What firefighters need to know about this element of building construction
BY GREGORY HAVEL
In recent years, use of precast concrete wall panels has become more common and is supplanting the use of concrete block and other masonry materials in large buildings such as office buildings, factories, gymnasiums, movie theaters, and large retail stores. Although precast wall panels may be more expensive than masonry materials, the greatly reduced amount of on-site labor and construction time required can make the total cost of the project the same or less than that of masonry.
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The original concrete wall panels were called “tilt-ups” or “tilt-slab” construction. The builder would level the ground, pour the concrete floor slab of the building, and set up on it the wall panel forms, next to the foundation wall. After reinforcing steel and wire were placed in the forms, the concrete would be poured and finished like a floor or a driveway. After the concrete was cured, the forms would be stripped from the panels. Each panel would then be tilted up into position on the foundation wall and supported by temporary diagonal braces until permanently connected to the foundation and the building frame.
A more recent development is the manufacture of the wall panels at a fixed facility, as has commonly been done with prestressed concrete floor, roof plank, and “double-Tee.” This procedure allows for the use of heated forms to speed curing of the concrete and to better control the texture, color, and quality of the finished product. This better quality control includes compliance with consensus standards like those of the American Concrete Institute (ACI),1 the International Conference of Building Officials (ICBO),2 the Portland Cement Association (PCA),3 the Prestressed Concrete Institute (PCI),4 the American Welding Society (AWS),5 and the American Society for Testing and Materials (ASTM).6
BUILDING CONSTRUCTION
These wall panels are loaded onto trucks and transported to the building site (photo 1), where each panel is transferred from the truck to its place by crane (photo 2) and connected to the foundation, to the building frame, and to the adjoining wall panels.
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A further refinement in these precast wall panels is the inclusion of insulation. A modern wall panel is usually eight or 12 inches thick. Once in place, it will look like solid concrete from inside or outside the building. However, it is actually a thin concrete face on each side with foamed plastic insulation (usually polystyrene or polyethylene billets) sandwiched between them (photo 3). Or, panels may be manufactured with oval or round cores like concrete floor plank and the voids pumped full of liquefied plastic foam. The only solid concrete part of these panels will be at the edges and solid load-bearing ribs connecting the foundation anchor plates and the connections for girders and joists at floor and roof levels.
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Each panel is labeled at the time it is manufactured, including codes and barcodes, to show to which building it belongs and where in the building it is to be placed (photo 4).
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Use of this type of manufactured insulated panel results in lighter weight and shorter construction schedules than are possible using a tilt-up system or masonry walls. This type of panel also reduces long-term energy costs for building operations.
These panels are usually transported by semitrailer in eight-foot widths. Wider panels, such as 10 and 12 feet wide, are transported as oversize loads. They may be 30 feet or more in length for a warehouse, factory, or “big box” store and may exceed 50 feet for a three-story building (photo 5).
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These wall panels most often are connected to the foundation by welding the connection plates on the wall panel to a large steel angle attached to plates set in the foundation (photo 6). They are usually connected to girders and joists supporting floors and roofs and to each other by welding (photos 7, 8).
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Joints between panels are packed with insulation and caulked (photo 9). Even though these panels are filled with combustible plastic foam, they can be manufactured for use as fire-rated partitions, with advertised ratings up to four hours.7 If the wall is fire rated, the joints between panels are packed with the proper grade of mineral wool or ceramic fiber insulation and caulked with an approved fire-resistant material.
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Precast concrete wall panels may be bearing or nonbearing walls. If they are used as nonbearing walls, they will be supported by a steel or concrete building frame resting on a concrete foundation and attached to it by welds. On the outside of a building, it will perform like a curtain wall (photo 10).
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There are two methods for using these panels as bearing walls. One method pours a concrete footing below grade in an excavation and a concrete foundation wall on top of it, with steel plates and steel angles set into the top of the wall at points where wall panels will be attached. After the concrete is cured, the foundation is backfilled. The wall panels are set in place on the foundation wall and welded to it (photo 11).
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If the wall panels are erected before the steel or concrete building frame, they will have temporary diagonal supports anchored into the ground (photo 12), which will be removed after the building frame is complete. If the building frame is erected before the wall panels, they will be connected to it as they are set, without temporary supports.
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The other method for using these panels as bearing walls pours a concrete footing below grade but with a “keyway” in the top. The wall panels are set directly into this keyway, shimmed to make them plumb and level, supported by diagonal braces (photo 13), and grouted in place (photo 14). The wall panels are then backfilled to grade, making them their own foundation wall. After this, the building frame is erected and connected to the wall panels. Or, the steel building frame can be set in place with temporary columns supporting the girders at the walls until the panels are set in place and connected (photo 15).
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Door and window openings may be cast into the panels when they are made (see photo 13) or may be cut in with diamond saws after the walls are erected. Large openings like overhead doors and wide storefront entrances may result in the complete removal of the lower part of the panel, leaving the upper part of the panel supported entirely by its connections to adjacent panels (see photo 8).
These buildings are usually considered Type II (noncombustible) construction and may enclose several acres with steel columns throughout supporting the roof, like your local “big-box” store (photo 16). Or, they may enclose a large area with clear roof spans of 100 feet or more, like a gymnasium, a fieldhouse, or an auditorium (photo 17). Frequently, they are large enough that building codes require installation of automatic fire sprinkler systems. Structural steel is often left exposed and unprotected.
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CONNECTIONS
In this type of structure, as in any other, the weak points are the connections between the parts of the building. Most of the connections in this type of structure are welded. If the welds were properly done by qualified workers, they can be stronger than the steel members they join. Substandard welds or welds that have become stressed, corroded, or rusted are weak points and are liable to early failure from ordinary loads or in a fire.
Even if all of the connections are of high quality, every building deteriorates with age. This normal aging is accelerated by lack of maintenance, changes in use that load the structure beyond its designed capacity, and maintenance that is only cosmetic (as in regular repair and painting after the roof leaks but no attention is given to the roof or the structural steel that supports it).
FIRE CONDITIONS
Even if there are no weak connections, a fire in one of these buildings is likely to cause an early collapse of the roof (usually lightweight steel on lightweight steel bar joists). Since all parts of the building are tied together as part of a system, a roof collapse is more likely to tip walls inward than outward. The roof supports are usually more securely connected to the walls, hence the roof may collapse before the wall connections fail. Failure and collapse of any part of the structure will affect the stability of other parts.
A building with precast wall panels often has high ceilings and heavy fire loading, as in retail stores, factories, and warehouses. A fire in one of these occupancies will require large volumes of water from large lines and early and continuous support of the automatic sprinkler system. If the fire is not controlled quickly with these methods, master stream appliances will be needed.
Ventilation will be difficult. The roof will be an unsafe place for firefighters to perform vertical ventilation if the structural steel and roof deck have been heated by the fire. Horizontal ventilation will also be difficult, since there are few windows and the walls are difficult to breach.
As in any structure with a large fire load or life load, a firefighter’s best approach to a problem in this type of building is to become familiar with the building and its construction, its fixed fire detection and suppression systems and their quality of maintenance, the fire prevention practices enforced by the management, the emergency plans prepared and practiced by the occupants, and regularly updated preplans. These actions may not eliminate the risk when fighting a fire in one of these buildings, but they can reduce the risk to firefighters to a level that can be considered acceptable if lives or high-value property need to be protected.
Endnotes
1. American Concrete Institute (ACI): Farmington Hills, MI: www.aci-int.net, www.aci-int.net/general/free_resources.htm, www.aci-int.net/general/CI04_Committee.pdf, and ACI 318, Building Code Requirements for Reinforced Concrete.
2. International Conference of Building Officials (ICBO): Codes available at http://webstore.ansi.org/ansidocstore/dept.asp?dept_id=23.
3. Portland Cement Association (PCA): Skokie, IL: www.cement.org/basics/index.asp.
4. Prestressed Concrete Institute (PCI): Chicago, IL: www.pci.org; see especially MNL-116.
5. American Welding Society (AWS): Miami, FL: www.aws.org; see D1.1, Structural Welding Code, Steel.
6. American Society for Testing and Materials (ASTM): West Conshohocken, PA: www.astm.org Codes are available at http://webstore.ansi.org/ansidocstore/astm.asp.
7. Fire rating data from Fabcon USA, Savage, MN: www.fabcon-usa.com/products/page_8a.php. Other manufacturers provide similar products.
GREGORY HAVEL is deputy chief and training officer of the Burlington (WI) Fire Department and a 29-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College and 30 years of experience in facilities management and building construction.
