowed various intensities of blast loads to be investigated. The blast analysis was based on using the SDOF system and the idealised blast curve is illustrated in the image below.
The concept of this invention is protected by a patent. It is also being developed as a full military construction system. This system will enable the swift response to operational needs and will facilitate the fast erection of military units, including field administrative facilities, ammunition storage facilities, bunkers and other premises.
I believe that the introduction of this system will significantly enhance the safety of domestic and industrial buildings alike.