Precast Concrete Advances Aid Military Projects
New research, tests and studies are expanding the capabilities of precast concrete in key ways that aid military structures, including blast, earthquake and storm resistance
By Brian Miller, P.E., LEED AP
Tests conducted by the Air Force Research Laboratory on precast concrete sandwich wall panels are helping to improve designs for blast resistance of conventional, off-the-shelf, precast concrete panel construction. Photo courtesy of Clay Naito, Lehigh University
The high performance attributes of precast concrete offers a resilient building system for military structures. As an engineered system, precast concrete can be cast to adjust its attributes to fit specific criteria outlined in any Request for Proposal. In recent years, major strides in the testing and evaluation of precast concrete building systems have helped extend its capabilities in key ways that benefit military structures. Some of these capabilities include providing efficient and resilient protection against blast, storm, fire, progressive collapse and seismic events—they also are helping to meet sustainable design goals.
Tests conducted by the U.S. Air Force Research Laboratory and the Portland Cement Association provided comprehensive results on the capabilities of precast concrete panels to withstand blast impacts. The research—supported by Lehigh University, Auburn University, the University of Missouri, and the Precast/Prestressed Concrete Institute (PCI)—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 30-ft span. The second phase, completed in 2010, provided more 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 initial evaluations, which have been released by the Air Force, show that precast concrete performed well under all conditions.
Several related research programs have been conducted for solid and insulated precast panels with prestressed and nonprestressed reinforcement. These results help designers utilize precast concrete to protect against threats often required in the design of government facilities. Results also show that the minimum required stand-off distance can be reduced when using precast concrete compared to traditional design requirements. PCI is finalizing a related committee report, Blast-Resistant Design Manual.
PCI also recently published a PCI Journal article entitled “Structural Integrity and Progressive Collapse in Large-Panel Precast Concrete Structural Systems,” encouraging designers and precasters to provide redundancy and integrity through static-design methods that meet existing guidelines. These guidelines include those incorporated into the Department of Defense Unified Facility Criteria—Design of Buildings to Resist Progressive Collapse, and the General Services Administration’s Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects. The National Institute of Standards & Technology also is conducting research on the disproportional collapse of precast concrete, beam-column assemblies, which falls under a much larger ongoing progressive-collapse research program.
SEISMIC RESEARCH AND DESIGN
PCI and a consortium of industry and academic centers have performed landmark tests that verify innovative design methodologies for precast concrete lateral load resisting systems designed to withstand high-seismic forces. Evaluated throughout the 1990s, several of these concepts have since been adopted into building codes such as ACI-318, which is referenced by the International Building Code (IBC). These systems now are used by developers across the country, including in high-seismic zones like California.
The Precast Seismic Structural Systems research was sponsored by PCI, as well as the National Science Foundation, and the Precast/Prestressed Concrete Manufacturers Association of California (now PCI West). Tests were conducted on four ductile-frame systems and a shear-wall system using a five-story, 60 percent scale shake-table structure.
From this evaluation and subsequent design analysis, a Precast Hybrid Moment Frame system was created that provides seismic resistance in both the longitudinal and transverse directions. The system uses unbonded, post-tensioned strands running through a duct in the center of the beam and through the columns. Mild steel reinforcement runs through ducts at the top and bottom of the beam in the connection region and is sleeved through the column and grouted.
The design allows the moment-resistant frame to absorb seismic energy independent of the structural members’ integrity. The post-elastic performance is concentrated in the connection region rather than a structural member. This “rubber-band” approach allows the building to self-right itself after a seismic event, as the building flexes with the earthquake force and then pulls back into its original position with little to no structural damage. The design helps control locations where damage may occur, and ultimately reduces the likelihood of having to replace a structure in its entirety after an earthquake.
In addition to this work, the Diaphragm Seismic Design Methodology project has addressed floor systems and horizontal diaphragms that more effectively contribute to a structure’s seismic performance. The goal is to create an appropriate seismic design methodology for eventual code integration.
The work is being conducted by a consortium led by PCI and conducted at Lehigh University, the University of Arizona, and the University of California-San Diego (UCSD). The project involves a series of computerized models that were verified on scale models at the UCSD Network for Earthquake Engineering Simulation shake table and examined the performance of multiple-diaphragm connection details.
Another project at UCSD investigates the performance of architectural precast concrete cladding during a seismic event. As part of a larger project, the top two stories of a five-story building have been clad in precast panels and subjected to stresses on the university’s shake table. The test examined the behavior of bolted sliding connections and flexing connections under inter-story drift.
In recent years, PCI has advanced the science of forensic design by sending reconnaissance teams to visit the sites of devastating recent earthquakes in Concepcion, Chile; Christchurch, New Zealand; and Tohoku, Japan. These findings have been consistent in determining that precast concrete structures, especially those built to modern building codes and designed by following advanced seismic guidelines, performed very well. PCI’s eLearning Center (elearning.pci.org) offers an educational program on the reconnaissance teams and their findings. In response to the evolving knowledge in this field, PCI has released the Second Edition of its Seismic Design of Precast/Prestressed Concrete Structures. This manual incorporates seismic-design provisions of the 2006 IBC as well as other recent innovations.
The impact of hurricanes, tornadoes and other extreme weather events can wreak havoc with buildings that cannot withstand wind loads and other forces inflicted upon them. Studies have shown, however, that precast concrete can endure such conditions. This can be especially significant for military buildings that often must continue operating through crisis and serve as safe havens during storms. These design techniques also can be applied to residential housing often connected to military bases.
A demonstration by PCI and the Portland Cement Association in 2008 showed the protection that precast concrete construction can provide. During the test, lengths of 2x4s were fired at 100-mph toward various wall samples, including brick- and siding-covered, wood-framed walls, a reinforced brick wall, and an insulated, precast concrete wall. In all cases except the precast concrete wall, the wooden projectile caused severe damage penetrating the wall. The precast wall not only withstood the 2x4, only a slight mark was left from its impact.
Recent investigations of the damage done by Hurricanes Katrina and Rita along the Gulf Coast showed that structures clad with precast concrete panels performed well because of their durability and the strength of the connections. Upgrades to building codes and higher awareness of the need for more protection have encouraged the use of precast concrete. Key needs for protection include resistance to high waves and scour, formed by waves rushing beneath a slab on grade.
Precast concrete also provides significantly enhanced passive fire protection due to its inherent inorganic composition. Buildings that rely on code-approved sprinklers leave themselves vulnerable to damage to the water supply and other defects.
Compartmentalizing the design with a precast concrete structural system limits damage and gives occupants more time to evacuate the premises. PCI recently published its updated fire manual including an ICC-ES Evaluation Report, which allows the manual to be used as an alternate to the code provisions.
As an engineered system, precast concrete can provide specific capabilities that meet the unique design requirements of military buildings. Consulting a local precaster during the design process can ensure the potential of these capabilities is fully realized. That input allows engineers to provide the most cost-efficient, constructible, resilient and aesthetically pleasing design possible.