Introduction: Brittle vs. Ductile Failure in Structures
In engineering, brittle failure refers to a sudden fracture of a material or component with little or no prior deformation or warning. This lack of warning makes brittle failures especially dangerous. By contrast, a ductile failure typically involves yielding and visible distortion, allowing for warning. Engineers generally prefer ductile behavior so that if a component overloads, it will deform gradually rather than shatter catastrophically, providing a margin of safety.
In structural engineering design, avoiding brittle failure is crucial for safety. Design codes explicitly require that critical elements fail in a ductile manner. If brittle failure modes are likely, the connection must be carefully detailed to prevent them through proper spacing, reinforcement, and material selection. In short, understanding and preventing brittle failures is key to resilient, safe structures.
Brittle Failure Modes in Timber Connections
Reference: prEN 1995-1-1:2023
Timber connections often use dowel-type fasteners (bolts, nails, screws, rods) to hold members together. These connections may fail in two broad ways: a ductile mode where the steel fasteners yield or the wood embeds around them, or a brittle mode where the wood itself fractures (splits or shears) suddenly. Insufficient spacing and edge distances, the grain direction of timber, larger fastener groups, and loading conditions, for instance, are some of the factors that lead to brittle failure.
Figure 1: Brittle and ductile failure modes in timber connections, respectively
The new draft code emphasizes identifying several brittle failure mechanisms for connections with multiple fasteners. The ductility of a connection can be achieved through yielding of fasteners, but when many fasteners act together, the wood can break before the fasteners yield. Brittle failure checks may be neglected if yield mode (f) in equation 11.15 of this code (in which the fasteners fail in double plastic and embedment failure in timber member) governs, and if the minimum spacing requirements are met. Clause 11.5.1(1) of prEN 1995-1-1:2023 specifies that if multiple dowel-type fasteners are used in laterally loaded steel-to-timber and timber-to-timber connections, designers must consider brittle failure modes caused by row shear, block shear, plug shear, net tensile failure, and splitting.
Block shear, row shear, plug shear, and net tensile failures typically occur when timber members are subjected to loading parallel to the grain. In contrast, splitting occurs when timber members are loaded perpendicular to the grain. For loading at an angle to the grain, splitting caused by the tensile force component perpendicular to the grain should be taken into account.
For timber members with a single row of fasteners, the design brittle failure capacity should be the minimum resistance of splitting, row shear, and net tensile failure modes. But for multiple fastener connections, the minimum of all five failure modes should be considered.
Row Shear Capacity
Row shear failure is caused when a row of fasteners tears out a thin strip of wood in shear along the grain.
Figure 2: Row shear failure mode
In this case, the wood between that row of fasteners and the loaded end shears off. The row shear capacity can be determined by using the formula provided by the code.
n90 is the number of fasteners in a row perpendicular to the grain
Fv,la,d is the design shear resistance per side shear plane in the timber member
Block Shear Capacity
A block shear failure occurs where fasteners penetrate through the member, causing the block of wood containing the fasteners to punch out from the member.
Figure 3: Block shear failure mode
The code provides a check for block shear for fully penetrated connections, essentially comparing the capacity of the two shear planes versus the end tension plane.
Where,
Fbs,Rd is the design block shear capacity of a timber member
Fv,la,d is the design shear resistance per side shear plane in the timber member
Ft,d is the design tensile resistance parallel to the grain of the head tensile plane
Figure 4: Block shear failure surfaces and loads acting on it
For reinforced connections, the formula may be replaced by
Plug Shear Capacity
Plug shear is a failure mode similar to block shear, but it occurs in partially penetrating fastener connections—such as nails or screws that do not pass through the entire thickness of the wood. In this case, the failing wood chunk looks like a “plug” being pried out from the face of the member.
Figure 5: Plug shear failure mode
The design plug shear capacity of a timber member should be taken as:
Where,
Fv,la,d is the design shear resistance per side shear plane in the timber member
Ft,d is the design tensile resistance parallel to the grain of the head tensile plane
Fv,b,d is the design shear plane resistance of the bottom shear plane in the timber member
Net Tensile Failure
This mode is essentially a net-section break of the timber member due to the reduced cross-sectional area from fastener holes or notches.
Figure 7: Net Tensile Failure mode
The code advises checking the net tension capacity of the member, considering the reduction in cross-section due to the pre-drilled holes for the fasteners and possible slots for metal plates, using wood’s tension strength parallel to the grain. Refer to prEN 1995-1-1:2023, Clause 11.5.8, for more information.
Splitting Capacity
Splitting is caused when the timber member cracks along the grain due to tension perpendicular to the grain. The code states that when a force in a connection acts at an angle to the grain, splitting caused by the tensile force or force component, perpendicular to the grain, shall be taken into account.
Figure 8: Splitting of timber members
The design splitting capacity of one connection should be taken as:
Design Brittle Failure Capacity of the Connection
Clause 11.5.3 of prEN 1995-1-1:2023 states that the design brittle failure capacity of a connection consisting of several timber members should be obtained as the sum of the capacities of the timber members transferring the load.
Conclusion
Brittle failures in timber connections tend to occur when the connection geometry or load is applied in the wood’s weakest orientation. It is a critical limit state that must be addressed to ensure the safety and serviceability of structures. In summary, engineers should ensure adequate spacing, edge, and end distances for fasteners; limit the number of fasteners in a row to reduce risks of splitting or row shear; use the effective number of fasteners to account for reduced group action; explicitly check all brittle failure modes (splitting, row shear, block/plug shear, net tension) and design to the lowest capacity. Additionally, tension perpendicular to the grain should be avoided wherever possible.
