Prefabricated timber modules are increasingly used as load-bearing structures in multi-storey residential buildings. Unlike traditional applications where they serve as non-load-bearing elements within superstructures such as steel frames, these modules must now support not only their own dead weight but also imposed loads, snow loads, wind loads, and more. This means higher need of more accurate predictions of the degree of utilization for both ultimate and serviceability limit states in various structural elements. In this study, an effective structural element based 3D finite element (FE) model initially developed and experimentally validated for small prefabricated modules has been further refined. The paper aims to validate the enhanced FE model, analyze inter-modular connection slip and shear deformations under varying loads, and identify key parameters influencing racking behavior in different module types. The model is experimentally validated against two full-size modules — one designed by platform framing and the other by balloon framing — and used to simulate various load scenarios in parametric studies. The model demonstrated satisfactory prediction of the racking stiffness and strength compared to experimental results. Furthermore, simulations revealed the influence of door opening placement and differences between platform and balloon framing on the non-linear racking behaviors. Balloon framing, in particular, offers advantages for reducing shear deformations within the module. The study also investigates the structural behavior of the inter-modular connections. The observed slip deformations in these connections can significantly affect the global racking behavior of a multi module structure. For a horizontal load F = 63.7 kN, the slip deformation of the inter-modular connections become larger than the shear displacements within the test modules.

This study addresses the instability of wooden trusses assembled with punched metal plates. The instability of compressed wooden elements is a complex problem due to the specific boundary conditions, the timber orthotropy, and the difficult quantification of the defects. This research presents an analytical framework based on the Eurocode approach for predicting the instability of compressed wooden elements, considering the effect of boundary constraints representative of punched metal plates. The general aspects of this research are twofold: (i) proposing an analytical approximate expression for assessing the theoretical buckling load of compressed beams with elastic boundary constraints; (ii) deriving the buckling design curves as a function of the geometric imperfection of the structural element. The authors refer to the constraints exerted by punched metal plates, experimentally characterized to determine the response along the six degrees of freedom. The experimental results were used to generate a high-fidelity finite element (FE) model of the connection, validate it using digital image correlation, and estimate by extrapolation the stiffness properties of a selection of punched metal plates. Additionally, a secondary FE model was developed to simulate the out-of-plane deflection of structural elements with different types of punched metal plates, predict the failure load from static incremental analysis, and estimate the buckling design curves. In conclusion, the research aims to specialize the design method of compressed members according to the Eurocode, taking explicitly into account the boundary constraints representative of punched metal plates. It is found that while the theoretical instability load of beams with elastic constraints closely approximates that of the clamped condition, the instability load under imperfections resembles the pinned condition more closely. This observation leads to systematically higher imperfection coefficients for elastic constraints than pinned conditions.

Prefabricated timber modules can help make the building sector more sustainable by reducing greenhouse gas emissions. However, structural challenges, like vertical relative deformations and the buckling of timber studs on timber rails still limit the height of tall timber buildings. These challenges are affected by how studs and rails interact. This study aims therefore to investigate this interaction by experimental tests and finite element (FE) modelling of five-layer Cross Laminated Timber and structural timber bottom rails under compression loads applied perpendicular to the rails via structural timber studs. Results from the conceptual compression tests with centric and eccentric loads show that CLT bottom rails have a much higher loading-bearing capacity compared to structural timber bottom rail. Additionally, local penetrations were observed in the contact zone between stud and rail which were included in the FE models allowing to estimate the contact stiffness.

I samband med byggandet av ett större timmerhus i Västra Ämtervik 2024 väcktes frågan om kvaliteten hos grova timmerbalkar. Dessa används ofta som bärande delar där materialets styvhet och hållfasthet är avgörande, men kunskapen om deras egenskaper och hur sorteringsmetoder fungerar är begränsad. Visuell sortering placerar ofta balkarna i klass C24, trots att kvaliteten tycks vara högre. För att bättre utnyttja materialets potential inleddes därför en pilotstudie i samarbete mellan Wermlands Byggnadsgille, Rinns såg, Sweco, Klara arkitekter och Karlstads universitet. 

Tre furubalkar med tvärsnitt 155 × 257 mm testades statiskt och dynamiskt. Balkarna hade en fuktkvot på omkring 15–20 % vid leverans, tät årsringstruktur (1,2 mm), hög densitet (508 kg/m³) och en elasticitetsmodul motsvarande den höga klassen C35 (E₀ = 13 000 MPa). Böjhållfastheten var något lägre än normalt, troligen på grund av fukt, och en balk brast i skjuvning till följd av torksprickor. De dynamiska metoderna visade god förmåga att uppskatta styvheten även för dessa testade grova balkar.

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.