Bolts vs Dowels: Choosing the Fastener

Same shape, different fastener

On a drawing, a bolt and a dowel can look almost the same - a circle marking a steel pin through the timber. In Eurocode 5 they are treated as genuinely different fasteners, with different spacing rules, different capacity contributions, and different behaviour under load. Choosing the wrong one rarely causes a dramatic failure. It quietly costs you stiffness, capacity, or buildability, and the cost usually shows up late - as a frame that deflects more than the model said, or a connection that will not assemble on site.

This sheet sets the two side by side and gives a straightforward way to decide between them. The mechanism detail for each fastener lives in its own sheet - this one is about the choice.

The one difference that drives the rest

Almost everything that separates a bolt from a dowel comes from a single feature: the hole.

A bolt sits in a hole drilled 1 to 2 millimetres larger than its diameter. It has to slide into bearing before it carries any load - until the load is high enough to push the bolt against the edge of its hole, the joint moves freely.

A dowel is driven into a hole drilled to a close tolerance, near enough the same diameter as the dowel itself. There is no clearance to take up, so the dowel is in bearing from the first increment of load.

That is the whole story in miniature. The bolt's clearance buys easy installation and an anchored head; it costs stiffness. The dowel's tight fit buys stiffness and a clean profile; it costs installation tolerance and the head. Every entry in the comparison below traces back to this one difference.

Where the bolt wins

A bolt has a head, a nut, and a washer, and that hardware does real work.

The most useful consequence is the rope effect. Because the washer anchors the bolt against the timber surface, the bolt can carry some axial load as it bends under lateral force, and EC5 lets that contribute up to 25% extra lateral capacity. EC5 §8.2.2(2) caps it at 25% for bolts and at zero for dowels. For a capacity-governed connection sitting close to its limit, that 25% can be the difference between a design that works and one that needs a larger fastener.

The clearance hole also makes bolts forgiving. A slightly misaligned hole disappears into the clearance; the connection still assembles. Bolts can be fitted, adjusted, and inspected on site without a drilling jig. And because a bolt can be re-tightened, a bolted connection can be revisited over the structure's life - which matters, because timber shrinks as it dries and bolts loosen with it.

So the bolt wins where buildability, adjustability, inspectability, and raw capacity-per-diameter govern.

Where the dowel wins

The dowel's tight fit removes the slip that dominates timber moment connections.

Joint slip is the main reason a timber moment connection deflects more than a rigid model predicts, and the largest single component of that slip is the bolt's hole clearance. A dowel removes it. The connection is in bearing immediately, behaves more stiffly, and sits closer to the rigid assumption an engineer is tempted to make. It is still semi-rigid - embedment yield and fastener bending still add rotation - but it is meaningfully stiffer than the bolted equivalent. For frames where deflection governs, that decides the fastener.

The tight fit also lets dowels sit closer together. EC5 permits tighter spacing for dowels than for bolts - 3d perpendicular to grain against 4d for bolts, for instance - because the close-fit hole displaces fewer fibres and starts fewer splitting cracks than a pre-drilled oversized bolt hole. In a compact moment connection where you need many fasteners in a small area, that density is a practical advantage.

Finally, the dowel leaves nothing on the surface. No head, no nut, no washer. For exposed glulam frames - half the reason architects specify timber - that clean profile matters, and the buried steel performs better in fire because the surrounding timber and char layer insulate it rather than letting an exposed head heat up and conduct inward.

So the dowel wins where stiffness, fastener density, appearance, and fire performance govern.

The question that usually settles it

When the choice is genuinely open, one question resolves most of it: is the joint carrying moment, or just shear?

A shear connection has a well-defined load path and rarely lives or dies on stiffness. Buildability and capacity tend to govern, and that points to bolts. A moment connection distributes force radially around a fastener group, and its serviceability is usually governed by rotation - which points to dowels, because slip is what you are trying to eliminate.

That is a heuristic, not a rule. A heavily loaded shear connection that is capacity-tight still benefits from the bolt's rope effect; an exposed shear connection on a visible frame might still take dowels for appearance. But as a first cut, "moment leans dowel, shear leans bolt" gets you to the right answer most of the time.

Both fasteners share the same yield theory once bearing is engaged, including the same characteristic yield moment:

M_{y,Rk} = 0.3 · f_u · d^{2.6}

The Johansen mechanisms, the embedment strength, and the Hankinson angle correction are identical for both - the differences are entirely in the hole fit, the rope effect, and the spacing rules.

Using both

The choice is not always either-or. Many real connections use both fasteners together: dowels for the primary moment transfer, where their stiffness is doing the structural work, and a couple of bolts to hold the assembly in alignment during erection before the dowels are driven. The bolts handle the buildability problem - getting the steel plate and timber clamped square so the dowel holes line up - and the dowels handle the load. It is a common detail on shop-fabricated moment connections, and it sidesteps the dowel's main weakness without giving up its stiffness.

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. The bolt is the more forgiving fastener and the higher-capacity one for a given diameter; the dowel is the stiffer, cleaner, fire-safer one. When the connection carries moment and deflection governs, take the dowel. When it carries shear and buildability governs, take the bolt. When you need both qualities, use both.