Direct bearing is one of the most fundamental load-transfer mechanisms in timber structures. Loads can be transferred through a simple and readily understood load path and often with little additional connection hardware. Bearing can also develop significant local deformation and redistribution without an immediate loss of load-carrying capacity.

Bearing-based connections are therefore an often-overlooked “secret weapon” in the mass timber design arsenal. They can fully exploit the precision prefabrication capabilities of timber’s digital CNC-based manufacturing processes to deliver connections that are highly cost-effective, fast to install and structurally robust.

What makes timber bearing particularly interesting is that compression perpendicular to grain behaves quite differently from conventional strength considerations for structural materials such as concrete and steel. Rather than reaching a clearly defined resistance followed by loss of capacity, timber loaded perpendicular to grain can progressively compress and deform while continuing to sustain increasing load. Following initial yielding, the response can exhibit significant strain hardening, with resistance continuing to increase locally as the timber undergoes further compression.

This means that neither the end of the initial elastic response nor the onset of local crushing necessarily defines the design limit for bearing. Unlike many conventional ULS checks, there is no single physical point that universally represents failure. The appropriate resistance threshold is inherently linked to the deformation that can be tolerated by the particular structural system, bringing considerations normally associated with serviceability into determining bearing capacity.

This creates an unusual problem for structural design: if bearing resistance cannot be defined by a single inherent failure threshold, how should the engineer determine the level of resistance that can appropriately be utilised in a particular structural application?

The previous Eurocode 5 approach

The compression-perpendicular-to-grain material strength used by Eurocode 5, f_c,90,k, is itself determined from a deformation-based test criterion rather than from rupture or a subsequent loss of load-carrying capacity. Under EN 408, the strength is established from the load-deformation response using a defined offset criterion. The resulting value therefore represents a standardised point within the nonlinear compression response of the timber, rather than the occurrence of a structurally significant material failure.

The previous Eurocode 5 then used this material property within a prescribed framework for determining the resistance of an actual bearing condition. Considerations of the effective bearing length were used to account for the influence of bearing geometry, load spreading and the participation of timber adjacent to the nominal bearing area. Importantly though, reaching the resistance determined by the previous Eurocode 5 did not necessarily mean that a further increment of applied load would result in a material structural failure. In many bearing conditions, the timber could continue to deform, redistribute stress and develop additional resistance.

While this additional resistance existed physically, the previous Eurocode 5 did not generally provide a basis for utilising it in design. This is significant because, following initial yielding, relatively small additional bearing deformations can correspond to material increases in resistance. The allowable design capacity was therefore capped, meaning that in some applications the design could not access useful additional capacity that would result in only minor additional deformation, well within the required performance envelope of the particular connection or structural system.

The revised Eurocode 5 addresses this differently. The underlying material behaviour and reference compression-perpendicular-to-grain strength remain the starting point, but the resistance that can be utilised is now more directly related to the level of deformation that can be accepted by the particular structural system.

The revised Eurocode 5 approach

Where the previous Eurocode 5 took a relatively conventional approach to assessing the capacity of a bearing connection (determine the applied actions, determine the element resistance using the relevant material properties and modification factors, and compare the two), the revised Eurocode 5 departs from this sequence in a novel way, and one that is relatively unusual within structural design codes more generally.

The revised Eurocode 5 addresses this by more directly relating the bearing resistance that can be utilised to the deformation that can be accepted by the particular structural system. The engineer therefore considers the consequences of bearing deformation in establishing the appropriate design resistance.

• Where a structure can tolerate greater local bearing deformation without compromising its required performance, more of the timber’s bearing capacity can be utilised.

• Conversely, where relatively small bearing movements would adversely affect the structure, the acceptable resistance needs to correspond to a lower level of deformation.

The significance of the revised approach is that these differences can now be explicitly recognised in design, providing engineers with a rational basis for utilising bearing capacity that was not accessible under the previous Eurocode 5 framework. Also, this approach brings more engineering logic into the determination of bearing resistance. Rather than treating the codified resistance as though it represents a physical failure point, the engineer is required to consider what deformation is acceptable in the context of the particular structure and then establish usable bearing capacity accordingly.

In broad terms, the revised Eurocode 5 check follows a different sequence:

1. Determine the reference compression-perpendicular-to-grain properties of the timber.

2. Establish the deformation criterion appropriate to the bearing condition. This requires consideration of how much local bearing deformation can be accepted without compromising the required performance of the connection or the wider structural system.

3. Determine the bearing resistance corresponding to that deformation, accounting for both the material response and the geometry of the bearing arrangement.

4. Verify the design action against the resulting resistance, together with the applicable deformation requirements.

The third step is particularly important to understanding the revised method. The revised EC5 relates the resistance that can be utilised to the level of bearing deformation considered acceptable for the particular application. Through the material response factor, k_mat, greater permissible deformation can allow more of the timber’s continuing post-yield bearing capacity to be utilised in designs. The novel “twist” EC5 introduces is that the adopted deformation criterion is not simply checked after the resistance calculation as a separate serviceability consideration; serviceability considerations themselves form part of determining the timber’s capacity that is available for design purposes.

Making better use of bearing in timber design

The practical benefit of the revised approach is that it allows the design resistance to make better use of a characteristic that timber bearing already possesses: its ability to continue developing resistance through local compression and deformation beyond the initial reference strength.

The benefit is therefore not simply “more bearing capacity.” It is that the revised framework gives engineers a more rational basis for leveraging bearing as a structural mechanism, allowing the connection design to take advantage of timber’s actual load-deformation behaviour within parameters set by the wider structural system’s requirements.

The CEN/TC 250/SC 5 team who manage Eurocode 5’s development should be commended. This work represents a sophisticated approach to a complex design consideration, and achieves a result that can deliver genuine benefits to the sector by supporting the broader deployment of connection solutions that are simpler to detail and fabricate, more economical in their use of hardware, and potentially more robust through their ability to accommodate local deformation and redistribute load.

A greater role for structural logic

The revised treatment of bearing also illustrates a broader and important direction that emerged in the new Eurocode 5. The code increasingly places the engineer’s understanding of the structural mechanism at the foundation of the assessment, rather than relying heavily upon prescriptive resistance checks.

This is evident elsewhere in the treatment of timber connections. The new EC5 specifically notes that where different fasteners or load-transfer mechanisms contribute to resisting the same action, the engineer must consider whether their stiffness and deformation behaviour are compatible and, consequently, how load can realistically be distributed between them. The calculation therefore begins with the engineer establishing how the connection is expected to behave before they pull out their calculator.

Overall, this represents a mature development in structural design standards. Timber connections are often governed by interactions between material behaviour, geometry, stiffness, deformation and multiple load-transfer mechanisms that cannot always be adequately represented by a single prescriptive capacity check. Explicitly requiring these mechanics to inform the assessment allows the design method to better reflect the behaviour of the structure being designed.

The ability to conceive the fundamental structural logic of a connection has always been an inherent part of engineering design; however, many of today’s analysis approaches may mean that this skill is not as consistently developed or exercised across the sector.

The new Eurocode 5’s bearing capacity assessment is therefore not so much about giving engineers greater discretion to select favourable design parameters. Rather, it restores an engineer’s conceptual skills to the place where they belong: at the heart of any structural design.

So as a final note to any engineers reading this: you may now need to exercise some brain muscles that have not had a solid workout since your university days, as so many of today’s design approaches are quite prescriptively “guard railed”. But at least it makes it less likely that AI will take your job any time soon.