Wood Screw Design for NDS

Self-tapping screws are widely used in timber construction in North America for transferring axial and shear forces between structural elements. These fasteners provide high load-bearing capacity, efficient installation, and versatile connection configurations for timber-to-timber and timber-to-steel connections.

The SPEC Toolbox Screw Design module allows engineers to evaluate screw connections using AWC NDS (National Design Specification for Wood Construction) provisions together with manufacturer-specific screw data.

The calculator integrates supplier fastener libraries and structural verification procedures to evaluate the performance of screws used in timber structures.

The tool supports several timber and engineered wood materials commonly used in North American construction.

Supported materials include:

• Softwood

• Hardwood

• Glulam (GLT)

• Cross-Laminated Timber (CLT)

• Laminated Veneer Lumber (LVL)

• Plywood

• Steel plates for timber-to-steel connections

This allows engineers to design screw connections for both traditional wood framing and modern mass-timber structures.

The calculator provides an interactive 3D visualization of the screw connection to help engineers understand the geometry and load transfer behaviour.

The visualization displays:

• member geometry

• screw orientation

• screw placement

• applied forces

Users can rotate and inspect the connection model to verify the configuration before performing structural checks.

This visual feedback helps ensure that the connection setup accurately reflects the intended structural scenario.

The screw connection is defined using two structural members:

• Member 1 – primary structural element

• Member 2 – secondary or connected element

For each member the following properties can be defined:

• member type

• member dimensions

• elastic modulus

• tensile strength

• shear strength

These properties are used in the structural verification of the connection according to AWC NDS provisions.

The calculator also allows engineers to define:

• grain orientation (side grain or end grain)

• connection type (discrete or continuous)

These parameters influence the capacity of the fasteners and the failure modes evaluated in the connection.

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

The calculator allows engineers to define the design loads acting on the connection:

• Fx – shear force in the x-direction

• Fy – shear force in the y-direction

• Fz – axial force along the screw axis

The resulting shear force V is automatically calculated from the defined load components.

These forces represent the design actions acting on the fastener system according to the selected design philosophy.

The applied loads are visualized within the connection model to clearly illustrate the direction and magnitude of the forces.

The Screw Design module integrates product data from recognized fastener manufacturers.

Engineers can define:

• screw supplier

• screw family

• specific screw type

Each screw type includes predefined mechanical properties provided by the manufacturer, such as:

• screw diameter

• thread configuration

• embedment characteristics

• withdrawal resistance parameters

This integration ensures that connection calculations are performed using realistic fastener properties.

Different screw configurations can be selected depending on the structural application.

Available options include:

• Partially threaded screws

• Fully threaded screws

The thread configuration influences how forces are transferred between the connected members and determines whether the fastener primarily resists:

• axial tension

• shear forces

• combined loading conditions

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

Screw connections in the United States are evaluated according to the AWC National Design Specification for Wood Construction (NDS).

The calculator supports the following design methods:

1. AWC NDS 2018

   The standard design procedure defined by the American Wood Council for wood connections. This method evaluates screw resistance using NDS provisions for dowel-type fasteners and wood fasteners.

2. AWC NDS 2018 (Supplier ICC)

   This option allows connection design using manufacturer-verified fastener properties certified through ICC-ES evaluation reports. These values may differ from general code equations and reflect proprietary screw performance.

Design Philosophy

Two design philosophies are available for connection verification:

LRFD — Load and Resistance Factor Design

LRFD applies load factors and resistance factors to ensure structural reliability under ultimate limit states.

ASD — Allowable Stress Design

ASD evaluates connection capacity using allowable stresses and service load combinations.

The chosen design philosophy determines how loads and resistances are combined during structural verification.

The geometric arrangement of screws significantly influences the strength and failure modes of timber connections.

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

Manual Arrangement

Engineers can manually define the fastener layout including:

• number of screws

• row configuration

• edge distances

• end distances

Minimum Distance and Spacing Rules

Alternatively, the calculator can automatically apply minimum spacing and edge distance requirements according to AWC NDS provisions.

These geometric limits help prevent brittle timber failure modes such as splitting or tear-out.

The parameters evaluated include:

• spacing along rows

• spacing between rows

• edge distances

• end distances

• fastener position relative to the member surface

After defining the connection geometry, material properties, and applied loads, the calculator performs structural verification according to AWC NDS provisions.

The output summary includes the following checks.

Brittle Failure

This verification evaluates potential brittle failure modes in the timber member including:

• row tear-out capacity

• net tension capacity

• group tear-out capacity

These checks ensure that the timber surrounding the screws can safely transfer the applied forces without splitting or tearing.

Geometry Check

Verifies that screw spacing, edge distances, and end distances satisfy the geometric requirements defined by the design standard.

Shear Capacity

Evaluates the shear resistance of the screw connection based on timber properties and fastener resistance models.

Axial Capacity

Evaluates the axial resistance of the screw connection 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 that the overall connection capacity is not exceeded.