Design of Timber Connections to Eurocode 5: A Comprehensive Guide with Worked Example

This article discusses the design of timber connections to Eurocode 5. It contains worked example illustrating the design of bolted timber connections.

Timber connections form the backbone of any wooden structure, ensuring stability and strength in construction projects. These integral components actively bind different timber elements together, transforming individual pieces of wood into cohesive and robust frameworks. Without proper connections, even the highest quality timber would fail to achieve its full potential in structural applications. Thus, the choice and implementation of timber connections demand meticulous planning and precision.

Timber connections are essential for transferring loads between timber elements in a structure. Properly designed connections ensure that the forces are adequately distributed, maintaining the structural integrity and safety of the timber construction. Here are some key reasons justifying the importance of timber connections:

1. Load Distribution: Timber connections ensure that loads are transferred efficiently between different structural elements, preventing localized failures and ensuring uniform load distribution.
2. Structural Integrity: Robust connections enhance the overall stability and integrity of the structure, reducing the risk of collapse under various loading conditions.
3. Durability: Well-designed connections contribute to the durability of the timber structure by preventing issues such as joint degradation, moisture ingress, and biological attack.
4. Aesthetics: Thoughtfully designed timber connections can enhance the visual appeal of the structure, especially in exposed timber constructions.
5. Ease of Construction: Properly planned connections facilitate easier and faster construction, reducing labor costs and construction time.

Types of Timber Connections

Timber connections can be broadly classified into mechanical and adhesive joints. Each type has its own set of advantages and applications.

Mechanical Connections

Mechanical connections use fasteners such as nails, screws, bolts, and dowels to join timber elements. These connections are popular due to their ease of installation and high load-bearing capacities.

1. Nailed Connections: Commonly used for light-load applications, nailed connections are easy to install and provide sufficient strength for many structural applications.
2. Screwed Connections: Screws offer higher load-bearing capacities than nails and provide better resistance to withdrawal forces. They are often used in applications where higher strength and stability are required.
3. Bolted Connections: Bolts are used for heavy-load applications and provide high load-bearing capacities. They are suitable for joining larger timber elements and can be easily disassembled if necessary.
4. Dowelled Connections: Dowels, often used in glulam structures, provide high stiffness and load-bearing capacities. They are commonly used in timber frame construction and heavy timber structures.
5. Metal Connectors: Brackets, hangers, and plates are used to connect timber elements, providing high load-bearing capacities and easy installation. These connectors are often used in modern timber construction to simplify assembly and improve structural performance.

Adhesive Connections

Adhesive connections use various types of glue to bond timber elements together. These connections are popular in engineered wood products and prefabricated timber structures.

1. Glued Laminated Timber (Glulam): Layers of timber are glued together to form strong and durable structural elements. Glulam connections are used in applications where high strength and long spans are required.
2. Cross-Laminated Timber (CLT): Layers of timber boards are glued together at right angles to form large, solid panels. CLT connections provide high strength and stiffness, making them suitable for multi-story timber buildings.

Design of Timber Connections to Eurocode 5

Designing timber connections according to Eurocode 5 involves several steps, including the selection of suitable connection types, calculation of load-bearing capacities, and consideration of factors such as moisture content, temperature, and duration of load.

Selection of Connection Type

The choice of connection type depends on several factors, including the type of load (tension, compression, shear), the size and orientation of the timber members, and the required level of performance. Common connection types include nails, screws, bolts, dowels, and metal connectors.

Calculation of Load-Bearing Capacities

Eurocode 5 provides formulas and tables for calculating the load-bearing capacities of various connection types. The main parameters involved in these calculations are:

• Density of Timber: The strength and stiffness of the connection are influenced by the density of the timber.
• Diameter of Fasteners: Larger diameters generally provide higher load-bearing capacities.
• Embedment Strength: Depends on the type of fastener and timber density.
• Spacing and End Distances: Adequate spacing and end distances are crucial to avoid splitting and ensure the connection’s strength.

Factors Affecting Connection Design

1. Moisture Content: Timber’s mechanical properties vary with moisture content. Eurocode 5 provides adjustment factors for connections in varying moisture conditions.
2. Temperature: Elevated temperatures can reduce timber strength. The code includes factors to account for temperature effects.
3. Duration of Load: Timber’s strength decreases with prolonged loading. Eurocode 5 provides modification factors for different load durations (permanent, long-term, medium-term, short-term, and instantaneous).

Safety and Serviceability

Eurocode 5 emphasizes safety and serviceability, ensuring that timber connections can withstand loads during their intended lifespan without excessive deformation or failure. Partial safety factors for materials and loads are provided to ensure a high level of reliability.

Worked Example

Let’s walk through a detailed example to design a bolted timber connection according to Eurocode 5.

Problem Statement

Design a bolted connection for a timber beam-to-beam joint subjected to a characteristic load of 30 kN. The beams are made of C24 timber and are used in Service Class 1 conditions. The bolts are M16 with a tensile strength of 400 MPa. The connection is double shear, meaning the bolt passes through both beams and a steel plate.

Step 1: Material Properties

• Timber Grade: C24
• Characteristic Density of C24 Timber: ρ = 350 kg/m³
• Design Strength of C24 Timber in Compression Parallel to Grain: f_c,0,d = 17 MPa
• Bolt Material: Steel with tensile strength f_u = 400 MPa

Step 2: Determine the Diameter and Spacing of Bolts

The bolt diameter (d) is given as M16 (16 mm).

According to EC5, the minimum spacing between bolts (a₁) in the direction parallel to the grain should be 7d, and in the direction perpendicular to the grain should be 4d.

• Parallel Spacing (a₁): 7 × 16 mm = 112 mm
• Perpendicular Spacing (a₂): 4 × 16 mm = 64 mm

Step 3: Calculate the Load-Carrying Capacity per Bolt

The load-carrying capacity of a bolt in double-shear connection is calculated using the formula:

F_{v,Rk}=min(f_uA_{bolt};  \frac{2f_htd}{1.3})

Where:

• f_u​ = ultimate tensile strength of the bolt (400 MPa)
• A_bolt​ = cross-sectional area of the bolt (201 mm² for M16)
• f_h​ = embedding strength of timber (17 MPa for C24 timber)
• t = thickness of the timber member (assume 50 mm for each beam)
• d = bolt diameter (16 mm)

First, calculate the bolt’s tensile capacity:

F_{v,Rk,bolt}=400\times201\times10^{-6}=80.4kN

Next, calculate the timber’s embedding capacity:

f_h=0.082(1-0.01d)\times p_k\\=0.082(1-0.01\times16)350=9.7Mpa

Therefore, the embedding capacity per bolt:

F_{v,RK,timber}=\frac{2\times 9.7\times50\times16}{1.3\times10^3}=11.96kN

Step 4: Determine the Number of Bolts

n=\frac {30}{11.96}=2.51

Rounding up, we need at least 3 bolts to safely carry the load.

Step 5: Check Spacing and Edge Distances

Ensure the spacing and edge distances comply with EC5:

• Parallel Spacing (a₁): 7d = 112 mm (OK)
• Perpendicular Spacing (a₂): 4d = 64 mm (OK)
• End Distance (a₃): 10d = 160 mm (OK)

Step 6: Design Resume

• Number of Bolts: 3
• Bolt Diameter: M16
• Parallel Spacing: 112 mm
• Perpendicular Spacing: 64 mm
• End Distance: 160 mm

Also See: Designing a Cross Laminated Timber Floor | Worked Example

Conclusion

Designing timber connections in compliance with Eurocode 5 requires a thorough understanding of material properties, load conditions, and environmental factors. By following the systematic approach outlined in EC5, engineers can ensure the safety and durability of timber structures. The worked example above demonstrates the practical application of these principles, providing a clear guide for designing bolted timber connections. With the right design and careful execution, timber structures can offer exceptional performance and longevity.