Parallel-strand lumber (PSL), sold under the brand name Parallam, is made by Trus Joist from lower-grade, defect-laden wood veneers. The Doug fir, southern pine, or yellow poplar veneer is chopped into 1-inch by 8-foot-long strips, then coated with glue. The strands are oriented with their grain parallel and extruded in billets up to 11 inches thick by 19 inches deep and theoretically unlimited length. The billets are then cut into standard beam sizes. The manufacturer has produced Parallam beams up to 54 inches deep by laminating extrusions together. Common Parallams are available in 1 3/4-, 2 11/16-, 3 1/2-, 5 1/4-, and 7-inch thicknesses. The regular, uniform, "exotic" appearance of the bonded strands in a PSL beam is considered by many to be aesthetically pleasing, and these beams are often left exposed in architectural applications. This, combined with the fact that PSL has a higher E-value than LVL, may influence the choice of Parallam over the LVL.
Outdoor use. PSL can be ordered in pressure-treated southern pine. Because of the effects of treatment and exposure (surface checking), pressure-treated Parallams come with their own set of (lower) published design values, depending on application. Level 1 pertains to material used in dry service (19% maximum moisture content). Level 2 covers material in above-ground use and a minimum-maximum moisture range of 19% to 28%. Service level 3 applies to beams used in ground contact or saturated use — for example, in dock construction.
Similar to Parallam PSL, Timberstrand, also from Trus Joist, is made of chopped, parallel-oriented wood strands and is generically called laminated strand lumber (LSL). The wood used is either fast-growing aspen or yellow poplar and the strand length is typically around 12 inches. (By contrast, the strands in OSB, a related product, are about 6 inches long. Although several companies produce OSB, Trus Joist is currently the only manufacturer that converts lumber strands into structural framing lumber.) Timberstrand headers come in 3 1/2-inch widths and ten depths from 4 3/8 inches to 18 inches. LSL is a good choice for short-span headers. It's also used in the production of engineered studs, plates, posts, and rim joists. Because it's reliably straight, strong, long, wide, and stable, many builders have taken to making their stair stringers from engineered lumber. Although you can use LVL for stringers, 1 3/4-inch-thick Timberstrand is cheaper and is said to hold nails better without splitting. Trus Joist markets Timberstrand LSL specifically for this application.
Taking a minimalist approach to beam design, Superior Wood Systems's lightweight I-joist headers (SWII) replace conventional headers in 2x4 and 2x6 walls. Pre-installed expanded polystyrene insulation between the OSB webs provides a claimed R-value of 18 (for the 2x6 version). The header flanges are machine-stress-rated (MSR) lumber, and the dimensionally stable header depths match standard 2-by dimensions at 7 1/4, 9 1/4, and 11 1/4 inches. Said to cost about the same as the materials for a standard lumber header, the SWII headers eliminate the labor of site fabrication. The company's general manager, Jim Lamb, points out that if a worker cuts and preassembles all of the lumber headers for a frame at once, it might take an entire day. The same number of SWII headers would take that worker about an hour to cut. For this reason, SWII headers are popular with modular builders and frame panelizers.
The manufacturer discourages use of their header over garage door openings and limits available lengths to 14 feet as a direct precaution. Uninsulated headers can be used as clear-span joists where a double joist is specified. The 2x4 SWII, with an R-value of about 9, is also suggested for use as an insulating rim joist — not a bad idea, although to provide nailing for the floor joists, you'll have to pack out the interior web.
Sandwich Plates and Pack-Outs
A big benefit of any engineered beam can be the on-site labor it eliminates; old standby built-up and flitch-plate beams take a lot of work to assemble, and the assembly has to be done according to a specific fastener schedule to achieve reliable results. The Better Header company factory-assembles a variety of flat-stock and I-beam headers with the wood members already attached. Recessed bolt heads eliminate the problem of locating joists and hangers on layout, as well as the need for furring, and permit easy go-over with drywall or wood trim. According to Better Header's Tom Gallina, a primary advantage of this product lies in its size-to-strength ratio relative to LVLs and glulams: "A lot of designs call for 2x8 and 2x10 floor joists, and people don't want a dropped girder in there. An engineered wood girder of comparable strength is going to stick out of the ceiling. We sell more 8- and 10-inch I-beams than any other size." With the wood facings and plates already applied, these beams won't slow down the framing process, but they will require a crane or plenty of manpower to set in place. A packed-out W8x24 beam with a top plate weighs 34.1 pounds per linear foot. Although the solid 2-by facing contributes to the strength of the beam, its value is disregarded in the application tables; the company considers only the known performance value of the steel. However, its sandwich-plate LVL beam is rated for the combined values of the steel and the facing.
All manufacturers offer sizing and specification assistance in their product brochures and on their Web sites. Sizing software is often available online for downloading or on disk by request. Be careful, though. These calculations are only for uniform loading applications, not point-load scenarios. If you're confronting a special application, don't wing it — consult an engineer, who can direct you to the best beam for the job. Modifications. You can drill holes in some of these beams, but not just anywhere it suits you. Just as with fastener schedules, where and how you drill affect beam integrity, so it's important to consult the manufacturer's literature or technical support before you modify the beam in any manner, whether by end-notching, beveling, ripping to width, or drilling even one little hole for one little wire.
Dave Holbrook is an associate editor atThe Journal of Light Construction.