Photo 1 - Wood I-Joist, Photo 2 - Wood I-Rafter, Photo 3 - Wood I-Rafter
Photo 4 - Wood I-Joist, Photo 5 - Built-up joist, Photo 6 - Built-up joist.
Photo 7 - Built-up beam, Photo 8 - Flitch beam, Photo 9 - Flitch beam.
Photo 10 - Flitch beam, Photo 11 - Wood box beam, Photo 12 - Steel box beam.
Photo 13 - Pre-cast box beam, Photo 14 - Steel W-beam, Photo 15 - Steel W-beam.
Photo 16 - Pre-cast concrete I-beam, Photo 17 - Hollow core floor panels, Photo 18 - LiteSteelbeam.
Photo 19 - LiteSteelbeam, Photo 20 - Faux rafter tails, Photo 21 - Faux beams.
Photo 22 - Faux timber trusses, Photo 23 - Faux knot.
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 sawn wood and four structural composite lumber (SCL) beams:
2. Glue laminated
3. Laminated strand lumber (LSL)
4. Parallel strand lumber (PSL)
5.Laminated veneer lumber (LVL)
This time, we'll finish with the final eight common beam types:
6. Wood I-joist
10. Wide-flange steel
11. LiteSteelbeam (LSB)
12. Pre-cast concrete
13. Faux (fake)
6. Wood I-Joist
A wood I-joist is an interesting piece of engineering. Most of the material you see — the vertical web — is made with plywood, LVL or oriented strand board (OSB) and thus cannot be used independently as a beam. Plywood and OSB are not used in the building hierarchy as load-bearing structural members (review page 100 in the April 2009 issue of Firehouse®); plywood and OSB are never used as a column, girder, purlin or joist. Plywood and OSB are classified as "wood structural panels"; plywood and OSB are used independently as wall sheathing and decking for floors and roofs. However, combined with a structural composite lumber (SCL) top flange and bottom flange, a strong and stable joist is produced. Non-structural engineered wood can be used for the web of a load-bearing beam because the vertical web serves just one purpose: the web keeps the top flange separated from the bottom flange; as long as the web can keep the flanges separated, the I-joist will do its job.
Wood I-joists are a structural engineered wood product often used as floor joists and occasionally as rafters. I-joists are made by gluing sawn or laminated veneer lumber (LVL) flanges to the top and bottom of a plywood or OSB web. Engineered wood I-joists are twice as strong as a conventional solid sawn wood beam and are much easier to handle at the jobsite.
Although featherweight, wood I-joists exhibit remarkable strength and rigidity consistent along the entire length of the beam. I say featherweight because of the strength-to-mass ratio of a typical engineered I-joist. Consider this: an I-joist 26 feet long and 9½ inches deep weighs around 55 pounds (depending on the size of the flanges). That is not just "lightweight"; wood I-joists are truly featherweight. For developers, wood I-joists offer a cheap alternative to open web wood trusses or steel (bar) joists.
I-joists can be manufactured with web "knockouts" that allow running utilities through the joist rather than under or around the joist. Knockout holes also provide ventilation when the joists are used in cathedral-type ceilings. Remember: As mentioned, the primary purpose of an I-beam web (stem) is to keep the flanges separated. Even without knockouts, penetrations for plumbing and mechanical ductwork can be drilled through the web; however, so that structural integrity is not compromised, all penetrations must be located and sized according to manufacturer recommendations.
Although you can penetrate with the web, do not mess with flanges. Wood I-joist flanges should never be notched or drilled and all special cuts, such as bearing cuts, must also abide by manufacturer recommendations.
Several different wood I-joists are available and feature a variety of designs and materials. The critical part of any wood I-joist is the joint that connects each flange to the web. This critical joint is often patented by the manufacturer. Along with LVL and visually graded sawn lumber, flanges are often made of machine stress-rated (MSR) lumber. This is dimension lumber that has been evaluated using mechanical non-destructive stress-rating equipment. Mechanical stress-rating equipment measures material stiffness and sorts the lumber into strength classes.
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.
As with all structural steel, rule-of-thumb elongation is one inch for each 10 feet in length when the steel (not the ambient temperature) is heated to 1,000 degrees Fahrenheit. The rule-of-thumb for structural steel failure is between 1,200°F and 1,300°F. Structural steel is unable to support its own weight when heated to 1,500°F. The cooling of super-heated structural steel should be a tactical priority. What's interesting is that structural steel gets stronger as it heats. The compressive strength of structural steel increases until heated to around 300°F; its tensile strength increases until heated to around 600°F.
11. LiteSteelbeam (LSB)
New (to me anyway) is the "LiteSteelbeam," or LSB. Note that the LSB manufacturer borrowed the "lite" moniker from the beer industry; not only are Lite Steelbeams strong and efficient, apparently they are low calorie as well. On average, LSB is 40% lighter than a hot rolled steel beam or an engineered wood I-beam. Because carpenters can manipulate an LSB without a crane or other special handling equipment, LSB in installation is comparable with lumber. LSB can be assembled by carpenters using a circular saw equipped with a special blade and traditional nailing. LSB can also be assembled using self-drilling screws and can be welded.
The good news is that steel is inherently non-combustible and thus will not contribute fuel to a fire. That said, it should be expected that Lite Steel products can fail quickly when exposed to high temperature — with or without direct fire exposure. Comparing the fire performance of unprotected Lite Steel to that of structural steel is similar to comparing the performance of a wood I-joist to a solid sawn timber.
12. Pre-Cast Concrete
When you gaze upon a large concrete beam, you can be certain that it is a composite of concrete and steel. A concrete beam without reinforcement does not exist. For concrete to function as a beam, it must be a composite of concrete to resist compression and steel to resist tension. This reinforcement can be steel reinforcing bars ("rebar"), steel cables or a combination.
13. Faux Beam
Don't be fooled by contemporary buildings that feature what appear to be conventional timber beams. Faux timbers look impressive but they are charlatans; faux beams are decorative and non-structural. (Faux, pronounced "foe," is an adjective that means artificial, imitation or fake; not genuine or real.) Faux beams are not part of the building construction structural hierarchy, add no additional strength or fire resistance, and should not be relied on to support firefighters. I've seen buildings that feature impressive faux timber trusses. Be careful not to classify a building featuring faux timber as Type IV, heavy timber.
Faux beams are featured in both residential and commercial applications. Common commercial applications include restaurants, hotels, offices and places of worship. Often made of extruded polyurethane or high-density Styrofoam, faux beams are molded and painted to mimic wood timbers — right down to hand-painted faux knots. Occasionally, gypsum board is finished and painted to look like a timber beam; although gypsum sheetrock will not burn it is not load bearing and should not be considered an operating platform for firefighters. (Just to clarify, no faux beam should be considered as an operating platform for firefighters.)
Faux timber beams typically are not fire rated; however, depending on the type of occupancy and your local code, they can be manufactured to provide a Class A fire rating or finished with a fire-retardant coating.
MARK EMERY, EFO, is a shift battalion chief with the Woodinville, WA, Fire & Life Safety District. He is a graduate of the National Fire Academy's Executive Fire Officer program and an NFA instructor specialist. Emery received a bachelor of arts degree from California State University at Long Beach and is a partner with Fire Command Seattle LLC in King County, WA. He is in no way affiliated with or an advocate for the truss manufacturing or building construction industries. He may be contacted at email@example.com or access his website www.competentcommand.com.