Part 4 — More Types of Beams You Are Likely to Encounter Welcome back to the "Behold the Beam" series of articles. In part three ( Firehouse ®, September 2010) of the series, we discussed the first five of the 13 common types of beams that firefighters can expect to encounter: 1. Solid...
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The maximum length of a wood I-joist is limited to about 66 feet; such long-length I-joists feature LVL flanges that are finger-jointed. (Although I haven't seen one, I have been told that an I-joist can reach 80 feet.) The I-joist webs are spliced using butt-jointing, tongue-and-groove or "scarf" configurations. As with SCL, wood I-joists offer reduced dead load, fewer beams (due to wider spacing), dimensional stability and a "quiet floor."
It is no fire service secret that unprotected wood I-joists will fail early when exposed to fire. As with all structural components, it is essential that fire officers complete a master craftsman size-up before letting firefighters enter a building offensively. I am amazed that firefighters continue to be harmed after falling through floors into basements with fire "hidden" below their feet. For additional wood I-joist information, navigate your browser to the following websites:
• http://www.woodaware.info/PDFs/I-Joists_FirefighterSafety_0509.pdf (nice PowerPoint presentation)
• http://www.uluniversity.us/catalog/display.resource.aspx?resourceid=187716 (you must register with UL)
• http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/DHS_Report_Modeling_Floors_1208_final.pdf (UL report of fire-performance testing of floor assemblies
A built-up beam gains its strength by increasing beam width rather than depth. Don't confuse a built-up beam with a flitch beam (see beam 8.). Dimensional lumber is connected side by side using nails or bolts. Although not as strong, pound for pound, as an engineered beam, a built-up sawn wood beam can be assembled by carpenters in the field. Occasionally, plywood is sandwiched between the dimensional lumber. Glue is often applied between each layer to increase shear resistance. A strategic benefit of a built-up beam is the added fire resistance due to the added mass. Unless overloaded, when exposed to fire, a built-up beam should perform as well — or better — than solid sawn lumber. That said, don't forget that connections represent the weak point in any structural system.
8. Flitch Beam
At first glance, a flitch beam looks similar to a built-up beam; however, by placing a steel plate between two wood sections, a flitch beam is a composite sandwich of wood and steel. The purpose is to add strength and rigidity without having to increase depth or significantly increase width. The composite flitch assembly is usually bolted together. The relationship of this wood-steel composite is straightforward: the steel provides structural strength; the wood provides a nailing and finishing surface. An un-designed strategic benefit of a flitch beam is that the wood can protect the steel from heat.
9. Box Beam
A box beam can be manufactured using steel, concrete and steel, or wood. When using wood, two-by-four-inch lumber is sandwiched between two vertical layers of plywood. The assembly is nailed and glued. The advantage of using a wood box beam rather than a solid sawn wood beam of the same dimension is the additional strength without adding significant weight (dead load) — and it has the appearance of a substantial beam.
Short stubs of wood I-joist are sometimes used as an alternative to solid sawn lumber between the plywood layers. For appearance, decorative wood (or synthetic) box beams are sometimes used as "faux (fake) timbers" (see beam 13). Although a wood box beam may look like a timber, it is not solid sawn wood; wood box beams are hollow and feature connections that solid beams do not. Heavy-duty box beam girders are always made of steel or a composite of steel and pre-stressed concrete. Although a wood box beam may look like timber, it too is hollow.
10. Wide-Flange Steel
Easily confused with a steel I-beam, a wide-flange steel beam is referred to as a W-beam. Wide-flange beams differ from I-beams in that the wide flange sections are formed by welding together three metal plates (two for the flanges and one for the stem), whereas I-beams are formed by a rolling or an extrusion process. As a result, the flanges in wide flange sections are typically not tapered. As a reminder, the top and bottom portions of an I-beam are called the flanges. Separating the flanges is the job of the vertical portion of a steel I-beam called the web. (The web is sometimes referred to as the stem.) Pound-for-pound (mass-to-strength), a wide-flange steel beam is the strongest of the 13 beam types discussed in this article.