Understanding the Challenge
CLT consists of multiple layers of timber stacked in alternating directions. The grains run parallel to their length, providing excellent structural integrity against forces parallel to the grain. However, when it comes to compression perpendicular to the grain, careful consideration must be given to the location of the applied load or support. This calculator accounts for various load/support scenarios, including center, edge (parallel and perpendicular to the grain), corner, and continuous options.
Compression Perpendicular to grain in CLT
Compression perpendicular to the grain refers to a compressive force applied perpendicular to the grain direction in cross-laminated timber. In CLT panels, this type of loading commonly occurs at:
- Point loads from columns or any concentrated loads
- Bearing supports (e.g. walls or beams under floor panels)
- Connection zones where concentrated stresses occur
Understanding how this compression behaves in CLT is necessary because CLTs are significantly weaker in this direction, making the bearing area and location critical for safe design.
Why Location of Load or Support Matters
The location and size of the applied load or bearing area on a CLT panel significantly influences the distribution of compressive stresses and the panel’s effective bearing strength perpendicular to the grain. Different support conditions such as loading at the center, edge, or corner lead to varying stress distribution across the panel surface and through its layers.
To account for these effects:
- The draft Eurocode 5 (EN 1995-1-1:2023) introduces a modification factor, kc,90,CLT, which adjusts the design strength based on the contact area and location of the load or support. It reflects the interaction between stress distribution and CLT layup configuration. The characteristic compressive strength perpendicular to grain, “fc,90,k” is used for checking the stress for such cases.
- The current Eurocode 5 (EN 1995-1-1:2004) provides a more general kc,90 factor for compression perpendicular to the grain, but does not explicitly differentiate between CLT-specific conditions like panel layup or localized support. Similarly, the characteristic compressive strength perpendicular to grain, “fc,90,k” is used for checking the stress for such cases.
- AS 1720.1 (Australian Standard for Timber Structures) also addresses bearing strength perpendicular to the grain, using empirical equations that consider bearing length and member geometry, but it lacks specific provisions for CLT, requiring engineering judgment or supplementary guidance. The characteristic value in bearing perpendicular to grain, “f’p” is used for checking the design capacity for such cases.
- proHolz offers detailed design guidance tailored to CLT, including recommended reduction factors for different support locations (e.g., center, edge, corner) and emphasizes the importance of accounting for rolling shear effects and layer stiffness variation in design.
This calculator integrates these considerations to provide accurate assessment of compression perpendicular to grain strength for CLT panels under realistic support conditions.
Here are the typical cases in CLT:
1. Center of the Panel
Load applied within the panel boundaries, away from all edges and supported in a similar manner.
2. Edge – Parallel to Grain
Load located near an edge where the grain runs parallel to that edge. and supported in a similar manner.
3. Edge – Perpendicular to Grain
Load positioned near an edge perpendicular to the grain direction. and supported in a similar manner.
4. Corner of the Panel
Load applied at the intersection of two edges (e.g., at a corner column or post support).
5. Continuous Load/Support
Load is applied continuously along a line (e.g., wall, beam) of the CLT panel. The Edge Parallel and perpendicular to Grain options are also available for the continuous case.
How CLT Toolbox Simplifies Timber Connection Design
CLT Toolbox offers a suite of tools and resources designed to address timber connection challenges in the design of CLT – Compression Perpendicular to Grain with advanced analysis capabilities. The user interface is simple to use, which features a dynamic graphic that allows you to observe how the inputs change accordingly.
The input section allows you to choose the CLT layer thickness among the various suppliers or allow the user to input custom layups, select design codes, column dimensions, and also insert the magnitude of the compressive load, which is the design load for this calculator.
It also has options to easily toggle among the various load/support locations.
The output summary displays the major check for this calculator which is the compression check.
The material property tab shows the respective material properties of the selected CLT.
This tab performs the actual calculation for such type of timber connections. The calculations are performed in a brief manner, avoiding any possible black boxes, and are carefully referenced to their corresponding design codes, making them easily understandable, and educational.
