Dowels in Timber Connections

A pin and nothing else

A dowel is the simplest fastener in the Eurocode 5 catalogue - a smooth steel cylinder, no head, no nut, no washer, no thread. You drive it into a hole and that is the whole connection. The simplicity is misleading. The dowel is the fastener of choice for the connections that matter most - exposed glulam frames, moment-resisting joints, anywhere stiffness and appearance both count - and getting it right depends on details that the smooth profile hides.

The tight fit that changes everything

The single feature that separates a dowel from a bolt is the hole. A bolt sits in a hole 1 to 2 millimetres larger than its diameter and has to slide into bearing before it carries load. A dowel is driven into a hole drilled to a close tolerance - typically the same nominal diameter as the dowel, give or take a fraction of a millimetre.

That tight fit means there is no slip. Where a bolt slides freely until it reaches the edge of its oversized hole, a dowel is in bearing from the first increment of load. The connection responds immediately and stiffly.

This is the defining mechanical difference between the two fasteners, and almost everything else about dowel design follows from it. The tight fit is also what makes dowels harder to install - you cannot wiggle a dowel into place the way you can a bolt. The hole has to be drilled accurately and the dowel driven home, often with the steel plate and timber clamped in alignment first.

Why no slip matters: dowels and moment connections

Joint slip is the dominant source of deflection in timber moment connections. A small rotation at the joint - caused by hole tolerance, embedment yielding, and fastener bending - produces a large displacement at the far end of the connected member. For a bolted moment connection, the 1 to 2 millimetres of hole clearance alone can add several millimetres of swing at a member's end.

Dowels remove the hole-clearance component of that slip. Because the dowel is in bearing from the start, the joint is stiffer and the connection behaves closer to the rigid model an engineer might assume. This is why moment-resisting connections - portal frame eaves, ridge joints, rigid beam-to-column connections - are almost always detailed with dowels rather than bolts when stiffness governs.

It does not make the connection fully rigid. Embedment yielding and fastener bending still contribute rotation, so a dowelled moment connection is still semi-rigid and should be modelled as such. But it is meaningfully stiffer than the bolted equivalent, and for frames where deflection governs the design, that difference decides the fastener.

Same yield theory, same yield moment

Once bearing is engaged, a dowel carries load exactly as a bolt does - by pressing on the timber, with the timber resisting through embedment. The Johansen yield mechanisms are identical, and so is the characteristic yield moment:

M_{y,Rk} = 0.3 · f_u · d^{2.6}

Dowels are typically smooth bar of mild or higher-grade steel, with f_u in the same 400 to 800 N/mm² range as bolts. The embedment strength and Hankinson angle correction apply identically - both are covered in the Timber Connection Design overview.

EC5 §8.6 treats fasteners between 6 mm and 30 mm diameter as dowels. Below 6 mm the rules shift toward the nail provisions; above 30 mm the dowel provisions no longer apply.

No washer, no rope effect

The price of the dowel's clean profile is that it has no way to anchor axial load. A bolt carries some tension through its washer and nut, and EC5 §8.2.2(2) lets that contribute up to 25% extra lateral capacity through the rope effect. A dowel has no head and no washer, so it cannot develop axial resistance at all. For dowels, the rope-effect contribution is taken as zero.

In practice this means a dowel of a given diameter in a given timber has a lower design capacity than the equivalent bolt, because the bolt gets the rope-effect bonus and the dowel does not. If your connection is capacity-governed and you are close to the limit, a bolt may carry more for the same diameter. If your connection is stiffness-governed - as moment connections usually are - the dowel's lack of slip outweighs its lack of rope effect.

Spacing - tighter than bolts

Eurocode 5 allows dowels to be placed closer together than bolts. The headline minima for solid timber:

• a₁ parallel to grain: (3 + 2|cos α|) d
• a₂ perpendicular to grain: 3 d
• a₃,t loaded end: max(7d, 80 mm)
• a₄,t loaded edge: max((2 + 2 sin α) d, 3d)
• a₄,c unloaded edge: 3 d

Compare the perpendicular spacing - 3d for dowels versus 4d for bolts. The tighter dowel minima are allowed because the close-fit hole displaces fewer fibres and starts fewer splitting cracks than a pre-drilled oversized bolt hole. A tighter, denser fastener group is one of the practical advantages of dowels in a compact moment connection where you need many fasteners in a small area.

The full set is in EC5 §8.6.

The clean-connection advantage

A dowel leaves nothing on the surface. No bolt head, no protruding nut, no washer. For exposed timber - the visible glulam frames that are half the reason architects specify timber in the first place - this matters. A row of bolt heads reads as industrial hardware; a dowelled connection, especially one with the dowel ends concealed by timber plugs, reads as clean structure.

The buried steel also performs better in fire. A bolt head sitting on the timber surface heats quickly and conducts heat inward, weakening the connection from the head. A dowel entirely embedded in timber is insulated by the surrounding wood and the char layer that forms around it. For exposed timber structures with a fire-resistance requirement, concealed dowels - or glued-in rods, which take the idea further - are often the only detail that achieves the rating without added protection.

Installation is where dowels are won or lost

The tight fit that gives dowels their stiffness also makes them unforgiving to install. The hole has to be drilled to a close tolerance, square to the face, and aligned through the timber and any steel plates before the dowel is driven. On a bolted connection a slightly misaligned hole is hidden by the clearance; on a dowelled connection there is no clearance to hide it, and a misaligned dowel either will not drive or splits the timber as it is forced through.

In practice this means dowelled connections are often shop-fabricated or assembled with drilling jigs, particularly where steel plates are slotted into the timber and the dowels driven through pre-aligned holes. The precision is part of the cost - dowels buy stiffness and appearance at the price of fabrication tolerance.

When to choose a dowel over a bolt

The decision usually comes down to what governs the connection.

Choose a dowel when stiffness matters - moment connections, frames where deflection governs, anywhere joint slip would otherwise dominate the serviceability check. Choose a dowel when the connection is exposed and appearance matters, or when fire performance requires the steel to be buried.

Choose a bolt when capacity is tight and you need the rope-effect bonus, when the connection has to be assembled or adjusted on site without precision drilling, or when you need to be able to inspect and re-tighten the connection over the structure's life.

Many real connections use both - dowels for the primary moment transfer and a couple of bolts to hold the assembly together during erection before the dowels are driven.

Pulling it together

A dowel is a bolt without the head, the washer, or the slip. You give up axial capacity and easy installation; you gain stiffness, a clean exposed surface, and better fire performance. For the moment connections at the heart of a timber frame - the joints where slip would otherwise govern - that trade is almost always worth making.