currently available to quickly and accurately sense the presence of toxins in HVAC systems and building shafts and automatically initiate responses. Smoke detectors in use today can initiate certain actions, such as shutting down the HVAC. A more sophisticated approach would involve developing new sensors and installing them in HVAC systems that could isolate dangerous toxins in one area of building as soon as the threat is recognized. These sensors could use the same core element that was described earlier to protect first responders.

Implementation of Existing Technology

Historically, the blast engineering of buildings evolved in response to the most recent destructive event. For example, explosions producing extensive amounts of flying glass led to better glazing systems that include robust frames and mullions, films, and composite glazing. The main barrier to wide application of this latter technology, which has two broad categories, is cost. Structures such as courthouses use standard glazing with laminations to resist shattering, and robust frames and mullions; the cost of these systems is typically 25 percent more than glazing with no blast resistance. State Department criteria lead to glazing approximately two times thicker than conventional systems for the lower 10 to 15 stories; the cost is typically 100 percent greater than glazing with no blast resistance. Another component of cost is conservatism arising out of approximations in CONWEP and BLASTX, the most commonly used software for predicting blast pressure. These approximations are often accepted in preference to undertaking costly three-dimensional, computational-fluid-dynamics (3D CFD) models. Recalibration of BLASTX is needed.

Close attention has been given to the blast engineering of column design, especially for steel column splices, which are typically built to resist global structural but not local bending. Blast loading requires splices to resist local bending as well. Implementation of this technology is hampered by construction cost, magnified by uncertainty in the requisite analysis.

Better knowledge of the engineering properties of masonry (such as that employed to build the U.S. Capitol) and of aged reinforced concrete (such as that at the Pentagon) is needed to exploit advanced analytical techniques. Another benefit would be to introduce new materials such as Linex, a spray-on, self-bonding elastomer that has been tested in Israel with U.S. participation. Linex increases the ductility of masonry walls, such as the inside surface of the brick at the Pentagon.

In crowded urban areas, where adequate standoff distance or blast walls are impractical, new structures should consider new materials such as stainless steel curtain walls. Also, louvers and plenums for air-conditioning may occupy up to 20 percent of the lower-floor wall-surface area, creating a soft spot in the building skin. Alternative designs might reduce such vulnerability.

Fire resistance ratings currently in use in the United States should be cor-



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