Although engineered wood products such as glued laminated timber (glulam) and cross-laminated timber (CLT) have successfully eliminated the flaws inherently exist in conventional wood products, they are still not comparable with steel and concrete in terms of strength and stiffness. Among all different options for reinforcement, Carbon Fibre is relatively popular due to its high tensile strength, low weight, and easy installation. This study presents an analysis of flexural stiffness and stress distributions of CLT panels reinforced with carbon fibre mats, based on an analytical method and finite element method (FEM).
Design methods of cross-laminated timber elements subjected to bending is considered. The methods are based on LVS EN 1995–1–1. The presented methods were checked by the experiment and analytically. Two cross-laminated timber plates with the total thickness of 95 mm were tested under action of static load. The considered cross-laminated timber plates were analysed by FEM method, which is based on the using of computational program ANSYSv14. The comparison of stresses acting in the edge fibres of the plate and the maximum vertical displacements shows that the considered methods can be used for engineering calculations so as the difference between the experimentally and analytically obtained results does not exceed 20%.
The deregulation of timber for use in large scale constructions has seen the addition of new innovative timber-based products to a category of products referred to as engineered wood products. A now well established addition to these products is cross laminated timber, or CLT for short. CLT products use a form of orthogonal layering, where several parallel wooden boards are arranged in a number of layers, each layer being orthogonal to the previous. The use of orthogonal layering allows for increased stiffness in the two plane directions, resulting in a lightweight construction product with high load bearing capacity and stiffness.
To evaluate the dynamic behaviour of structures, engineers commonly apply the finite element method, where a system of equations are solved numerically. Given a sufficient amount of computational power and time, the finite element method can help to solve most dynamical problems. For sufficiently large or complex structures the amount of resources needed may be outside the scope of possibility or feasibility for many. Therefore, evaluating the usage of certain design simplifications, such as omitting to models aspects of the geometry, or alternative forms of analysis for CLT panels may help to reduce the time and resources required for an analysis.
In this Master's dissertation, a seven-layer CLT-panel has been created. In the model, each individual board and the gaps between the boards are modelled. The seven-layer model is used as a reference to evaluate the possibility of using less detailed alternative models. The alternative models are created as a layered 3D model and a composite 2D model, both models omit the modelling of the individual laminations, resulting in the layers being solid.
The results show small errors for the alternative models when using modal analysis. Concluding that the modal behaviour and dynamic response of a CLT panel can be evaluated using a composite 2D model or a less-detailed layered 3D model. This significantly reduces the amount of time and computational power needed for an analysis, and clearly indicates the benefit of using alternative less detailed models.
Project contact is Sylvain Ménard at Université Laval
Summary
Designers of large buildings generally want floor systems with large spans (9 m). These floors are often sized by the requirement of vibratory performance and, correlatively, deflection. The composite wood-concrete floors allow large spans with reduced static height. They are a promising alternative to simple concrete slabs. Objective 1 - Determine the evolution of the natural frequency of the CLT-concrete composite floor as a function of the stiffness of the connector, and correlate the experimental results with the model by the finite element method. Objective 2 - Parametric study of the vibration performance of the CLT-concrete composite floor. The impact of several parameters on the dynamic performance of the floor will be determined, especially the characteristics of the constituent materials, connector and the creep of the floor. Objective 3 - Build the metamodels for the study of multi-objective optimization optimization of a wood-concrete composite floor solution in relation to a regional problem in Aquitaine.
Finite-Element-Based Prediction of Moisture-Induced Crack Patterns for Cross Sections of Solid Wood and Glued Laminated Timber Exposed to a Realistic Climate Condition
Moisture may significantly influence the dimensions and behavior of wooden elements and, thus, it is important to consider within both serviceability as well as ultimate limit state designs. Dimensional changes, also called swelling (during wetting) and shrinkage (during drying), are non-uniform due to the direction-dependent expansion coefficients of wood and usually lead to eigenstresses. If these exceed certain strength values, cracking may occur, which reduces the resistance to external loads, especially to shear stresses. The current standard Eurocode 5 takes these circumstances very simplified into account, by so-called service classes, defined based on the surrounding climate and average moisture levels over the course of a year. Accordingly, reduction factors for strength values and cross section widths are assigned.
For a better understanding of the climate-induced changes in wooden beams, we exposed 18 different beams with varying cross sections to a representative climate of Linz, Austria, within the framework of a finite element simulation and investigated the resulting moisture fields and crack patterns. For this purpose, expansions and linear-elastic stresses were simulated by using the thermal and moisture fields obtained in the first simulation step and expansion coefficients. Using a multisurface failure criterion, two critical points in time were determined for each cross section, at which advanced crack simulations were carried out using the extended finite element method. The resulting crack lengths showed that the Eurocode 5 assumption of a linear relationship between crack-free and total width could be verified for both drying and wetting cases.
In future, the obtained crack patterns might also be used to investigate the actual reduction of load-bearing capacities of such cross sections, since the position of a crack and, for example, the maximum shear stress may not coincide. For the first time in this work, a consistent concept is presented to estimate the resulting crack formation in a wooden element from any moisture load based on a mechanical well-founded simulation concept. For this reason, this work is intended to lay a basis for a more accurate consideration of climate-related loads on wooden elements up to timber constructions.
Cross-laminated timber (CLT) is well known as an interesting technical and economical product for modern wood structures. The use of CLT for modern construction industry has become increasingly popular in particular for residential timber buildings. Analyzing the CLT behavior in high thermal environment has attracted scholars’ attention. Thermal environment greatly influences the CLT properties and load bearing capacity of CLT, and the investigation can form the basis for predicting the structural response of such CLT-based structures. In the present work, the finite element method (FEM) is employed to analyze the thermal influence on the deformation of CLT. Furthermore, several factors were taken into consideration, including board layer number, hole conformation, and hole position, respectively. In order to determine the influence, several numerical models for different calculation were established. The calculation process was validated by comparing with published data. The performance is quantified by demonstrating the temperature distribution and structural deformation.
The report aims to investigate norms, standards, guidelines and experience within the industry for how to design CLT (cross-laminated timber) regarding vibrations induced from humans. The following is being researched, ISO137, KL-trähandboken, Eurocode 5 and a new unpublished working draft of Eurocode 5 final working draft, Canadian CLT handbook and Cross-laminated timber structural design according to Eurocode from Austria.The conclusion is that the literature for CLT is non-existent in the current Eurocode 5 which only addresses timber floors with joists, however the new Eurocode draft suggests an update to include CLT which is similar to the norm CLT from Austria.The report contains a calculation part in which an analysis is conducted for a real project with calculations based on Eurocode 5 and the Eurocode 5 final working draft, the design tool Calculatis and FEM program RFEM. The calculations are compiled and evaluated.The calculation results show differences between the different standards. The natural frequencies are typically the same. The biggest difference is between the accelerations which is in direct relation to the modal mass, and the modal mass differs a lot between the calculations. It is understandable how Eurocode 5 final draft and RFEM calculate the modal mass, but not so for Calculatis as it doesn’t show any calculations in the technical documentation.There is a difference of the modal mass between Eurocode 5 final draft and RFEM, likely because EK5 calculate the modal mass for a rectangular floor simply supported at two or four sides. Whereas the RFEM model is not strictly rectangular nor is it simply supported everywhere, instead there are beams in some places. This suggests that caution should be regarded in calculations where floor structures have been simplified.
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