High performance, high expectations: Ensuring resiliency with precast concrete walls

by Catherine Howlett | June 1, 2013 9:48 am

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by Brian Miller, PE, LEED AP

Recent code changes, more stringent sustainability requirements, and a challenging economy are some of the factors increasing demand for high-performance structures that integrate and optimize several attributes on a lifecycle basis. This definition carries with it several shifts from ‘business as usual.’

The concept of ‘high-performance’ encompasses sustainability and its related concepts and practices. However, it goes beyond a ‘this-or-that’ approach by requiring optimization of all relevant attributes for a project. Hence, characteristics such as energy and water conservation, safety, security, and durability are no longer just options, but requirements that must be integrated into the overall design, construction, and performance of a structure. (This definition of ‘high-performance buildings’ is paraphrased from the 2007 Energy Independence and Security Act [401 PL 110-140].)

At the same time, ‘high-performance’ also focuses on a structure’s long-term performance, which is a marked change from the first-cost-driven approaches often seen. One vital attribute of long-term performance is ‘resiliency.’ A natural extension of sustainability, it focuses on the structure’s ability to resist natural and man-made disasters such as earthquakes, fires, hurricanes, tornados, floods, and terrorist attacks. It also encompasses how quickly the structure can be back in service after such events occur, and the resources required. Essentially, high-performance structures must be resilient against various forces, and protect occupants and property within.

Once the mindset is shifted to this bigger picture, traditional materials and approaches may need to be rethought. To build high-performance structures, one needs high-performance materials that are versatile, efficient, resilient, and able to meet multiple requirements and challenges. One example is precast concrete, which offers the strength, durability, and fire resistance of concrete, but with additional benefits such as offsite manufacturing, which offers speed of construction, minimal site impact, and a high degree of quality and versatility.

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Wall systems
A major component of a high-performance structure is the exterior wall system. Precast concrete wall systems can be categorized into three main groups: solid, thin-shell, and insulated sandwich wall panels (ISWPs). The first two types offer traditional, durable wall systems that can be integrated with other interior wall systems (e.g. steel-stud and cavity walls), while ISWPs offer additional benefits.

ISWPs are made up of two wythes of concrete sandwiching an internal layer of rigid insulation. Inner and outer wall thicknesses vary based on project needs, design requirements, degree of composite action required, or how much the two walls work together to resist loads.

These are face-sealed (rain barrier) systems with the outer wythe serving as the primary defense against bulk rainwater intrusion. It also provides the exterior architectural finish and a continuous air barrier, which is now required in certain climate zones by the 2012 International Energy Conservation Code (IECC). Typically a minimum of 76 mm (3 in.) thick, the outer wythe also acts as a vapor retarder. However, this is usually not a concern since the layer of rigid insulation typically serves as a vapor barrier.

For this inner core, several types of insulation can be used, such as polyisocyanurate (polyiso), expanded polystyrene (EPS), or extruded polystyrene (XPS). Each has different properties and advantages. The thickness of this insulation layer is selected to provide the R-value to meet energy requirements. Usually, the insulation is run edge-to-edge meeting continuous insulation (ci) requirements of both IECC and American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings.

The concrete inner wythe often provides the structural capacity, but it can be designed to work compositely with the outer wythe. The two wythes are typically connected by composite material ties, which eliminate thermal bridging. The degree of composite action between the wythes, determined by the structural designer, is controlled by the type and frequency of the connectors. Precast fabricators can assist in the design of these wall systems and help ascertain which is most appropriate for the project.

When an ISWP’s interior wythe is used as the exposed interior wall surface, it provides a durable surface and saves the need for additional materials such as drywall, interior furring, and wall compound (along with the trades and labor to install them). Precast concrete also helps improve the overall indoor environment, as it neither contains volatile organic compounds (VOCs) nor offers a food source for mold or mildew.

Specifying ISWPs also maximizes the thermal mass effect, which essentially provides an increased effective R-value, reducing heating and cooling loads. The thermal mass effect also helps maintain a uniform temperature (improving occupant comfort), and often shifts peak energy demand to times when energy costs are lower. This can help reduce HVAC equipment’s required capacity (i.e. size), improve energy efficiency, and lower operational costs since less energy will be required to heat and cool the structure.

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Storm protection
The increasing rate at which newer buildings are being demolished, and the rising costs of repairing or replacing more-recent structures related to natural disasters, highlights an unfortunate reduction in durable construction. High-performance structures provide the resiliency needed to resist natural disasters, protect occupants, and return to full service with minimal resources.

Most of the United States is susceptible to earthquakes, fires, or severe storms. Hurricanes and tornados cause billions of dollars of damage annually. The former can produce sustained winds in excess of 240 km/h (150 mph), while the latter yields shorter-duration, but intense, winds of more than 400 km/h (250 mph). On their own, these winds can do extensive damage, but they also carry the potential for ‘missiles’ of debris that can have further impact. High-performance structures should inherently be built to withstand these types of forces and exposures.

The strength of precast concrete systems provides protection from such threats. Precast concrete’s impact resistance has been proven via several demonstrations[4] by the Precast/Prestressed Concrete Institute (PCI) and Portland Cement Association (PCA). During tests, lengths of 2x4s were fired at 160 km/h (100 mph) toward wall samples made with various construction methods. The construction included wood-framed and steel-stud walls covered with brick veneer and vinyl siding, and an insulated, precast concrete sandwich wall. Except for the precast concrete wall, the wooden projectile always penetrated the assembly, causing severe damage. The wall not only prevented the 2×4 projectile from penetrating the assembly, but also left only a slight blemish from the impact.

Precast concrete systems have been used to meet the storm-resistance requirements of Federal Emergency Management Agency (FEMA) 361, Design and Construction Guidance for Community Safe Rooms. Devastating Midwest storms have prompted incorporation of tornado safe rooms in schools and other facilities. Depending on location, these facilities must resist winds of 210 to 400 km/h (130 to 250 mph), as well as meet flying debris and missile impact criteria. Some other requirements include a 4.8-kPa (100-psf) roof live load and modified load factors for combinations, including wind effects.

For example, the Dallas County School District (Buffalo, Missouri) created a FEMA safe room for its gymnasium. This way, the room serves a dual purpose and is in close proximity for students in the event of a tornado. The project employed precast concrete insulated sandwich wall panels and double tees for its roof to meet FEMA 361 requirements. The interior side of the ISWPs had a steel-troweled, painted finish serving as the finished interior surface.

Other examples include the Coast Transit Authority (CTA) comfort stations in Biloxi, Mississippi. The elevated structures were designed to replace facilities destroyed by a hurricane. Using funds from FEMA and Mississippi Emergency Management Agency (MEMA), the owner built four redesigned comfort stations engineered for Category 5-type wind and storm surge.

The structures use a complete precast structural and architectural system. Even the roof panels are constructed with precast concrete and have an integral terra cotta color and water-repellant admixture. Upkeep is minimal regardless of weather conditions—no painting is required and no roofing or siding needs to be replaced if high winds occur. The naturally ventilated restrooms were designed to be hosed down for cleaning, so rainwater getting inside via absorption is not an issue.

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Earthquake resistance
After the seismic design provisions were revised in the International Building Code (IBC), more structures must be designed for greater seismic forces. Along with National Science Foundation and a consortium of industry and academia, PCI has conducted extensive research on seismic design methodology of precast concrete over the past two decades. This research includes the Precast Seismic Structural Systems (PRESSS) and Diaphragm Seismic Design Methodology (DSDM) research programs.

Both programs included scaled mockups of structures that were tested under varying seismic conditions. The PRESSS concepts have now been codified within American Concrete Institute (ACI) 318, Building Code Requirements for Structural Concrete and Commentary, and are accepted within IBC. The more recent DSDM project employed the world’s second-largest shake table at the University of California San Diego. Results are currently going through the codification process. This research has resulted in an innovative precast hybrid, moment-frame structural system that is being used throughout California and other heavy seismic regions.

In designing the Independent System Operator headquarters in Folsom, California (pictured on page 30), structural engineers capitalized on this innovative precast hybrid, moment-frame structural system for the office wing. According to Brian Reil, principal with Buehler & Buehler Structural Engineers, precast hybrid moment-resisting frames were employed to meet the site’s seismic demands, as well as the project’s sustainability goals.

“The structure is designed for seismic conditions, with beam-column joints utilizing both post-tensioning cables and mild steel reinforcing,” Reil explains. “In a seismic event, the reinforcing steel dissipates energy in a ductile manner while the post-tensioning cables running the length of the frame will provide elastic action that creates a self-righting force to pull the joint and frame back towards its original position.”

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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[7] 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[8] 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.

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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[10] 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[11].

Source URL: https://www.constructionspecifier.com/high-performance-high-expectations-ensuring-resiliency-with-precast-concrete-walls/

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