The impact of the horizontal shrinkage in CLT elements

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Introduction

Modern building design is characterized by an increasing trend towards larger scales and greater spans, particularly within the domain of timber construction. The adoption of advanced engineered wood products, such as CLT (Cross-Laminated Timber), has facilitated the construction of mid- and high-rise buildings and long-span floor systems that were historically dominated by concrete and steel.

The use of CLT to achieve large horizontal spans (e.g., L > 40 m) means that the absolute linear deformation (∆L) caused by even small moisture content changes (∆M) becomes critical. The shrinkage that occurs in the longitudinal direction is relatively small, but a longer floor results in a larger total potential movement. These CLT floor panels are frequently connected to rigid cores. When the CLT shrinks but is restrained by these cores, the movement is converted into large internal stresses within the timber elements and significant reaction forces at the connections or causing large deformations at the top of these cores.

Problem definition

The adoption of the shrinkage coefficients for CLT defined in the upcoming FprEN 1995-1-1:2024 marks a significant shift in structural design practice. These updated values directly influence the calculation of dimensional changes in the in-plane direction of CLT panels. As companies increasingly pursue larger buildings with bigger horizontal spans, the impact of these prescribed shrinkage values is critically amplified. In such large structures, the total linear deformation potential becomes substantial. This deformation can either enter the connection, creating large forces, or act as an additional horizontal force on the core, creating more deformation.

The research subject, investigating the impact of shrinkage in horizontal CLT elements in mass timber buildings, was selected due to its direct relevance to the host company, Lüning. The company’s prior experience with challenges related to this specific subject provided the essential practical context and motivation for an in-depth study.

Hygrothermal Simulation

To evaluate the long-term performance and durability of the building envelope, this research employs WUFI® (Wärme und Feuchte instationär), developed by the Fraunhofer Institute for Building Physics (IBP). Unlike traditional, steady-state Glaser methods, WUFI® conducts dynamic numerical simulations of coupled heat and moisture transport under realistic, transient boundary conditions.

A critical and complex variable in this simulation is determining the hygrothermal impact of the adhesive layers within the CLT elements. Specifically, the study investigates how these bonding agents alter internal moisture behaviour, moisture accumulation, and vapor resistance across the assembly’s cross-section.

The figure below illustrates the simulated moisture content within the layers of the CLT element over time, comparing the model with adhesive interfaces against a model without adhesives. By analysing these temporal moisture gradients, the overall drying duration of a CLT element can be determined.

This data allows for the quantification of how much the adhesive layers have influence on the drying process, providing a clear timeline for when the core wood layers will successfully reach safe equilibrium moisture content.

Field Measurements

To gain deeper insight into the actual dynamics of moisture accumulation within CLT elements, in situ field measurements were conducted on an active construction project. The empirical data revealed significant localized moisture entrapment, specifically concentrated within the core layers of the CLT assembly. The measurements in the pictures below are taken in different elements but the depth of around 10cm are the same. The one on the left displays a 14.3% but the right one displays a 29.6%, which is quite high.

What is next?

In the upcoming phase of this research, the field measurements and hygrothermal simulations will be integrated to quantify the total shrinkage of the CLT elements.

By correlating the simulated moisture loss with the material’s specific coefficients of shrinkage, the resulting internal structural stresses within the CLT cross-section will be calculated. Ultimately, the objective is to develop engineering solutions or design guidelines to mitigate these hygrothermal stresses and prevent structural failure.