Overview
ISO 8970:2020 - "Timber structures - Testing of joints made with mechanical fasteners - Requirements for timber density" specifies a density‑based method for selecting wood specimens used to determine the strength and stiffness of timber joints with mechanical fasteners. Intended to be used alongside a test standard, it ensures that specimen density distribution matches the timber to which test results will be applied. The standard assumes specimens are conditioned to target climate, that density is normally distributed, and that deviations are reported.
Key topics and requirements
- Conditioning: Wood shall be at equilibrium at (65 ± 5) % RH and (20 ± 2) °C at testing; report whether conditioned by drying or moistening.
- Density measurement: Density for actual moisture content is determined per ISO 13061-2.
- Sampling criteria:
- Mean density of selected pieces (ρm,sel) must meet: 0.95·ρm,tar ≤ ρm,sel ≤ 1.05·ρm,tar.
- Coefficient of variation of selected pieces (CVρ,sel) must satisfy: CVρ,sel ≥ CVρ,tar (target variation).
- Recommended density coverage: pieces should span [(1 − k)ρm,tar ; (1 + k)ρm,tar] with statistical factor k (see standard).
- Corrections (Annex A): If sampling criteria are not met, observed resistances are corrected:
- Mean value correction using exponent c: Ri,cor = Ri · (ρm,tar / ρm,sel)^c.
- Variation correction modifies the coefficient of variation for resistance using a corrected CV formula.
- Annex B provides background and methods to estimate exponent c; example c values for European softwood are listed for common failure modes (e.g., withdrawal, embedment).
- Test report content: Must include reference to ISO 8970:2020, wood species, conditioning method, per-piece densities, mean and CV, any moisture content measured, evidence of conformity or deviations, and other influencing information.
Applications and users
ISO 8970:2020 is practical for:
- Structural testing laboratories performing joint strength and stiffness tests with mechanical fasteners.
- Timber product manufacturers validating connector and fastener performance for design use.
- Structural engineers and researchers needing reproducible, comparable test results tied to target timber density distributions.
- Standards developers and certification bodies who reference density-based sampling and correction procedures.
The standard improves the reliability and applicability of joint strength data by ensuring density representativeness or by providing validated correction procedures.
Related standards
- ISO 13061-2 - Physical and mechanical properties of wood: Determination of density for physical and mechanical tests (normative reference).
- Product test standards for timber joints (used in conjunction with ISO 8970:2020) - consult the specific test method for the joint under evaluation.
Keywords: ISO 8970:2020, timber density, timber structures, mechanical fasteners, testing of joints, wood sampling, density correction, ISO 13061-2.
