by Stephen P. Ward, Eur Ing, CEng, MICE, MIExpE
All western democracies are now acutely aware of the apocalyptic consequences of a well-orchestrated attack on high-profile government facilities and other related targets. Many of these buildings are historical, ornate, listed and constructed using traditional techniques with masonry elevations. Many of the modern retrofitted reinforcement techniques used to protect these structures against terrorist attacks are unsightly, intrusive or inappropriate. However, security specialists are well aware that while there might be little that can be done to defend a building against an aircraft attack, much can be done to defeat the more traditional car bomb and bullet. This article will focus attention on some of the methods available to the structural engineer to strengthen existing masonry structures and provide resistance to the effects of a blast attack.
In strengthening an existing masonry building to resist the effects from blast, structural engineers have to consider a number of conflicting requirements. Some of these include:
Prevent the Blast Wave Entering the Building. Apart from the direct effects of an explosion on the structure, much of the damage caused is due to the effects of the blast wave entering the internal parts of the building. The relatively fragile components of modern offices offer little resistance to high-energy blast waves and yet are critical to the efficient functioning of the workplace. Partitions, false ceilings, lighting, heating and ventilation ductwork and trunking are very vulnerable, not to mention computer systems, telecommunications and security apparatus. By keeping the blast wave out of the building, damage to the internal fabric and equipment is minimized, and recovery accelerated.
Set-back. Set-back is the pre-requisite for all blast mitigating solutions and is often the cheapest. It can take many forms: open areas, car parks, pedestrian-only zones or even sacrificial buildings, to name a few. It may be also possible to create sufficient set-back using low-level walls, perimeter fences or barriers. However, in locations where adequate stand-off cannot be achieved, it will be necessary to either reduce the size of the threat (e.g. by restricting the size of the delivery vehicle) or provide some measure of structural protection1.
Windows. The most vulnerable parts of any building are the windows. Considerable research and development has taken place around the world to determine the best methods of protecting them. For many years, one of the most expedient measures has been to apply Anti-Shatter Film (ASF) combined with Bomb Blast Net Curtains (BBNC). However, manufacturers only guarantee ASF for 10 years, and BBNC requires regular cleaning and obstructs the view. Removal of ASF is time-consuming and labor-intensive.
Protect Occupants. The most important function of structural protection is to safeguard the people who work and live in the building. Every building owner has a duty of care; there is a need to make the place safe from terrorist attack. This can take several forms depending on a whole spectrum of parameters. Smaller buildings with robust exterior walls can be strengthened to resist attack. For large buildings, it may be more economical to establish "safe havens" within the structure instead of strengthening the whole of a vulnerable facade. Properly trained security personnel, appropriate surveillance systems and well-rehearsed emergency procedures will all help to protect occupants in the event of a crisis.
Prevent Structural Failure. Regrettably, there have been many explosive incidents in which the victims have survived the initial attack only to lose their lives when the building subsequently fell down. Two of the most important factors structural engineers have to consider are robustness and redundancy. Robustness is a measure of the buildings ability to cope with hazards in an acceptable way. Redundancy is a condition relating to the ability of a structure to transfer loads into alternate areas. Buildings that are robust and structurally redundant are capable of surviving blast loads well; buildings that are not tend to suffer badly.
Reinforcing Existing Masonry Walls. The philosophy behind reinforcing existing masonry walls is to provide increased strength along with improved ductility and/or "catcher" (restraint) systems wherever possible. There are several ways of achieving this depending on the size of the threat, the type of wall (load-bearing or infill) and degree of fenestration.
- Steel Column and Plate. In this robust form of retrofit technique, a number of steel columns are secured behind the wall and connected into the building frame at the floor and ceiling level. Steel plates connect the flanges of the columns together, producing an in-situ tensile membrane capable of resisting loads of up to 50 psi. Ideally suited where load-bearing walls must give support to the floor above, the internal surface preparation is minimal. However, the engineering is demanding and the installation process intense, particularly as each connecting weld must be sound, and construction details can be problematic. The technique is therefore relatively expensive.
- Steel Stud Partition. Vertical steel studs are fixed between floors and support reinforced gypsum board or laminated glass. This partition is then placed at least 300mm inside the existing non-load-bearing wall to act as a catcher screen. The system is easy to install, requiring no surface preparation but can only be used for relatively light blast loads.
- Elastomeric Spray. Elastomeric spray uses a urea or polyurea-based coating up to 15mm thick applied directly to the rear face of an existing masonry wall. Once dry, the coating forms a tensile membrane enhancing the flexural capacity of the masonry, thus significantly reducing spalling. The coating is relatively inexpensive but the wall must be prepared very thoroughly and considerable attention paid to the cleanliness of the masonry surface. The system has been exposed to blast pressures up to 35 psi and impulses of 215 psi-ms, successfully reducing spalling, but cannot be used on load-bearing walls without the support of another load-bearing system.
- Geotextiles. Tests where geotextiles have been secured to the rear of masonry walls and subjected to blast loads have been conducted. The fabrics2 have either been mechanically attached to the floors above and below or glued to the internal face of the masonry wall. In so doing, they act as a "catcher system" restraining spalled and broken masonry from entering the building envelope. While effective, considerable attention must be paid to securing fabric top and bottom or ensuring there is an effective bond between the fabric and the masonry. Further, special arrangements must be made for load-bearing walls and for walls with windows.
- Retrofitted Reinforced Masonry. Reinforced masonry is stronger and more ductile than unreinforced and is capable of resisting relatively high out-of-plane loads3 depending on the level of reinforcement. Retrofitted reinforced masonry4 uses techniques developed in the building restoration industry where existing structural masonry is diamond-core drilled from the roof to the foundation and specially designed grout inflated masonry anchors are installed and allowed to cure. The system has been tested to125 psi 284psi-ms (440 lbs at 41 feet) and can also be used to secure blast-proof windows within masonry walls, combining window security with masonry strengthening. Research has also shown that masonry walls with high levels of internal vertical loads (e.g. in multi-story buildings) resist spalling better than those that are not load bearing (e.g. infill panels or single-story construction). Retrofitted reinforced masonry can also be post-tensioned after installation to increase the internal vertical stress and maximize spalling protection in low-level masonry structures. The anchors are easily installed even in occupied buildings within the plane of the wall and are not visible once installation is complete. Further, retrofitted reinforced masonry can also be used in areas of high seismic risk where dynamic loads due to ground movement have to be resisted.
- Internal Concrete Skin There are certain situations where the blast load is so large that it is not possible to provide the required level of protection using the conventional retrofitted techniques described above. In such cases, the only solution is to retrofit the building with an internal concrete skin. This is an effective but expensive solution. A full structural analysis is required to determine whether it is necessary to underpin the foundations to resist the additional dead loads. Further structural reinforcement may be required to strengthen the building frame to resist the huge dynamic loads likely to arise and prevent building collapse. Further, there will be loss of space inside the building equivalent to the thickness of the concrete skin and the necessary "air gap" behind the existing wall5.
- Durisol Block. A proprietary product known in the United States as Durisol Block provides a variation on the internal concrete skin. Durisol is basically a hollow concrete block made of mineralized wood shavings as the aggregate, instead of sand and stone. This mixture is used to make stay-in-place wall forms for concrete structures, freestanding sound barriers and other products. Subject to the limitations above, Durisol Block provides an expedient solution to the problem of retrofitting masonry structures to resist the effects of explosions.
Existing masonry walls can be strengthened in a variety of ways to resist the effects of explosions. Comparisons between each method are summarized in the table above. Often, it is possible to combine two or more systems into a hybrid scheme capable of resisting loads greater than the sum of the individual systems. Fundamentally, there is no substitute for set-back and security arrangements must be tested against the ease with which it possible to deliver and detonate an explosive device. At this stage, it is often possible to mitigate against the effects of explosions by adopting changes to established procedures without making alterations to the building structure. However, if all those procedures are circumvented, there is no substitute and existing masonry buildings must be retrofitted in some way.
1 Experience shows that structural protection is usually only provided when all other means of mitigation have been considered and exhausted. Structural alterations are expensive and building owners are unlikely to spend money unless they are forced to do so by appropriate legislation.
2 Commercial names include Aramid, Kevlar and Geofabric.
3 Out-of-plane loads act perpendicularly to the surface of the wall.
4 The Blastec System manufactured by Cintec International Ltd.
5 In such designs the existing outer wall is assumed to fail under blast load and in doing so it will deflect inwards significantly. The remains of the masonry wall and the blast wave then impact on and are resisted by the internal concrete wall.
Stephen P. Ward is the division manager of CINTEC America in Washington, D.C. CINTEC America. has associated offices in Newport, Wales, UK; Newcastle, Australia; Ottawa, Canada and in India. CINTEC specializes in the restoration, strengthening, and refurbishment of masonry structures worldwide. For more information, please email [email protected], call 1 (800) 363-6066, or go to www.cintec.com.