9.4 Engineered Wood, Trusses, and Load Paths

9.4 Engineered Wood, Trusses, and Load Paths

Engineered wood products (EWP) and prefabricated wood trusses are CSI Division 06 17 00 (Shop-Fabricated Structural Wood). They are governed by IBC Chapter 23, the manufacturer's evaluation report (ICC-ES ESR), and the Truss Plate Institute (TPI) ANSI/TPI 1 standard for metal-plate-connected trusses. EWP design values come from the maker's tables, not generic lumber tables.

Engineered Wood Product Types

EWP advantages: longer spans, dimensional stability, less waste. They are highly heat-sensitive in fire — engineered I-joists can fail faster than solid sawn lumber, which is why IRC R501.3 requires a membrane (e.g. 1/2-in gypsum) or sprinklers under engineered floors.

Glulam Grading and Camber

Glulam beams are specified by stress class, e.g. 24F-V4 (24 = allowable bending Fb of 2400 psi; F = bending member; V4 = layup combination). The high-grade laminations go in the tension zone at the bottom of a simple span — installing a glulam upside-down is a serious field error. Glulam is usually fabricated with camber (a slight upward curve) to offset dead-load deflection, and comes in appearance grades — Industrial, Architectural, Premium — selected for how exposed the member will be.

I-joists are governed by APA/ASTM D5055; they must never be cut through the flange, and web holes are limited to the manufacturer's hole chart (large holes only near mid-span where shear is low). Rim board and squash blocks transfer bearing loads around I-joist ends and prevent rotation at supports.

Trusses and TPI 1

Metal-plate-connected wood trusses are engineered by the truss design engineer and shipped with sealed truss design drawings. Key field rules per TPI 1 and BCSI (Building Component Safety Information):

• Never cut, drill, or notch a truss member or chord in the field without engineer approval — it voids the design.
• Trusses require permanent and temporary bracing to prevent buckling; the lateral restraint and diagonal bracing are part of the engineering.
• The truss layout/placement diagram is for installation only — it is not an engineering document.

Load Path and Lateral Connectors

A continuous load path carries gravity and lateral (wind/seismic) loads from roof to foundation: roof/truss to wall, wall to floor, floor to wall below, wall to foundation. Each transfer needs a positive mechanical connection. Common metal connectors (e.g. Simpson Strong-Tie, USP) include:

• Joist hangers (face-mount/top-flange) for joist-to-beam.
• Hurricane/seismic ties (H-clips, twist straps) for truss/rafter-to-top-plate uplift.
• Hold-downs and anchor bolts for shear-wall-to-foundation.

Connectors must use the manufacturer-specified nails (often 10d x 1-1/2 in in hangers) — substituting shorter or fewer nails cuts the rated capacity.

Uplift Worked Example

Wind uplift creates net upward force on the roof. If a truss carries 400 lb of dead load but wind produces 650 lb uplift at the bearing, the net uplift = 650 - 400 = 250 lb. The chosen hurricane tie must have a listed uplift capacity (e.g. H2.5A ≈ 415 lb) exceeding 250 lb with the code load factor. Selecting a connector below the net uplift demand is a classic exam and field failure.

Lateral resistance flows through shear walls (sheathed with rated wood structural panels) and diaphragms (roof and floor sheathing acting as horizontal beams). Sheathing nailing — edge and field spacing, e.g. 8d at 6 in edges / 12 in field — sets the shear capacity per the IBC nailing/shear-wall tables; tightening edge nailing to 4 or 3 in increases capacity. Drag struts (collectors) carry diaphragm forces into shear walls, and hold-downs resist the resulting overturning at wall ends, completing the lateral load path to the foundation.

Common Exam Traps

Use the manufacturer's ESR tables for EWP, not generic lumber values. Protect engineered floors with a gypsum membrane (IRC R501.3) because they fail fast in fire. Never field-modify a truss without engineer sign-off, and brace per TPI 1/BCSI. For load path, every connector needs the specified nails, and uplift design uses net (not gross) uplift.

Engineered Wood Products

Engineered lumber outperforms sawn wood for spans. LVL (laminated veneer lumber) and PSL (parallel strand) make strong beams/headers; glulam is a large laminated beam; I-joists (flange + OSB web) span long floor distances light. Engineered products must not be field-notched or drilled outside the manufacturer's allowable hole chart — cutting an I-joist flange destroys it. They require specified bearing length and web stiffeners at concentrated loads.

Common Exam Traps

• Trap: You can notch an I-joist flange like a 2×10. Never cut the flange; webs only per the chart.
• Trap: Field-modifying a truss (cutting a web/chord). Trusses are engineered — any change needs the truss engineer's approval.
• Trap: Ignoring uplift — the load path must be continuous from roof to foundation with hurricane ties/hold-downs.
• Trap: Treating LVL bearing length as optional — concentrated loads need specified bearing and stiffeners.