Article and photos by Gregory Havel
During the past 150 years, engineering and materials testing have proved that only certain parts of a beam of a rectangular cross-section are essential to carrying the load to be imposed on it. As a result, much of the material contained in the beam or column of rectangular cross-section is omitted without reducing its strength, resulting in I-beams and columns with an H-shaped cross-section (photo1). This great reduction in mass reduces costs and inherent fire resistance. Since buildings are designed for “normal” conditions, the loss of inherent fire resistance by reducing mass is not considered significant except by firefighters and fire protection engineers.
(1)
Further engineering and materials testing showed that much of the web of an I-beam was not needed for load-bearing. Removal of this “unnecessary” web material resulted in steel trusses (photo 1). Additional testing showed that the solid steel rods originally used as web members in steel trusses were not needed and could be replaced by steel tubes and sheet-metal C-channels, as shown in photo 1.
The same principles were applied to wood beams, resulting in the parallel-chord wood trusses and the wood I-joists that are common today. These beams also fail more rapidly under fire conditions. For ease in installing ducts, pipes, and cables, some manufacturers now produce I-joists with many triangular openings in the web, turning them into trusses.
Engineering established the principle of shaping a two-dimensional structural member to make it more three-dimensional and increase its strength when it created the I-beam. This principle has been applied to shaping thin sheet metal into steel studs and joists. Forming grooves and ridges into steel studs (photo 2) and joists parallel with the corners makes the thin sheet metal stud more rigid and better able to carry its vertically imposed load.
(2)
The most recent development in steel joists is the removal of triangular sections of the web, turning it into a truss (photo 3). The remaining parts of the web are formed into C-channels to increase their strength, resulting in a joist with a significant reduction in weight and without a significant reduction in strength when it is properly installed and braced.
(3)
Lightweight construction components like shaped steel studs and modified C-channel steel joists can be used to support wall assemblies and floor-ceiling assemblies that have been listed as “fire-rated” after testing to the National Fire Protection Association (NFPA) Standard 251 (ASTM E119, UL 263), Standard Methods of Tests of Fire Endurance of Building Construction and Materials.
Please note that the individual structural members like columns, beams, studs, and joists are not listed or rated for fire resistance. To be listed and to pass the test in the NFPA 251 standard, they must be protected within a wall assembly or floor-ceiling assembly by noncombustible materials or by a listed fire-resistant insulation material that is applied directly to the structural member.
Also please note that the test in the laboratory is conducted with fire on only one side of the assembly and no fire inside the assembly. A “fire-rated” assembly with fire or hot products of combustion inside its concealed space will not perform as it did during its testing in the laboratory and is not considered fire-rated under those conditions.
The Underwriters Laboratories Fire Protection Directory is available online at http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/fireressrch.html or at www.ul.com. Links to a “Quick Guide” to the Directory and to fire-resistive assemblies can be found on these Web sites.
Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 30-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; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.
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Subjects: Building construction for firefighters
FE Category: Prevention and Protection, Building Construction
