Mitigation of Terrorist Attacks on Buildings (Part 6): Structural Roof Systems

If the government knows how to blast-proof their buildings to protect their judges and politicians, don’t we deserve the same right to know how to protect our homes and businesses? We feel that the general public needs the same quality information as the federal government. So we are excerpting and summarizing the cumbersome FEMA design documents into bite-size chunks for easy reading. Right now we’re focusing on FEMA 426, which can show you how to (at least partially) blast-proof your homes and businesses.

This week we bring you part six of our discussion on Building Design to Mitigate Terrorist Attacks (or other disasters). The information is appropriate when considering any type of blast, whether intentional or accidental, as well as small arms fire. Today we are focusing on structural roof systems.

ROOFS — Cast in Place Concrete is the Answer Once Again

When it comes to blast resistance and even high wind resistance, building heavy and building on-site (as opposed to pre-manufactured materials) is preferable.

Beam Shown with Stirrups that Make a Box Around the Longitudinal Steel, Prior to Concrete Placement

The preferred roofing system is cast-in-place reinforced concrete with beams in two directions. If this system is used, beams should have continuous top and bottom reinforcement. Stirrups (steel bars wrapping around the main steel, perpendicular to the beam direction) closely spaced along the entire span are recommended.

Cast in Place Concrete Roof Being Formed

Somewhat lower levels of protection are afforded by conventional steel beam construction with a steel deck and concrete fill slab. The performance of this system can be enhanced by use of normal-weight concrete instead of lightweight fill, or increasing the gauge of welded wire fabric reinforcement (WWF).

Precast and pre-/post-tensioned systems don’t work as well. Concrete flat slab/plate systems are also less desirable because of the potential of shear failure at the columns. When flat slab/plate systems are used, their punching shear resistance should be improved. Do this by providing continuous bottom reinforcement through columns in two directions to retain the slab in the event that punching shear failure occurs. Edge beams should be provided at the building exterior.

Precast Concrete Roof Being Installed

Lightweight systems such as untopped steel deck or wood frame construction, are considered to afford minimal resistance to air-blast. They are light-weight systems, and light-weight construction has minimal blast or high-wind resistance. These systems are prone to failure due to their low capacity for downward and uplift pressures.