Mass timber buildings incorporate a diverse array of fastening systems, including screws, nails, bolts, and staples. Screws are gaining favor due to their significant engineering and architectural benefits.
With the availability of fully threaded and longer screws, these fasteners are becoming increasingly popular in timber construction. The primary advantage of screws is their exceptional holding power, stemming from their threaded design. These threads enhance the contact surface area with the wood, improving overall grip. This engagement allows screws to more effectively resist both axial forces and lateral loads, making them a highly dependable option in timber construction.
Moreover, screws reduce the risk of timber splitting and can be installed at various angles to the grain. Where reassembling might take place, their removal causes minimal damage to the timber. Screws also demand less tooling and preparation for installation, adding to their practicality. Architecturally, when there is a need to conceal connections, screws offer an advantage as they can be embedded within the timber, maintaining aesthetic integrity.
Figure 1 Typical timber to timber floor connection using screws
Introduction to the Approaches of Screw Design
The European Yield Model
In Eurocode, the design calculations for laterally loaded screw are based on Johanson’s formulas. For screws that are laterally loaded, Eurocode incorporates Johanson’s formulas, accounting the rope effect. Commonly referred to as the European Yield Model equations, these formulas address six distinct failure modes pertinent to screws in shear connecting two timber elements. These modes encompass embedment failure of the timber members, yielding failure of the fastener, and a blend of both.
Figure 2 EYM diagrams for timber-to-timber connections
Figure 3 Approaches to screw connections design
In situations involving steel-to-timber connections subjected to lateral loads, up to five potential failure modes are identified. These include two modes specific to thin steel plates and three for thicker plates. Echoing the timber-to-timber connection scenarios, the failure modes range from embedment failure in the timber, yielding failure of the fastener, or a combination of both.”
In the Eurocode, the design approach for screws under lateral loads varies based on their diameter. Screws with a diameter less than 6mm are treated as nails. Screws with a diameter greater than 6mm are designed as bolts.
Figure 4 EYM diagrams for steel-to-timber connections
Lateral Loads at an Angle to the Grain
For loads at an angle to the grain direction, the capacity of the timber connections must be checked with the applied lateral load. Additionally, due to the load angle, there will be a component of the load perpendicular to the grain direction and that must be checked against the splitting capacity of the timber.
Figure 5 load at an angle to the grain
In AS1720 screw design the process involves referring to a characteristic capacity table. The design capacity is then found by multiplying the characteristic capacity by different factors such as the grain orientation factor, head fixity factor and shear plane factor.
For coach screws, there are distinct characteristic capacity tables for loads parallel and perpendicular to the grain. The capacity for loads at an angle is determined by interpolating between these two tables.
Axially Loaded Screws
For axially loaded screws, the Eurocode stipulates evaluating three key capacities: the axial withdrawal capacity of the screw, the head pull-through capacity, and the tensile capacity. The design of axially loaded screws is governed by the minimum of these three values.
CLT Toolbox’s screw design calculator under Eurocode can accommodate both axially and laterally loaded screws. Notably, screws can be installed at angles deviating from 90 degrees relative to the grain direction. However, in such scenarios, Johanson’s yield line method becomes inapplicable.
Screws inherently possess withdrawal resistance along their axis. When inclined screws face lateral loads, their axial withdrawal resistance contributes to countering these loads. In these cases, the European Yield Model (EYM) equations are not used.
The Australian code takes a similar approach for axially loaded screws and laterally loaded screws. The characteristic capacity of axially loaded screws is directly read from tables and their capacity is modified based on factors to account for the grain orientation.
End Grain
Regarding the use of screws in end grain, general recommendations advise against fixing screws in end grain. However, when it comes to specifics, the Eurocode and Australian code differ in their approaches. In the Eurocode, restrictions are primarily based on the angle of fixing. This implies that there are certain angles at which screwing into end grain is more acceptable or less risky. On the other hand, the Australian code introduces a factor of 0.6 for screws used in end grain. This factor essentially modifies the calculated capacities of the screws, reflecting the reduced effectiveness and reliability of screw connections in end grain scenarios.
The CLT toolbox’s screw design calculator is an advanced tool specifically tailored for optimizing screw connections in mass timber buildings. It facilitates the design of both axially and laterally loaded screws. The inclusion of inclined screws, a significant aspect of timber design, enhances the versatility of the calculator. Users have the unique option to input the screw inclination angle, both along the grain direction and perpendicular to the grain direction, accommodating various design scenarios in mass timber structures. Loads applied at an angle to the grain can also be input and our calculator can calculate the capacity of the connection.
The calculator incorporates the two design codes: AS1720 (Australian Standards) and Eurocode. AS1720 employs tables to account the characteristic capacities of connectors. For Australian users who prefer the European yield model (EYM) approach, the calculator offers the flexibility to incorporate EYM calculations, thus aligning with diverse design preferences and standards.
Moreover, the CLT toolbox’s screw design calculator supports the use of combined screws. This feature is particularly beneficial for designs involving screws with varying angles of inclination or different properties, allowing for a high degree of customization in screw selection and placement. Users can input these variances directly into the calculator, ensuring that all aspects of the screw design are precisely tailored to the specific requirements of the mass timber building.
Conclusion
Connections design is one of the main challenges of timber structures. Out of the many possible ways to connect members of mass timber buildings, usage of screws with varying thread length are gaining a wide range of acceptance due to their availability, ease of use, aesthetic value, and high strength. Solving screw connections design would therefore be a great help to the mass timber industry.
The CLT toolbox’s screw design calculator is a comprehensive and user-friendly tool for professionals in timber design. It combines functionality with accuracy, adhering to international standards and accommodating a wide range of design scenarios in the evolving field of mass timber construction. Our users can design using both the Australian code and Eurocode for axially and laterally loaded screws and for screws that are either partially threaded or fully threaded.
