Wood Screw Design Software to Eurocode 5

Self-tapping screws are widely used in modern timber engineering for transferring axial and shear forces between structural elements. These fasteners provide high load-bearing capacity, simple installation, and versatile connection configurations for timber-to-timber and timber-to-steel joints.

The SPEC Toolbox Screw Design module allows engineers to evaluate screw connections in a wide range of timber structures using manufacturer-specific fastener data. The calculator integrates real screw geometries and product libraries from leading suppliers, enabling accurate modelling of screw behaviour and load transfer.

The tool supports different timber materials and connection types commonly used in timber construction.

Supported member materials include:

• Softwood

• Hardwood

• Glulam (GLT)

• Cross-Laminated Timber (CLT)

• Laminated Veneer Lumber (LVL)

• Plywood

• Steel plates for timber-to-steel connections

This flexibility allows engineers to evaluate connections in both traditional timber structures and modern engineered timber systems.

To assist with connection configuration, the calculator provides an interactive 3D visualization of the screw arrangement and connected members.

The model shows:

• screw orientation

• load directions

• member geometry

• fastener placement

Users can rotate and inspect the connection to clearly understand the load transfer mechanism and screw positioning.

This visual feedback helps engineers verify the geometry of the connection before performing structural verification.

The calculator allows engineers to define the structural members involved in the connection.

Two members can be configured:

• Member 1 – primary structural element

• Member 2 – secondary element or connected member

For each member, the following properties can be defined:

• member material type

• member dimensions

• characteristic timber density (ρk)

• mean density (ρmean)

The density values are used to calculate the embedment strength of the timber and therefore influence the resistance of the screw connection.

Additionally, the grain orientation relative to the screw can be defined to correctly represent load transfer behaviour in the timber.

The screw connection may be subjected to axial and shear forces acting in multiple directions.

The calculator allows engineers to define the applied loads acting on the connection, including:

• Fx – shear force in the x-direction

• Fy – shear force in the y-direction

• Fz – axial force acting along the screw axis

These load components represent the design actions acting on the connection according to the selected design standard.

The combined shear force is automatically calculated and used for connection verification.

The applied loads are visualized in the connection model to clearly illustrate their direction and point of application.

The calculator integrates screw data from recognized manufacturers, allowing engineers to select fasteners directly from supplier product libraries.

Users can define:

• screw supplier

• screw family

• specific screw type

Each screw type includes predefined mechanical properties such as:

• diameter

• thread configuration

• strength properties

• withdrawal resistance parameters

This integration ensures that the design calculations are based on realistic fastener properties and certified manufacturer data.

Different screw configurations can be selected depending on the connection requirements.

Available options include:

• Partially threaded screws

• Fully threaded screws

Thread configuration affects the load transfer mechanism and determines whether the screw primarily resists shear forces, axial forces, or combined loading.

The calculator also supports smart screw length selection, helping engineers choose appropriate fastener lengths based on the connection geometry.

Screw connections are evaluated according to Eurocode 5 design provisions for timber connections.

The calculator currently supports three design methodologies:

1. EN 1995-1-1:2004 (Eurocode 5)
   Standard design approach for timber connections using dowel-type fasteners.

2. prEN 1995:2023
   The upcoming revision of Eurocode 5, introducing updated design provisions and calculation models.

3. EN 1995-1-1:2004 (Supplier ETA)
   Design approach based on manufacturer-specific European Technical Assessment (ETA) values for proprietary fasteners.

These design methods allow engineers to evaluate screw performance according to either general Eurocode rules or manufacturer-certified design data.

The geometric arrangement of screws strongly influences the resistance and failure modes of timber connections.

The calculator allows engineers to configure screw placement using two approaches:

Manual Arrangement

Engineers can manually define the screw layout, including:

• screw spacing

• edge distances

• number of screws along different directions

Minimum Distance and Spacing Rules

Alternatively, the calculator can automatically apply Eurocode minimum spacing requirements, ensuring that screw placement satisfies geometric design rules.

Parameters considered include:

• spacing parallel to grain

• spacing perpendicular to grain

• edge distances

• end distances

• fastener positioning relative to member geometry

These geometric checks ensure that brittle timber failure modes such as splitting are avoided.

After defining the screw configuration, timber properties, and applied loads, the calculator performs structural verification of the connection.

The output summary includes the following checks:

Geometry Check

This verification ensures that screw spacing and edge distances satisfy Eurocode geometric requirements for timber connections.

Shear Capacity

The shear resistance of the screw connection is evaluated according to Eurocode dowel-type fastener models.

Axial Capacity

The axial resistance of the screw is evaluated based on withdrawal resistance and fastener properties.

Combined Actions

When axial and shear forces act simultaneously, the calculator verifies the combined loading condition to ensure the connection capacity is not exceeded.

Slip Modulus

The module also provides the slip modulus of the screw connection, representing the stiffness of the fastener under load. This parameter is important for structural modelling and deformation analysis.