6.3.2 Building Structural Systems

In the selection of the structural system, consider both the direct effects of air-blast and the potential for progressive collapse in the event that a critical structural component fails.

The characteristics of air-blast loading have been previously discussed. To resist the direct effects of air-blast, the structural characteristics listed below are desirable.

• Mass. Lightweight construction is unsuitable for providing air-blast resistance. For example, a building with steel deck (without concrete fill) roof construction will have little air-blast resistance.
• Shear Capacity. Primary members and/or their connections should ensure that flexural capacity is achieved prior to shear failure. Avoiding brittle shear failure significantly increases the structure’s ability to absorb energy.
• Capacity for Reversing Loads. Primary members and their connections should resist upward pressure. Certain systems such as prestressed concrete may have little resistance to upward forces. Seated connection systems for steel and precast concrete may also have little resistance to uplift. The use of headed studs is recommended for affixing concrete fill over steel deck to beams for uplift resistance.

To reduce the risk of progressive collapse in the event of the loss of structural elements, the structural traits listed below should be incorporated.

• Redundancy. The incorporation of redundant load paths in the vertical-load-carrying system helps to ensure that alternate load paths are available in the event of failure of structural elements.
• Ties. An integrated system of ties in perpendicular directions along the principal lines of structural framing can serve to redistribute loads during catastrophic events.
• Ductility. In a catastrophic event, members and their connections may have to maintain their strength while undergoing large deformations.

Historically, the preferred material for explosion-mitigating construction is cast-in-place reinforced concrete. This is the material used for military bunkers, and the military has performed extensive research and testing of its performance. Reinforced concrete has a number of attributes that make it the construction material of choice. It has significant mass, which improves response to explosions, because the mass is often mobilized only after the pressure wave is significantly diminished, reducing deformations. Members can be readily proportioned and reinforced for ductile behavior. The construction is unparalleled in its ability to achieve continuity between the members. Finally, concrete columns are less susceptible to global buckling in the event of the loss of a floor system.

Current testing programs are investigating the effectiveness of various conventional building systems; however, in general the level of protection that may be a achieved using these materials is lower than what is achieved using well-designed, cast-in-place, reinforced concrete. The performance of a conventional steel frame with concrete fill over metal deck depends on the connection details. Pre-tensioned or post-tensioned construction provides little capacity for abnormal loading patterns and load reversals. The resistance of load-bearing wall structures varies to a great extent. More information about the response of these systems is described in the subsection on structural elements and in the section on exterior cladding in Section 6.4, Exterior Envelope.