Compression strength perpendicular to grain, also known as transverse compression strength, is a fundamental mechanical property that governs the bearing performance of wood at supports, connections, and contact surfaces. This article explains the standardized test methods defined in GB/T 1939, ISO 3132, ASTM D143, and EN 408, covering specimen preparation, loading procedures, determination of proportional limit stress, and typical engineering applications. It serves as a practical technical reference for wood product manufacturers, structural engineers, and accredited third-party laboratories performing transverse compression testing.
Technical Overview
Wood is an orthotropic material, meaning its mechanical behavior differs significantly along its three principal axes: longitudinal (parallel to grain), radial, and tangential. When a compressive load is applied perpendicular to the grain direction, the material exhibits a response that is fundamentally different from compression parallel to grain. The transverse compressive strength of wood is typically only 10% to 25% of its longitudinal compressive strength, which makes it a critical design parameter wherever concentrated loads bear against wood surfaces.
Unlike compression parallel to grain, which usually produces a distinct failure event, wood under transverse compression does not fracture abruptly. Instead, the cell structures progressively buckle and collapse, and the load–deformation curve shows a gradual transition from an initial linear elastic region into a plateau and eventual densification stage. For this reason, most standards specify the compressive stress at the proportional limit, rather than an ultimate crushing stress, as the primary reported result for small clear specimens.
- Transverse compression strength is a bearing-related property, not a member strength property.
- Results are strongly influenced by load direction (radial versus tangential), density, and moisture content.
- Primary reference standards include GB/T 1939-2009, ISO 3132, ASTM D143, and EN 408.
- Characteristic values derived from these tests feed directly into structural design provisions such as EN 1995 (Eurocode 5).
Testing Samples
Small Clear Specimens (GB/T 1939 / ISO 3132 Method)
For laboratory characterization of clear wood, standard practice uses small prismatic specimens free of knots, decay, and grain deviation. In the Chinese national standard GB/T 1939-2009, the standard specimen measures 20 mm by 20 mm in cross section and 30 mm along the grain direction. The load is applied across the full radial or tangential face, and radial and tangirectional results are recorded separately because ray structure and earlywood/latewood layout produce measurably different stiffness and strength. Dimensional tolerances are typically held within plus or minus 0.5 mm, and specimens must be conditioned to equilibrium at approximately 12% moisture content before testing.
ASTM D143 Perpendicular-to-Grain Specimens
ASTM D143, the long-established North American method for small clear timber specimens, specifies a 50 mm by 50 mm by 150 mm block loaded through a 50-mm-wide steel bearing plate centered on its length. This configuration represents partial (local) bearing, simulating conditions such as a beam resting on a narrow support plate or a bolt washer bearing on a wood face. The crosshead speed is specified at 0.305 mm per minute (0.012 in/min), and the reported result is the fiber stress at the proportional limit.
Structural-Size Specimens (EN 408 Method)
For solid timber, glued laminated timber, and cross-laminated timber used in construction, EN 408 prescribes testing at full structural cross sections. Specimens are loaded through bearing plates of defined width, and compressive strength perpendicular to grain is evaluated either at maximum load or at a defined strain limit, commonly 10% relative compression, when no distinct maximum occurs beforehand. This method captures the influence of size, growth characteristics, and manufacturing features that small clear specimens cannot represent.
Testing Items
Depending on the specified standard and the purpose of the evaluation, a transverse compression test program may include the following measurable parameters:
- Compressive stress at the proportional limit, perpendicular to grain, in radial and tangential directions.
- Maximum compressive stress or stress at a defined strain limit (for example, 10% strain under EN 408).
- Apparent modulus of elasticity in compression perpendicular to grain, determined from the initial linear portion of the load–deformation curve (see also GB/T 1943 for dedicated modulus testing).
- Full load–deformation curves documenting yield behavior, plateau response, and densification.
- Moisture content and air-dry density of each specimen at the time of test.
- Moisture-corrected strength values normalized to 12% moisture content.
Testing Methods
Apparatus
Testing is performed on a calibrated universal testing machine equipped with a load cell of appropriate capacity, rigid parallel steel platens, and, where required, a bearing plate of specified width for partial bearing configurations. A spherical seat or self-aligning head is recommended to ensure uniform load application, and displacement is captured with a precision transducer or extensometer capable of resolving deformations to at least 0.01 mm.
Specimen Conditioning and Measurement
Specimens are conditioned in a controlled environment, typically 20 degrees Celsius and 65% relative humidity, until constant mass is reached, targeting a moisture content of approximately 12%. Before loading, the width and length of the bearing surface are measured to the nearest 0.1 mm at multiple positions, and the loaded area is calculated to the nearest square millimeter. Any specimen showing visible defects, distortion, or non-uniform grain angle in the test zone is rejected and replaced.
Loading Procedure
- Center the specimen on the lower platen with the designated face (radial or tangential) aligned upward for loading.
- Apply an initial seating load and verify uniform contact across the bearing surface.
- Load continuously at the constant rate prescribed by the applicable standard, without shock or interruption, while recording load and deformation data.
- Continue loading until the specified end point is reached: the proportional limit region is well past for small clear specimens, the defined strain limit for EN 408, or the specified maximum deformation.
- Retain a representative portion of each specimen for immediate moisture content determination after the test.
Determination of the Proportional Limit
The proportional limit stress is identified graphically as the point at which the load–deformation curve first departs from its initial straight-line portion. Because visual identification can be subjective, standards provide deviation criteria, such as a defined offset of deformation from the extended linear line, to standardize the judgment. Consistent interpretation is essential, since the proportional limit, rather than a peak load, is the value used for material characterization in most small-specimen methods.
Calculation and Moisture Correction
The compressive strength perpendicular to grain is calculated as the governing load divided by the bearing area. For small clear specimens, the governing load is normally the proportional limit load; the stress is then corrected to a reference moisture content of 12% using a standard linear correction of the form sigma-12 equals sigma-M multiplied by the quantity 1 plus alpha times the quantity M minus 12, where M is the actual moisture content and alpha is a species-group coefficient, commonly taken as approximately 0.035 for transverse compression in GB practice. All corrected values, load–deformation curves, failure descriptions, and specimen moisture and density data are compiled in the final test report.
Applications
Compressive strength perpendicular to grain is one of the most frequently invoked wood properties in structural detailing and product qualification. Typical engineering and quality-control applications include:
- Bearing design of beams, joists, and trusses resting on plates, studs, or concrete supports.
- Sill plates and wall plates subject to compressive loading from superimposed structures.
- Bolted and dowelled connections, where washers and steel plates bear against the wood face.
- Notches, bearings, and haunches where load transfer occurs across the grain.
- Railway sleepers, bridge decks, and timber mats subject to concentrated transverse loading.
- Derivation of characteristic values and design stresses for solid timber, glulam, and cross-laminated timber under EN 1995 and related frameworks.
- Species characterization, forestry research, and comparison of modified or heat-treated wood against untreated references.
- Factory production control and incoming material verification for engineered wood product manufacturers.
FAQ
Why is proportional limit stress reported instead of ultimate strength?
Wood loaded perpendicular to grain does not fail by fracture; the cells fold and densify progressively, so the load continues to rise without a true breaking point. The proportional limit represents the stress at which permanent cell-wall collapse begins, making it the most meaningful and reproducible index of service-relevant bearing capacity. Standards such as GB/T 1939, ISO 3132, and ASTM D143 therefore base their small-specimen results on this parameter.
What is the difference between radial and tangential results?
Radial loading, where force acts from bark side toward pith, generally yields higher stiffness and slightly higher proportional limit stress, particularly in hardwoods with prominent rays. Tangential loading produces lower values because the relatively soft earlywood layers are loaded directly in series. This is why standards require the load direction to be recorded and the two directions to be reported separately.
How does partial bearing differ from full bearing?
In full bearing, the entire loaded face of the specimen is compressed, as in the GB/T 1939 small clear specimen. In partial bearing, such as the 50 mm bearing plate used in ASTM D143 or the specified plate widths in EN 408, only part of the surface is loaded. Restraining effects from the surrounding unloaded material raise the apparent compressive resistance, so partial-bearing results are not directly interchangeable with full-bearing results.
Which standards are most commonly specified for testing?
GB/T 1939-2009 governs small clear specimen testing in China and is progressively being consolidated into the GB/T 1927 series of physical and mechanical test methods. ISO 3132 provides the corresponding international small-specimen method, ASTM D143 covers North American practice for small clear timber, and EN 408 applies to structural-size timber and glulam. The choice depends on product type, target market, and the governing design code.
How should results be interpreted for design?
Transverse compression results are used as inputs to characteristic value derivation and allowable or design stress assignments, typically in combination with statistical treatment of sample populations as outlined in standards such as ASTM D245 and ASTM D2555 for clear wood data or EN 384 for structural timber. Design provisions such as EN 1995 also permit increased design stresses where deformation is tolerated or where the loaded area is effectively larger than the contact area. Accredited third-party laboratories should always state the applied standard, load direction, moisture correction, and evaluation criterion when reporting results.