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Moisture induced internal stresses in structural wood elementsby Professor Sven Thelandersson project leader and supervisor
Dr. Staffan Svensson co-supervisor, Post graduate student Johan J?nsson, M.Sc., http://woodtech.ce.luth.se/proj.sv/projkat/9835-9919-LTH, Lund University of Technology, Division of Structural Engineering
Failures of timber structures in practice are very often caused by stresses in the direction perpendicular to grain. The low strength of wood perpendicular to grain requires special considerations in design of timber structures. Structural engineers, generally unfamiliar with strongly anisotropic materials like wood, can easily make mistakes. But the problem is not only educational, the mechanisms and the governing factors behind perpendicular to grain failure in timber structures are not sufficiently known. This makes it also difficult to provide engineers with clear and correct information.
State of art
Considerable research efforts have been focussed on the risk for perpendicular to grain failure the last decades. One line of research has been to determine the perpendicular to grain stresses due to external mechanical loads for different types of structural glulam elements such as curved beams, frame corners, beams with varying cross section, arch beams and joints. The results from such studies are often presented in the form of simplified design rules for stresses perpendicular to grain to be included in codes or manuals. In many cases a simple stress criterion is not relevant for predicting failure in transverse tension. To predict failure more correctly, a fracture mechanics approach is needed. Fracture mechanics have been applied for wood with good result by several researchers for e.g. notched beams and beams with holes, see e.g. [1-2]. Researchers dealing with timber engineering have now recognised the major role of climatic actions not only as a cause for moisture movements and enlarged deformations under load, but also as strength reducing factor in the cross grain direction of wood. Recent research on the behaviour in cross grain directions has been focussed on the effects of climate, climate variation and climate history, see e.g. [3,4,5]. This research field is still in its beginning and systematic research efforts have to be made to predict effects of internal moisture induced stresses and to develop design methods where climatic actions are accounted for in a rational way.
The main objective of the proposed project is to develop and verify methods to predict moisture induced internal stresses generated in timber structures under the action of arbitrary varying climate. More specifically the goal is to clarify/determine
? the development of moisture induced internal stresses under short and long time exposure to variable climate (including cyclic moisture)
? the influence of sustained stress from external load on the development of moisture induced stresses for short and long time exposure
? the influence of internal stresses on basic strength in loading perpendicular to grain
? the statistical variability of internal stresses for a given moisture exposure
In the first 3 year phase of the project, two-dimensional moisture induced stresses will be investigated. Extensive tests will be performed on thin slices of glulam or solid timber beams, sealed on all but two sides so that moisture transfer can take place in only one direction. Released deformations caused by internal stresses are measured by a slicing technique and using digital image processing. The experimental program is divided into two main parts:
? Short term behaviour and exposure to single moisture change
? Long term behaviour and exposure to variable climate conditions
The tests will be simulated by computer based modelling in two steps:
1. calculation of moisture distribution,
2. calculation of moisture induced stresses based on the first step.
Later in the project, stress states formed in regions with three-dimensional moisture distribution fields, will be studied. This situation is typical near the end of a structural element or in the vicinity of holes and notches.
The project is expected to produce systematic knowledge about the relation between climatic actions and internal moisture induced stresses, making it possible to predict internal stresses from arbitrary short- and long-time climatic variations. The effect of stresses from externally applied load during moisture exposure on the development of moisture induced stresses will be demonstrated.
? Leicester, R. H. Some aspects of stress field at sharp notched orthotropic materials. Div. of Forest products, Paper 57, CSIRO, Austrailia, 1971.
? Gustafsson, P. J. A study of the strength of notched beams. Proceedings in CIB 18A paper 21-10-1, Parksville, Canada. beams. Part2: Long term tests and analysis. VTT Publications 334, Espoo, Finland, 1998.
? Aicher, S., G. Dill-Langer and A. Ranta-Maunus, Duration of load effects in tension perpendicular to the grain of glulam in different climates. Holz als Roh- und Werkstoff 56, 1998.
? Gustafsson P. J., P. Hoffmeyer and G. Valentin, DOL effect on end-notched-beams. Holz als Roh- und Werkstoff 56, 1998.
? Morlier, P and A. Ranta-Maunus, DOL effects of different sized timber beams. Holz als Roh- und Werkstoff 56, 1998.
More info of this article can be found on the web at: http://woodtech.ce.luth.se/proj.sv/projkat/9835-9919-LTH