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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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.