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- Frühwald, Eva, et al.
(author)
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Design of safe timber structures - How can we learn from structural failures in concrete, steel and timber?
- 2008
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Conference paper (peer-reviewed)abstract
- During recent years, several spectacular collapses happened in large timber structures and the question is what can be done to prevent future failures. The main hypothesis for this project has been that quality assurance, control systems and improved training may be necessary, since the unwanted events are primarily related to human errors. A survey of failures in 127 timber structures has been made and results with an analysis of the underlying causes and associated conclusions and recommendations are presented. The most common failure causes are poor strength design (41%), poor principles during erection (14%), on-site alterations (13%) and poor design with respect to environmental actions (11%). Wood quality, production methods and production principles only cause a small part (together about 11%) of the failures. The problem is therefore not the wood material, but designers and workers in the building process.
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| 3. |
- Frühwald, Eva
(author)
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Effect of high-temperature drying and restraint on twist of Norway spruce
- 2007
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In: Drying Technology. - : Informa UK Limited. - 1532-2300 .- 0737-3937. ; 25:1-3, s. 489-496
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Journal article (peer-reviewed)abstract
- During drying, timber distortion is a major defect mainly due to shrinkage anisotropy, differences in longitudinal shrinkage, and spiral grain. The distortion can be reduced by external restraint and use of appropriate kiln schedules. The research presented here is part of a project on the improvement of shape stability of Norway spruce by high-temperature drying. The effects of drying temperatures between 80 and 170 degrees C and restraint on the extent of twist in Norway spruce were investigated on 30 cm lengths. Results show that significant twist reduction could be achieved in restrained specimens sawn from core wood. This effect was permanent even after exposure to subsequent moisture cycling. In addition to high-temperature drying, heat-treated material (Thermowood (R)) was also investigated. As for the high-temperature dried material, twist amplitude in moisture cycling was highest for pith-in specimens. Increased heat-treatment temperature reduced the twist amplitude, thus improving the shape stability. However, for the material dried in this study, shape stability was not influenced by the drying temperature.
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| 4. |
- Frühwald, Eva
(author)
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Effect of high-temperature drying on properties of Norway spruce and larch
- 2007
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In: European Journal of Wood and Wood Products. - : Springer Science and Business Media LLC. - 0018-3768. ; 65:6, s. 411-418
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Journal article (peer-reviewed)abstract
- In timber drying, mechanical properties may be changed due to treatment temperature and treatment duration. In general, when increasing the kiln temperature, drying time is decreased and some timber properties are negatively affected. In this study, the effect of different drying temperatures (80, 120 and 170 degrees C) on equilibrium moisture content and sorption rate, on bending strength and stiffness was investigated for Norway spruce and larch from four proveniences and the results were compared to those obtained for heat-treated wood (Thermowood(R)). The experiments confirm earlier research that both treatment temperature and treatment duration affect the properties. High-temperature drying or treatment can be optimized for several applications, yielding strong but not so durable timber or vice versa.
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| 5. |
- Frühwald, Eva
(author)
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Effect of high-temperature drying on spruce and larch Shape stability, mechanical properties and mould growth
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Shape stability, the ability of the wood to retain its shape, is one of the most important parameters when using timber as a building material and for other applications. Generally, timber undergoes changes in dimensions and shape when its moisture content is changed. However, this can often not be tolerated in constructions and thus, methods to improve the shape stability have to be found. In this thesis, the influence of high-temperature drying on the immediate shape stability after drying but also on the long-term behaviour under in-service conditions was studied for Norway spruce and different larch species. Special focus was on twist, as this is the predominant distortion type leading to downgrading of Scandinavian timber species. Small-scale specimens were dried at high temperatures (120?C and 170?C) and compared to conventional drying (80?C). All drying schedules, independent of temperature, included pre-steaming and steaming phases at saturated conditions (98?C, 98 % relative humidity). The effect of restrained drying or top-loading was investigated by restraining half of the boards during the drying process. After completed drying, all specimens were moisture-cycled to investigate the permanency of shape stability and distortion reduction attained during restrained drying. The influence of drying temperatures on shape stability of spruce was also compared to commercial heat-treatment. The results showed that high-temperature drying is an alternative for Scandinavian timber species such as Norway spruce and larch. Distortions, especially twist, were successfully reduced by restrained drying and the obtained shape stability was permanent, even in changing humidity conditions. Drying temperature did not influence shape stability, but similar results were found for shape stability for drying at temperatures levels 80, 120 and 170?C. The good shape stability for low-temperature drying (80?C) is most probably achieved by the pre-steaming and steaming phases, which constituted a short high-temperature treatment with corresponding softening and creep behaviour. Excessively twisted boards were heat-treated at 95?C to investigate the possibility to reduce twist. Twist could be effectively reduced by heat-treatment, however, twist increased slightly in subsequent moisture cycling. These results confirm earlier studies in that excessively twisted boards can be straightened by heat-treatment, as mechano-thermal creep helps to attain the desired shape of the board. High-temperature drying influences both the chemical and physical properties of timber. In this study, sorption behaviour, stiffness (static and dynamic modulus of elasticity in bending), bending strength and toughness were investigated. It could be shown that stiffness is more or less unaffected by drying temperature, whereas bending stress decreases with increasing drying temperature. The toughness tests did not show a clear tendency of strength loss with increasing temperature. Finally, the effect of different drying temperatures and heat-treatment on the susceptibility for mould growth was studied. It could be stated that all kiln drying temperatures resulted in about equal mould growth levels, whereas air-drying had lower and heat-treatment substantially lower mould levels. Other factors influencing the amount of mould growth were presence of sapwood or heartwood and the type of surface ? original drying surface or re-sawn surface.
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| 8. |
- Frühwald, Eva, et al.
(author)
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Image analysis study of mould susceptibility of spruce and larch wood dried or heat-treated at different temperatures
- 2008
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In: Wood Material Science & Engineering. - : Informa UK Limited. - 1748-0272 .- 1748-0280. ; 3:1-2, s. 55-61
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Journal article (peer-reviewed)abstract
- In this study, mould growth on wood was investigated by image analysis. The studied parameters were drying and heat-treatment temperatures (20-210°C), original and resawn surface and different wood species (spruce and larch). Small specimens—some of which were inoculated with a spore suspension—were stored under humid conditions and photographed once a week. Mould growth was assessed by image analysis. In general, results found in earlier studies regarding the influence of several parameters could be confirmed. Image analysis was found to be a useful method to quantify mould growth in an objective and reproducible way.
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| 9. |
- Frühwald, Eva
(author)
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Improvement of shape stability by high-temperature treatment of Norway spruce - Effects of drying at 120 degrees C with and without restraint on twist
- 2006
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In: European Journal of Wood and Wood Products. - : Springer Science and Business Media LLC. - 0018-3768. ; 64:1, s. 24-29
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Journal article (peer-reviewed)abstract
- During drying, timber changes its shape mainly due to shrinkage anisotropy, radial differences in longitudinal shrinkage and spiral grain. The warp, causing severe downgrading of the timber, can be reduced by restraint and appropriate climate treatments of different types. The research presented here is part of a larger project on the improvement of shape stability of Norway Spruce (Picea abies) by high-temperature treatment. In this part, a method for determining twist as well as results for a drying temperature of 120 degrees C are presented. The effects of presteaming, drying and steaming with and without restraint on the size of twist in Norway Spruce were investigated in laboratory scale. Short-term twist-reducing effects as well as the permanency of the reduced distortions in subsequent moisture cycling were investigated. Results show a clear dependency of twist on the distance from the pith. Furthermore, the twist is reduced in restrained specimens sawn close to the pith. This effect is permanent even after exposure to subsequent moisture cycling.
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| 11. |
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| 12. |
- Frühwald, Eva, et al.
(author)
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Teachers’ view on combining assessment elements into a holistic assessment concept
- 2008
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In: [Host publication title missing].
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Conference paper (peer-reviewed)abstract
- Abstract in UndeterminedAssessments of students’ knowledge and skills are sometimes habitually performed, but the design of proper assessment concepts – beneficial to both teaching and learning – ought to be a central issue for all course planners in higher education. The main purpose of this paper is to discuss a variety of assessment elements, as well as ways in which to combine them. It uses experiences from Lund University’s Faculty of Engineering (LTH) to provide suggestions and advice for course planners. The discussion is based on the combination of a literature review in the field of teaching and learning, of empirical material obtained from surveys and group discussions with 22 teachers at LTH, and of the authors’ own experiences in their capacity as course leaders and teachers. The study reveals a preference for projects and written assessment, as compared with the less preferred individual oral assessment. The paper indicates potential for quality enhancements through the adapted tailoring of assessment elements into holistic assessment concepts.
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