wood protection by design

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Wood protection by design – concepts for durable timber bridges.
Conference Paper · September 2015
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Christian Brischke
Thuenen Institute of Wood Research
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Linda Meyer-Veltrup
Heinz-Piest-Institut für Handwerkstechnik an der Leibniz Universität Hannover
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Sven Thelandersson
Lund University
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Norwegian University of Science and Technology
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WOOD PROTECTION BY DESIGN –
CONCEPTS FOR DURABLE TIMBER BRIDGES
Brischke, C. 1 , Meyer-Veltrup, L. 2 , Thelandersson, S. 3 , Malo, K.A. 4
ABSTRACT
Various abiotic and biotic agents have a negative effect on the service life of materials
used for bridge constructions. Corrosion of steel, carbonization of concrete and decay of
timber are limiting their service lives. It is a common perception that the expected
lifetime of a timber structure is only a fraction of that of a concrete or steel structure. In
spite of this there are numerous timber structures remaining since centuries such as
Norwegian stave churches, half-timbered oak houses, and covered bridges in
Switzerland – commonly accepted as consequence of proper design and workmanship.
In contrast, many untreated timber structures show severe decay after only a few years
in service due to wetting.
The WoodWisdomNet research project DuraTB (‘Durable Timber Bridges’) aims
therefore to significantly improve the durability of timber bridges. A modern and
innovative concept is sought to provide design solutions being among the best
alternatives with respect to environmental friendliness, initial and life-cycle costs,
showing excellent results in life-cycle analyses.
The paper gives an overview about objectives and research activities of the project.
Work on the development of performance and climate models performed at Lund
University, Sweden, and Leibniz University Hannover, Germany, are presented in the
latter part of the paper. Various details were evaluated regarding their moisture-induced
risk for decay, which was identified as key element for service life prediction of bridge
structures in the absence of chemical wood preservation.
Key words: Decay modelling, glulam, moisture risk, Network arch bridge, Norway
spruce
1 Leibniz University Hannover, Faculty of Architecture and Landscape Sciences, Institute of Vocational
Sciences in the Building Trade, Herrenhäuser Str. 8, D-30419 Hannover, Germany, Tel: +49 511 762
5829, E-mail: brischke@ibw.uni-hannover.de
2 Leibniz University Hannover, Faculty of Architecture and Landscape Sciences, Institute of Vocational
Sciences in the Building Trade, Herrenhäuser Str. 8, D-30419 Hannover, Germany, Tel: +49 511 762
4598, E-mail: meyer@ibw.uni-hannover.de
3 Lund University, Division of Structural Engineering, Box 118, Paradisgatan 5c, SE 22100 Lund,
Sweden, Tel: +46 46 222 4885, E-mail: thelandersson@kstr.lth.se
4 Norwegian University of Science and Technology, NTNU, Department of Structural Engineering, Rich.
Birkelandsv 1a, NO-7491 Trondheim, Norway, Tel: +47 7359 4784, E-mail: kjell.malo@ntnu.no

INTRODUCTION
Timber is a traditional building material for a huge variety of applications, such as
fencing, decking, and cladding, but also for load bearing structures like jetties,
dwellings, and other functional buildings. Its outstanding strength-weight ratio allows to
use wood even for heavy load bearing bridges (Ritter 1990). However, whenever wood
is exposed directly to the weather it requires protective measures against weathering and
biological attack, e.g. by fungi, insects and bacteria. In many parts of the world timber
bridges have therefore been made of wood treated with preservatives such as creosote
and copper-chromium arsenate (CCA, Fig. 1, Bigelow et al. 2009). Alternatively, very
durable wood species can be used, which are usually available only from tropical
resources and therefore controversially discussed in respect of unsustainable forest
management. Likely due to this limitations timber lost significant market shares to steel
and reinforced concrete since the late 19 th century, and the latter became the most
important bridge building materials today.
More recently, there is an increasing demand for renewable resources fulfilling high
performance criteria. In this respect timber can play a key role again due to its high
potential of storing carbon. Nevertheless, using timber for load bearing structures in the
outdoor environment has become a challenging task more than ever. Many highly
effective wood preservative have been or will be banned in the near future (Hundhausen
et al. 2014). Traditional methods of wood protection by design, e.g. through the
application of roof shelters, are considered old-fashioned and frequently not accepted by
architects and designers. The research project DuraTB aimed therefore on developing
design concepts for durable timber bridges without any chemical wood preservation.
Fig. 1. Monitoring of timber bridges. Left: Daleråsen bridge, Norway, creosote treated
glulam. Right: Essing bridge, Germany, untreated glulam. Left bottom: Humidity
sensor. Right bottom: Electrical resistance sensors.

OBJECTIVES AND STRUCTURE OF THE PROJECT
The research within the WoodWisdomNet project DuraTB aims to significantly
improve the general standing and applicability of wood as structural material in bridges.
Therefore various technical and scientific objectives were followed in five different
work packages and numerous tasks as shown in Table 1 and Fig. 3.
Table 1. Project topics divided into work packages and tasks.
Work package Task Description
1. Coordination 1.1 Project management
2. Performance 2.1 Collection of field data from existing instrumented bridges
based service life 2.2 Development of climate exposure model for bridge structures
design of timber
bridges
2.3 Tests of climate exposure (moisture content, temperature) in
structural details
2.4 Development of suitable dose-response model for fungal
decay
3. Hygro-thermal
effects in wooden
members
4. Design
concepts for
durable timber
bridges
2.5 Methodology for service life design of bridges
3.1 Numerical models relating rain, spray, RH, T, to distribute
material climate effects in members
3.2 Moisture distribution, moisture induced stress and risk of
cracking in members in connections
4.1 Wooden bridge decks
4.2 Design concepts for short to medium span bridges
4.3 Design concepts for medium to long span bridges
4.4 Splicing of large glulam members
4.5 Fatigue of axial-carrying connectors in wooden members
4.6 Performance evaluation of design concepts (structural
performance, lifetime, LCC, LCA)
4.7 Maintenance practices and repair techniques for extending
service life of timber bridges
5. Dissemination 5.1 Produce a book or report on design of durable timber bridges
5.2 Arrange open workshops
5.3 Prepare proposals to CEN TC 250 SC 5 to the new generation
of EN 1995-2 Timber bridges
5.4 Publication in scientific papers, journals and conferences
Within WP 2 data from weather and material climate monitoring (examples shown in
Fig. 1) on field trials and real structures in service are collected and used for modelling
decay and consequently performance of bridge components and details. Finally, an
engineering design concept will be developed on the basis of previous guidelines for
timber used in arbitrary above ground conditions (Isaksson et al. 2014). The doseresponse
models are going to be used to evaluate durability and expected service life of
the bridge design concepts proposed in WP 4.
WP 3 focusses on the hygro-thermal behaviour of wooden members. Numerical
modelling (FEM) and simulation are applied and adapted to large-size glulam
structures. Therefore data from long-term recording of bridge structures in different
Nordic countries were used to verify and further develop existing models. Design
concepts for durable bridges with a span range of 10 to 150 m will be developed in
WP 4. Therefore network arch bridges with hangers crossing each other at least twice
located in one plane are considered as a promising starting point. The so-called ‘spoked
wheel’ configuration may further improve this concept with pairs instead of single

hangers having common fastening points on the arch level and separate fastening points
on the deck level, displaced in the transverse direction (Malo and Ostrycharczyk 2014,
Fig. 2).
Fig. 2. Design concepts. Left: Network bridge. Right: Bridge with spoked hangers
(taken from: Malo and Ostrycharczyk 2014).
Fig. 3. Flow of essential information.
DEVELOPMENT OF DECAY MODELS
A key issue for service life prediction of timber structures in outdoor use is the
development of decay models. Due to the biological nature of the most relevant
degrading agents, i.e. wood-destroying fungi, the use of dose-response models suggests

itself. In Europe several decay models were developed during the last decade (Brischke
and Thelandersson 2014). The models suggested so far differ in experimental data
source and prevailing decay types considered. Within the DuraTB project a logistic
dose-response model has been selected using basically wood moisture content and
temperature as input and fungal decay as output variables. The model is based on field
tests data, where white and soft rot fungi were the predominant decay organisms. In
contrary, brown rot fungi are the most prominent hazard for Norway spruce glulam if
exposed to the weather as intended in this project. Hence, the existing model needed
verification to predict brown rot decay and thus performance of timber elements. As
shown in Fig. 4 brown rot preceded clearly faster than white and soft rot at a given
dosage and thus at given time. Furthermore, wood exposed to shade tended to decay
slower which might be caused by the formation of biofilms consisting of algae and other
non-degrading organisms. Further field test data from running trials at different
locations in Europe will be used to further validate and improve the model if needed.
Mean decay rating [0-4]
4
y = 4*EXP*(-EXP(1.564-(0.0054*x)))
R² = 0.8681
3
2
1
Brown rot - No shade
Brown rot - Shade
White and soft rot
0
0 200 400 600 800 1000 1200 1400
Dose
Fig. 4. Preliminary model for brown rot decay in different softwoods with clear
differentiation between unshaded and shaded conditions. Grey diamonds indicating
white and soft rot decay in earlier field tests. Mean decay rating and dose calculated
according to Brischke and Thelandersson (2014) based on wood MC and temperature.
Besides wood moisture content and temperature, and thus besides the ambient climatic
conditions, the formation of cracks might have a significant effect on durability. Cracks
serve as entry ports for both, moisture and fungal spores. Since the dimension and tree
ring orientation of a wooden element can have a significant impact on its dimensional
changes and thus on the formation of cracks, this potential influence parameter was
examined separately. Specimens of different size were exposed above ground for five
years and monitored with respect to moisture and fungal decay. In summary, a clear
effect of component dimension was neither found on moisture content nor on decay
development. However, it was confirmed that cracks can serve as starting point for
fungal decay, but alternative ways of infection must not be neglected (Fig. 5).

50 x 50 mm
35.4 x 35.4 mm
25 x 25 mm
Fig. 5. Decay and cracks in differently sized Norway spruce specimens exposed above
ground for five years.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge support from the WoodWisdom-Net+ and all
partners involved.
REFERENCES
Bigelow, J., Lebow, S., Clausen, C.A., Greimann, L., Wipf, T.J. 2009: Preservation
treatment for wood bridge application. Transportation Research Record: Journal of the
Transportation Research Board 2108 (1): 77-85.
Brischke, C., Thelandersson, S. 2014: Modelling the outdoor performance of wood
products – a review on existing approaches. Construction and Building Materials 66:
384-397.
Hundhausen, U., Mahnert, K.-C., Gellerich, A., Militz, H. 2014: CreoSub – New
protection technology to substitute creosote in railway sleepers, timber bridges, and
utility poles. International Research Group on Wood Protection, IRG/WP/14-30644.
Isaksson, T., Thelandersson, S., Jermer, J., Brischke, C. 2014: Beständighet för
utomhusträ ovan mark. Guide för utformning och materialval. Rapport TVBK-3066.
Lund University, Division of Structural Engineering, Lund, Sweden.
Malo, K.A., Ostrycharczyk, A.W. 2014: I: COST Timber Bridge Conference - CTBC
2014: Bern University of Applied Sciences, Switzerland; Institute for Timber
Construction, Structures and Architecture: 81-86.
Ritter, M.A. 1990: Timber bridges: Design, construction, inspection, and maintenance.
US Forest Service, Washington.
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