Wednesday, August 2, 2023

Parallel Strand Lumber (PSL) Deck Beam

Parallel Strand Lumber (PSL) Means and Methods

An Analysis of Weyerhaeuser's 5 1/4" x 16" 2.0E Parallam® Plus Parallel-Strand Lumber (PSL) 2.0E on Townhome Back Deck

INTRODUCTION

In February 2021, after doing some research on my deck beam, I noticed that something wasn't quite right.  In addition, I noticed neighbors having repair work performed on their beam, which raised some questions and concerns. I enjoy construction and learning new things related to construction, so I made the decision to dig a little deeper.  First, I took a look at the structural construction drawings (available to the public via Henrico County Building Inspections), which were labeled with a 5 1/4" x 16" Parallel-strand Lumber (PSL) 2.0E Wolmanized (SL-2). Below is a plan view of the back deck from the Structural Construction Drawings, Sheet S-114 as well as the deck beam to post connection detail from Sheet S-303.  The beam is highlighted in yellow, spanning approximately 20 feet.

Figure 1.  Screenshot from structural drawing for my townhome obtained from the Henrico County Building Inspection Department.  The plan view of the back deck of the townhome is 20 ' x 12'-5 1/2".  It is drawn from a horizontal plane looking down from above.

Figure 2. Deck Beam to Post Connection, Sheet S-303.  The bearing length is approximately 6 inches.  This is where the beam comes into contact with the Wood Post (6"x6" member per the Post Schedule).

Note: SL-2 means it was designed for Service Level 2, or wet use with an allowable moisture content > 16% and <= 28%.  Service Level 3 (SL-3) is Weyerhaeuser's designation for "Saturated Use with an allowable moisture content >28% that is exposed to continuous wet conditions.  Per the construction drawings, the beam was not designed for Service Level 3, exposure to continuous wet conditions.  More information on Service Levels at the bottom of this blog.

The beam is an Engineered Wood Product (EWP), manufactured by Weyerhaeuser.  Parallel Strand Lumber allows for the support of heavy loads and longer spans. After digging into old photos that I took during the construction of the home, I verified that it was, in fact, a Weyerhaeuser Parallel-strand Lumber (PSL) 2.0E Wolmanized beam, as labeled on the member in the photo (below) and the construction drawings.

   Figure 3.  Picture of deck beam taken during construction of home (November 17, 2018).

Figure 4.  Picture of deck beam taken on September 28, 2021, after removing wrapping and before repair.  The beam is labeled as complying with ICCES ESR 1387 and CCMC 11161-R and meeting a minimum stiffness of 2.0E, constructed with Eastern species (Southern Pine and/or Yellow Poplar). According to the markings, the beam was manufactured on November 30, 2017, at 13:37 and was most likely manufactured at the Buckhannon, West Virginia facility.

Also noted on Sheet S-001 by the registered design professional, are the Design Loads and Minimum Allowable Design Value; (100% load duration):

Figure 5. Design Loads on Construction Drawings, Sheet S-001.

Figure 6. Screenshot from the General Notes on Construction Drawing Sheet S-001.

WHAT DOES THE MANUFACTURER SAY?

  1. I contacted Weyerhaeuser and presented my questions/issues and received a reply with the following (Weyerhaeuser was not involved in the design or engineering of the home):  
    • The details presented suggest that the cladding did not follow recommendations as stated in TJ-7102. Cladding should be installed with furring strips and allow drainage to prevent moisture entrapment (as shown on page 10). Without this drainage, the beam will trap moisture, which may lead to performance issues.
    • The issue is primarily structural since the beam now needs to be designed for wet use – it cannot be dried.
  2. Weyerhaeuser provides a Specifier's Guide (#TJ-7102) at the following link, which provides specifications and warranty information.
  3. Cladding Information Parallam® Plus PSL is suitable for exposed conditions with wet-dry moisture cycles. However, it may be desirable for aesthetic reasons to wrap the product in a decorative cladding. As with all treated wood products, avoid applications that can trap moisture. If decorative cladding is used, the following considerations are critical for Parallam® Plus PSL to perform satisfactorily:
    • The designer, builder, and owner must share responsibility for ensuring that the assembly is designed, installed, and maintained in a way that will prevent water from entering and being trapped.
    • If the prevention of water intrusion over the life of the structure cannot be assured, then cladding must be designed and installed in a manner that allows adequate drainage and sufficient air-flow to facilitate drying. Suggested beam and column details are shown at right.
    • Water runoff from repeated wetting may transfer residue, such as excess preservative, natural wood extractives, and dirt, which can result in discoloration of architecturally sensitive surrounding construction. Where this is a concern, detailing that limits the potential for repeated water exposure should be used.
    • Metal cladding materials should not be used, as the preservative treatment can react with the metal and lead to corrosion of the cladding and fasteners.
    • All fasteners, furring strips, and other materials used in the cladding assembly must be corrosion-resistant, treated, or otherwise resistant to decay.
    • Vented cladding, such as a soffit or drilled cladding material, should be used to allow proper drainage. Routine maintenance is also required to ensure that vent holes remain open and free of debris.
    • For column bases with ground contact, maintain a 3" (minimum) gap between cladding and finish grade for drainage. For bases with patio or deck surface contact, maintain a 1" (minimum) gap between cladding and surface.
    • Weyerhaeuser goes on to explicitly state the following along with an illustration of cladding on beams:
Figure 7. Page 10 of the Weyerhaeuser Specifier's Guide.


Figure 8. Page 10 of the Weyerhaeuser Specifier's Guide, Cladding on Beams.

WHAT DOES THE BUILDING CODE SAY?

Virginia Uniform Statewide Building Code.  Virginia Code 13VAC5-63-120. Section 112 Workmanship, materials and equipment, D. Section 112.3.1 Conditions of listings (https://law.lis.virginia.gov/admincode/title13/agency5/chapter63/section120/), states: 

Where conflicts between this code and conditions of the listing or the manufacturer's installation instructions occur, the provisions of this code shall apply. 

Exception: Where a code provision is less restrictive than the conditions of the listing of 
the equipment or appliance or the manufacturer's installation instructions, the 
conditions of the listing and the manufacturer's installation instructions shall apply.  

In addition to the VUSBC and the Specifier's Guide, the Construction Drawing's General Notes (Section C. Engineered Lumber, Sheet S-002) state that "All engineered lumber shall conform to the minimum product specifications and installation requirements as provided by Truss Joist MacMillan manufactured products.

Figure 9. Screenshot of Structural Construction Drawings, Sheet S-002

THE WARRANTY

Weyerhaeuser provides a 30-year limited warranty (link), which does not cover the following:
  • Damage to Product resulting from fires, floods, natural disaster or any other cause beyond Weyerhaeuser’s control.
  • Damage to Product resulting from misuse, improper product applications or installation of the Product, or noncompliance with the installation instructions, applicable building code or generally accepted construction practices.
  • Defects in the structure due to construction, installation or manufactured sub-assembly.
  • Any alterations to the Product after the original installation.

THE OUTCOME

Ultimately, as an owner, I wanted to ensure that the assembly was installed in a way that will prevent water from entering and being trapped and allow the beam to maintain its structural integrity throughout the home's lifetime (or 30-year warranty).  Weyerhaeuser states in its Weyerhaeuser Engineered Wood Products and Oriented Strand Board Technical Bulletin that "strength and stiffness reductions occur at elevated moister contents and stiffness degradation at elevated moisture contents can approach 50% in some cases.  When in doubt, replace the EWP or get it evaluated by a building design professional."  I was fortunate enough to have the cladding removed, the beam professionally evaluated, and finally repaired with proper cladding that included furring strips and vented cladding at the bottom that allows for drainage and the member to breathe.  Below, are the before and after pictures:

    Figure 10. Picture of wrapped PSL beam from the driveway.  Beam is wrapped with cladding on both sides and the bottom of the beam.

                       Figure 11. Picture of the top of the beam that is covered by metal flashing.

    Figure 12. Picture of repair with a vented cladding on the bottom, furring strips attached to the beam, and cladding then attached to the sides with the furring strips providing 3/8" of "breathing" room, similar to that of the manufacturer's recommendation.

    More on Service Levels

    The service level of a piece of PSL lumber is a measure of how wet it is expected to get in use. SL-1 is the driest service level, and SL-3 is the wettest. The service level affects the structural loads and design of the lumber in a few ways.
    • The allowable stresses for the lumber are lower for SL-3 than for SL-1. This means that the lumber can support less weight in SL-3 conditions.
    • The connections between pieces of lumber must be designed differently for different service levels. For example, withdrawal connections (where a piece of lumber is pulled out of another piece of lumber) must be stronger for SL-3 than for SL-1.
    • The moisture content of the lumber must be controlled in order to maintain the desired service level. For example, if the lumber is exposed to rain, it may need to be dried out before it can be used in SL-1 conditions.
    In general, the higher the service level, the more restrictive the design requirements. This is because wet lumber is weaker and less stable than dry lumber.

    Here is a table that summarizes the effects of service level on the structural loads and design of PSL lumber:


    Forte Web Report - Reactions, Shear, Moment, Deflection

    Using the calculation provided by TJ-7102 for Deflection,

    Figure 13. Deflection Calculation in TJ-7102

    I created code in Python to calculate the deflection, which differs by 0.06564 inches or 9% more than ForteWeb's results.  See below for the code and results:

    Figure 14. Python Code to calculate the deflection as provided in the TJ-7102 calculation for Total Deflection.

    REFERENCES:

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