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Cross Laminated Timber (CLT) in Bridges

The advantages and disadvantages of Cross Laminated Timber application in Bridges AR0531 Innovation & Sustainability AR1B025-D3 BT Research Methodology

Margarita Kyanidou Faculty of Architecture, Urbanism and Building Sciences / Building Technology 4743911 [email protected] 6 November 2017 Number of words: 2493 Focus and restrictions – Specific focus: Studying the use of CLT applications In bridges. Restricted to decking System.

Abstract – This paper aims to investigate, the application of Cross Laminated Timber elements in Bridges. Exploring the use of Cross Laminated Timber in bridges, regarding its properties and limits to determine the advantages and the disadvantages of this solution to replace common construction methods in the industry.

Key words – Structures, CLT, Cross Laminated Timber, Bridge, decks, slabs. AR0531

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1 Introduction

2 Methodology

The constant expansion of wood construction in architecture coincides with the development of new building materials and new building systems. Since more than ten years, CrossLaminated Timber (CLT) a new generation of engineered wood product, developed initially in Europe, has experienced considerable popularity with architects and civil engineers. By crosswise gluing of individual board layers from the straightened raw wood material is formed used for load bearing walls, ceilings and roof constructions in multi storey residential building or in special constructions such as bridges.(Mohammad, Gagnon, Douglas, & Podesto, 2012) The present paper will focus to the application of Cross Laminated Timber in bridges, aiming to answer the following question: Which are the advantages of Cross Laminated Timber application in Bridges? in comparison with the dominant construction system? Sub-questions are formulated in order to answer the main question : -Which is the definition of Cross Laminated Timber and what about its manufacturing process? -How Cross Laminated timber is applied in bridge construction. -What are the advantages of Cross Laminated timber decking system in bridges, in general and in comparison with the dominant structure system? The paper begins with a brief introduction to cross-laminated timber and its manufacturing process. Thereafter, the application o CLT in bridge construction is declared and some advantages are presented in chapter 4. Afterwards the advantages and disadvantages of CLT application in bridges are analysed in chapter 5. The 6 chapter consist the advantages of CLT decking system in comparison with steel- concrete composite slab. Finally, the paper ends with Discussion of the results, the conclusion and the reflection of the research in 7,8 and 9 chapters repsectively.

The latest years an adequate number of books, articles and research papers concerning Cross Laminated Timber in general and its application in Bridges, have been compiled and published. A variety of them, have been reviewed to ensure an equitable result for this paper. The sources are organised due to their correlation with the research question and sub-questions. The used keywords during the literature research where: “Cross Laminated Timber”,”Cross Laminated Timber advantages”,”Cross Laminated Timber and Bridges” ,”Cross Laminated Timber in Bridges”, “Cross Laminated Timber and CO2 footprint”. The paper “Introduction to Cross Laminated Timber” carried out from M. Mohammad, Sylvain Gagnon, Eng., Bradford K. Douglas and P.E., Lisa Podesto, the book Massive Timber Construction Systems. (2014). WoodSolutions” provide basic information on the various attributes of CLT as a product and as structural system in general. The articles “Heavy-duty bridge, with a 68 m span in northern Canada” by Mathias Oberholzer and “Design and construction of a 160-Metre-Long Wood Bridge in Mistissini, Québec” by Denis Lefebvre and Grégoire Richard, provide detailed information about the construction method of the timber Bridge in Maicasagi constructed by Nordic Wood Structures and for the Wood Bridge in Mistissini, Québec, constructed by Stantec Structures respectively. Also both articles present the advantages of CLT application as decking system and justify this choice. The paper “Design of timber structures” by Pousette, Anna ,the books “Timber bridges” by Mettem, and “Building with cross laminated timber- Load-bearing solid wood components for walls, ceilings and roofs” present detailed information about timber bridges and timber constructions in general. The papers “A New Bridge Proposal – Road Bridge with a Cross-Laminated Timber Slab” by Hannes Behrens and Per Benner and the “Cross Laminated Timber-Civil 510 - Term Project” by Arshia Mandegarian and Svetoslav Milev provide information about CLT slab applicated as a decking system in bridges.

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3 Cross Laminated Timber: Definition and Manufacturing Process Cross Laminated Timber (CLT) panels consist of several layers of structural lumber boards stacked crosswise (typically at 90 degrees) and glued together on their wide faces and, sometimes, on the narrow faces as well. A crosssection of a CLT element has at least three glued layers of boards placed in orthogonally alternating orientation to the neighbouring layers to obtain specific structural capacities. Cross Laminated Timber products are usually fabricated with three to seven layers and even more in some cases. Thickness of individual lumber pieces may vary from 16 mm to 51 mm and width may vary from about 60 mm to 240 mm (2.4 inch to 9.5 inch). In order to get the desired length of boards, they are finger-jointed. (Mandegarian & Svetoslav Milev,2011)

Figure 1: CLT panels consist of several layers of structural lumber boards stacked crosswise (typically at 90 degrees) (Mandegarian & Svetoslav Milev,2011)

By adding pressure to the stacked layers, often by using a hydraulic press device, they are formed into one solid element. After that, precise CNC machines are used to cut the part into its final shape (Wood Solutions, 2014). Lumber is visually-graded or machine stress-rated and is kiln dried. Additionally, cross-laminating provides relatively high in-plane and out-of-plane strength and stiffness properties in both directions, giving panels a two-way action capability similar to a reinforced concrete slab. (Mohammad et al., 2012). Because the product is relatively new, there is not any standardized design approach in Eurocode. In countries part of the European

Union, technical approvals for construction products are issued by members of the European Organisation for Technical Assessment, EOTA. The definition of a European Technical Assessment (ETA) is formulated in the Regulation (EU) No 305/2011 of the European Parliament and of the Council of 9 March 2011.000:(Vegas, 2013)(Behrens & Benner, 2015)

4 Cross Laminated Timber in Bridges Nowadays, the main structures of bridges are commonly made of steel, concrete or timber, and the decking system from composite steelconcrete slab.(Lefebvre, Richard, 2014) Although, the latest years the Cross Laminated timber slabs are preferred as an alternative method of decking system, in bridge construction. The decking system of a bridge can be defined as the surface which the traversing traffic drives upon. This is the surface which is used as a continuation of the driving surface on either side of the bridge. It is a fact that, strength analysis of CLT, presents technical challenges in that the properties of wood are very poor in the direction perpendicular to grain, leading to issues in the inter-lamina behaviour of CLT panels when subjected to linearly varying strain conditions such as bending. For this reason, other structural solution than CLT is preferred for beams in bridges. (Behrens & Benner, 2015) As a result, as far as it concerns the Cross-Laminated Timber in Bridges, most of the times it consists only the decking system. Some examples of bridges with CLT decking system are the following.

Figure 2: Maicasagi Bridge River,of Chibougamau, Quebec(Lefebvre, Richard, 2014) 3

5 Advantages and disadvantages of Cross Laminated Timber decking system in Bridges

Figure 3: Hundorp Bridge,north of Lillehammer, Oslo. (Rune B & Nyløkken, 2010)

Figure 5: Mistissini (Oberholzer,2012)

Bridge

,

Québec

Figure 4: Street bridge in Kössen (Wiegand, Seidel, Mestek, Werther, & Winter, 2011)

In a first stage, it is important to consider the advantages of Cross Laminated Timber (CLT) as a construction material in bridge construction. One of the main advantages of Cross Laminated Timber is that the cross-laminations gives the wood panels a level of isotropy, in terms of being subjected to loads in several directions. Also it is light and has a high strengthto-weight ratio. Furthermore, it contains very few defects and flaws because of its process of production. The flawed sections are cut out so a continuous panel can be formed. Also, It is light and has a high strength-to-weight ratio (Lefebvre, Richard, 2014). . Another one of the main advantages of CLT as a structural material is that the cross structure of CLT components guarantees integral stability. Through the crosswise arrangement of the board layers and the blocking effect connected with this, humidity changes in the panel layers only lead to minor swelling and skrinkage. The cross laminated timber construction members therefore remain, also with the usual humidity changes, very accurately fitted and dimensionally stable.(Behrens & Benner, 2015) As far as it concerns the environmental impact of Cross laminated timber, is manufactured using softwood from sustainably managed forests. Compared to other solid construction methods the manufacture and processing of cross laminated timber components requires very little energy. It contributes to sustainable carbon storage and thus to the minimization of the greenhouse effect through the fusion of CO2.(Wiegand, Seidel, Mestek, Werther, & Winter, 2011) Moreover, at the end of its use cross laminated timber can be, materially or thermically, recycled. In the event of thermical recycling of reinstated cross laminated timber elements, only the CO2 which was stored during the growth period of the timber is discharged into the atmosphere. (Behrens & Benner, 2015) However, there are some disadvantages related to CLT elements. First of all,timber is susceptible to damages caused by moisture and endanger its durability. Hence, extra measures 4

have to be taken in the details of the bridge in order to minimize water infiltration. Traditionally moisture protection of timber has been made with either biological treatment, preservative treatment or structural weather protection (Mettem, 2011).For instance, the slab could be protected with a waterproof membrane. Furthermore, the driving surface should be paved on a welded insulation mat above the waterproof membrane (Mettem, 2011).Using a bituminous coating it is possible to reduce to a maximum the possibility of water penetration and stagnation in the wood. (Lefebvre, Richard,, 2014) Furthermore, there are some difficulties in applying design code calculations for CLT, since the interaction between individual wood lamina in a CLT panel is relatively complex and would require extensive testing and analysis before simplified code equations become available. (Arshia Mandegarian and Svetoslav Milev)

6 Advandages of Cross Laminated Timber decking system in bridges, in comparison with steel-concrete composite slab. In a second stage it would be efficient to name the advantages of CLT as decking system in comparison with the prevalent decking system in bridges, which is steel-concrete slab. The main reason that a cross-laminated timber slab is used for a bridge decking system instead for the dominant steel – concrete solution is the possibility to utilize large prefabricated elements to facilitate a fast erection. Short construction time could be a key issue where site conditions are difficult, e.g. when launching a bridge over a stream or working at large heights etc. Since a local resource is used and prefabrication is applied, bridges can be designed, manufactured, and erected in a matter of weeks. As a result, also the construction cost is reducing. (Oberholzer,2012) During the service life of the bridge, the concrete slab stabilizes the steel girders against instability phenomena such as lateral torsional buckling and local buckling of the upper angle. However, the stabilizing properties of the concrete deck develop as the concrete hardens.

Thus, steel girders are vulnerable to buckling during the casting and hardening processes. The risk for instability failure during the construction phase can be decisive for the structural design of the steel girders, and it can be required to use temporary bracing. Furthermore, the falsework and casting procedure is both labour intensive and time consuming which could lead to considerable costs. Furthermore, it can be shown that a considerable portion of the moment capacity of a steel-concrete composite cross-section is consumed by the moment that is caused by the large self-weight of the concrete. Since the weight of wood is only a fifth of that of concrete, it is believed that a non-composite bridge with a CLT deck would not require much larger main girders than a steel-concrete composite bridge. A lighter bridge deck would not subject the girders to such high loads. Thus, the risk of local buckling of the compressed angle during construction is decreased. (Lefebvre, Richard,, 2014) Another advantage of a light bridge deck is that the load on the foundation is reduced. Therefore, smaller and cheaper abutments should be required. Also, since the falsework and casting process is eliminated a shorter overall construction time of the bridge is facilitated. Finally, compared to concrete, a timber structure puts less load on the transport of the structural elements. (Lefebvre, Richard,2014) It is interesting to compare the carbon footprint of the cross laminated wood deck structure and a structure with a mixed steel/concrete deck. Stantec Construction conducted a research comparing two reference bridges of each category. (Lefebvre, Richard,2014)

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Table 1: CO2 equivalent decking bridge

emission

for

Table 2 : CO2 equivalent emission steel-concrete decking bridge

wood

for

Overall, the total carbon emissions for the wood bridge were negative, which is a very good result. The total difference between the two solutions is 1,472 tons of CO2, equivalent emissions, which is equal to the CO2 emitted in combustion 640,000 litres of gas.

7 Discussion Looking at the advantages that CLT offers in bridge deck construction, its environmental impact and its comparison with the major deck construction, it is obvious that the application of CLT slab as a bridge decking system is positively prevail. Notwithstanding, some challenges are still present in terms of code acceptance. There is a need to continue and support research and development activities in order to adopt a codes and standards road map to ensure a wide acceptance of the CLT product and systems.

fabrication, faster erection quick assembly, and its positive environmental impact, could address CLT decking system as a preferred choice by designers and clients in this type of construction, as it also results from the comparison with the major decking system (steel-concrete slab). Also, it seems that the true benefit of CLT as a sustainable building product is easily realized when it replaces steel and concrete in construction. It’s a potentially powerful way to monetize the ability to restore our forests and remove some of the fuel load . Although there are disadvantages of CLT in bridges that they cannot be disregard. The sensitivity of CLT against moisture requires that the design process should consider multiple lines of defense to prevent potential moisture entrapment.Moreover the lack of an applying general design code, requires further research on design code concerning CLT constructions in order to be specified the difficulties on calculations for its elements, A research must be done on the Bending Strength, Shear Strength and Rolling Shear Strength of the material depending on the number of panels. These values would be required in order to perform the various checks necessary to determine the capacity of a CLT cross-section.

9 Reflection This paper is based on the general review of the advantages of the CLT decking system in bridge construction. A more accurate research by taking more construction and mechanical variables under consideration should be done. By comparing in detail several different bridge examples and each step of their construction by using exact measurement results and taking under consideration different climate conditions and also manufacturing location,could lead to a more accurate, certain and deductive conclusion.

8 Conclusions CLT is a high-quality wood product and has great potential applicated in bridges due to its strength and mechanical properties and its environmental impact. The superior attributes of the CLT panels and assemblies such ease of 6

References Avdelningen för Konstruktionsteknik .H. Behrens , P. Benner (2015). Lunds Tekniska Högskola Lunds Universitet, Building with cross laminated timber - Loadbearing solid wood components for walls, ceilings and roofs (2011). T.Wiegand,P. Seidel, N. Mestek,S.Werther,N.Winter.Studiengemeinschaf t Holzleimbau e.V CROSS LAMINATED TIMBER- Civil Term Project. (2015) A. Mandegarian , S.Milev . University of British Columbia Design of timber structures. (2011). Pousette, Anna. Swedish Forest Industries Federation, ISBN 9789163700552. HUNDORP BRIDGE-Bridge deck rehabilitation using cross-laminated timber. (2010). R.B Abrahamsen, T.E. Nyløkken Introduction to Cross Laminated Timber. (2012). M. Mohammad, S. Gagnon, K. Bradford K. Douglas, P.E. Lisa Podesto . Massive Timber Construction Systems. (2014). WoodSolutions. Mettem, Christopher J (2011). Timber bridges. Spon Press.95 ISBN-10: 0415577969

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