Retrofitting existing masonry buildings to resist explosions: there are several approaches available, and the choice depends on the size of the threat, type of wall, and degree of fenestration - retrofitting masonry

Masonry Construction, Dec, 2002 by Stephen P. Ward

The tragic events of September 11 served to highlight the vulnerability of existing structures to terrorist attack. All Western democracies are now acutely aware of the apocalyptic consequences of a well-orchestrated attack on high-profile facilities.

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, 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 focuses 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 delivered using a vehicle-borne improvised explosive device (IED).

Design considerations

Structural engineers have to consider a number of conflicting requirements when strengthening an existing masonry building to resist the effects from blasts.

Setback. A setback is the prerequisite for all blast-mitigating solutions and is often the cheapest. It can take many forms, including open areas, parking lots, pedestrian-only zones, or sacrificial buildings. It may be possible to create sufficient standoff using low-level walls, perimeter fences, or barriers.

In locations where adequate standoff cannot be achieved, it is necessary to reduce the size of the threat by restricting the size of the delivery vehicle or providing some measure of structural protection. Structural protection is usually only provided when all other means of mitigation are considered and exhausted. Structural alterations are expensive, and building owners are unlikely to spend the money unless they are forced to do so by legislation.

Prevent the blast wave from entering the building. Apart from the direct effects of an explosion on the structure, much of the damage 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.

Partitions, false ceilings, lighting, heating and ventilation ductwork, computer systems, telecommunications, and security apparatus are vulnerable. Keeping the blast wave out of the building minimizes damage to the internal fabric and equipment and accelerates recovery.

Windows. The most vulnerable part of any building is the windows. Considerable research and development have taken place to determine the best methods of protecting this vital part of a structure.

For many years, one of the most expedient measures was 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, and studies have shown that once two cycles of ASF and BBNC are applied and removed, it may have been more cost-effective to install blast-resistant glazing from the outset.

Protect occupants. The most important function of structural protection is to keep the people safe from terrorist attack. This protection 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 entire vulnerable facade. Properly trained security personnel, appropriate surveillance systems, and well-rehearsed emergency procedures help to protect occupants in the event of a crisis.

Prevent structural failure. Regrettably, there have been many explosive incidents in which the victims survived the initial attack only to lose their lives when the building subsequently fell down, such as with the World Trade Center. Two of the most important factors structural engineers have to consider are robustness and redundancy.

Robustness is a measure of the building's ability to cope with hazards in an acceptable way. Redundancy relates to a structure's ability to transfer loads into alternate areas. Buildings that are robust and structurally redundant are capable of surviving blast loads; 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 strength depending on the size of the threat, type of wall (loadbearing or infill), and degree of fenestration.

Steel column and plate. This particularly robust form of retrofit uses a number of steel columns secured behind the wall and connected to the building's frame at the floor and ceiling level. Steel plates connect the flanges of the columns, producing an in-situ tensile membrane capable of resisting loads up to 50 psi.