Timber Connection Design: Steel Plates, Dowels & Modern Fasteners

The connection is where timber engineering becomes most demanding. While timber members can span impressive distances with elegant efficiency, the joints between those members concentrate forces into small areas of a material with limited bearing and shear capacity. Successful timber connection design requires understanding both the material's anisotropic behaviour and the mechanics of load transfer through fasteners.

This guide covers the full spectrum of structural timber connection types, from simple nailed joints to sophisticated moment-resisting systems, with practical guidance on selection, design, and fire performance.

Fundamentals of Timber Connection Behaviour

Load Transfer Mechanisms

All timber connections transfer load through one or more of these mechanisms:

1. Bearing — direct compression of timber against timber or steel
2. Embedment — a fastener pressing into the timber fibres
3. Withdrawal — a fastener being pulled along its axis from the timber
4. Adhesion — chemical bond between timber and adhesive/resin

The European Yield Model (EYM), codified in EN 1995-1-1 (Eurocode 5), predicts connection capacity based on the interaction between fastener bending and timber embedment. The model identifies multiple failure modes — from fastener yielding to timber splitting — and takes the lowest capacity as the design value.

Timber Properties Affecting Connections

For C24 structural construction timber with characteristic density ρk = 350 kg/m³, the embedment strength parallel to grain is approximately:

fh,0,k = 0.082 × (1 − 0.01d) × ρk

Where d is the fastener diameter in mm. For a 12 mm dowel in C24 timber: fh,0,k = 0.082 × (1 − 0.12) × 350 = 25.2 N/mm²

Dowel-Type Fastener Connections

Nails

Nails are the simplest and most common timber fastener, used primarily for:

• Stud-to-plate connections in timber frame walls
• Joist hangers and restraint straps
• Sheathing attachment (OSB, plywood)
• Secondary structural connections

Design capacity (single shear, EN 1995-1-1):

Key design rules for nails:

• Minimum penetration into receiving member: 8d (smooth shank) or 6d (ring shank)
• Pre-drilling required for timber density >500 kg/m³
• Staggered pattern to avoid splitting along grain
• No load-bearing capacity in withdrawal for smooth shank nails

Screws

Modern self-tapping screws (STS) have transformed timber connection design. Unlike traditional coach screws, STS feature:

• Continuous thread along full shank length
• Self-drilling tip eliminating pre-drilling
• High withdrawal capacity (axial loading)
• Ability to be installed at angles (inclined screw connections)

Typical STS capacities (single shear, C24 timber):

Inclined screw connections exploit the high withdrawal capacity of STS by installing screws at 30–45° to the shear plane. This creates a combined shear-tension load path that can achieve 2–3× the capacity of screws loaded purely in shear.

Bolts

Bolts remain the standard for heavy timber connections, particularly where:

• High single-fastener capacity is required
• Connections must be demountable
• Visual inspection of tightening is needed
• Large-section BSH glued laminated beams are being connected

Bolt capacities (double shear, steel-timber-steel, C24):

Design considerations for bolted connections:

• Holes drilled 1 mm oversize (bolt diameter + 1 mm)
• Washers required under head and nut (minimum 3d diameter, 0.3d thickness)
• Tightening to snug-tight (not high-strength bolt torque)
• Check for block shear and row shear failure modes
• Minimum end distance: 7d (loaded end), 3d (unloaded end)

Dowels

Steel dowels are smooth cylindrical pins driven into pre-drilled holes with zero tolerance (hole diameter = dowel diameter). They offer:

• Higher capacity than bolts of the same diameter (no hole oversize)
• Cleaner aesthetic (no protruding heads or nuts)
• Better fire performance (no exposed steel)
• Suitability for concealed connections

Dowels are the preferred fastener for high-quality exposed timber structures and are standard in glulam portal frames and multi-storey timber buildings.

Steel Plate Connections

External Steel Plates

External (exposed) steel plates are bolted or screwed to the outside faces of timber members. They are:

• Simple to fabricate and install
• Easy to inspect
• Suitable for site connections
• Visible (may require architectural treatment)

Typical configurations:

• Single-sided plate (asymmetric — introduces eccentricity)
• Double-sided plates (symmetric — preferred for primary connections)
• Gusset plates at truss nodes
• Bracket connections for beam-to-column joints

Internal (Slotted) Steel Plates

Internal plates are concealed within a slot cut into the timber member. The slot is typically cut by CNC router to tight tolerances (plate thickness + 1 mm). Dowels pass through the timber and plate simultaneously.

Advantages of slotted plate connections:

• Concealed metalwork — clean timber aesthetic
• Inherent fire protection — timber covers the steel
• Symmetric load path — no eccentricity
• High capacity — multiple dowels in double shear

Design parameters for slotted plate connections:

Knife Plates

Knife plates are thin steel plates (typically 5–6 mm) inserted into a saw cut in the timber. They are used for:

• Light truss connections
• Bracing connections
• Secondary member attachments

The saw cut is made on site or in the factory, and the plate is secured with nails or small-diameter bolts on each side.

Moment Connections

Moment-resisting connections in timber are more complex than in steel construction because timber's limited embedment strength means large numbers of fasteners are needed to develop significant moment capacity.

Dowel Group Moment Connections

A circular or rectangular pattern of dowels through a slotted steel plate can resist moment through the couple created by dowels in tension (withdrawal or bearing) and compression (bearing):

Design approach:

1. Calculate the moment demand at the connection
2. Determine the lever arm between tension and compression dowel groups
3. Calculate the force per dowel: F = M / (n × z) where n = number of dowels in one group, z = lever arm
4. Verify each dowel has sufficient capacity for the combined shear and axial force
5. Check timber for block shear, row shear, and splitting

Glued-In Rod Moment Connections

Glued-in rods are the most efficient method for creating moment connections in timber. A cluster of threaded rods bonded into the end grain of a beam can develop moment capacities comparable to the beam's bending strength.

Design parameters:

Capacity of glued-in rods (axial, C24 timber):

Quality control for glued-in rods:

• Holes drilled by CNC to ensure alignment and depth
• Rods degreased and cleaned before bonding
• Adhesive injected from bottom of hole to avoid air pockets
• Cure time minimum 24 hours at 20°C before loading
• Pull-out testing of production samples (minimum 3 per batch)

Fire Design of Connections

The Critical Weakness

Connections are often the weakest point in a timber structure during fire. While timber chars at a predictable rate (0.65 mm/min for softwood), steel components within connections lose strength rapidly:

• At 300°C: steel retains ~100% of ambient strength
• At 400°C: steel retains ~90% of ambient strength
• At 500°C: steel retains ~78% of ambient strength
• At 600°C: steel retains ~47% of ambient strength
• At 700°C: steel retains ~23% of ambient strength

Protection Strategies

Strategy 1: Concealment (preferred)

Recess all steel components below the timber surface and plug with timber:

• Minimum timber cover: charring rate × required fire resistance + 7 mm (zero-strength layer)
• For 60 minutes fire resistance: 0.65 × 60 + 7 = 46 mm timber cover
• For 90 minutes: 0.65 × 90 + 7 = 65.5 mm → 66 mm timber cover

Strategy 2: Oversizing

Increase the steel plate thickness and fastener diameter to account for strength loss at elevated temperature. This approach is less elegant but simpler for site connections.

Strategy 3: Intumescent protection

Apply intumescent coating to exposed steel plates. The coating expands when heated, forming an insulating char layer. Requires:

• Minimum dry film thickness (DFT) per manufacturer's fire test data
• Regular inspection and maintenance
• Not suitable for concealed locations

Strategy 4: Timber cladding

Attach sacrificial timber boards over exposed steel connections. The cladding chars during fire, protecting the steel beneath. Minimum cladding thickness calculated as for Strategy 1.

Connection Fire Design Table

Modern Fastener Systems

Proprietary Connector Systems

Several manufacturers offer engineered connection systems that simplify design and installation:

Concealed beam hangers: CNC-routed pockets in the beam end receive a steel shoe that transfers load in bearing. No visible metalwork. Typical capacity: 50–200 kN.

Moment connector plates: Pre-engineered steel plates with pre-determined dowel patterns, supplied with design tables. Reduces engineering time for standard connections.

Column base connectors: Steel shoes that connect timber columns to concrete foundations while maintaining a moisture break. Available for columns up to 500 × 500 mm.

Innovative Connection Technologies

Cross-laminated dowels: Hardwood dowels inserted perpendicular to the connection shear plane, creating an all-timber connection with no steel. Used in sustainability-focused projects.

Friction-based connections: Large-diameter bolts with controlled pre-tension create friction connections that resist load without embedment. Inspired by steel construction practice.

Shape memory alloy (SMA) connections: Research-stage technology using nickel-titanium alloy fasteners that can self-centre after seismic loading, returning the structure to its original geometry.

Practical Design Workflow

Step 1: Define Connection Forces

Extract forces from the structural model:

• Axial force (tension or compression)
• Shear force (vertical and horizontal)
• Moment (if applicable)
• Load duration class (permanent, medium-term, short-term, instantaneous)

Step 2: Select Connection Type

Step 3: Preliminary Sizing

Use capacity tables to estimate the number and size of fasteners required. Allow for:

• Load duration factor (kmod): 0.6 (permanent) to 1.1 (instantaneous)
• Partial safety factor (γM): 1.3 for connections
• Service class factor: Class 1 (dry), Class 2 (moderate), Class 3 (wet)

Step 4: Check Geometry

Verify that the required fastener pattern fits within the available timber cross-section:

• End distances and edge distances satisfied
• Spacing requirements met
• No clash with other connections or services
• Sufficient timber remaining after slots/holes for structural adequacy

Step 5: Detail for Fabrication

Produce connection details showing:

• All dimensions and tolerances
• Fastener type, size, grade, and quantity
• Steel plate dimensions, thickness, grade, and finish
• Slot dimensions (for concealed plates)
• Assembly sequence
• Fire protection requirements

Step 6: Specify Quality Control

Define inspection and testing requirements:

• Torque values for bolts
• Pull-out test requirements for glued-in rods
• Visual inspection criteria
• Documentation requirements

Connection Detailing Best Practice

Avoiding Splitting

Timber splitting is the most common connection failure mode. Prevent it by:

• Maintaining minimum end and edge distances
• Pre-drilling for all fasteners in hardwood and dense softwood
• Staggering fasteners to avoid loading the same grain line
• Using self-tapping screws as reinforcement perpendicular to grain
• Specifying reinforcement screws at notches and holes

Allowing for Movement

Timber shrinks and swells with moisture content changes. Connections must accommodate this:

• Slotted holes for bolts in the perpendicular-to-grain direction
• Flexible connections between timber and rigid materials (steel, concrete)
• Separation between timber and moisture sources (DPC at column bases)
• Allowance for 1% shrinkage across the grain for green timber, 0.3% for kiln-dried

Durability

Protect connections from moisture and corrosion:

• Stainless steel fasteners for Use Class 3 and above
• Hot-dip galvanised steel plates for external connections (minimum 85 μm coating)
• Drainage paths to prevent water pooling at connection points
• DPM or DPC between timber and concrete/masonry

Related Resources

• Structural Timber Guide — comprehensive reference for timber construction
• C24 Structural Construction Timber — standard structural timber specifications
• BSH Glued Laminated Beams — engineered timber for large-span connections
• Timber Fire Resistance — fire design principles for timber structures