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CLT Compression – Perpendicular to Grain

By February 6, 2024No Comments

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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 subjected to forces perpendicular to the grain, CLT becomes vulnerable to compression, which can lead to structural failure if not properly addressed.

Compression Perpendicular to grain in CLT

One of the most prevalent types of timber connection in mass timber buildings occurs when CLT panels are supported by GLT columns or when the CLT panels support a GLT column, which is typically apparent in flat panels. Such type of connections result compression perpendicular to grain in Cross-Laminated Timber (CLT) panels, which requiring a particular attention. The CLT panels in these cases are subjected to a compressive force perpendicular to grain on either the top or bottom sides of the CLT member. This type of loading may result in the bearing failure of the CLT members, thus, should be checked for tensile and compressive stresses. An illustrative figure on the loading of such CLT member is shown below.

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The load distribution due to the compressive force applied perpendicular to the grain is calculated using the strut and tie approach which provides a clear and effective means to calculate stress distribution in timber structures. The strut and tie model is a simplified analysis method used in structural engineering to represent complex stress paths as linear struts (compression) and ties (tension). In such cases, this approach helps in visualizing and calculating how the compressive force spreads within the timber elements. The angle used for the calculation of the load distribution for this approach, θ=35o, is taken from prEN 1995.1.1:2023, which suggests a stress spreading gradient of 35o for CLT. The choice of a 35o angle for stress spreading in CLT, is based on extensive research and testing. This angle reflects a balance between structural efficiency and safety, ensuring that the stress distribution is accurately represented for optimal structural design as suggested by the code.

 

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CLT Toolbox can simulate these stress distributions using the strut and tie model, allowing designers to visualize and adjust their designs accordingly. By integrating prEN 1995.1.1:2023 and AS1720.1 guidelines into the design software, it ensures adherence to the latest standards and best practices.

Timber Failure Checks

Timber connections are fundamental in the structural integrity of timber buildings. Ensuring these connections withstand various stresses—compression, tension, and shear is crucial.

Compression Check:This check ensures if the respective connection can withstand compressive forces without buckling or crushing. The key consideration in compression checks lies in the timber’s compressive strength, which varies significantly with the grain orientation. Compression perpendicular to the grain often requires more attention due to lower strength in this direction. Compression failure arises when compressive forces exceed the connection’s ability to withstand them, causing crushing, yielding, or buckling of the timber.

Tension Check: The purpose of this check is to confirm that the connection can resist forces trying to pull it apart. The tensile strength of the timber, which varies significantly with grain orientation, is the most critical concern in tension checks. Because of its lower strength, tension perpendicular to the grain requires more attention than tension parallel to grain check. Tension failure occurs when the connection is subjected to tensile forces that surpass its resistance. This can lead to a gradual pulling apart or abrupt rupture of the timber elements.

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.

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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.

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The output summary displays the major checks for this calculator which are the compression and tension checks.

 

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The material property tab shows the respective material properties of the selected CLT.

 

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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.

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