Masonry Magazine February 2005 Page. 27
IN RECENT YEARS, MANY BUILDINGS IN THE U.K. HAVE HAD THEIR WINDOWS PROTECTED OR STRENGTHENED to resist the effects of explosions in an attempt to reduce the level of casualties associated with terrorist bomb attacks. While the windows form the most vulnerable parts of a building, occasionally it becomes necessary to strengthen the walls as well, particularly in older and weaker historical structures or where the blast loads are high. This article examines a number of different techniques available that can be employed to make existing masonry walls stronger and more capable of safely resisting the effects of explosions.
For nearly 20 years, Cintec International, Ltd., based in Newport, South Wales, and its sister company in the U.S., Cintec America, Inc., have manufactured specialized anchors for reinforcing, strengthening and repairing all types of existing masonry structures around the world. Retrofitted, reinforced masonry support anchors are comprised of stainless steel sections, a grouting sock and an engineered grout. Installation is performed by precisely drilling holes using wet or dry diamond coring technology. More recently, Cintec anchors have been used to improve the load-carrying capacity of masonry arch bridges and the impact resistance of parapets, as well as to provide seismic protection in multi-story masonry structures.
The tragic events of September 11 have served to highlight the vulnerability of existing structures to terrorist attack in the United States. 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, inelegant, intrusive and 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 strengthen existing masonry structures and provide resistance to the effects of a blast attack.
Limitations
FOR SPACE REASONS it is not possible to consider the full spectrum of possible terrorist attacks (e.g., biological, chemical and nuclear) or methods (e.g., ballistic and vehicle impact). Therefore, this article will only consider defense against blast attacks delivered using vehicle-borne Improvised Explosive Device (IED).
Considerations
IN STRENGTHENING an existing masonry building to resist the effects from blasts, structural engineers have to consider a number of conflicting requirements. Some of these include:
Stand-Off
Stand-off, or to use the U.S. term "set-back," is the prerequisite 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 also be possible to create sufficient stand-off using low-level walls, perimeter fences or barriers. However, in locations where adequate stand-off cannot be achieved, it is 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 protection.
Prevent the Blast Wave from 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, and heating and ventilation ductwork are very vulnerable, as are 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.
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 these vital parts of a structure. 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-inten-
Figure 1.
Widespread damage to conventional glazing
The Voice of the Mason Contractor
February 2005
Masonry 25
Sth
Photo courtesy of G.C. Mays and PD