6.2.1 Building Exterior

This section discusses the building shape, placement, and exterior ornamentation. For a discussion of exterior cladding, see Section 6.4, Building Envelope.

At the building exterior, the focus shifts from deterring and delaying the attack to mitigating the effects of an explosion. The exterior envelope of the building is most vulnerable to an exterior explosive threat because it is the part of the building closest to the weapon, and it is typically built using brittle materials. It also is a critical line of defense for protecting the occupants of the building.

The placement of the building on the site can have a major impact on its vulnerability. Ideally, the building is placed as far from the property lines as possible. This applies not only to the sides that are adjacent to streets, but the sides that are adjacent to adjoining properties, since it is not certain who will occupy the neighboring properties during the life of the building. A common, but unfortunate practice is to create a large plaza area in front of the building, but to leave little setback on the sides and rear of the building. Though this practice may increase the monumental character of the building, it also increases the vulnerability of the other three sides.

The shape of the building can have a contributing effect on the overall damage to the structure (see Figure 6-3). Re-entrant corners and overhangs are likely to trap the shock wave and amplify the effect of the air blast. Note that large or gradual re-entrant corners have less effect than small or sharp re-entrant corners and overhangs. The reflected pressure on the surface of a circular building is less intense than on a flat building. When curved surfaces are used, convex shapes are preferred over concave shapes. Terraces that are treated as roof systems subject to downward loads require careful framing and detailing to limit internal damage to supporting beams.

Generally, simple geometries and minimal ornamentation (which may become flying debris during an explosion) are recommended unless advanced structural analysis techniques are used. If ornamentation is used, it is preferable to use lightweight materials such as timber or plastic, which are less likely than brick, stone, or metal to become lethal projectiles in the event of an explosion.

Soil can be highly effective in reducing the impact of a major explosive attack. Bermed walls and buried roof tops have been found to be highly effective for military applications and can be effectively extended to conventional construction. This type of solution can also be effective in improving the energy efficiency of the building. Note that if this approach is taken, no parking can be permitted over the building.

Interior courtyards or atriums are other concepts for bringing light and a natural setting to the building without adding vulnerable openings to the exterior.