Specialized Fasteners for Timber Structures

I. New Requirements for Connections in Modern Timber Structures

1. High Load-Bearing Capacity: To transfer greater tension, compression, shear forces, and bending moments.
2. High Stiffness and Deformation Control: To ensure overall structural stiffness and serviceability comfort.
3. Durability and Fire Resistance: Connection nodes must account for anti-corrosion, moisture resistance, and necessary fire protection measures.
4. Aesthetics and Construction Convenience: Suitable for factory prefabrication and rapid on-site assembly.

II. Types of Major Specialized Fasteners and Their Mechanical Principles

• Principle: Loads are transferred through the mechanical interlocking (withdrawal resistance/grip) between the threads and wood fibers. These are the most widely used connectors for tension and shear resistance.
• Specialized Types:
  • Fully Threaded Screws (Self-Tapping Screws): Ranging from a few millimeters to over a meter in length. They often feature specialized threads (e.g., square threads) to enhance grip, with self-drilling tips. Used for CLT panel-to-panel connections and additional reinforcement.
  • Dowel-type Bolts (Glulam Bolts): Typically made of high-strength alloy steel with continuous deep threads along the shank, used with washers and nuts. Ideal for heavy-duty tension and shear connections in glulam beams and columns.
• Key Point: Minimum edge distance, end distance, and spacing must be determined based on wood density and thickness, and the effective anchorage length must be calculated.

• Principle: Steel pins or plates are partially or fully embedded into pre-cut slots in the timber, primarily relying on the bearing capacity of the wood to transfer shear forces. Commonly used in the tension and compression zones of bending-resistant joints.
• Specialized Types:
  • Split Rings / Shear Plates: Circular toothed steel rings placed in pre-drilled holes with a bolt passing through the center. The ring embeds into the wood, providing excellent withdrawal and shear resistance.
  • Embedded Steel Plate Connectors: Toothed or perforated steel plates embedded into slots in timber members and secured with bolts or screws, achieving concealed and efficient force transfer.

• Principle: Utilizing punched tooth plates or the group effect of numerous small-diameter nails to distribute forces into the timber. Primarily used for light-frame timber truss nodes and prefabricated wall panel connections.
• Specialized Types: Truss Plates, angle brackets, hold-down anchors, etc. Mostly made from galvanized steel sheets formed by stamping.

• Principle: In structures like bowstring trusses and spatial timber grids, high-strength steel tie rods and stainless steel cables are used with specialized end anchors to connect with timber members, enabling large spans.

III. Material, Anti-Corrosion, and Fire Protection Considerations

1. Materials: Fasteners are typically made from high-strength carbon steel, alloy steel, or stainless steel. Stainless steel (grades 304, 316) is used in outdoor, humid environments, or with chemically treated wood due to its superior corrosion resistance, though at a higher cost.
2. Anti-Corrosion Treatment: Carbon steel fasteners must undergo effective anti-corrosion treatments, such as hot-dip galvanizing, Dacromet, or mechanical galvanizing. For highly corrosive environments, composite bolts with "stainless steel bodies + carbon steel threads" can be used.
3. Fire Protection: In structures with high fire-resistance requirements, connection nodes may need to be wrapped in fireproof boards, or special fasteners that maintain strength for a certain period at high temperatures should be used. Additional loads caused by bolt exposure after wood charring must also be considered.

IV. Design, Installation, and Future Trends