Modeling of nailed timber connection : Displacement path dependency in sheathing-to-framing connections

Connections in wood have been investigated and advanced ever since the ground-breaking work of Johansen in the early nineteenth century. Nevertheless, not much investigation has been undertaken on the existence of load-displacement path dependency in a sheathing-to-framing connection. Herein, a sheathing-to-framing connection is investigated in relation to displacement path dependency. This work uses 3D Finite Element beam-on-foundation models of an Oriented Strand Board (OSB/2) sheathing nailed to a C24 wood framing, to study possible strategies to numerically simulate the displacement path dependency. The models are used to study if non-linear elastic or elastic-plastic embedment properties of an annular-ringed shank nail in the wood-based materials bring about the path dependency using Connector elements in combination with different material models in the FE software Abaqus. Numerical results are compared with corresponding experimental test results of the connection together with the Eurocode 5 approach. The outcome of the numerical study both; confirms the existence of displacement path dependency and shows that this property in the connection can be described by plasticity properties in nail, sheathing material and the wood framing

Flerfaskontinuummodellering av trä baserat på blandningsteori

Wood has been used as a construction material for a very long time. The development of efficient industrial production processes of wood has expanded the use of the material with the introduction of new products, such as engineered wood products. Considering the adversely changing climate, the use of wood in construction is advocated due to its environmental benefits, such as its low carbon footprint. As a naturally growing material, however, wood has a high moisture content when harvested. Additionally, the chemical composition of wood fibers together with its porous structure, gives wood a strong affinity towards moisture, throughout the whole lifecycle of the material. The moisture content in wood strongly influences its physical and mechanical properties, such as strength, stiffness, shape stability and durability properties. Further, it requires energy-intensive drying processes to bring wood to the desired moisture content for structural use. The task of predicting the moisture content and transport of moisture in wood is challenging. It involves multiple phases, i.e., liquid water, gaseous vapor and the solid wood fibers, and it also engages a number of physical processes such as evaporation/condensation, adsorption/desorption, diffusion and seepage of the fluids, heat conduction and swelling/shrinkage of the wood fibers. This thesis investigates the interplay between heat, moisture and their associated transport mechanisms in wood. The mechanics of the solid wood material is also studied. The primary goal of this thesis is to develop a thermodynamically consistent continuum model that is capable of predicting the macroscopic behavior of wood subjected to varying climate conditions and mechanical loading. The hybrid mixture theory is used todevelop a multiphase continuum model for wood, in which, at the macroscale, the wood material is considered to contain immiscible solid, liquid and gaseous phases. Constitutive relations are derived by fulfillment of the entropy inequality at the macroscopic scale. Interaction processes involving phase changes through sorption and evaporation/condensation, and diffusive transport mechanisms are described using the macroscale chemical potential as defined by the hybrid mixture theory. The thesis starts with introductory chapters describing the overall properties of wood of importance in this context and the interactions between wood and moisture. A summary of the mixture theory as applied to this work is also presented. The thesis contains four attached papers, Paper I, Paper II, Paper III and Paper IV. In Paper I a model describing moisture transport and sorption processes in wood below the saturation point of the wood fibers is presented. The model is developed further, in Paper II and Paper III, to incorporate wood-water interactions below and above the fiber saturation point. Shrinkage/swelling and non-linear elastic deformations are also implemented. A drying test simulation of wood starting from the green state is performed and compared to experimental results. The model presented in Paper II and Paper III is complemented in Paper IV by considering damage associated with anisotropic cracking of the solid wood material. The phase field fracture modeling approach is used for this purpose. The resulting non-linear coupled partial differential equations governing the macroscopic behavior of the material are solved numerically using the finite element method. Simulations are performed to check the overall performance of the theoretical framework behind the proposed models and they are compared to experimental results for the identification of some of the material parameters of the models.Trä har använts som byggnadsmaterial under mycket lång tid. Utvecklingen av effektiva industriella förädlingsprocesser har gjort att nya produkter, såsom korslimmat trä, ständigt utvecklas. Användningen av trä i byggnader har vissa miljöfördelar, till exempel dess relativt låga koldioxidavtryck och dess kollagrande förmåga. Då trä är ett naturligt växande material så har det en hög initial fukthalt. Dessutom ger den kemiska sammansättningen av träets fibrer samt dess porösa struktur materialet en stark affinitet till fukt under hela materialets livscykel. Fukthalten i trä påverkar starkt dess fysiska och mekaniska egenskaper såsom styrka, styvhet, formstabilitet och beständighetsegenskaper. Vidare krävs det mycket energikrävande torkningsprocesser för att göra materialet lämpligt för användning i bärande konstruktioner. Att beskriva en fysikaliskt stringent modell som beskriver fukthaltsfördelning och transport av fukt i trä är utmanande. Trä består bland annat av cellulosafibrer som i sin tur innehåller vattenånga och vatten i vätskefas. Ett antal olika fysikaliska processer såsom absorption/desorption, ångdiffusion, kapillärsugning, värmeledning och svällning/krympning av träfibrerna är centrala i detta sammanhang. Denna avhandling undersöker samspelet mellan värme, fukt och tillhörande transportmekanismer i trä samt hur dessa processer kopplas till solidmekaniska töjningar och spänningar. Det primära målet med denna avhandling är att utveckla en termodynamiskt stringent kontinuummodell som kan förutsäga det makroskopiska beteendet hos trä som vid mekanisk belastning även utsätts för varierande fukt- och temperaturförhållanden. En så kallad blandningsteori (Hybrid Mixture Theory) används för att ta fram de styrande ekvationerna på makroskopisk nivå där materialet antas innehålla distinkta fasta, flytande och gasformiga faser. Konstitutiva relationer härleds genom att på olika sätt se till att entropi-olikheten på makroskopisk nivå alltid uppfylls. Interaktionsprocesser genom förångning/kondensation i materialet (sorption) och transportmekanismer beskrivs med hjälp av den makroskopiska kemiska potentialen så som den vanligen definieras i blandningsteorin. Avhandlingen inleds med en övergripande beskrivning av träets egenskaper och dess uppbyggnad. Den använda teorins övergripande struktur och hur den tillämpas för trä i detta arbete presenteras också. Avhandlingen innehåller fyra bifogade artiklar, Paper I, Paper II, Paper III och Paper IV. I Paper I presenteras en modell som beskriver fukttransport och fuktfixering i fuktområden under fibermättnadspunkten. Modellen vidareutvecklas i Paper II och Paper III för att även kunna ta hänsyn till fukttransport och fuktfixering över fibermättnadspunkten. I detta sammanhang implementeras icke-linjära elastiska deformationer och dess koppling till krympning- och svällningsprocesser. Torksimuleringar av träprover som initialt är över fibermättnadspunkten utförs och jämförs med experimentella resultat. Modellen som presenteras i Paper II och Paper III kompletteras i Paper IV där även sprickbildning av trä beaktas. En speciell typ av brottmekanikmodell används för detta ändamål (Phase-field crack model). De styrande icke-linjära kopplade partiella differentialekvationerna, som utgör modellen för det makroskopiska beteende hos trä i detta arbete, löses numeriskt med hjälp av finita element metoden. Diverse simuleringar utförs för att visa på modellens övergripande möjligheter. Olika aspekter av det föreslagna teoretiska ramverket testas med hjälp av numeriska simuleringar, där de teoretiska resultaten analyseras och jämförs med en rad experimentella observationer för identifiering av några av modellens materialparametrar

A model for multiphase moisture and heat transport below and above the saturation point of deformable and swelling wood fibers – I : Mass transport

A thermodynamically consistent model for heat and mass transfer in deformable wood fibers is developed. Thehybrid mixture theory is used to model the material as a mixture of three phases, consisting of a solid, a liquidand a gas phase. The solid phase consists of dry fibers and bound water constituents, whereas the gas phasehas dry air and water vapor constituents. Emphasis is put on the mass flow and mass exchange of moisture inthe material both below and above the saturation point of the solid wood fibers. Generalized forms of Fick’s,Darcy’s and Fourier’s laws are derived, and the chemical potential is used as a driving force for mass flow.Mass exchange due to sorption and evaporation/condensation processes is implemented in the model, wherehysteretic properties both within and above the hygroscopic moisture range are described using Frandsen’shysteresis model. Moisture induced swelling/shrinkage is included where the porosity of the material canvary. A large strain setting formulated for general orthotropy is adopted for the mechanical deformations. Toshow the performance of the resulting model, it is implemented in a finite element method framework andused to simulate the processes of heat and moisture transport dynamics of a wood sample subjected to dryingfrom an over-hygroscopic moisture state

A model for multiphase moisture and heat transport below and above the saturation point of deformable and swelling wood fibers-II : Hygro-mechanical response

A non-linear elastic constitutive model for a swelling/shrinking orthotropic wood matrix is proposed. Themodel is thermodynamically consistent, derived based on the principles of continuum mechanics and thehybrid mixture theory. Moisture induced strains are introduced considering finite deformations by assuming amultiplicative split of the deformation gradient tensor into a swelling and an elastic part. Novel definitions ofthe Cauchy stress tensor and the moisture-dependent elastic material tangent matrix are obtained. The modelis coupled to a multi-phase transient mass and heat transport model developed in Part I of this work. Inthis part of the work 2-D and 3-D test examples are used to describe the ability of the model to simulatemoisture-induced distortions when drying wood within the hygroscopic and also from the over-hygroscopicmoisture ranges. Despite deriving the model considering wood, the obtained constitutive relations can besuitably adopted to other orthotropic porous materials displaying properties similar to that of wood

Three-dimensional orthotropic nonlinear transient moisture simulation for wood : analysis on the effect of scanning curves and nonlinearity

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