Photo courtesy Gate Precast
Defense against blasts
Another area of resilient design includes protection from explosions, whether accidental or deliberate (e.g. a terrorist attack). These events often damage, or render useless, part of the structure’s primary support system. Hence, the structure could experience progressive collapse as loads are increased on the remaining structural elements, causing them to fail.
The goal of structural-integrity design is to provide enough strength and redundancy to the structure so the failure of one component does not result in a disproportionate collapse of the remaining building. Most structures in this risk category (e.g. Department of Defense [DOD] and General Services Administration [GSA]) are designed in order to meet the Anti-Terrorism Force Protection (ATFP) requirements of the Unified Facilities Criteria (UFC). This requires different analysis techniques than are often used by structural designers. For example, the response of the system must be analyzed by dynamic response and not an equivalent static load. All structures are also designed for a Level of Protection (LOP), with each level having defined allowable limits of deformation and damage.
Designers must also take into account how different materials resist and react to explosive forces. Recent tests conducted by the Air Force Research Laboratory and PCA provide comprehensive results on the capabilities of precast concrete panels to withstand forces generated from blasts.3 Supported by PCI, Lehigh University, Auburn University, and the University of Missouri, the research examined the blast resistance of insulated precast concrete sandwich wall-panel construction under full-scale blast conditions in a multi-story structure.
The initial tests, conducted in 2007, used panels with a 9-m (30-ft) span. The second phase, completed in 2010, offered further comprehensive results, including static testing of more than 50 single-span and multi-span panels that contained varied wythe-thicknesses and types of connectors. The final evaluations, released by the Air Force, show precast concrete performed well under all conditions. These results help designers employ precast concrete to protect against threats as required in the design of government facilities. Results also show the minimum required standoff distance can be reduced when using precast concrete compared to traditional design requirements.
The United States Attorney’s office for the Western Division of Kentucky recently leased space in a new building in Louisville that had to meet GSA’s requirements. A six-story government building located in a high-profile city district, it required a blast-resistant design with an appearance complementing the look of an adjacent historic federal building constructed of natural stone. The LOP was determined to be ‘medium.’ Precast concrete was used for the envelope system, as well as for portions of the parking structure occupying the building’s first two levels.
Photo courtesy the Bristol Group
Fire safety
Fire safety is an example of where codes have reduced their overall performance requirements. IBC now allows for substantial building areas and heights to be built with Type V construction (which can include wood or light-gage steel framing) where an approved sprinkler system is provided. This shift toward active fire protection systems as the primary life safety component, and the decrease in containment and passive fire-resistant construction, does not result in resilient construction. In other words, structures must either be completely rebuilt or have extensive reconstruction after a major fire event. For example, wood and light-gage steel framing assemblies do not react well to water, and often require complete replacement after exposure.
Precast concrete provides significantly enhanced passive fire protection due to its inherent inorganic composition. Buildings relying solely on code-approved sprinklers leave themselves vulnerable to damage to the water supply and other defects. Containment by compartmentalizing the design with a precast concrete structural system limits damage and allows occupants more time to evacuate the premises. PCI recently published its updated third edition of the Design for Fire Resistance of Precast/Prestressed Concrete handbook, including an International Code Council Evaluation Service (ICC-ES) report that allows its use as an alternate to code provisions.5
Conclusion
High-performance structures must be designed for resilient construction and therefore should employ the appropriate materials and systems. Precast concrete can provide the inherent versatility, efficiency, and resiliency needed to meet the demands of today’s projects.
Brian Miller, PE, LEED AP, is the managing director of business development at the Chicago-based Precast/Prestressed Concrete Institute (PCI). He has more than 25 years of experience in the precast concrete, concrete materials, and construction industries. Miller can be reached at www.pci.org.
