Holzforschung Austria

Climate change affects the growth conditions for Norway spruce (Picea abies) which is the most important species for the European construction timber industry. As a countermeasure, the species mix in European forests is being changed to include more hardwood, but also more drought-resistant softwood species, for example Douglas-fir (Pseudotsuga menziesii). Previous research has shown that Douglas-fir can on the one hand be suitable to produce high-strength material. On the other hand, however, Douglas-fir wood can also have a considerably lower strength than what is expected from spruce. Therefore, there is need for improved strength prediction and strength grading methods for Douglas-fir, to enable the correct allocation of Douglas-fir roundwood to the most suitable target product. In the present study, computed tomography (CT) image reconstructions of 53 Douglas-fir logs were used to predict the strength of the sawn timber and thus to identify logs which are suitable to produce glulam lamellas. To achieve this, statistical modelling was combined with Finite Element (FE) modelling of destructive tensile tests, based on simulated fibre orientations for the boards.

In Europe, fifth percentile values are required for the calculation of characteristic values of strength and density. The European standard EN 14358:2016 defined three ways to calculate a 75% lower confidence bound (LCB) for such fifth percentile values, based either on a lognormal parametric approach, on a normal parametric approach or on a non-parametric approach. Using simulated data with different sample sizes and with different underlying distributions, this paper studied the effects of using each of the three approaches of EN 14358. As the third approach in EN 14358 did not seem to be fully non-parametric, the simulation study included, as a fourth approach, a fully non-parametric calculation of the LCB for the fifth percentile. The simulation study confirmed that both non-parametric approaches led to acceptable results for some important distributions, although the non-parametric approach defined in EN 14358 seemed to be more conservative especially for data with a non-normal distribution. The study also confirmed that the use of an incorrect parametric assumption can lead to systematically misleading LCB values for the fifth percentile. The authors recommend replacing the non-parametric approach currently defined in EN 14358 by a fully non-parametric approach. This approach can easily be implemented in a standard.

There are four inherent problems when predicting the airborne sound insulation of cross-laminated timber (CLT) plates. (1) The internal loss factor of wood products is high and therefore difficult to measure using the structural reverberation time technique. (2) Solid wood is a low-density building material compared to reinforced concrete or masonry construction. (3) Timber is orthotropic. (4) Orthotropic behaviour includes low shear moduli. CLT plates, similarly to thick masonry or concrete plates, exhibit thick plate behaviour in the high frequency range (f>1600Hz). The high frequency sound insulation data of nine plates is examined. Transitions to a thick plate seems to occur in all measurements. However, a clearly observable plateau is only visible for six of the nine plates. Simple methods are applied to extract information about the CLT plates from this high frequency (f>1600Hz) measured data. The data extracted from the plate is compared to typical values for equivalent isotropic plates, where the bending stiffness is assumed to be B_iso=√(B_x B_y ). The results are discussed in the context of laboratory measurement of sound insulation.

A global increase in the wood fuel pellet market requires knowledge of new biomasses pelleting abilities. As large-scale industrial tests of new materials are costly, tests in e.g., a single pellet press (SPP) are desirable. SPPs have many different configurations and it typically produces one pellet at a time and can give results of its pelletability. This review has surveyed the research that has been carried out of SPPs to ascertain the feasibility of comparing their obtained data and the results. The results show that it is almost impossible to compare the data and results of the various different SPP studies, e.g., some information from the data used was missing, resulting in that only 27 out of 70 papers were comparable. One solution could be the introduction of a common SPP testing method using a determined set of data that enables a reference pellet to be produced in every study.

This contribution discusses recent progress in development of windows equipped with vacuum glass. Vacuum glazing is commonly understood as glass products consisting of two parallel glass panes with a very narrow gap. Moreover, they feature a tight edge seal and house a grid of distance pillars. During production the gap is evacuated. As a result, vacuum glazing products widely eliminate convective heat transfer and minimize conductive heat transfer. As such, they represent highly-insulating glass products that regularly feature U-values in the range of triple-glazing, or even below (about 0.2 to 0.3 W.m ⁻² .K ⁻¹ ). While the research pertaining to the development of vacuum glass can be traced back to the first patent of 1913 [1], relatively few research efforts have been conducted regarding the application of vacuum glazing in window constructions. In this context, the present contribution focuses on the application of vacuum glass products in window constructions. Toward this end, two application cases are addressed in detail. One effort addresses the application of vacuum glass in new window constructions. The major objective of this effort is the reduction of heat flow through the window construction. Thereby, innovative paradigms involving multiple operation, size, and construction options are considered. The second case deal with the potential of vacuum glazing products in view of deployment in thermal retrofit of existing buildings. This application case does not only focus on the thermal performance of the windows, but also on the necessity to protect the appearance of heritage building facades (including those of the historical window constructions). As such, vacuum glazing products provide an alternative to replacing existing windows with high-insulating triple-glazed products.

The suitability of common analytical methods for the determination of active substances from wood preservatives in aged wood samples was investigated during an interlaboratory study. Permethrin, propiconazole and tebuconazole were quantified in 1.5 and 8 year-old wood samples by gas chromatography and liquid chromatography. Generally, the applied methods yielded reliable results for these samples. However, wood components can coelute with propiconazole and tebuconazole during liquid chromatography. Optimization of separation might be required if UV detection is applied.

Tree bark is an interesting source for various building and furniture materials. It holds a high number of volatile components—resulting in emissions whose effects on building residents have to be considered. Spruce and larch barks were dried using different methods and prepared as loose particles and panels. The VOC (volatile organic compound) emissions after 3 and 14 days were measured in a small chamber using gas chromatography and coupled mass spectroscopy. The influence of bark type, drying method, hot pressing and time on the emissions was quantified. The total volatile organic compounds (TVOC) emissions from spruce bark were higher than those from larch bark. High-temperature treatment and time significantly decrease TVOC emissions from the investigated barks. Terpenes, aldehydes and acids were analysed in the emitting gases. The high temperatures whilst pressing panels are problematic due to furfural constitution. Its emissions and partly those of 2-octenal and acetic acid are relevant for present and upcoming evaluation schemes. Aldehydes are the critical substance when using bark in the interior and must be controlled in product development.

Indoor air quality (IAQ) was investigated in 13 newly-built, occupied timber houses on a long-term basis. The study included a comparison of the construction types timber-frame (TF) and solid wood (SF), in addition two different ventilation types, controlled vs. window ventilation, were contrasted. The emission progression of volatile organic compounds (VOCs) including formaldehyde, was recorded and compared with the subjective well-being of the residents, which was identified by use of standardized questionnaires. This was completed with toxicological assessment and repeated measurements of specific medical parameters of health indicative character. It was found that VOC-emissions were initially elevated regardless of construction and ventilation type. However, after a period of up to 8 months emissions mostly decreased to an average level. Whereas, the SW constructions released distinctly more terpenes compared to the TF houses, there was no significant difference regarding the total concentration of emitted VOCs. The use of controlled ventilation systems resulted in lower VOC-concentrations and thus in higher IAQ compared to window ventilation. From a toxicological point of view the major part of the investigated houses were unobtrusive and IAQ was considered as “high” or “satisfactory.” Residents were continuously very satisfied with their health and quality of life. This perception was confirmed by the results gained from the accompanying medical examinations, giving no indication for physical impairments.

For the proper simulation of hygrothermal processes in roof constructions with ventilation layers the knowledge of climate conditions within the ventilation layer is requisite. In this work a model for the assessment of temperature and air humidity has been developed using multiple regression analysis. Therefore, the climate conditions inside the ventilation layers of differently covered and oriented roofs have been monitored for one year. Relevant outside climate parameters for the calculation of ventilation layer climates have been identified. The comparison between measured and calculated values indicated an adequate accuracy of the developed model with limitations for the use in snow fall periods.

This contribution reports on recent advances in the utilization of vacuum glass in contemporary window construction. Generally speaking, vacuum glazing consists of two glass panes with an evacuated interstitial space. To maintain the functionality of the glazing, a vacuum-tight edge seal and a grid of distance-holding pillars are required. Vacuum glazing features a first-rate thermal performance as it significantly reduces conductive and convective heat transport rates. In comparison to multi-pane insulation glasses of comparable thermal performance, vacuum glass products feature a reduced weight and construction depth. However, the application of vacuum glass in windows requires a critical rethinking of the current practice of window construction, especially with regard to thermal bridges and the related surface condensation risk at the glass/frame-construction joints. Specifically, the glass edge seal, which can be considered to be the weak spot of vacuum glass in terms of heat transfer, requires an insulating cover that is not provided in typical insulation glass frame configurations. Further relevant aspects to be considered include the structural stability of window constructions with vacuum glass, the acoustical performance, and issues regarding usability. In this context, the present contribution highlights the methodology and findings of two recent research projects (MOTIVE, FIVA) that addressed window construction requirements with regard to vacuum glazing deployment.

Aim The aim was to decipher Europe‐wide spatio‐temporal patterns of forest growth dynamics and their associations with carbon isotope fractionation processes inferred from tree rings as modulated by climate warming. Location Europe and North Africa (30‒70° N, 10° W‒35° E). Time period 1901‒2003. Major taxa studied Temperate and Euro‐Siberian trees. Methods We characterize changes in the relationship between tree growth and carbon isotope fractionation over the 20th century using a European network consisting of 20 site chronologies. Using indexed tree‐ring widths (TRWi), we assess shifts in the temporal coherence of radial growth across sites (synchrony) for five forest ecosystems (Atlantic, boreal, cold continental, Mediterranean and temperate). We also examine whether TRWi shows variable coupling with leaf‐level gas exchange, inferred from indexed carbon isotope discrimination of tree‐ring cellulose (Δ¹³Ci). Results We find spatial autocorrelation for TRWi and Δ¹³Ci extending over a maximum of 1,000 km among forest stands. However, growth synchrony is not uniform across Europe, but increases along a latitudinal gradient concurrent with decreasing temperature and evapotranspiration. Latitudinal relationships between TRWi and Δ¹³Ci (changing from negative to positive southwards) point to drought impairing carbon uptake via stomatal regulation for water saving occurring at forests below 60° N in continental Europe. An increase in forest growth synchrony over the 20th century together with increasingly positive relationships between TRWi and Δ¹³Ci indicate intensifying impacts of drought on tree performance. These effects are noticeable in drought‐prone biomes (Mediterranean, temperate and cold continental). Main conclusions At the turn of this century, convergence in growth synchrony across European forest ecosystems is coupled with coordinated warming‐induced effects of drought on leaf physiology and tree growth spreading northwards. Such a tendency towards exacerbated moisture‐sensitive growth and physiology could override positive effects of enhanced leaf intercellular CO2 concentrations, possibly resulting in Europe‐wide declines of forest carbon gain in the coming decades.

As an effect of changing forest management—away from softwood monocultures to more robust mixed stands—the availability of hardwood on the European timber market increases. Thus, a more diversified spectrum of hardwood products is required between the established uses in furniture and energy production. Glued timber products are a promising option in this respect. One important prerequisite for efficiently producing glued hardwood products is to establish hardwood strength grading. To this end, the current paper explored the potential of microwave scanning, stand-alone or combined with the measurement of dynamic stiffness, to estimate the tension strength of ash, beech, sweet chestnut and oak lamellas. In this preliminary study, combining microwave and dynamic stiffness measurement showed much potential for hardwood strength grading for all four species; for beech and sweet chestnut, coefficients of determination (\({r}^{2}\)) beyond 60% could be achieved, which is on a level with established softwood grading principles. For ash and oak, \({r}^{2}\approx 45\%\) was observed, which is acceptable for machine strength grading. The paper also considered measuring density using microwaves. Such a density measurement was found to be as accurate for hardwoods as for softwoods.

This study deals with the influence of electron beam irradiation (EBI) on the wettability of Norway spruce surface samples. To evaluate this possible effect in detail, the changes in chemistry and wood structure were analysed using the methods of IR spectroscopy and Scanning Electron Microscopy (SEM). The surface wettability was determined by the contact angle measurement method. The results of infrared spectra show decreased relative numbers of available hydrophilic groups (e.g. hydroxyl and carbonyl groups) in the wood surfaces due to the electron beam irradiation. This behaviour is consistent with the values from the surface wettability measurements. Structural differences of the wood surfaces after electron beam irradiation were not detectable. Based on these findings pre-treatment of wood surfaces for industrial usages can be a potential field of application of this technology.

A successful coupling of architectural design with multi-aspect building performance assessment is a complex, but necessary requirement for today’s building planning- and retrofit-activities. Architects are required to not only possess the vocabulary and basic knowledge in multiple fields, but must also work in collaborative design teams, composed of different domain specialists (e.g., structural engineers and building simulation experts). However, training in collaborative work is rarely provided in academic surroundings. In this contribution, we describe an educational effort toward interdisciplinary work on a specific and clearly defined architectural design task, which strongly necessitates the consideration of performance mandates. The task is the retrofit and redesign of an existing building façade from the 1950s. “Rationalist” buildings of this period often display reasonable functional solutions and good daylight availability, but they have performance shortcomings in other areas. These encompass, for instance, poor thermal performance of the envelope, lack of sufficient indoor environmental control, and unsatisfactory overall appearance. In a combined design studio and project course for building performance modelling, students from different disciplinary backgrounds formed interdisciplinary design teams. These teams worked together on façade retrofit ideas for the aforementioned building, considering both aesthetic aspects and performance issues from the very first design sketch. This led to the development and performance evaluation of a number of original façade retrofit ideas. In addition, the students were asked to devise the building process management. They thus had to consider not only design issues, but practical matters of building construction. The present contribution illustrates the scope, the applied approaches, and the concrete results of this interdisciplinary academic effort.

To introduce the research question a brief overview of existing computational models is given. Up till now, there is merely a guideline but no European standard for structural‐sealant‐glazing systems. Consequently, the focus will be on the experimental large‐sized specimens with load‐bearing adhesives, after that an optimized specimen based on them will be analyzed. This optimizations are just basis of theoretical considerations. The main aspect is to evaluate and interpret the deformation mechanisms over a long‐term observation period and unsteady climatic conditions. The feasibility studies, performance assessments and optimization of bracing timber‐glass‐composite (TGC)‐façades is a main component. Therefore, ideal four‐story buildings are exposed to horizontal wind force. To increase their stiffness a horizontal and vertical connection (coupling) between the TGC‐elements is analyzed. In addition to the commonly used silicone adhesive a stiffer epoxy is used as an alternative. In this way, it is also thinkable to reach higher stiffness. TGC‐façades should consequently replace the stiffening walls and take all the bracing function. To calculate their utilization and resistance, stress and stability analysis are required. Safety factors and strength values based on scientific knowledge are the basis for that. Finally, various component‐adaptations are carried out step by step. This illustrates the complex relationship in the stiffening behavior.

Semitransparently coated wooden building elements such as facades and windows undergo discolouration during exposure to sunlight, depending on the environmental conditions and protective properties of the coatings. The present paper aims at the development of numerical prediction models of photo-induced discolouration as a basis for texture synthesis for CAD applications. Wood samples of European larch and Norway spruce were coated with several (semi-) transparent coatings with varying colours and dry film thicknesses and exposed to Xenon arc light with outdoor cut-off filter to simulate outdoor irradiation conditions. Colour prediction models were developed from the obtained colour measurement data in the CIE XYZ colour space, since the progression of corresponding values showed a monotonous curve characteristic of each coating system. A novel colour measurement procedure was developed to estimate the colour of early- and latewood in the annual rings of wood. This method made it possible to generate colour-consistent textures for use in rendering software, as demonstrated on two examples.

This paper aims to sketch the research results on machine grain angle determination on six indigenous hardwoods and spruce as reference. The system widely used for softwood grain angle determination (tracheid effect) is only restrictedly suitable for hardwoods. Therefore, the systems microwave scanning and electrical field strength measurement were tested as well as the above-mentioned tracheid effect with optimized settings. The measured grain angles were compared to visually determined splitting angles (reference angles). All three machine systems give reliable grain angle values for spruce. For oak, the tracheid effect is not applicable. The reference angle splitting contains weaknesses. Nonetheless, high coefficients of determination (R2) indicate a functioning automatic measurement system. Thus, it can be concluded that for the majority of the tested species automatic grain angle determination is possible. Ash does not show satisfactory results.

For a reduction of heat energy losses much effort has been undertaken during the last years to improve insulation, heating and ventilation systems. Despite, wind-tightness is often disregarded, even though the importance of heat losses through wind-washing has already been shown by research findings. Especially the realization of the wind-tightness at the intersection of wall and rafters proves to be challenging due to difficult geometrics at this position. The presented study clarifies the influence of different wind-tightness qualities at the wall-roof intersection on the heat flow through a rafter roof. For that purpose laboratory tests were carried out. The gained data shows that the insulation material has a very strong effect on the increased heat flow through the roof caused by wind-washing. The results also point out that a certain duration and value of the pressure difference (wind load) between eaves and attic is necessary to cause an effect. The influence of the pressure difference is more pronounced compared to the influence of the gap geometry at the wall-roof intersections. While the projecting roof and the fixation of the roof underlay have a very strong effect, the inclination of the roof has no significant influence on the additional heat flow due to low wind-tightness.

Due to their bio-based character, oil-based coatings become more and more prevalent in wood surface finishing. These coatings impart appealing optical and haptic properties to the wood surface, but lack sufficient protection against water and mechanical influences. The present study reports a simple green route to improve the performance of linseed oil coating by the addition of nanofibrillated cellulose (NFC). In order to achieve surface chemical compatibility with linseed oil, NFC was chemically modified with acetic anhydride and (2-dodecen-1-yl)succinic anhydride, respectively, using propylene carbonate as a solvent. NFC/linseed oil formulations were prepared and applied to wood substrates. The wear resistance of oil-coated wood surfaces was assessed by a newly developed test combining abrasive loading with subsequent contact angle measurement. As revealed by infrared and nuclear magnetic resonance (NMR) spectroscopy, as well as X-ray diffraction (XRD), NFC has been successfully modified without significantly affecting the structure of cellulose. In abrasion tests, all NFC-modified oil coatings performed better than the original oil. Interestingly, NFC only suspended in propylene carbonate, i.e., without chemical modification, had the strongest improvement effect on the coating's wear resistance. This was primarily attributed to the loose network structure of this NFC variant which effectively prevents the oil from penetration into the wood surface, thus forming a protective NFC/oil composite layer on the wood surface.