Mitigation of Terrorist Attacks on Buildings (Part 4): Building Material Selection

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 has the right to the same quality information as the federal government. Therefore we will be excerpting and summarizing the cumbersome FEMA design documents into bite-size chunks which may be of interest to the concerned layperson. Right now we’re focusing on FEMA 426, which can show you how to (at least partially) blast-proof your homes and businesses.

Image Courtesy FEMA 426

This week we bring you part four 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. We are focusing on structural design considerations today. In short, what is the best structural material to use on your buildings? The short answer: conventionally reinforced concrete. Generally speaking the heavier your floors and walls, the more blast and small arms-resistant they will be.

Given the evolving nature of the terrorist threat, it is impossible to predict what threats may be of concern during the lifetime of the building; it is therefore prudent to provide protection against progressive collapse initiated by a localized structural failure caused by an undefined threat. Because of the catastrophic consequences of progressive collapse, incorporating these measures into the overall building design should be given the highest priority when considering structural design approaches for mitigating the effects of attacks.

Deciding ahead of time to mitigate the direct effects of air-blast enhances life safety by providing protection against localized failure, flying debris, and air blast entering the building. It may also facilitate evacuation and rescue by limiting the overall damage level and improving access by emergency personnel.

WHAT SHOULD WE BUILD IT OUT OF?

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.

To resist the direct effects of air-blast, the structural characteristic
listed below is usually the most important.

❍ MASS. Lightweight construction like wood or metal stud framing 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.

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.

Cast in Place (CIP) Concrete

Reinforced concrete 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 achieved using these materials is lower than what is achieved using well-designed, cast-in-place, reinforced concrete. For example, 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.