Ola Dahlblom

On Latin hypercube sampling for structural reliability analysis

Latin hypercube sampling is suggested as a tool to improve the efficiency of different importance sampling methods for structural reliability analysis. In simple importance sampling, where the sampling centre is moved from the origin to the design point, standard Monte Carlo sampling can be replaced by Latin hypercube sampling. The efficiency improvement is then highly dependent on the choice of sampling directions. Different versions of Latin hypercube sampling are also successfully employed to improve the more efficient axis orthogonal importance sampling method. By means of different numerical examples, it is shown that more than 50% of the computer effort can be saved by using Latin hypercubes instead of simple Monte Carlo in importance sampling. The exact savings, however, are dependent on details in the use of Latin hypercubes and on the shape of the failure surfaces of the problems.

A numerical study of the shape stability of sawn timber subjected to moisture variation: part 3: influence of annual ring orientation.

Summary Numerical simulations were performed to investigate how the annual ring orientation affects the shape stability of sawn timber. The influence of radial variations in the basic properties and the spiral grain is also studied. The knowledge obtained can contribute to more effective use of the raw material through allowing boards having properties that would yield bad shape stability to be sorted out. Possibilities for improving shape stability through gluing pieces of wood together are examined as well.

A Numerical Study of the Shape Stability of Sawn Timber Subjected to Moisture Variation, Part 2, Simulation of Drying Board

Abstract A theory for analysing the shape stability of sawn timber was implemented in a finite element program. To illustrate the types of results that can be obtained, the behaviour of a board during drying was simulated. The simulation yields information about unfavourable deformations and stresses during the drying process. To investigate factors that influence drying deformations, a parameter study was performed in which the influence of different constitutive models and different material parameters was studied. In addition, the influence of the spiral grain angle was examined.

A numerical study of the shape stability of sawn timber subjected to moisture variation: Part 1 Theory

A three-dimensional theory for the numerical simulation of deformations and stresses in wood during moisture variation is described. The constitutive model employed, assumes the total strain rate to be the sum of the elastic strain rate, the moisture-induced strain rate and the mechano-sorption strain rate. Wood is assumed to be an orthotropic material with large differences between the longitudinal, radial and tangential directions in the properties found. The influence of the growth rings, the spiral grain and the conical shape of the log on the orthotropic directions in the wood is taken account of in the model. A finite element formulation is used to describe the deformation process and the stress development during drying.

Finite element study of growth stress formation in wood and related distortion of sawn timber

Lack of straightness in timber is the most frequent complaint regarding solid (and laminated) timber products worldwide. Nowadays, customers demand higher quality in the shape stability of wood products than they did earlier. The final distortion of timber boards is mostly caused by moisture-related stresses in wood (drying distortions) and growth-related stresses (distortions appearing when logs are split up to timber boards by sawing). To get more knowledge on how these distortions can be reduced in wooden products, there is a need for improved understanding of this material behaviour through good numerical tools developed from empirical data. A three-dimensional finite element board distortion model developed by Ormarsson (Doctoral thesis, Publ. 99:7, 1999) has been extended to include the influence of growth stresses by incorporating a one-dimensional finite element growth stress model developed here. The growth stress model is formulated as an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The simulation results presented include how stresses are progressively generated during the tree growth, distortions related to the redistribution of growth stresses during log sawing, and distortions and stresses in drying reflecting the effects of growth stresses. The results show that growth stresses clearly vary during tree growth and also form a large stress gradient from pith to bark. This in itself can result in significant bow and crook deformations when logs are sawn into timber boards. The distortion results from the simulations match well with the results observed in reality. The parametric study also showed that the radial growth stress distribution is highly influenced by parameters such as modulus of elasticity, micro fibril angle and maturation strain.

Evaluation of Parameters of Bond Action between FRP and Concrete

This paper presents the evaluation of parameters describing bond action between fiber-reinforced polymer (FRP) and concrete in RC structures strengthened in flexure. The parameters were evaluated on the basis of fitting nonlinear finite-element results to experimental results from literature. Three-dimensional (3D) finite-element simulations were used to account for transversal effects without introducing any geometry-related correction coefficients in the material model. The parameters defining the material model, describing bond action, were initial stiffness, shear strength, and fracture energy of the FRP-concrete interface. Bilinear, trilinear, and exponential curve shapes were evaluated. The parameters were related to concrete tensile strength and shear stiffness of the adhesive. According to the analysis, the models provide a good estimation of ultimate load and strain distribution in FRP compared with test results. The results showed that whether the bond-slip curve was assumed to be bilinear, trilinear, or exponential has a minor effect.

Numerical study of how creep and progressive stiffening affect the growth stress formation in trees

It is not fully understood how much growth stresses affect the final quality of solid timber products in terms of, e.g. shape stability. It is, for example, difficult to predict the internal growth stress field within the tree stem. Growth stresses are progressively generated during the tree growth and they are highly influenced by climate, biologic and material-related factors. To increase the knowledge of the stress formation, a finite element model was created to study how the growth stresses develop during the tree growth. The model is an axisymmetric general plane strain model where material for all new annual rings is progressively added to the tree during the analysis. The material model used is based on the theory of small strains (where strains refer to the undeformed configuration which is good approximation for strains less than 4%) where so-called biological maturation strains (growth-related strains that form in the wood fibres during their maturation) are used as a driver for the stress generation. It is formulated as an incremental material model that takes into account elastic strain, maturation strain, viscoelastic strain and progressive stiffening of the wood material. The results clearly show how the growth stresses are progressively generated during the tree growth. The inner core becomes more and more compressed, whereas the outer sapwood is subjected to slightly increased tension. The parametric study shows that the growth stresses are highly influenced by the creep behaviour and evolution of parameters such as modulus of elasticity, micro-fibril angle and maturation strain.

NUMERICAL AND EXPERIMENTAL STUDIES ON INFLUENCE OF COMPRESSION WOOD TIMBER DISTORTION

ABSTRACT In timber exposed to moisture variations, drying distortions is a serious problem that might result into sawn timber and other wood products unsuitable for construction purposes. Two characteristics of wood are that its behaviour is strongly orthotropic and that it is very sensitive to variations in moisture content. In addition, wood is characterised by variation in its properties from pith to bark. A further important property of wood, which affects its behaviour, is its spiral grain. For timber containing much compression wood the drying distortions are also highly dependent upon where the compression wood is located in the sawn boards. The present study concerns an experimental investigation of density, grain angles, shrinkage parameters and longitudinal elastic modulus in a number of spruce boards containing much compression wood. On the basis of the data obtained, numerical simulations have been carried out in order to determine the deformations that developed in the boards during changes in moisture content.

Optimization, a tool with which to create an effective drying schedule

A method for defining effective schedules for kiln drying of wood is presented. The method is designed in such a way that it proposes an optimized variation of temperature and humidity which yields ...

Using CORBA middleware in finite element software

Distributed middleware technologies, such as CORBA can enable finite element software to be used in a more flexible way. Adding functionality is possible without the need for recompiling client code. Applications and libraries can expose their functionality to other applications in a language neutral way, enabling a more direct and easy transfer of data, without the need for intermediate input and output files. The CORBA software components can be easily configured and distributed transparently over the network. A sample structural mechanics code, implemented in C++ is used to illustrate these concepts. Some future directions, such as placing CORBA enabled finite element software on HPC centres are also discussed.