7.4 Framing & Structural Wood Systems

7.4 Framing & Structural Wood Systems

Light-frame wood construction dominates California residential and light commercial building. Contractors must understand lumber grading, moisture limits, engineered wood assemblies, load-bearing wall framing, roof truss bracing, high-seismic shear wall load paths, and code-mandated notch/hole limits.

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Lumber Grading, Moisture Content & Engineered Wood

Softwood Grading & Moisture Standards

Framing lumber is visually or mechanically graded under rules established by agencies like the Western Wood Products Association (WWPA) or West Coast Lumber Inspection Bureau (WCLIB). Primary species utilized in California include Douglas Fir-Larch (DF-L) (highest modulus of elasticity and bending strength $F_b$), Hem-Fir (HF), and Spruce-Pine-Fir (SPF).

Under CRC Section R301.2, structural framing lumber must have a maximum moisture content of 19% at the time of installation, stamped as S-DRY (Surface-Dry) or KD (Kiln-Dried):

$\text{Moisture Content (MC)} = \left( \frac{\text{Wet Weight} - \text{Oven-Dry Weight}}{\text{Oven-Dry Weight}} \right) \times 100\%$

Installing unseasoned green lumber (S-GRN $> 19%\text{ MC}$) causes severe radial shrinkage as wood dries in service, leading to nail pops, cracked drywall, warped wall plates, and structural squeaks.

Engineered Wood Products (EWP)

1. I-Joists (e.g., TJI Joists): Structural OSB webs glued between solid sawn wood or LVL flanges. Offer high span-to-weight ratios. Web hole knockouts must strictly adhere to manufacturer charts; never cut joist flanges.
2. Glued Laminated Timber (Glulam): Dimensional lumber laminations bonded with waterproof structural adhesives. Used for heavy structural beams and header spans.
3. Laminated Veneer Lumber (LVL) & Parallel Strand Lumber (PSL): High-strength composite wood headers and posts that eliminate warping and splitting.
4. Wood Structural Panels (Plywood vs OSB): Rated under DOC PS 1 (Plywood) or PS 2 (OSB). Subfloors utilize $3/4\text{ inch}$ tongue-and-groove panels rated for Exposure 1.

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Wall & Floor Framing Systems

Platform Framing vs Balloon Framing

• Platform Framing: Standard modern technique where each story is constructed as an independent platform. Subfloor shear diaphragms provide a flat work surface for framing exterior walls.
• Balloon Framing: Continuous studs extend from foundation to roof rafter plates. Requires mandatory horizontal fireblocking (2x lumber or $3/4\text{ inch}$ plywood) at floor levels per CRC Section R602.8 to prevent chimney-effect fire spread inside concealed wall cavities.

Sole Plate Anchoring & Decay Protection

Under CRC Section R317.1, pressure-treated lumber (treated with ACQ or CA preservatives) or naturally durable heartwood (redwood/cedar) is mandatorily required for bottom (sole) plates resting directly on concrete or masonry foundations in contact with earth.

[!CAUTION] Corrosion Risk: Preservative chemicals in ACQ-treated wood severely corrode standard carbon steel fasteners. All nails, screws, and anchor bolts in contact with pressure-treated plates must be hot-dipped galvanized (ASTM A153) or 304/316 stainless steel.

Sill Plate Anchor Bolt Standards (CRC Section R403.1.6):
 ➔ Diameter         : Minimum 1/2-inch (or 5/8-inch in high seismic zones)
 ➔ Spacing          : Maximum 6 feet on center (4 feet o.c. for 3-story structures)
 ➔ End Clearance    : Installed within 12 inches of plate ends
 ➔ Embedment Depth  : Minimum 7 inches embedded into concrete
 ➔ Plate Washers    : Minimum 3" x 3" x 0.229" thick steel washers (CA Seismic Category D/E)

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Roof Framing & Pre-Engineered Wood Trusses

Conventional Rafter Framing

• Ridge Board: Must be at least the same depth as the full plumb cut of the rafter.
• Ceiling Joists / Rafter Ties: Installed in the lower third of rafter height, running parallel to rafters, to resist outward lateral thrust exerted by rafters against exterior top plates.
• Collar Ties: Installed in the upper third of rafter height (maximum 4 feet on center) to resist wind uplift forces at the ridge.

Engineered Roof Trusses

Pre-manufactured roof trusses utilize triangulated wood members joined by metal connector plates (gang-nail plates).

[!IMPORTANT] Strict Truss Integrity Rule: Trusses are engineered to transfer axial loads through specific web geometries. Under no circumstances may a contractor cut, notch, drill, modify, or alter truss chords or web members on site without prior written approval from a registered Professional Engineer (PE).

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Seismic Load Paths, Shear Walls & Hold-Downs

Because California sits in high Seismic Design Categories ($D_0, D_1, D_2, E, F$), light-frame wood structures must maintain a continuous lateral load path to transfer earthquake-induced inertial shear forces down to the foundation.

Continuous Seismic Load Path Flow:
 [ Roof Diaphragm Sheathing ] 
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 [ Upper Story Wood Shear Walls ] 
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 [ Floor Diaphragm Sheathing ] 
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 [ Lower Story Wood Shear Walls ] 
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 [ Hold-Down Anchors & Sill Anchor Bolts ] 
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 [ Reinforced Concrete Foundation ]

Wood Structural Panel Shear Walls

Wood shear walls consist of OSB or Plywood panels nailed directly to wall framing studs. Shear wall capacity is governed by:

1. Structural panel thickness ($3/8\text{ inch}$, $15/32\text{ inch}$, $19/32\text{ inch}$).
2. Fastener size (8d common [$0.131" \times 2.5"$] or 10d common [$0.148" \times 3.0"$]).
3. Edge Nailing Schedule: Fastener spacing along panel edges (6", 4", 3", or 2" on center). Field nailing on intermediate studs is standard at 12 inches on center. Edge distance from nail center to panel edge must be a minimum of 3/8 inch to prevent edge blowout under racking loads.

Hold-Down Anchors (Hold-Downs)

Lateral seismic shear acting at the top of a shear wall creates an overturning moment ($M = V \times H$) that attempts to lift the leading post of the wall off the foundation. Contractors install heavy steel hold-down anchors (such as Simpson Strong-Tie HDU series) bolted to double end posts and anchored directly into heavy foundation J-bolts or continuous threaded rods to resist uplift forces exceeding 10,000 lbs.

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Joist & Stud Drilling and Notching Code Limitations

Modifying framing members to route plumbing pipes, electrical cables, or HVAC ducts must strictly comply with CRC Sections R502.8 (Joists) and R602.6 (Studs) to prevent structural collapse.

Floor Joist Notching & Drilling Rules (CRC R502.8)

• Notches: Prohibited in the middle third of the joist span!
  • Maximum notch depth at joist ends: 1/4 of joist depth.
  • Maximum notch depth in outer thirds of span: 1/6 of joist depth.
  • Maximum notch length: 1/3 of joist depth.
• Holes: Maximum bored hole diameter cannot exceed 1/3 of joist depth. Hole edges must be kept at least 2 inches from joist top/bottom edges and 2 inches from any notch.

Wall Stud Drilling & Notching Rules (CRC R602.6)

2x4 Stud Dimension Reference (Actual Width = 3.5 inches):
 ➔ Bearing Stud Max Notch  : 3.5" x 0.25 = 0.875 inches
 ➔ Bearing Stud Max Hole   : 3.5" x 0.40 = 1.40 inches
 ➔ Non-Bearing Max Notch   : 3.5" x 0.40 = 1.40 inches
 ➔ Non-Bearing Max Hole    : 3.5" x 0.60 = 2.10 inches

Worked Board Feet & Material Estimating Math

Calculate the total board feet (BF) for 20 pieces of $2 \times 8$ dimensional lumber, each 16 feet long.

1. Board Feet Formula: $\text{Board Feet (BF)} = \frac{\text{Thickness (in)} \times \text{Width (in)} \times \text{Length (ft)}}{12}$

2. BF per Piece: $\text{BF}_{\text{piece}} = \frac{2 \times 8 \times 16}{12} = \frac{256}{12} = 21.33\text{ BF}$

3. Total Order (20 pieces): $\text{Total Board Feet} = 20 \times 21.333 = 426.67\text{ Board Feet}$