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.

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 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. 

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. 

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 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 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.  

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