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D2718-24 – Standard Test Method Technical Guide
📐 Test Method Overview: Understanding Planar Shear (Rolling Shear)
ASTM D2718-24, formally designated as the Standard Test Methods for Structural Panels in Planar Shear (Rolling Shear), provides two distinct procedures for evaluating the shear properties of wood-based structural panels. These methods specifically target shear distortion occurring in planes parallel to the edge planes of the panel. The primary properties determined under this standard are shear strength and the modulus of rigidity. Materials covered include structural plywood, oriented strand board (OSB), and composites of veneer and wood-based layers.
💡 Significance in Design: Planar shear characteristics are essential for the rigorous design of glued wood-panel structural components, such as box beams, folded plate roofs, and stressed skin panels. The standard also notes that planar shear may govern design at low span-depth ratios encountered in floors, concrete forms, and bulk storage structures.
⚙️ Test Procedure and Speed Selection: Choosing Between Method A and B
The choice between the two test methods is dictated by the purpose of the test and the equipment available. Both methods determine shear strength, but they apply load in fundamentally different ways to suit various intended applications.
| 🟦 Feature | 📏 Method A: Plate Loading | 📏 Method B: Five-Point Bending |
|---|---|---|
| 📐 Specimen Configuration | Rectangular panel section adhered between steel plates with protruding knife edges | Panel section loaded over two continuous spans |
| ⚙️ Loading Mechanism | Load applied at the panel faces via knife edges on the steel plates | Planar shear stress induced by transverse bending load |
| 🎯 Key Property Outputs | Shear strength; Modulus of rigidity | Shear strength consistent with transverse loading |
| ⚡ Primary Use Case | Confirming design values for plywood and OSB; evaluating composite modulus | Determining shear strength in panel applications under transverse loading at any moisture condition |
⚠️ Standard Unit Requirements: Per Section 1.4 of the standard, values stated in inch-pound units are regarded as the standard. Any values provided in parentheses (SI units) are mathematical conversions for information only and are not considered standard.
📊 Key Measured Properties and Supporting Standards
Test Method A is uniquely capable of determining the modulus of rigidity, which is a composite property of the entire specimen. While the method does not produce pure shear, the specimen length is prescribed so that secondary stresses have a minimum effect. For plywood panels where the ratio of shear moduli between parallel and perpendicular plies is known, this method allows for the calculation of the rolling shear modulus.
Test Method B emphasizes shear strength consistency with real-world panel applications under transverse loading. A key advantage is its ability to evaluate shear strength at any moisture condition.
To fully characterize test specimens, this standard references ASTM D2395 (Test Methods for Density and Specific Gravity of Wood and Wood-Based Materials) and ASTM D4442 (Test Methods for Direct Moisture Content Measurement of Wood and Wood-Based Materials) for determining complementary physical properties of the panels.
❓ Frequently Asked Questions
🔍 What is the primary difference between Test Method A and Test Method B?
Test Method A uses steel plates to load a rectangular panel section directly, measuring both shear strength and modulus of rigidity. Test Method B induces planar shear through five-point bending over two continuous spans, focusing on shear strength under practical transverse loading conditions.
💡 What materials are covered by ASTM D2718-24?
The standard applies to structural panels, including but not limited to structural plywood, oriented strand board (OSB), and composites of veneer and wood-based layers.
⚡ When is planar shear (rolling shear) critical in design?
It is critical for designing components like box beams, folded plate roofs, and stressed skin panels. It often governs at low span-depth ratios, such as floors bearing high concentrated loads, concrete forms under high pouring pressure, and bulk storage structures.
📌 What are the standard units for reporting results?
Per Section 1.4, inch-pound units are the standard for these test methods. SI units provided in parentheses are for information only and are not part of the standard.