OSB sheathing-to-wood framing connection, as typically used in light-frame shear walls, was experimentally examined in a novel biaxial test setup with respect to possible path dependence of the load-displacement relation. The connection with an annular-ringed shank nail was loaded under displacement control following nine different displacement paths within the sheathing plane, which coincided at a number of points. In intersection points, resultant connection force, its orientation and work performed on the connection system to reach the specific point were calculated and compared. Evaluation of experiments revealed significant path dependence with respect to orientation of force resultants at path intersection points. However, magnitude of the forces and the work carried out showed relatively small dependence of the displacement path undertaken. Comparison of uniaxial connection tests with the European yield model demonstrated strong contribution of withdrawal resistance of the ringed shank nail to its lateral strength. Results of this type are a valuable basis to build better models when simulating such connections in wood structures.

The empirical test developed as validation for a newbeamelementmodel that can account for bothmechanical and environmental load action in finite element analysis is presented. The testing protocol allowsfor the identification and analysis of contributing deflection components in bending under varying MCconditions, including mechano-sorption. The components of deflection in the shear-free span of a four-pointbending test and their responses to varyingmoisture are evaluatedwith an analytical procedure. The experimentwas conducted on clear, straight-grained sapwood and heartwood specimens of Norway spruce (Picea abies)(30 15 640 mm3). The program consisted of three phases: 1) long-term (LT) experiments under constanttemperature of 60°C and RH cycles between 40% and 80%, 2) a short-term static experiment to determine thevariation in the sample set and the load level of the LT experiment on end-matched specimens, and 3) creeptests at 60°C and constant humidity at either 40%or 80%to determine the effect ofmoisture on the viscoelasticcreep. Mass changes and hygro-expansion measured on matched specimens were used in the analyticalmethod. Constitutive models used for describing the material-level response to loads and moisture changeswere applied to the shear-free segment of the specimens disregarding actual moisture gradients and fiberorientation inside the test specimens. A successful identification of each deflection component and isolation ofmechano-sorption component was accomplished. In the 90 da of testing, the dominant component of the totaldeflection was the elastic component, followed by the mechano-sorptive component. Creep was found to benonnegligible and important in the correct description of mechano-sorption. The effect of moisture on theviscoelastic behavior showedmost important during loading and first stages of decreasing deflection rate phase.

Timber boards manufactured with a traditional sawing pattern often contain both heartwood andsapwood. In such boards, internal constraints can occur during drying because of a radial variation in greenstate(GS) MC between the heartwood (30-60%) and sapwood region (120-200%). Despite such knowledge,the initial MC is seldom considered when evaluating kiln-drying schedules. The effect of GS MC on thedevelopment of tangential tensile stress during drying is studied for four types of timber boards. A numericalmodel was developed that can simulate transient nonlinear orthotropic moisture flow and moisture–inducedstress and distortion in wood with the use of the finite element method. The stress analysis considers elastic,hygroscopic, and mechano-sorptive strain. The study shows that the GS MC does not significantly influencethe maximum stress state, but that it does influence the time at which the maximum tangential tensile stressoccurs at different exchange surfaces. This results in several periods in the drying schedule where unfavorablehigh stress situations in the tangential direction arise, which could lead to crack propagation.

The European design standard for timber structures provides, besides obligatory safety requirements, a set of general serviceability requirements. Despite their generality, they have been proven important in design of timber structures, especially in varying climatic conditions, where the time dependent deflection can have a dominant role in long term performance. The total deformation consists of instantaneous elastic deformation, hygroscopic deformation, time dependent creep and mechano-sorptive deformation. The three latter deformations are influenced by climate, and when the change in climate is considerable over time, the deflection will significantly increase. In this paper a test-setup is created to study the effect of mechano-sorption on the global deflection of clear wood samples. The samples are loaded in a three-point bending test subjected to a constant mechanical load combined with a cyclic climatic load. The moisture induced stress and bending distortion were simulated by taking into account elastic, hygroscopic and mechano-sorptive strain. The non-linear moisture flow was simulated using Fick’s law. A parametric study was performed to obtain a better understanding of the constitutive equation, especially the term related to the moisture and temperature dependent diffusion coefficient. In addition to the simulations, an experiment was performed to verify the global deflection and mass change. The obtained results show that the mechano-sorption behaviour of the tested clear wood samples can be modelled, but an improvement of both the experimental setup and the model is required to come to more accurate conclusions on this type of long-term material behaviour.

This paper introduces, with the development of user-subroutines in the finite-element software Abaqus FEA (R), a new practical analysis tool to simulate transient nonlinear moisture transport in wood. The tool is used to revisit the calibration of moisture simulations prior to the simulation of mechanical behaviour in bending subjected to climate change. Often, this calibration does not receive sufficient attention, since the properties and mechanical behaviour are strongly moisture dependent. The calibration of the moisture transport simulation is made with the average volumetric mass data experimentally obtained on a paired specimen of Norway spruce (Picea abies) with the dimensions 30x15x640 mm(3). The data, from a 90-day period, were measured under a constant temperature of 60 degrees C and systematic relative humidity cycles between 40 and 80%. A practical method based on analytical expressions was used to incorporate hysteresis and scanning behaviour at the boundary surface. The simulation tool makes the single-Fickian model and Neumann boundary condition readily available and the simulations more flexible to different uses. It also allows for a smoother description of inhomogeneity of material. The analysis from the calibration showed that scanning curves associated with hysteresis cannot be neglected in the simulation. The nonlinearity of the analysis indicated that a coherent set of moisture dependent diffusion and surface emission coefficient is necessary for the correct description of moisture gradients and mass transport.

A three-dimensional numerical model was employed in simulating nonlinear transient moisture flow in wood and the wood's hygro-mechanical and visco-elastic behaviour under such conditions. The model was developed using the finite element software Abaqus FEA, while taking account of the fibre orientation of the wood. The purpose of the study was to assess the ability of the model to simulate the response of wood beams to bending and to the climate of northern Europe. Four-point bending tests of small and clear wood specimens exposed to a constant temperature and to systematic changes in relative humidity were conducted to calibrate the numerical model. A validation of the model was then performed on the basis of a four-point bending test of solid timber beams subjected to natural climatic conditions but sheltered from the direct effects of rain, wind and sunlight. The three-dimensional character of the model enabled a full analysis of the effects of changes in moisture content and in fibre orientation on stress developments in the wood. The results obtained showed a clear distinction between the effects of moisture on the stress developments caused by mechanical loads and the stress developments caused solely by changes in climate. The changes in moisture that occurred were found to have the strongest effect on the stress state that developed in areas in which the tangential direction of the material was aligned with the exchange surface of the beams. Such areas were found to be exposed to high-tension stress during drying and to stress reversal brought about by the uneven drying and shrinkage differences that developed between the outer surface and the inner sections of the beams.

At times dowelled glulam timber connections experience crack development in the fibre direction. The main reason for this is moisture variation in the timber elements which induces a stress perpendicular to the fibre direction. The aim of this paper is to study the influence of different moisture conditions and vertical dowel spacing on crack development through numerical simulations by use of the finite element method in three dimensions. A transient non-linear Fickian moisture diffusion model is implemented to simulate the moisture state within the glulam beam. The moisture gradient in the diffusion model was created by adopting a physical scenario by assuming what conditions the considered glulam beam will go through, from the factory up to installation. Further, an extended finite element method (XFEM) for two different vertical dowel spacing, 100 mm and 300 mm, with a linear elastic fracture mechanics (LEFM) approach was applied for the crack simulation. The results reveal that the moisture variation in combination with unfavourable placement of dowels can cause a crack to develop in the glulam timber beam. Moreover, it was shown that a moisture induced crack development may be modelled successfully by use of an Extended Finite Element Method (XFEM) approach.

This study investigates the impact of sheathing panel cracks on the structural performance of light-frame, modular-based timber buildings, focusing on the racking stiffness and strength of the individual timber walls in the modules. Previous research has investigated such walls for decades and lead to practical design methods in the harmonized European design code, Eurocode 5. Such hand calculation methods are effective for simple geometries but for walls with openings or complex forms, a correct prediction of stiffness and strength is considerably harder to achieve and load levels where cracks initiate are almost impossible to predict. The paper presents both experimental and numerical studies to investigate how significant cracking in sheathing panels affects the load-carrying capacity of various light-frame timber walls. Finite element simulations using Abaqus are conducted to model the cracking of sheathing panels with the extended finite element method. Moreover, an orthotropic elasto-plastic connector model is introduced for the nail joints. The results indicate that significant cracking of the sheathing panels influences the stiffness and the load-carrying capacity of the wall elements and that the crack initiation and propagation is strongly affected by factors such as the location of openings, the shape of the sheathing panels and the type and position of sheathing-to-framing connections. The numerical results presented align satisfactory with the experimental data particularly regarding load levels at crack initiation and propagation. Furthermore, a parametric study investigates how cracks, orthotropic connector properties and vertical constraint of bottom rails influence the racking strength of different timber walls. 

Building with prefabricated light-frame modules is an innovative construction method whose market share is increasing in Sweden. It is often used to build low and mid-rise residential buildings where the modules are pre-manufactured in factories with high efficiency and accuracy. This building method has many advantages, though more research regarding wind stabilization of both individual and assembled modules is still needed. To study the racking stiffness and strength of such timber modules, a numerical model of their shear walls was created and verified based on experimental results from specially prefabricated test modules. A new test facility was built for this work, where several potentiometers and a digital image correlation system collected the data used to verify the simulation model. The results from the simulations agree well with the experimental findings, which indicate that the three-dimensional finite element model works with reasonable accuracy for this application.

Light-frame timber shear walls have been used as load-bearing elements in buildings for several decades. To predict the performance of such structural elements under loading, numerous analytical and numerical models have been developed. However, little focus has been on the prediction of the plastic damage behaviour and unloading of the walls. In this paper, a parametric Finite Element (FE) model is further developed by introducing elasto-plastic connectors to simulate the mechanical behaviour of the sheathing-to-framing connections. To verify the accuracy of the elasto-plastic model, full-size walls were tested and compared with results from simulations. The numerical results, from a few loading cycles, indicate that the model achieves reasonable accuracy in predicting both the nonlinear elastic and plastic deformations. Both experimental and simulation results demonstrate the importance of opening locations relating to the external racking force. The results also indicate that for a double-layer wall, its racking strength can be achieved by summation of the separate contribution from each layer. Furthermore, the internal layer was observed to contribute significantly less than the external layer since its nail pattern was based on the sheathing pattern of the external layer. © 2022 The Authors

Structures built with prefabricated timber modules have been recognised as an innovative construction method and have been implemented in several countries and regions. In recent years, there have been considerable research activities directed towards these types of structures. However, most of the studies have focused on modules made of steel and concrete in their load-bearing structures and only a few of them are exploring light-frame timber modules. This study focuses on the racking behaviour of light-frame timber modules through experimental and numerical investigations. Full-size tests were performed to examine the global and local structural behaviours of several test modules. A novel finite element model of the modules is also presented. It is a parameterised structural model with high flexibility concerning the generation of different module geometries, materials, fastener types and assembly methods etc. The numerical model was developed in the commercial finite element software ABAQUS, and the numerical results obtained were validated against results from experimental tests. The validation results indicate that the model is capable of achieving satisfactory accuracy in predicting both the global and local structural behaviour of light-frame timber modules. Furthermore, several parametric studies are conducted and discussed to examine how certain parameters affect the structural response of the modules. 

In recent decades, there is a trend in Scandinavian countries to build multi-storey residential houses using prefabricated timber modules. It is a highly efficient construction process with less environmental impact and less material waste. A significant building element in the timber modules is the light-frame timber wall, which has to be carefully analysed and optimized in this process. This paper presents a new parametric Finite Element (FE) model that can simulate both in-plane and out-of-plane deformations in the light-frame walls. A new and flexible (Eurocode based) approach to define the properties of the mechanical connections is introduced. A numerical model is presented through simulations of several walls that were verified with full-scale experiments. The results indicate that the numerical model could achieve fairly reasonable accuracy with the new approach. Furthermore, several parametric studies are presented and discussed from global and local points of view, to investigate the effects of certain parameters that are not considered in the design method according to Eurocode 5.

Prefabricated timber modules are being increasingly used in the load-bearing structure of entire residential buildings reaching heights up to six stories. The development is driven by the demand of high-quality housing that remains affordable while fulfilling tough environmental requirements imposed on modern construction. To enable further development of this type of buildings additional research is needed despite the considerable number of studies previously performed. This study provides an extensive experimental investigation by subjecting three modules to three different load cases. In each load case, the modules were initially loaded with dead-load placed atop of the module. Thereafter the modules were laterally loaded at the top using a servo hydraulic piston in displacement control. The main aim of the study was to assess the structural behavior of these modules under combined lateral and vertical loading, and also to generate experimental data suitable for verification of finite element models. Results from the test series reveal significant variation in racking stiffness and racking strength depending on the module’s design. Furthermore, in some cases more stiff and stronger mechanical inter-module connections are needed to enhance their global structural performance. Finally, the experimental results reveal that the modules are relatively ductile in their shear response when subjected to horizontal load.

From an environmental sustainability point of view, modern construction practices increasingly favorcarbon neutral buildings including those made from timber. Prefabricated timber modules havebecome popular due to their efficient in-house production followed by systematic and rapid on-siteinstallation. Construction companies often use these lightweight modules for residential buildings upto six story when feasible. While several studies are available that simulate stiffness and strength ofshear walls, a major component of the module responsible for transferring shear load and acting asa load bearing wall for vertical loads, e.g [1] and [2] for the EC5 design principles, relatively little workhas been done to analyze the structural performance of entire modules. This is likely due to limitedtime span the construction type has been common practice, practical challenges associated withexperimental tests and numerically demanding simulations of large structures. However, there aresome exceptions, e.g [3].This study introduces the concept of “super elements”, which are developed by condensing theinternal degrees of freedom (DOF:s) of a whole timber module to specified parts of its boundary. Theaim of this study is to reduce the number of DOF:s by using substructuring so that an entire structurecan be analyzed while subjected to external loading. Substructuring is a method of dividing a wholemodel into user defined parts (super elements) and coupling these together to create a global model[4]. The internal DOF:s of the super elements are “condensed” using static condensation, and thesuper elements are then connected to the rest of the model along selected restrained DOF:s [5].Figure 1(a) shows an example of a building with timber modules, while Figure 1(b) illustrates partsof a full-size timber module. Figure 1(c) represents a super element of the module. A simple, linearFE super element is developed for analyzing a part of a whole timber structure and it is coupled tothe rest of the structure only at designated pre-selected nodes. The model is then used to analyzethe response during various load cases applied to the whole structure.

The properties of sheathing-to-framing joints considerably affect the load carrying capacity of a light-frame timber shear wall. A fastener with isotropic or kinematic hardening properties is modelled for the sheathing-to-framing joints with a zero-length element, with coupled properties in two perpendicular (orthogonal translational) directions to avoid the overestimation achieved with an uncoupled alternative. A single fastener experiment is performed to determine the elastic and plastic properties. For both fastener level and wall level modelling, monotonic as well as cyclic loading scheme is analysed. A concept of modelling the elasto-plastic coupled behaviour with hardening of the connector model for the fasteners is suggested. A damage response of the fastener is also studied to estimate the failure in load capacity of the connector model and decrease in the wall capacity after the maximum loading.

The building of low and mid-rise modular-based timber buildings is a growing industry sector worldwide. Today, the use of prefabricated light-frame volume modules (sometimes with CLT walls) is a prevalent and innovative construction method. This building method has many advantages, though some of the technical solutions and design methods are quite challenging and should be optimised without risking structural safety. The paper presents results from an ongoing research project that deals with testing and numerical analysis of light-frame timber test modules. It focuses on the testing and simulation of global racking stiffness of different types of test modules and the testing of a friction-based joint between two test modules. The modules showed larger shear load carrying capacity than expected. They also showed a ductile failure behaviour and large variation in racking stiffness between different types of test modules.

Instability failures of timber elements and timber structures are reported relatively frequently although there are some suggestions available how to prevent such failures. These types of failures are characterized by sudden deformations that typically lead to failure in a single loadbearing element or collapse of the entire structure. This paper deals with buckling analysis and geometric nonlinear stress analysis of pitched roof structures of wood. A FE- model has been developed and used to study how different parameters influence the buckling modes and force distribution in the lateral bracing system of the roof structure. The simulated forces in the bracing system are also compared with results based on a simple design method given in Eurocode 5 (EC5) and a method where the compressed top chord is treated as a beam on a continuous elastic foundation. The buckling simulations showed the out-of-plane buckling to be the critical failure mode for the truss structure studied and the geometric nonlinear analysis showed the bracing stiffness and the bracing forces to be significant lower than those calculated by hand according to EC5.

It is vital to find smart use for cut-offs from the CLT-production, which is typically 5-10% ofthe volume, due to e.g. window and door openings. One application is to make CLT-boards anduse them in timber frame walls as horizontal rails. The cross layers have fibres in the loadingdirection which enhance the load-bearing capacity. This pilot study indicates that the short termstrength and stiffness of a rail made of 5 ply CLT is 4 times higher compared to that of structuraltimber. Long-term tests showed that CLT-rails (45x95 mm) hardly deformed at all with time,while rails from structural timber deformed substantially, after 6 months of loading with 20kN

It is vital to find smart use for cut-offs from the CLT-production, which is typically 5-10% ofthe volume, due to e.g. window and door openings. One application is to make CLT-boards anduse them in timber frame walls as horizontal rails. The cross layers have fibres in the loadingdirection which enhance the load-bearing capacity. This pilot study indicates that the short termstrength and stiffness of a rail made of 5 ply CLT is 4 times higher compared to that of structuraltimber. Long-term tests showed that CLT-rails (45x95 mm) hardly deformed at all with time,while rails from structural timber deformed substantially, after 6 months of loading with 20kN

Production of CLT-panels typically results in 5-10% cut-offs due to window and door openings. CLT-boards can be made by slicing these cut-offs and finger-jointing them. This paper presents bending properties of 100 narrow CLT-boards (45x95x1800) made from 5-ply CLT panels made of Norway spruce. Alimited variation in E-modulus (CV=9%) and bending strength (CV=20%) was found with gross section values not far from typical structural C24-timber. The tests indicate that narrow CLT-boards have sufficient bending properties for being used as structural components, also where the bending capacity is formally utilized. However, the variation was slightly higher than for typical tests on larger CLT-elements (CV=8-16%), probably due to the smaller homogenization effect when fewer subparts carry the load. The rolling shear modulus was estimated to be Gr= 65MPa, which is to be expected due to the distance from the pith of the cross layerboards. The surprisingly high net flatwise bending strength (fm,05=49 MPa) can likely be attributed, for the most part, to the reinforcing effect from the cross layers that limit the slope of grain cracking near knots in the longitudinal layers. DIC-tests revealed an indication of a non-plane normal strain distribution over the beam depth in the shear-free zone between the inner loading points. This might lead to an under estimation of the shearfree local E-modulus according to EN408.

The ambient movement of three multi-storey cross-laminated timber (CLT) buildings have been measured and used to determine natural frequencies, mode shapes and damping ratios. This information, obtained by a simple, unobtrusive series of tests, can give insights into the structural performance of this form of building, as well as providing information for the design of future, taller timber buildings for dynamic loads. For two of the buildings, the natural frequency has been related to the lateral stiffness of the structure, and compared with the stiffness based on simple calculation. In future tall timber buildings, a new design criterion is expected to become important: deflection and vibration serviceability under wind load. Design standards give techniques for prediction and mitigation of wind-induced movement, but require an estimate of the mass, stiffness and damping ratio of the structure to make an accurate prediction. For multi-storey timber buildings there is no empirical basis to use for damping estimation, and there is little information for stiffness. This study therefore provides an insight into the modal properties for lateral vibration of multi-storey CLT construction which could inform the design of taller buildings in the future.

The influence of the loading orientation with respect to the grain direction of wood and the influence of the lateral dowel displacement boundary condition on the embedment behaviour of steel dowels in laminated veneer lumber (with parallel-laminated veneers) are investigated in this study. For limit states of the lateral boundary condition, the load-displacement behaviour was experimentally studied by means of full-hole embedment tests on screw-reinforced laminated veneer lumber, for two dowel diameters and up to large dowel displacements. A novel biaxial test set-up is proposed for embedment tests with constrained lateral dowel displacement boundary condition, in order to quantify laterally evoked reaction forces. Corresponding forces were found to change orientation with increasing dowel displacement and amounted to about 20% and 40% of the vertical reaction force for dowel displacements of 5 mm and twice the dowel diameter, respectively. The influence of the lateral displacement boundary condition was highlighted by comparison of the test data with a previously established data set for unconstrained embedment testing. Constrained loading showed a stiffer response and higher nominal embedment stresses, as well as a more pronounced displacement hardening, compared to unconstrained loading.

According to the harmonized European design code for timber structures, Eurocode 5, all pitched timber trusses used in load bearing roofs are designed as in-plane structures which means that a bracing system must be designed and put in place to prevent the out-of-plane instability. Results from numerical 3D stability analyses of the whole roof structure indicate that the out of plane stability is often the critical factor. Therefore, influence of stiffness properties of that system is studied in detail herein for long-span timber roofs. Focus is put on how the stiffness of the mechanical connections in the roof structure influences the load carrying capacity of the roof. The punched metal plate connections are modelled as non-coupled spring elements connecting the various beam elements in the timber truss respectively. The spring stiffness of the connections is derived from full-scale tests, which were made for all in- and out-of-plane degrees of freedom. To evaluate the experimental testing, a digital image correlation method was used. The results from the digital image correlation tests were compared with numerical simulations of the experimentally tested connections to check the potential of using the numerical simulations instead of the experimental testing to get the stiffness properties of various connections used in the whole roof stability analysis. Based on such analysis, punched metal plate fasteners showed to be an important contributor to the roof stability because of its relatively high stiffness in all six degrees of freedom.

According to the harmonized European design code for timber structures, Eurocode 5, all pitched timber trusses are designed as an in-plane structure, meaning that the bracing systems used are assumed to prevent the out-of-plane failure of the truss if sufficient strength and stiffness are provided. The present paper studies how the stiffness of a wooden bracing system contributes to the out-of-plane stability of a trussed roof structure. Results from numerical simulations indicate that significant bracing forces may occur in compressed structural members for long-span timber structures. As well, the values obtained from the calculations according to Eurocode 5 are occasionally far from the results obtained by numerical simulations.

The design of spatial connections in load bearing timber structures with steel angle brackets has insufficient support in the existing design standards. Therefore, research has been necessary to improve this state of the art. In the current paper an experimental study on two designs of angle brackets is presented and the results from full-scale experiments are compared to numerical and analytical computational models.

In a multi-storey residential housing project comprising of four 8-storey timber buildings, the bottom storeybeing designed with concrete and storeys 2-8 in timber, the vertical relative displacement, the temperature and the relativehumidity (RH) along one vertical channel in the external wall of one building has been monitored. Measurements startedduring construction and presented herein are results of 6.5 years of in-situ measurements. Displacement data was monitoredstorey-by-storey, with a sampling frequency of 1 measurement every 10-60 minutes. In another of the four buildingsadditional temperature and relative humidity measurements have been ongoing for about 5.5 years. These temperature andRH measurements were performed at six different locations in the building, at each location in eight positions through theexterior wall with a sampling frequency of 1 measurement every 15 minutes. The results show that the total verticaldisplacement over six storeys after 6.5 years of service life is approximately 23 mm as a yearly average, and over the yearthe displacement varies from this value by approximately ±2 mm. The main cause for the relative displacement is thedecrease of moisture content in the wood material leading to shrinkage after completion of the building. The resultsobtained show also that the exterior wall design of the building behaves well in terms of not comprising a general risk fordamp or mould in the timber core of the external walls.

Four buildings with clt-panels in their load bearing structure were built at the block Limnologen in Växjö, Southern Sweden. Their architecture is an example of the new ar-chitecture possible with this building system. Properties of these new structures are sought, one of these being their relative vertical displacement over time. These displacements are measured continuously for six storeys, up to now for just over two years. So far, the total measured dis-placements have reached a maximum of 21.1 mm over the 17.95 meter measuring length. Annu-lar variations of the displacements corresponding to the varying climate may be observed in the data.

This study relates to the topic of anchorage of shear walls. At times, eccentric forces between the sheathing and the anchoring devices may be introduced in the sill plate. In severe cases, such forces may cause the sill plate to split and to fail in a brittle manner. In this study, fracture mechanics are applied to develop a simple closed-form hand-calculation expression for estimation of the ultimate load capacity of the sill plate. Finite-element analyses using both linear elastic fracture mechanics (LEFM) theory and a nonlinear fictitious crack model are also used to predict the ultimate load-bearing capacity of the sill plate. The hand-calculation model is compared with the finite-element models, and good agreement is obtained. The results obtained with the various fracture mechanics models are compared with results available from previously performed experimental tests, and again good agreement is obtained. A general conclusion is that the LEFM theory is an adequate approach for the case studied and that the hand-calculation expression developed could be useful for structural design.

Paper-based materials are being alternative candidates to build load-bearing components for the high demanding building sector to be committed to the green transition, but more knowledge of structural mechanics of such components is needed. In this work, three categories of innovative load-bearing sandwich beams with cup-box core fully made of different paper materials were produced, tested and analyzed in four-point bending. The first failure mode was observed at the top facesheets due to the low compressive strength of paper materials; Beams with thin facesheets had premature buckling failure, whereas those with thicker facesheets exhibited ductility reaching higher deflection before the compressive failure. The developed finite element model, calibrated with the experiments for the equivalent bending and shear rigidities, provided figures of the modulus of the facesheet as well as the properties of the core. Furthermore, the compressive plasticity behavior of the facesheets was assessed by fitting the model with load-deflection curves from the tests. Using the model for the structural optimization of the thickness and height of the core, the work suggests optimal values 3 times higher than the original ones.

In the Värmland region, Sweden, an innovative sandwich structural element has been designed by the cooperation of pulp and paper companies for possible use in indoor products, such as tables, shelves, doors, and furniture in general [1]. The element, lightweight and strong, has facesheets of laminated paperboards made of recycled and/or virgin wood fibres. The core, made of paper pulp, has a unique shape like a cup box with staggered positions between the cups, see [1]). At the Department of Engineering and Chemical Science at Karlstad University, the mechanical performance and properties of the elements have been investigated via experimental tests and finite element (FE) models for different applications. Recently, four-point bending tests have been conducted in sandwich beams from the element, see Fig. 1(a). The facesheets of the beams were made of virgin and/or hybrid (virgin + recycled fibres) paper materials. Results from quasistatic experimental tests were evaluated by means of digital image correlation technique for accurate measurements of the beam deflection.

In this presentation, we will address aspects of flexural and shear rigidities of the tested samples [2, 3], as well as main observations of failures, which occurred at the top facesheets (c.f. Fig 1(b)). A parametric FE model will also be presented along with its calibration, capability of predicting the failure (c.f. Fig 1(c)), and possibilities for structural optimization.

References

[1]    Ecopals AB, Hållbar möblekonstruktion till en bråkdel av vikten/Durable furniture construction at a fraction of the weight. Retrieved March 03, 2024, https://ecopals.se/

[2]    Howard, G.A., Analysis and design of structural sandwich panels, Pergamon, (1969)

[3]    Lorna, J.G., Michael, F.A., The design of sandwich panels with foam cores. In: Cellular Solids: Structure and Properties. Cambridge University Press, 345-386, (1997)

This doctoral thesis addresses questions of how wind loads acting on multistoreytimber buildings can be dealt with by structural design of such buildings.The conventional use of sheathing either nailed or screwed to a timberframework is considered, together with other stabilizing structures such ascross-laminated timber panels.The finite element method was employed in simulating the structuralbehaviour of stabilizing wall units. A series of studies was carried out of walls inwhich the sheathing was nailed to a timber frame. Different structural levelswere studied starting with modelling the performance of single sheathing-toframingconnections, to the use of models for studying the overall structuralbehaviour of walls. The results of calculations using models for simulation ofwalls subjected to different loading agree reasonably well with experimentalresults. The structural properties of the connections between the sheathing andthe frame, as well as of the connections between the members of the frame,were shown to have a substantial effect on the simulated behaviour of shearwall units. Both these types of connections were studied and described inappended papers.Regarding cross-laminated timber wall panels, it was concluded that walls witha high level of both stiffness and strength can be produced by the use of suchpanels, and also that the connections between the solid wall panels can bedesigned in such a way that the shear forces involved are transmitted from onepanel to the next in an efficient manner.Other topics in the thesis include the properties of connections between shearwalls and the rest of the building. Typically high tension forces occur at specificpoints in a timber structure. These forces need to be transmitted downwards inthe structure, ultimately connecting them to the substrate. A lap-joint that maybe used for this purpose has been studied using generalized Volkersen theory.Finally the maximum capacity of a conventional rail to substrate connection hasbeen examined using linear and nonlinear fracture mechanics.

Wind loads acting on wooden building structures need to be dealt with adequately in order to ensure that neither the serviceability limit state nor the ultimate limit state is exceeded. For the structural designer of tall buildings, avoiding the possibly serious consequences of heavy wind loading while taking account at the same time of the effects of gravitation can be a real challenge. Wind loads are usually no major problem for low buildings, such as one- to two-storey timber structures involving ordinary walls made by nailing or screwing sheets of various types to the frame, but when taller structures are designed and built, serious problems may arise.

Since wind speed and thus wind pressure increases with height above the ground and the shear forces transmitted by the walls increase accordingly, storey by storey, considerable efforts can be needed to handle the strong horizontal shear forces that are exerted on the bottom floor in particular. The strong uplift forces that can develop on the wind side of a structure are yet another matter that can be critical. Accordingly, a structure needs to be anchored to the substrate or to the ground by connections that are properly designed. Since the calculated uplift forces depend very much upon the models employed, the choice of models and simplifications in the analysis that are undertaken also need to be considered carefully.

The present licentiate thesis addresses questions of how wind loads acting on multi-storey timber buildings can be best dealt with and calculated for in the structural design of such buildings. The conventional use of sheathing either nailed or screwed to a timber framework is considered, together with other methods of stabilizing timber structures. Alternative ways of using solid timber elements for stabilization are also of special interest.

The finite element method was employed in simulating the structural behaviour of stabilizing units. A study was carried out of walls in which sheathing was nailed onto a timber frame. Different structural levels were involved, extending from modelling the performance of a single fastener and of the connection of the sheathing to frame, to the use of models of this sort for studying the overall structural behaviour of wall elements that possess a stabilizing function. The results of models used for simulating different load cases for walls agreed reasonably well with experimental test results. The structural properties of the fasteners binding the sheathing to the frame, as well as of the connections between the members of the frame were shown to have a strong effect on the simulated behaviour of shear wall units.

Regarding solid wall panels, it was concluded that walls with a high level of both stiffness and strength can be produced by use of such panels, and also that the connections between the solid wall panels can be designed in such a way that the shear forces involved are effectively transmitted from one panel to the next.

Nailed connections are commonly employed for connecting sheathings to the framing used in shear walls. Although many aspects of such connections have been investigated thoroughly within the research community generally, the effect the loading direction has on connections of this sort has been much less investigated. In the present study experimental tests were carried out for determining in detail the effects the loading has on different sheathing-to-framing connections. The results obtained indicated the degree of loading to which a nail fastened to an oriented strand board (OSB) sheathing is subjected to not be strongly affected by which of the two main loading directions is involved, but that the effects of loading direction are found to be much greater if the timber element in question is also included in the testing carried out. The dependency of the loading effect on the loading direction at different loading stages and for different directions - parallel to the fibres, perpendicular to them, at some angle between these two main directions - was investigated here.

The use of cross-laminated structural timber elementsis becoming increasingly popular. The number of layersvaries normally from three upwards. The structural performanceof five-layer cross-laminated timber elements was investigated.The five layers consisted of 19mm thick boards,laid successively at right angles to each other and gluedtogether with PU-adhesive, layers 1, 3 and 5 lying in onedirection and layers 2 and 4 in the other. The stiffness andstrength of four cross-laminated timber elements (4955mmlong, 1250mm wide and 96mm thick) were studied duringin-plane bending. Two of the elements were first partitionedinto two parts that were reconnected in two different waysprior to testing. The influence of the way in which the crosslaminatedtimber elements were reconnected was studied,the behaviour observed being compared with the test resultsfor the unpartitioned specimens with respect to both strengthand stiffness. The experimental tests performed showed thecross-laminated timber elements to possess a high degree ofstiffness and strength. There was also found to be a markeddifference in behaviour between the two different ways inwhich the elements were connected to each other. One of thetwo connecting methods studied, being of less good designbut earlier frequently used in Sweden, showed as expectedpoor structural performance, whereas the other one appliedas a safer alternative performed well.

The use of structural timber in residential buildings has increased considerably in Sweden during the last decade. Concomitantly, there has been an emphasis on augmenting the level of prefabrication for such structures. A simultaneous need for education imparted in universities exists, and this paper contributes to this need by outlining a design example of a residential timber building with simple geometry using prefabricated volumetric elements. The serviceability limit state as well as the ultimate limit state have been defined and studied for some critical design situations. The results indicate, for example, that the degree of utilisation in the studied building system for compression perpendicular to the bottom rail in the first storey and buckling of the studs are in the same range - 91 % to 97 % - for the walls separating apartments.  

The present investigation focuses on evaluating the entire load displacement relationship, especially the softening part, of light-frame wall segments subjected to in-plane monotonic forces when the load-slip curves of the individual sheathing-to-framing fasteners are considered. Different sheathing-to-framing joint characteristics, including unloading behaviour, and stud-to-rail joint characteristics are incorporated in the analyses. Two loading cases are investigated: Horizontal loading resulting in uplift of the leading stud and diagonal loading representing a fully anchored wall. Two common types of finite element (FE) models for the sheathing-to-framing joints are used for the analyses: A single spring model and a spring pair model, where the joint characteristics valid for the timber properties perpendicular and parallel to the grain are used. The maximum capacity of the wall segments is somewhat overestimated when using the spring pair model compared to that of the single spring model. The softening parts of the load displacement curves are significantly affected, regardless of whether the perpendicular or parallel characteristics of the joints are used. The results from FE simulations using models with perpendicular and parallel characteristics are compared with full scale test results for walls with a single segment loaded horizontally and diagonally. The behaviour of the wall segments subjected to horizontal loading is dominated by fastener displacements perpendicular to the bottom rail. Hence, FE models including perpendicular characteristics should be used. For diagonal loading the behaviour of the wall segments is dominated by displacements parallel to the framing members, and FE models including parallel characteristics should therefore be used. The analyses were extended to multiple segment walls resulting in the same type of behaviour as single segment walls. (C) 2014 Elsevier Ltd. All rights reserved.

Over the last decade timber residential buildings with a prefabricated load bearing structure intimber have become increasingly common. Occasionally prefabricated timber modules up tosix stories are built and this accentuate among other issues that of large relative verticaldisplacements due to elastic and time-dependent displacements and displacements byshrinkage. In the current study vertical displacements were measured on three modules loadedvertically by a total of 151 kN over the time period of 90 hours. In average the verticaldisplacements measured by the displacement gauges of the modules were 3.17 mm includingdeformation in both a rail and an elastomer isolating block for sound and vibration isolation. 

Timber roof structures built up by several trusses designed with structural timber members and jointed together by punched metal plate fasteners are considerably effective structures for long spans. They are considered cost-effective and can serve their purpose well during the lifespan of a roof. Lately, the limits of how long spans they can withstand for a normal loading situation have been pushed. Failures that often relate to lateral instability in the top chord or in other compressed members of the truss have been reported. The effect of various parameters on the load carrying capacity of the top chord is studied. Analytical solutions based on the method available in Eurocode 5 and a method based on the assumption of an elastic foundation is used in combination with finite element simulations. Results show that, in a Nordic context, the approach suggested in Eurocode is problematic due to the suggested bay length which is supposed to develop between lateral supports. Simulation results also suggests that load-displacement curves based on experiments performed on the connections involved should be used in simulations of the top chord rather than an elastic spring.

Production of CLT-panels typically results in 5-10% cut-offs due to window and door openings. CLT-boards can be made by slicing these cut-offs and finger-jointing them. It is worth while from economic and climatic aspects to find applications for such boards as building products with long service life. It is preferred to find uses where the premium transverse properties are utilized. This paper present three novel ideas for re-use of cut-offs from CLT. (a) The re-use as REX laminations in CLT production is a smart concept that minimize waste in an economical way. (b) By using finger-jointed CLT-boards as horizontal rails in timber frame buildings, the problems with over-load and deformations in multi-storey houses are minimized. (c) The capacity of glulam beams to carry concentrated loads is greatly enhanced by integrating CLT-boards as laminations. 

Wood specimens to each of which alaminate of carbon fibre reinforcement polymers(FRP) was glued (creating a lap joint in each case)were loaded to failure. A total of 15 specimens ofthree types differing in the glued length (anchoragelength) of the FRP laminate (50, 150 and 250 mmrespectively) were tested, their strength, stiffness andstrain distribution being evaluated. Synchronizeddigital cameras (charge-coupled devices) used intesting enabled strain fields on surfaces they weredirected at during the loading procedure to bemeasured. These results were also evaluated bothanalytically on the basis of generalized Volkersentheory and numerically by use of the finite elementmethod. The lap joints showed a high level ofstiffness as compared with mechanical joints. A highdegree of accuracy in the evaluation of stiffness wasachieved through the use of the contact-free evaluationsystem. The load-bearing capacity of joints ofthis type was found to be dependent upon theanchorage length in a non-linear fashion. The experimental,analytical and numerical results were shownto be in close agreement with respect to the strengthand the strain distribution obtained.

Four different elastic models for sheathing-to-framing connections are presented and evaluated on asingle connection level and on a shear wall level. Since the models are elastic in their nature they aresuitable mainly for cases where the sheathing-to-framing connections are subjected to monotonicallyincreasing displacements. Of the four models one is uncoupled and the others are coupled with respect tothe two perpendicular displacement directions in a two-dimensional model. Two of the coupled modelsare non-conservative, while the third is conservative, indicating a path independency with respect to thework done to reach a defined state of deformation. When the different models are compared it is obviousthat the uncoupled model gives strength and stiffness values higher than the others; however it is notobvious which of the models to use in a shear wall analysis, each of the models having its advantages anddisadvantages. For the experimental data used as input in the analyses of this study however, a couplednon-conservative model seems the most appropriate.