Masonry Magazine February 2005 Page. 28
RETROFITTING MASONRY
sive, and studies have shown that once two cycles of ASF and BBNC has been applied and removed, it may have been more cost-effective to install blast-resistant glazing from the outset.
Protect Occupants
By keeping the blast wave out of the building, damage to the internal fabric and equipment is minimized, and recovery accelerated.
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, therefore, in addition to the requirements imposed by building regulations, 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 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 con
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Figure 2. Failure of the connections in a precast concrete frame.
sider 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 (Figure 2).
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, type of wall (loadbearing or infill) and degree of fenestration.
Steel Column and Plate
This is a particularly robust form of retrofit technique in which a number of steel columns are secured behind the wall and connected into the building frame at the floor and ceiling level (Figure 3). Steel plates connect the flanges of the columns together, producing an in-situ tensile membrane capable of resisting loads of up to 50psi. Ideally suited where loadbearing walls must give support to the floor above, the internal surface preparation for this retrofit is minimal. However, the engineering is demanding, and the installation process is intense, par-
Figure 3. Steel columns and plate.
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26 Masonry
February 2005
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