Trus Joist® Parallam® PSL and commodity 24F-V4 glued laminated timbers (glulam) are often thought of as interchangeable; however, these two beam technologies have distinct differences which are important to understand when specifying beam material. This technical blog highlights the advantages of using Parallam PSL over commodity 24F-V4 glulam for beam, header and column applications.
Anatomy
Parallam PSL beams are manufactured from long strands of wood that are oriented parallel to the length of the member. The strands are glued and pressed together to form one solid cross section with uniform properties so it does not have a “top” or “bottom”. As a result, Parallam PSL can be used in any beam application without concerns about a possible strength reduction or improper installation.
Glued laminated beams (glulams) are composed of individual, dimension lumber laminations, or “lams“, which are typically 2×4’s or 2×6’s. Lams are individually selected and positioned in the beam layup based on their performance characteristics, then bonded together flat-wise with adhesives. For maximum efficiency, high-strength lams are used in the tension zones of bending members where the most critical stresses occur.
Glulams are manufactured in balanced and unbalanced layups:
- A balanced layup is symmetrical in strength around the neutral axis of the beam depth, offering high strength in both the top and bottom of the beam. Balanced beams are best in applications that cause bending tension on both sides of the beam, such as cantilevers, or when a beam extends over multiple spans.
- In an unbalanced layup, one side of the beam has higher strength plies than the other. The side with the higher strength plies is intended to be the tension side of the beam. Unbalanced beams are primarily intended for simple span applications where only the bottom side of the beam experiences bending tension. To ensure proper installation of unbalanced beams, the top of glulams are stamped with the word “TOP.” If unbalanced glulams are used in applications where the lower strength side will experience tension, then a bending strength reduction must be applied.
Manufacturing and Performance
Camber
Weyerhaeuser’s manufacturing technology allows Parallam PSL beams and columns to be produced relatively straight and true. Parallam PSL beams are not manufactured with camber.
Commonly stocked glulams have a 5,000 ft camber. It is critical that cambered beams be installed with the “top” side up. If a cambered glulam is installed upside down, the beam deflection will be the sum of the deflection under load plus the beam camber. This may cause unacceptable and/or unsightly deflection, as well as problems with adjacent construction, such as plaster walls or ceilings.
Additionally, cambered members may lead to floor height difference when used as a rim or beam parallel to joists (more commonly seen in 2 hour fire wall assemblies). Likewise, cambered rim material may create additive elevation difference in shafts and elevator cores in multiple story application. It may also create challenges at the rim to plate connection.
Tolerances & Dimensional Compatibility
Weyerhaeuser manufactures Parallam PSL to within ±1/16″ of the published sizes at an expected end use moisture content of about 10%.
Per the Standard for Wood Products- Structural Glued Laminated Timber, manufacturing standards allow a range of -3/16″ per foot of depth, or up to + 1/8″ per foot of depth. These tolerances may present compatibility issues with other engineered lumber products (ELP). Likewise, be mindful of product depth when selecting glulam beams to use in conjunction with ELP. Glulams come in depths that may not be compatible with TJI joists and other Trus Joist SCL.
Moisture Content
Glulam beams are manufactured with a moisture content (MC) between12-14%. TJI Joists are manufactured with an intended end-use MC of 8-12%. I-Joist compatible glulams, while manufactured to be the same height as TJI joists, may shrink once in a closed up building. This can lead to uneven floors, which may cause finished surface problems and potential floor noise. Parallam PSL is manufactured to similar MC tolerances as TJI joists, which provides a more dimensionally stable and joist compatible beam solution.
Beam Modifications
Parallam PSL can be drilled and tapered-end-cut per the tables available in the Trus Joist Beam, Headers and Columns specifier guide. Likewise Forte®, Weyerhaeuser’s free single member sizing software, can analyze holes in Trus Joist SCL up to 2/3 the member depth. Additionally, untreated Parallam PSL can be field ripped to custom depths down to 3-1/2″ and widths to 1-3/4″ (per Weyerhaeuser’s Technical Bulletin #305 Re-Sawing Microllam® LVL, Parallam® PSL and TimberStrand® LSL).
Individual glulam manufacturers, as well as the APA, provide guidelines for cutting and notching glulam beams; however, glulam beams may not be field ripped. Any field modifications outside of published guidelines must be analyzed by a qualified design professional.
Column Applications
Parallam PSL can be used in column applications when both vertical and lateral wind loads are applied. Its homogeneous cross-section makes it a good solution for columns in tall walls. Allowable lateral and vertical loads for Parallam PSL can be found in TJ-9003, Weyerhaeuser’s US Wall Guide, or columns can be sized using Forte and Javelin software.
Balanced glulam beams can be used in column applications, however the commonly available 24F–V4 unbalanced glulam is not recommended for column applications due to the increased eccentricity which results from a non-homogeneous cross-section and camber. Additionally, using a cambered member as a column in a wall may cause issues with drywall installation.
Warranty and Service
Parallam PSL comes with a limited lifetime warranty, which warrants against manufacturing defects for the lifetime of the structure provided that the product is properly stored and installed per Weyerhaeuser’s instructions. Additionally, Weyerhaeuser has an experienced team of field and technical support representatives available to assist with member sizing or answer questions about installation or beam modifications. Email techsupport@weyerhaeuser.com or call 888-453-8358.
Conclusion
The unique anatomical characteristic of Trus Joist Parallam PSL beam technology provides design professionals and builders a high strength solution with maximum versatility in design and modification. Parallam PSL is ideal for beams, headers and columns and reduces the need for special design and installation consideration common to a non-homogenous glulam beam product. It is manufactured straight and true with uniformity and predictability. Trus Joist literature and software provides quick and easy guidance for field modification. Product is available in sizes 3-1/2″ to 7″ wide, and 9-1/2″ to 24″ deep. For use in exterior applications, Wolmanized® Parallam Plus PSL beam material is also available. Contact your local Trus Joist product representative for more information.
