ISO 27567:2009 – Laminated Veneer Lumber (LVL) – Measurement of Dimensions and Shape

1. Overview of ISO 27567

ISO 27567:2009, developed by ISO/TC 89 (Wood-based panels, Subcommittee SC 3, Plywood), establishes standardized test methods for measuring the dimensions and shape of structural laminated veneer lumber (LVL). This standard is referenced by ISO 18776, which specifies the general requirements for LVL products used in structural applications such as beams, headers, and scaffold planks.

LVL is an engineered wood product manufactured by bonding together multiple layers of thin wood veneers with adhesives, oriented with the grain running parallel. Unlike plywood where adjacent layers are cross-oriented, LVL’s parallel grain structure gives it exceptional strength along its length, making it a preferred material for load-bearing structural applications.

Accurate dimensional measurement is critical for quality control in LVL manufacturing. The standard covers seven key measurement parameters: thickness, length, width, spring (edge curvature), bow (face curvature), twist, section squareness, and cupping (surface curvature). These measurements ensure that LVL products meet the dimensional tolerances required for proper fit and structural performance in construction.

2. Test Apparatus and Measurement Procedures

2.1 Thickness Measurement

The standard specifies that thickness shall be measured at six specific locations on each test piece: approximately in the middle of each long side (25 mm from the edges) and at each corner (25 mm from the edge and 200 mm from the end). A screw micrometer or vernier caliper with flat, parallel measuring surfaces (contact area between 30 mm and 300 mm) is used, with readings recorded to the nearest 0.1 mm.

The test pieces must have a moisture content of (10 +/- 4) % as determined in accordance with ISO 16979. This is critical because wood-based materials expand and contract with moisture changes, and dimensional measurements are only meaningful when referenced to a standard moisture condition.

2.2 Length and Width Measurement

Length and width are measured to the nearest 1 mm along both long sides and both ends, yielding two measurements each. The apparatus must have an accuracy of 0.5 mm.

2.3 Spring, Bow, and Twist

Parameter	Description	Apparatus	Accuracy
Spring	Edge curvature – deviation from straightness	Wire/cord, thumbtacks, blocks, linear instrument	0.5 mm
Bow	Face curvature – deviation from flatness	Flat surface, masses (85 +/- 5) kg, linear instrument	0.5 mm
Twist	Out-of-plane deformation	Flat surface, linear measuring instrument	0.5 mm

For spring measurement, a wire or cord is stretched taut along the concave edge using blocks of equal thickness. The perpendicular distance from the midpoint to the wire is measured, and the spring value is calculated as the difference between this measurement and the average block thickness.

Bow measurement involves placing the piece convex-face-up on a flat surface with an (85 +/- 5) kg mass at the centre. The maximum gap between the piece’s underside and the flat surface is recorded.

2.4 Section Squareness and Cupping

Squareness of section is measured at three positions along the edge using a try square. The gap between the face at the opposite edge and the square leg is measured to the nearest 0.1 mm.

Cupping is measured at three positions using a straight edge placed across the surface at right angles to the long edge. A maximum moisture content gradient of 1 % through the thickness is required at the time of measurement.

Understanding these measurement parameters enables engineers to diagnose manufacturing process issues: consistent spring may indicate uneven adhesive application, bow often relates to pressing conditions, and cupping can signal moisture imbalance or inadequate consolidation pressure during lamination.

3. Engineering Design Insights

ISO 27567 provides the measurement framework essential for quality assurance in LVL production, but its real value to design engineers lies in understanding how these geometric deviations affect structural behaviour:

Spring and bow tolerances directly influence the ease of installation and the development of unintended bending stresses in continuous beam applications.
Twist is particularly problematic in long-span applications because it cannot be corrected during installation.
Cupping affects the surface flatness critical when LVL is used as formwork or substrate.
Thickness variation beyond specification may seem minor, but in multi-ply built-up beams, cumulative variations can lead to significant overall dimensional discrepancies.

Designers must understand that LVL’s mechanical properties are direction-dependent. All dimensional tolerances in ISO 27567 assume the grain direction is parallel to the length.

4. Frequently Asked Questions

Q1: Why does ISO 27567 require moisture content control?
Wood is hygroscopic; its dimensions change with moisture content. At (10 +/- 4) % MC, LVL is close to equilibrium with typical indoor environments, ensuring reproducibility across laboratories.

Q2: How does ISO 27567 relate to ISO 18776?
ISO 18776 provides overarching LVL specifications. ISO 27567 provides test methods for measuring those dimensions.

Q3: Can these methods be applied to other wood-based panels?
Many measurement principles are adaptable to glulam and CLT, but specific locations and loading conditions are calibrated for LVL.

Q4: What is the significance of the 85 kg loading mass?
The (85 +/- 5) kg mass simulates a typical uniform distributed load, providing a practical reference condition for measuring out-of-flatness.