8.5 Engineered Wood, I-Joists, Trusses & Connectors

8.5 Engineered Wood, I-Joists, Trusses & Connectors

Engineered Wood Products (EWP)

Engineered wood products are manufactured by bonding wood strands, veneers, or dimensional lumber with waterproof structural adhesives (phenol-resorcinol or polyurethane) under high heat and pressure.

1. Glued Laminated Timber (Glulam - ANSI A190.1)

Glulam members consist of individual 1-1/2" thick (nominal 2x) or 3/4" thick (for curved members) kiln-dried wood laminations oriented with grain running parallel longitudinally.

• Balanced vs. Unbalanced Layups:
  • Unbalanced Layup (Simple Spans): The highest-grade, highest-tensile wood laminations are concentrated on the bottom (tension) face, while lower-grade laminations occupy the compression zone. Unbalanced glulams are stamped "TOP" on the compression edge; installing an unbalanced beam upside down results in severe structural deficiency.
  • Balanced Layup (Continuous Spans & Cantilevers): Features equal high-grade tension laminations on both top and bottom faces, accommodating bending moment reversals over intermediate support columns.
• Appearance Grades:
  • Industrial Grade: Surfaced for utility; knots and imperfections remain open; used in concealed framing or industrial warehouses.
  • Architectural Grade: Surfaced smooth with open knots filled and voids puttied; standard choice for exposed commercial applications.
  • Premium Grade: Highest aesthetic grade with all knots filled and wood surfaces selected for uniform color; used in high-end civic and commercial showpieces.
• Manufacturing Camber: Glulam beams are built with an engineered upward curve (camber) to counteract dead load deflection, typically designed for 1.5 to 2.0 times calculated dead load deflection.

2. Structural Composite Lumber (SCL)

• Laminated Veneer Lumber (LVL): Produced by stacking thin wood veneers (1/10" to 1/8" thick) coated with waterproof adhesive, with the grain of all veneers running parallel in the longitudinal direction. LVL eliminates natural defects (knots, splits, grain slope), yielding high bending strength ($F_b$ up to 2,800–3,100 psi) and exceptional dimensional stability for headers, primary beams, and I-joist flanges.
• Parallel Strand Lumber (PSL / Parallam): Manufactured from long veneer strands (up to 8 feet in length) bonded parallel under pressure. Possesses superior compressive and tensile capacity; ideal for heavily loaded interior columns and long-span beams.
• Laminated Strand Lumber (LSL): Formed from shorter wood strands (4" to 12"); widely used for rim boards, wall studs, and headers.

3. Prefabricated Wood I-Joists & Strict Web Hole Cutting Rules

Wood I-joists combine high-strength LVL or solid lumber top and bottom flanges with a vertical oriented strand board (OSB) or plywood web.

                               WOOD I-JOIST HOLE CUTTING RULES

         ◄─── HIGH SHEAR ZONE ───►◄────── LOW SHEAR / MOMENT ZONE ──────►◄─── HIGH SHEAR ZONE ───►
         ┌────────────────────────────────────────────────────────────────────────────────┐
  FLANGE │ ██████████████████████████████████████████████████████████████████████████████ │
         ├──────────────────────────┐                  ┌──────────────────────────────────┤
         │  NO HOLES PERMITTED      │   ALLOWED HOLE   │  NO HOLES PERMITTED              │
     WEB │    NEAR SUPPORTS         │     (CENTER)     │    NEAR SUPPORTS                 │
         │  (High Shear Zone)       │    ○ Diameter    │  (High Shear Zone)               │
         ├──────────────────────────┘  ≤ Depth - 2*Flg └──────────────────────────────────┤
  FLANGE │ ██████████████████████████████████████████████████████████████████████████████ │
         └────────────────────────────────────────────────────────────────────────────────┘
         ▲                                                                                ▲
      BEARING                                                                          BEARING

  ┌─────────────────────────────────────────────────────────────────────────┐
  │                 STRICT I-JOIST FIELD MODIFICATION RULES                 │
  └─────────────────────────────────────────────────────────────────────────┘
     │
     ├─► 1. ABSOLUTE FLANGE INTEGRITY MANDATE
     │      NEVER cut, notch, drill, bevel, or alter top or bottom flanges under
     │      ANY circumstances. Flanges carry 100% of the bending tension and compression.
     │
     ├─► 2. WEB HOLE LOCATION (SHEAR VS. MOMENT)
     │      Large web penetrations (HVAC ducts, plumbing) must be cut in the CENTER THIRD
     │      of the span (low shear zone). Never cut large holes near bearing supports.
     │
     ├─► 3. MAXIMUM HOLE SIZE & SPACING
     │      Maximum round hole diameter = Joist Depth minus (2 × Flange Depth + 1/2").
     │      Maintain minimum spacing between holes equal to 2 × the larger hole diameter.
     │
     └─► 4. PRE-STAMPED KNOCKOUTS
            Factory-perforated 1-1/2" knockouts may be tapped out anywhere along the web
            for electrical wiring runs without structural calculation.

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Cross-Laminated Timber (CLT) & IBC Mass Timber Tall Buildings

Cross-Laminated Timber (CLT) panels consist of 3, 5, 7, or 9 layers of solid-sawn dimensional lumber ($2\times4$, $2\times6$, or $2\times8$) stacked flatwise with each layer oriented orthogonally ($90^\circ$) to adjacent layers and glued under hydraulic pressure.

IBC Mass Timber Tall Wood Building Classifications

The International Building Code incorporates three distinct Type IV sub-classifications for tall mass timber construction:

• Type IV-A (Maximum 18 Stories / 270 Feet): The most stringent mass timber classification. All CLT floor panels, mass timber columns, and bearing walls must be 100% encapsulated with noncombustible materials (e.g., multiple layers of Type X gypsum board) to achieve up to a 3-hour fire-resistance rating.
• Type IV-B (Maximum 12 Stories / 180 Feet): Allows a limited percentage of exposed mass timber on ceilings and interior partition walls (typically 20% of ceiling area or 40% of wall area), with the remainder encapsulated to provide a 2-hour rating.
• Type IV-C (Maximum 9 Stories / 85 Feet): Permits 100% exposed mass timber ceilings, columns, and walls, relying entirely on the inherent 2-hour fire-resistance provided by the massive charring cross-section of the wood.

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Structural Timber Connectors & Hardware

Modern mass timber and engineered wood structures utilize heavy-duty galvanized structural connectors:

• Heavy-Duty Joist Hangers (Top-Flange & Face-Mount): Engineered cold-formed or welded 10-to-12-gauge steel brackets transferring joist shear into headers or glulam girders.
• Concealed Beam Hangers: Internally routed aluminum or steel knife-plates with cross-drilled dowel pins that sit completely hidden inside mortised beam ends, providing a clean architectural look and protecting the steel connection from fire exposure.
• Post Bases & Standoffs: Must provide a minimum 1-inch standoff distance above concrete slabs or exterior grade to prevent capillary moisture absorption and fungal rot.
• Split-Ring & Shear-Plate Connectors: Metal rings embedded into pre-cut circular grooves in timber mating faces to transmit heavy shear loads across bolted joints.