Department of Architecture and Civil Engineering Division of Structural Engineering Lightweight Structures in collaboration with Architecture and Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Master’s Thesis ACEX30 Gothenburg, Sweden 2021 Timber Footbridge in Wendelstrand An Iterative Design Process Combining Architecture and Engineering Master’s thesis in the Master’s Programme Structural Engineering and Building Technology ALEXANDER ANGRÉN MARIA BRUZELL ROLL MASTER’S THESIS ACEX30 Timber Footbridge in Wendelstrand An Iterative Design Process Combining Architecture and Engineering Master’s Thesis in the Master’s Programme Structural Engineering and Building Technology ALEXANDER ANGRÉN MARIA BRUZELL ROLL Department of Architecture and Civil Engineering Division of Structural Engineering Lightweight Structures in collaboration with Architecture and Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2021 I Timber Footbridge in Wendelstrand An Iterative Design Process Combining Architecture and Engineering Master’s Thesis in the Master’s Programme Structural Engineering and Building Technology ALEXANDER ANGRÉN MARIA BRUZELL ROLL © ALEXANDER ANGRÉN & MARIA BRUZELL ROLL, 2021 Examensarbete ACEX30 Institutionen för arkitektur och samhällsbyggnadsteknik Chalmers tekniska högskola, 2021 Department of Architecture and Civil Engineering Division of Structural Engineering Lightweight Structures in collaboration with Architecture and Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Model in scale 1:20 of the proposed bridge design. Department of Architecture and Civil Engineering Göteborg, Sweden, 2021 I Timber Footbridge in Wendelstrand An Iterative Design Process Combining Architecture and Engineering Master’s thesis in the Master’s Programme Structural Engineering and Building Technology ALEXANDER ANGRÉN MARIA BRUZELL ROLL Department of Architecture and Civil Engineering Division of Structural Engineering Lightweight Structures in collaboration with Architecture and Engineering Chalmers University of Technology ABSTRACT Wendelstrand is a new residential area in Mölnlycke planned with sustainability and wood as overall concepts. Adjacent to the area is the lake Landvettersjön, which today is cut off by a highly travelled country road. A footbridge over the road will link the lake and Wendelstrand and create a safe crossing for both residents and visitors to the area. In this thesis a proposal for such a footbridge is developed with emphasis on the relation to the timber and sustainability-concept of Wendelstrand, as well as the integration between architectural and structural qualities. An iterative design method is used to examine and develop possible solutions for a bridge proposal. Reference projects of existing timber bridges are used in the development of feasible bridge designs. Site-specific prerequisites, intentions from the developer Next Step Group, regulations from the local municipality and the Swedish Transport Administration are considered and integrated into the design. The proposal suggests a straight bridge across the road leading to a floating structure on the lake. The bridge supports are integrated into the superstructure and designed to limit the impact on the site. Physical models are used to verify the assembly process, as well as the structural concept and spatial qualities of the design. The design corresponds with Swedish and European standards concerning structural response. Simplified calculations are performed to verify the global and detailed design of the design proposal. A solution for an efficient structure in addition to an appealing architectural appearance is achieved by applying the principles of active bending in the superstructure. Keywords: wood, timber, bridge, footbridge, conceptual, design, architecture, structure, active bending, Wendelstrand CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 II Contents 1 INTRODUCTION 1 1.1 Aim 1 1.2 Method 2 1.3 Limitations 3 1.4 Outline 3 2 DESIGN PROCESS 4 2.1 Step I: Contextualisation 4 2.2 Step II: Conceptual design phase 4 2.2.1 Intuitive phase 4 2.2.2 Intentional phase 4 2.2.3 Evaluation phase 5 2.3 Step III: Preliminary design phase 5 2.4 Step IV: Final design phase 5 3 STEP I: CONTEXTUALISATION – REFERENCE STUDY 6 3.1 Categorisation of timber bridges 6 3.2 Reference projects 8 3.2.1 Neckartenzlingen Pedestrian Bridge 8 3.2.2 Punt Staderas 11 3.2.3 Fussgängersteg Geheidgraben 14 4 STEP I: CONTEXTUALISATION – SITE 17 4.1 About the site 17 4.2 Clients demands 19 4.3 Site-specific boundary conditions 19 4.4 Summary of contextualisation 20 5 STEP II: CONCEPTUAL DESIGN 22 5.1 Design criteria 23 5.1.1 Spatial qualities 23 5.1.2 Bridge qualities 23 5.1.3 Structural concept 24 5.2 Intuitive phase 25 5.3 Intentional design phase 28 5.4 Evaluation phase 33 6 STEP III: PRELIMINARY DESIGN 35 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 III 6.1 Final design proposal 35 6.1.1 Overall bridge design 35 6.1.2 Structural concept 36 6.1.3 Experiment with active bending 37 6.2 Model development of structural concept 40 7 STEP IV: FINAL DESIGN 42 7.1 Bridge dimensions 45 7.2 Input data 47 7.2.1 Partial factors 47 7.2.2 Material properties 49 7.2.3 Loads 50 7.2.4 Load combinations 54 7.3 Global design 56 7.3.1 Superstructure in ULS 56 7.3.2 Superstructure in SLS 60 7.3.3 Support in ULS 61 7.3.4 Dynamic analysis 64 7.3.5 Torsional stiffness 68 7.3.6 Moisture induced deformation 70 7.3.7 Floating structure 71 7.4 Local design 72 7.4.1 Prestressed connections 72 7.4.2 Lamella joints 75 7.4.3 Column to superstructure connection 78 7.5 Production 78 8 DISCUSSION 80 8.1 The proposed bridge 80 8.2 The design process 81 8.3 Suggestions for future work 82 9 CONCLUSION 84 10 REFERENCES 85 APPENDIX CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 IV CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 V Preface In this study, an investigation of structural design for a site-specific bridge has been carried out. Our aim was, in addition to fulfilling the requirements of the client, to combine principles from structural engineering and architecture. We want to aspire to rethink and challenge standard solutions to contribute to the development of timber bridge design. First, we would like to thank our examiner Robert Jockwer for supporting our ambition for the bridge design, for the enthusiasm during the process, and for contributing with important knowledge regarding structural design of timber structures. Our supervisor Prof. Karl-Gunnar Olsson is highly appreciated for his understanding and commitment to combine architecture with engineering, for emphasising on the importance of reference analysis, and for daring us to think boldly. Thanks to Emelie Silverterna and Joakim Garfvé at Next Step Group, the developer of Wendelstrand, for the opportunity to collaborate and for insight in site-specific data, as well as valuable feedback on the design proposals. We would like to give thanks to Brosamverkan for the scholarship, who financed the material usage for our physical models. The scholarship enabled us to build models to verify the structural concept, validate the assembly method, and demonstrate the relation between the bridge design and Wendelstrand. Thanks to our opponent Vera Sehlstedt for numerous discussions and for support through the process. At last, we would also like to address a special thanks to Burkard Walther, Miebach Ingenieurbüro, Camathias SA, werk1, and Wendelstrand for allowing us to use their photographs and illustrations in the thesis. Göteborg June 2021 Alexander Angrén Maria Bruzell Roll CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 VI CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 1 1 Introduction Close to Mölnlycke, outside Gothenburg, the Bråta gravel pit will be phased out and changed into a new residential area named Wendelstrand. This will be Northern Europe’s largest residential area in wood and is developed by Next Step Group (E. Silverterna, personal communication, January 29, 2021). The area is situated right next to the lake Landvettersjön but the two are today separated by Boråsvägen, a heavily travelled country road (Wendelstrand, 2018). A timber bridge will facilitate a safe crossing, while the structure itself can be used to enhance the innovative ideas of Wendelstrand. Interplay between nature and the buildings is an important part in the design of Wendelstrand. An approach in the direction of environmental sustainability is taken by choosing timber as the main structural material of the buildings (Garfvé, n.d.). In 2017 the Swedish parliament decided that Sweden should reach zero net-emissions of greenhouse gases by 2045 at latest, proceeding with negative emissions (Proposition 2016/17:146 Ett Klimatpolitiskt Ramverk För Sverige, 2017). Life Cycle Analysis studies show that when comparing timber frame buildings with non-timber alternatives, including concrete and steel, the former requires lower energy and releases less greenhouse gases (Dodoo et al., 2016). In other words, one way to reduce the environmental impact of structures is to increase the use of timber. When designing timber bridges, emphasis must be laid on durable detail design preventing the superstructure from weathering, enabling the wood to dry out, and protecting the end-wood from exposure. Due to water absorption and the risk of rot, timber bridges need careful planning in order to limit the need for maintenance and frequent inspections than a bridge made out of steel or concrete (Pousette & Fjellström, 2004). Therefore, the maintenance costs of timber bridges are higher. On the other hand, timber has a high strength-to-weight ratio (Svenskt Trä, 2009). Consequently, a lighter construction can be achieved with less foundation work. However, the construction of timber bridge structures has evolved throughout the years. By learning from failure in timber bridges due to lack of careful detailing, poor choice of material, or lack of maintenance and cleaning, improved designs of modern timber bridges can be made (Pousette & Fjellström, 2004). 1.1 Aim The aim of this thesis is to develop a design proposal for a timber footbridge in Wendelstrand. Focus lies on combining structural engineering knowledge with architectural visions. The following research questions are formulated: “In what way can a bridge proposal be achieved that meets the requirements and ideas of the client Next Step Group for the planned residential area Wendelstrand?” “In what way can an iterative design method be used to develop a structural concept that enhances both engineering solutions and architectural qualities?” CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 2 1.2 Method An iterative design method consisting of four stages is applied to answer the two research questions. In the first stage of contextualisation, an overview of the task is achieved by research of reference projects and identification of site-specific requirements. The second stage is a Conceptual design phase where possible solutions are examined and developed into one suitable proposal. This design proposal is further developed in the third stage and validated in the fourth and last stage. Figure 1.1 illustrates the design process from start to end. Figure 1.1 Design process. To ensure fulfilment of the thesis aim, a set of evaluation criteria are formulated. These make the transition between the design phases, to ensure a correspondence between the final proposal and the thesis aim. The evaluation criteria include both external demands and requirements, and design criteria. The latter define expectations regarding both the visual appearance and the structural performance of the footbridge structure. Figure 1.2 illustrates the evaluation criteria, which are further explained in Chapter 5. Figure 1.2 Evaluation criteria. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 3 1.3 Limitations The project is limited to a specific site, Wendelstrand, with real boundary conditions that the bridge design must comply with. Next Step Group is considered as the client, to establish a realistic context for the design project. They have stated clear demands regarding the bridge which will be prioritised in the design. The bridge design must consider the drastic elevation change in the landscape, as well as fulfilling non- negotiable requirements regarding shore protection, geological conditions, and standards from the Swedish Transport Administration and Eurocode. The structural response of the final bridge design is roughly estimated, and dimensions are based on conservative calculations and available product dimensions. The chosen structural concept is based on experiments with physical models, combined with understanding gained from analysis of reference projects. A thorough investigation of the theory is outside the scope. Detailed investigation for optimisation of the connections and foundation elements is not performed. Moreover, the assembly method will only be tested in physical models, while considering the limitations of production, transport, and site conditions. To limit the thesis, no Life Cycle Analysis of the bridge proposal is performed. Groundwork and stabilisation of the hill is not considered in this thesis. 1.4 Outline In Chapter 2 Design process, the design methodology with its different steps is further explained. In Chapter 3 Step I: Contextualisation – Reference study, aspects when designing a timber bridge is stated and different timber bridge structures are introduced on a general level, as well as a detailed analysis of reference projects. Chapter 4 Step I: Contextualisation – Site, contextualises the site Wendelstrand and identifies the client’s and the Swedish Transport Administration’s requirements and specifications. This will work as input for the design process. In Chapter 5 Step II: Conceptual design, the iterative design process is initiated with a divergent exploration for possible concepts. Three possible solutions are the outcome of this process, which after consultation with the client and evaluation in relation to a set of criteria, are developed into one proposal. This proposal is further refined with a suitable structural concept and rough dimensions in Chapter 6 Step III: Preliminary design. Thereafter, the structural concept is validated through calculations and the assembly method confirmed by a physical scale model in Chapter 7 Step IV: Final design . At last, Chapter 8 Discussion, gives a critical evaluation and discussion of the work. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 4 2 Design process Iterative design is applied as methodology for the design study. The method is described in fib Model Code 2010 (International Federation for Structural Concrete, 2013) and adapted in the graduate course BOM170 Structural Design (created by Björn Engström and Morten Lund). The thesis methodology is based on the latter. To initiate the design process, the task is established in a specific context. A large variation of improvised, possible solutions is developed. By an evaluation considering identified demands and contextual requirements, the proposed solutions can be narrowed down to one suitable design proposal. This is in turn developed through a precise and detailed design by the means of structural analysis and physical models. 2.1 Step I: Contextualisation The intention of a contextualisation is to create an overview over the challenge and define the limitations of the task. This is achieved in two stages: firstly, an understanding of the current task is achieved by analysing solutions of similar challenges. Secondly, the specific problem is clarified by identifying challenges related to the site. This involves identification of different aspects of demands and definition of external requirements. The aim of the contextual stage is to create a foundation for the assessment of solutions in the following design process. An analysis of reference projects helps broaden the understanding of different specific structures. These are collected in a data bank to identify possible solutions in existing structures that demonstrate their feasibility. These references support the development of design proposals in the Conceptual design phase. 2.2 Step II: Conceptual design phase The Conceptual design phase is divided into three main steps: Intuitive phase, Intentional phase, and Evaluation phase (M. Plos, personal communication, September 2, 2020). By starting off in a broad and divergent generation of improvised concepts, qualities from these can then be merged into new and more specific concepts. The ideas are gradually developed and evaluated throughout the process to result in a final concept which fulfils the initially stated project intention. 2.2.1 Intuitive phase The aim of the Intuitive phase is to generate as many different concepts as possible and investigate any idea related to the general context and demands. Every idea is possible in this phase, without any critical review or evaluation. 2.2.2 Intentional phase The next phase aims to concretise the contextual requirements and to take the client’s demands into account in the development of the structural design. This design is in turn evaluated in relation to the governing demands and requirements. As a result, the improvised concepts are developed and modified into a few, potential concepts. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 5 2.2.3 Evaluation phase The last part of the Conceptual design phase includes a review of the contextual demands. Conflicting demands are identified and prioritised after feasibility, fulfilment of client’s intention and structural complexity. With a specified set of governing design criteria and demands, the most suitable structural concept can be defined and then brought into the next phase, the Preliminary design phase. 2.3 Step III: Preliminary design phase Based on the result in the Evaluation phase, the final proposal is developed into a complete structural concept. This includes identification of the structural behaviour, determination of material and preliminary sizing of main structural elements. Reference projects with similar challenges are used to develop the solutions. 2.4 Step IV: Final design phase The Final design phase includes a verification of the chosen structural concept. By detailed design of the structural elements, and calculations of the overall behaviour, the feasibility of the proposed concept can be proven. Physical models, in different scales, are also used to verify the structural design concept as well as the assembly method of the superstructure. This will allow for a verification of the detailed design as well as the relation between the overall structural design and its context. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 6 3 Step I: Contextualisation – Reference study As a literature study, reference projects of existing bridges are collected in a database and analysed. The reference projects will act as guidance for proper detailing, technical solutions, and ideas for structural concepts. An understanding of other qualities such structures hold is also gained from the reference analysis, such as the relation to the surroundings, consideration of pedestrians, or implementation of other functions. As these qualities are of subjective manner, the bridges are not analysed for this. Instead, it is used as inspiration in the next design step. Timber bridges make most of the studied bridges, but bridges in other materials are analysed as well. The reference data bank is illustrated in Figure 3.1, while the complete content can be found in Appendix A. Figure 3.1 An overview of the complete bridge database. 3.1 Categorisation of timber bridges The architectural appearance of a bridge is strongly linked to its structural concept. Each concept hols different benefits and limitations regarding maximum span length, superstructure dimensions, required foundation capacity, visual impact on the surroundings, material usage, and cost. The maximum span lengths vary considerably between the different bridge structures. Figure 3.2 illustrates different types of structural concepts applicable on timber bridges, categorised by span length. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 7 Figure 3.2 Different structural bridge types categorised by length. Different structural systems carry loads in different ways. For example, a slab and beam bridge carry the load by bending, while a truss bridge globally transfers load by axial forces and bending as the local load transfer. A king post truss bridge uses bending, tension, and compression to distribute the load to the foundation, while a superstructure in a stressed-ribbon bridge works only in tension. Depending on required span length, desired visual impact and specific boundary conditions, a suitable structural concept can be applied and adapted to the specific site. Approximate span length and beam height for different structural systems are shown in Table 3.1. Table 3.1 Approximate span lengths and beam heights for different timber bridge construction types. A compilation from (Gustafsson et al., 1996) and (Svenskt Trä, n.d.). Construction type Span length, L [m] Beam height, h [m] Slab bridge 20-30 (15) L/20-L/30 Beam bridge - Simply supported beam 8 L/15-L/20 - Simply supported round wood 10 L/15-L/25 - Simply supported glulam beam 30 (40) L/15-L/20 - Underspanned beam 10-50 L/8-L/12 - Combined cross-sections 50 L/15-L/20 - Timber plated structure 50 L/8-L/12 Strut frame bridge 40 L/15-L/20 Framework - Truss 30-75 L/12-L/18 - Beam or glulam 20-50 L/20-L/30 CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 8 King post truss bridge 10-50 L/4-L/12 Arch bridge - Arched walkway 10-60 L/25 - Hung walkway 20-30 L/4-L/6 - Post-supported walkway 20-50 L/4-L/6 Truss bridge 100 L/10-L/15 Stressed-ribbon bridge 20-100 L/120 Cable-stayed bridge 20-100 L/4-L/8 Suspension bridge 20-100 L/4-L/8 3.2 Reference projects In the following sub-chapters, an analysis of three structurally different and inspiring timber bridges are presented. The different studied aspects are: - Structural concept - Vertical load distribution - Horizontal load distribution - Rotational stability - Point load - Foundation - Principle connections - Production - Assembly - Material - Durability 3.2.1 Neckartenzlingen Pedestrian Bridge In Neckartenzlingen in south-west Germany a 96 m long cycle and pedestrian bridge spans over the Neckar river. The design and structure are created by Ingenieurbüro Miebach and completed in 2017. The bridge deck is 3 m wide and there are three spans where the mid span is 44.5 m and the other two 25.7 m (Miebach, n.d.). Figure 3.3 shows the continuous superstructure with its two parallel glulam beams. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 9 Figure 3.3 Neckartenzlingen Pedestrian Bridge (Burkhard, 2017). Used with permission. © www.hochbau-fotografie.de Structural concept The Neckartenzlingen pedestrian bridge is a cantilever bridge with Gerber joints. These hinges transfer shear forces only and are located between the supports where the bending moment is close to zero. The superstructure consists of two parallel block- glued and bent glulam beams that cantilever out from the supports. To utilise the material, the cross-section height decreases towards the mid of the span. The glulam blocks are 2.1 m wide in the top layer and tapers down to 0.8 m at the bottom (Brandt, 2018). The bridge stretches out in a gentle S-shape. Vertical load distribution Vertically distributed load is transferred in the glulam blocks through bending to the supports. Due to reduction of the cross-section height in the span midpoint, the self- weight in the middle is also reduced. The largest cross-section height is found over the supports where the bending moments are the largest. Horizontal load distribution Transversal horizontal distributed load is handled by horizontal bending of the glulam blocks, and through axial forces in the transversal beams underneath the concrete slabs. The fact that the bridge is slightly S-shaped also contributes to the horizontal load capacity. Longitudinal horizontal loads are transferred by shear in the adhesive between the glulam block layers, down to the foundation and through the columns. Rotational stability The S-shape of the bridge provides global rotational stability. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 10 Point load A vertical point load is longitudinally and transversally distributed by the concrete slab and transversal beams down to the two main parallel glulam beams. From there the load is transferred through bending to the supports and foundation. Foundation Concrete piers support the superstructure on each side over the river, while concrete foundations anchor each bridge end. The glulam beams are connected to the supports and foundations by steel profiles that penetrates the structure (Brandt, 2018). The supports are considered as simply supported. Rotational movement is allowed at each bridge end. Principle connections The continuous beam is simply supported over the supports, which results in that no bending moment must be led down to the supports. The prefabricated bridge parts are connected to each other by Gerber hinges. These are located where the bending moment along the bridge is close to zero. The glulam blocks are individually fastened to each other with adhesives. Steel profiles connects the two parallel glulam beams. An exploded view illustrates the individual bridge parts in Figure 3.4. Figure 3.4 Exploded view drawing of the Neckartenzlingen Pedestrian Bridge (Ingenieurbüro Miebach, 2017). Illustration created by Ingenieurbüro Miebach. Used with permission. Production Both the timber used for the glulam beams and the firm that manufactured the bridge components are local. Due to manufacturability the bridge cross-section has two parallel glulam beams. This creates a space where installation and electricity cables can be hidden (González, n.d.). The prefabricated wooden parts were transported to the site in different parts. The concrete slabs were pre-cast and transported to the site as well. Assembly The design allowed for sensible transport dimensions as well as a simplified assembly. The whole bridge took three days to assemble (Brandt, 2018). Two mobile cranes were used to lift the parts, standing on each side of the river. First the two outer spans were CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 11 lifted to its positions over the supports. Following, the beam over the mid span was lifted and connected with a Gerber hinge (Holzindustrie, n.d.). Material The superstructure is made of glulam beams while a pre-cast concrete slab with anti- slip surface constructs the decking. The railing and wires consist of stainless steel and the handrail is made of acetylated timber (Miebach, n.d.) Durability The 13 cm thick pre-cast concrete slab on top of the glulam beams gives a watertight protection (González, n.d.). The superstructure is protected by a 30 cm overhang of the concrete slabs. In addition, the glulam beams are tapered with 30° angle inwards which prevents rainfall from reaching the structure itself. Drainage channels made of steel are inserted under the concrete slab joints. To further protect the timber, a thin coat of glaze is applied (Brandt, 2018). 3.2.2 Punt Staderas In the municipality of Laax, Switzerland, the slender bridge Punt Staderas spans over the country road Oberalpstrasse N19 to create a safe passing for bicyclists and pedestrians. The bridge design and structure are made by Walter Bieler with help of Stephan Berni and was completed in 2015 (von Büren, 2016). The main idea was to reduce the amount of wood and use the same cross-sectional dimension in all structural elements. This resulted in a slender superstructure with a total length of 115 m with nine spans of varying lengths. The longest span is found over the road and measures 24 m. The pathway is 2.5 m wide which allows enough space for both bikers and pedestrians, and has a slope of 6% (Ekwall, 2017). Figure 3.5 shows the largest span from below and the integration between the V-shaped supports in the structure. Figure 3.5 Punt Staderas from below (Camathias SA, 2015b). Used with permission. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 12 Structural concept The superstructure is a Gerber girder built up as a Vierendeel girder. The webs in the Vierendeel frame builds up rectangular frames together with the upper and lower chords and with joints that can transfer bending moments into the webs. In contrary to a triangular truss with pinned connections to the upper and lower chord where the shear force is axially transferred through the diagonals (Wickersheimer, 1976). By using a Vierendeel girder a slender cross-section is achieved with a static height of 640 mm. The grid consists of two longitudinal layers with four beams each and an intermediate layer with transversal cross beams every 1.05 m (Ekwall, 2017). The concept is governed by the aim of using the same cross-sectional dimension for all structural elements, namely 160-by-240 mm and 14.5 m long (Guetg, n.d.). Since the span over the road is 24 m (longer than 14.5 m) glulam timber beams are used to ensure sufficient bearing capacity. The supports are V-shaped in both directions, with two considerable benefits. Firstly, the span lengths are decreased. Secondly, with an inwards V-shape, the bridge deck can provide weather protection of the support elements. In addition, the outer support beams stretch beyond the deck and enhance the railing design. The V- shaped supports improves the structural performance in two aspects. Firstly, the span lengths are decreased. Secondly, with a V-shape the V-shape allows for weather protection of the support elements by the bridge deck. The outer support beams stretch beyond the deck and enhance the railing design. Vertical load distribution A vertical, uniformly distributed load is lead through the Vierendeel girder to the supports by compression in the upper longitudinal layer and tension in the bottom longitudinal layer. At the supports the load is transferred through compression down to the foundation. Over the largest span the load is carried by bending moment through the glulam beams to the supports. Horizontal load distribution Horizontal capacity is provided by the transversal beams of the Vierendeel girder. A modest S-shape in the horizontal direction of the bridge path enhances the horizontal stability of the structure. The V-shaped supports are ordered in a 3-by-6 array with a steel bracing in the transverse direction. This geometry and additional bracing provide additional horizontal stability. For the longitudinal horizontal loads, the geometry of the V-shaped supports provide stability. Rotational stability The steel cross bracing inside the V-shaped supports provides global rotational stability (Figure 3.5). The local rotational stability comes from the stiff Vierendeel girder and connections between the members. With the use of a total of 2000 screws, each 80 cm long, the wooden elements are joined together (Oertli, 2018). Point load A vertical point load is transferred through the bridge deck down to the Vierendeel girder. The load is then transversally distributed by the cross beams to the longitudinal beams in the upper and lower layers of the girder. Then the force is carried by tension and compression through the girder to the supports, which in turn transfer the load through compression down to the foundation. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 13 Foundation Reinforced concrete abutments are placed at the two landing points. The V-shaped supports stand on concrete blocks. Principle connections Since the same cross-sectional dimension is used for all structural members, they match very well when connecting them together. The beam elements are assembled with fully threaded screws at an angle, which provides a shear proof behaviour. The connection stiffness was tested successfully at EMPA in Dübendorf beforehand (Oertli, 2018). The connections in the superstructure are covered by the bridge deck, while all other exposed connections are covered by boards that lead water away from the connection. Figure 3.6 shows the protecting boards and their influence in the visual appearance of the bridge. Figure 3.6 Side view of Punt Staderas (Camathias SA, 2015a). Note the cover board that leads water away from the structure. Used with permission. Production The bridge consists entirely of locally cut wood. Walter Bieler personally assisted the forester in the search for suitable trees that could provide 14.5 m long beams (Guetg, n.d.). Both the Vierendeel and glulam girders were prefabricated and then transported to the site for assembly. The foundation abutments were casted on site. Assembly First the V-shaped load bearing supports were installed on the foundations. Thereafter the prefabricated girders were lifted to their position. During the assembly of the glulam beam spanning over the road, the road was closed and the beam could be lifted to its position (Standardname, 2015). The girders were then connected by screws. To make sure correct positioning of the structural parts, laser tools were used (Oertli, 2018). Material Originally the idea was to build a wooden bridge in larch with material from the immediate vicinity, but since spruce is the most widespread species in the area, it was CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 14 chosen as the main structural material. Larch was instead used in the exposed details. For the main span, glulam beams of strength class C24 are used. The paving consists of mastic asphalt. Durability The expected life span for the bridge is set to 80 years and the wood will start to turn grey until it finally becomes silverish (Oertli, 2018). Careful detailing has ensured a properly airy structure. Covering boards acts like small canopies for water where the water can drip off, and sufficient distance around allow the structure to dry out. The superstructure is protected by the covered pavement and the supports are tilted inwards. Weather-exposed surfaces consists of larch while the protected wood is spruce (Guetg, n.d.). The exposed railing and canopies that protect the superstructure are easy to replace. 3.2.3 Fussgängersteg Geheidgraben In Olten, Switzerland, a conceptually interesting bridge is located. The design is made by the architecture firm werk1 with the engineers of Makiol Wiederkehr and was completed 2013. The bridge spans a little bit more than 7 m over a ditch which works as a retention basin, meaning that there seldom is any water in the ditch except from many days of rainfall (Makiol Wiederkehr, 2014b). The architectural idea is to have a wave-like pattern that is implemented both on the walking deck and railing, which can be seen in Figure 3.7. Figure 3.7 Fussgängersteg Geheidgraben with its wave-like superstructure and railing (Makiol Wiederkehr, 2014b). Used with permission. Structural concept Statically, the bridge works as a simply supported beam bridge where the wavy decking is the main structural part. The longitudinally beams in the superstructure are actively CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 15 bent, which creates a slab with good bending capacity both vertically and horizontally. It is assumed that the wavy handrails also to some extent contribute to the vertical bending moment capacity due to its height and actively bent members. As a simplification, the handrails can be read as a truss structure. Vertical load distribution Globally, a vertically distributed load is transferred through the wavy beams to the foundations by bending moment. The cross-sectional height of the wavy beams in the bridge deck provides the governing bearing capacity. Horizontal load distribution To create the waves, washers and nuts are used to spread and pull together the floorboards. They are mounted on two transversal steel rods above each other in a grid with 0.6 m spacing (Makiol Wiederkehr, 2014a). Together this makes a homogenous slab, which provides the horizontal capacity. Rotational stability The bridge has a relatively short span and a broad width, which provides global rotational stability. The railing with its integrated steel posts gives torsional stability. Point load A point load is transferred through one or two wavy beams towards the transversal steel rods, where the load then is transversally distributed to adjacent wavy beams and then finally longitudinally transported to the foundations. Foundation The bridge is resting on concrete foundations. Steel bearings of LNP profiles (120/120/10) connects the foundation and the timber superstructure. On one end of the bridge there are elongated holes in the connection between support and beam, which allows for swelling and shrinkage. Rotation is allowed in one bridge end. The steel bearings on respective side of the ditch have a height difference of 0.6 m (Makiol Wiederkehr, 2014b). Principle connections Washers and nuts provide the necessary distance between the thin beams to create a wave pattern in the deck. The distancers are mounted on threaded steel rods in tension. The railings are constructed by laying boards with similar wave pattern created by distances, see Figure 3.8. RRW steel profiled posts penetrate the laying boards. The distances also protect the steel posts. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 16 Figure 3.8 Railing structure with distancing (Makiol Wiederkehr, 2014a). Used with permission. Production The bridge is prefabricated, and the decking and railings are preassembled separately. The curvature of the waved boards is made by screwing the nuts on the threaded bars to wanted position, and then the nuts themselves work as distancing to the next layer board. Assembly The preassembled decking and railings are joined together, and then the bridge is lifted to its position on the concrete foundation in one piece (Makiol Wiederkehr, 2014a). Material Decking and railing is made of massive rough-sawn oak boards. Steel is used for the threaded rods, nuts, washers, posts in railing and supports. The foundation abutments are in concrete. Durability Due to the wavy character, water can drip through the structure, and the structure is also easily dried. A consequence of the weather exposed structure is an aged appearance with rust and weather torn wood (Makiol Wiederkehr, 2014a). The steel posts in the railing are protected by the distancing steels parts. An extra board is mounted on top of the railing to cover the fastenings. The fact that pedestrians walk directly on the unprotected superstructure, will shorten the service life of the bridge. It is also difficult to replace single elements without demounting the whole bridge. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 17 4 Step I: Contextualisation – Site When designing a bridge, there are several aspects to consider. The context of the specific site needs to be analysed and studied such as required span lengths, foundation possibilities, and surrounding scenery and buildings, and connection existing paths. When these aspects are formulated, a structural concept can be explored and developed. This concept should fulfil the identified requirements connected to the context. In the following chapter, general demands based on client’s expectations are formulated. These consider both the visual appearance of the bridge, the purpose of the footbridge, and the feasibility of the proposal. Geotechnical and topographical conditions outline the site-specific context. These are in turn are affected by requirements stated by the municipality and Swedish Transport Administration. The above-mentioned aspects are identified by a study of public documents and site-specific investigations. An early visit to the site contributed to a perception of the current site concerning characteristics in the terrain, scenery, and surroundings. The project background, main challenges, and governing demands are formulated in this chapter. 4.1 About the site The chosen site is a planned development area named Wendelstrand, situated east of Mölnlycke outside of Gothenburg (Figure 4.1). The current gravel pit will be transformed into a residential area by 2026-2030, initiated by Next Step Group. Wendelstrand is planned to hold 850 new residences and public buildings, such as a nursery school, elderly home, and shops (Härryda Kommun, 2020). The community building, named Lakehouse, will be in the centre of the area, housing services such as restaurants, co-working offices, and gym. The building outlines the main square of the area, which holds the most important social services in the area (Härryda Kommun, 2020). Figure 4.1 The specific site in relation to Gothenburg (Google maps, 2021). CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 18 The general intention of Next Step Group for Wendelstrand is to develop a sustainable residential area, where the buildings adapt to the landscape and the architecture involves a high presence of timber (Härryda Kommun, 2020). A view over Landvettersjön and adjacent nature reserve is found in the southeast corner of the main square, while the roof of Lakehouse offers the most spectacular view, see Figure 4.2. The architect aims to make Landvettersjön present and visible throughout the whole area. With an increasing building height further away from the lake, the residents can see the lake from their apartments, see Figure 4.3. Figure 4.2 Birds view of Wendelstrand (Wendelstrand, n.d.-b). Photo: Snøhetta. Used with permission. Figure 4.3 Wendelstrand from Landvettersjön (Wendelstrand, n.d.-a). Photo: Snøhetta. Used with permission. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 19 4.2 Clients demands As the architecture of Wendelstrand enables a view of the lake anywhere in the area, the only missing link to the lake is the physical one. The client, Next Step Group, envisions a bridge that connects Lakehouse and the main square of Wendelstrand with the lake, both physically and visually. The road Boråsvägen must be considered, with the priority of making the lake accessible for the residents and visitors of Wendelstrand. Furthermore, the client wishes for a bridge structure with a high presence of timber that correspond with the sustainability profile of the development project. In addition to this, the client envisions a design that offers qualities other than merely a connection between A and B, that can be combined with the expected experience of Wendelstrand and Lakehouse. 4.3 Site-specific boundary conditions The site-specific boundary conditions mainly concern the surrounding topography. Public regulations stated by the Swedish Transport Administration, and site-specific regulations regarding shoreline adds to the list of restrictions that must be considered in the bridge design. The required height clearance of 5.3 m over the road is stated by the Swedish Transport Administration (Härryda Kommun, 2019). Furthermore, the supports must be placed with a clearance of 2 m from the walkway (Trafikverket, 2021). The county administrative board in Västra Götaland has defined a shore protection area, stretching 100-200 m from the shore of Landvettersjön. The purpose of the expanded restriction is to keep the shorelines clear from permanent structures to enable public access to the water, in addition to protecting the geological conditions in and around the lake (Sektorn för samhällsbyggnad, 2020). The area between Landvettersjön and Wendelstrand lies within the shore protection and is thereby protected. Early site- investigations on behalf of Next Step Group has identified the possibility of excluding a short strip of land along the load, to make space for possible bridge foundations (E. Silverterna, J. Garfvé, personal communication, March 11, 2021). The further development of the timber footbridge will consider this explicit area as excluded from shore-protection regulations, that is, a 4.5 m wide strip along the road. As a compromise, the bridge design is limited to a maximum of two landing points on the lakeside of the road. In the initial planning process of the area of Wendelstrand, several technical investigations were performed on behalf of Härryda municipality. Important for the footbridge design is the investigation of geotechnical conditions of the site. The investigation proposes the supports to be constructed as either ground slab, drill pipe or a combination of both as the soil layers generally consist of sand with elements of silt, gravel, and solid rock (Norconsult, 2018). Furthermore, in the latest local plan, a risk of minor landslides in the steepest terrain southeast of the area right next to Boråsvägen, is identified (Härryda Kommun, 2020). The evaluation has estimated it as a non-critical situation with a ten-year risk. However, stabilisation of the terrain in question is outside the scope of the current project. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 20 Governing topographical conditions are identified as the following and illustrated together with the requirements from the Swedish Transport Administration in Figure 4.4: - 19 m height difference between Lakehouse and Landvettersjön. - Steep slope in the terrain (21°). - Boråsvägen has a varying width of 7-11 m including pavement. - Required height clearance of 5.3 m and 2 m width clearance. Figure 4.4 Governing boundaries of the site. 4.4 Summary of contextualisation From the site contextualisation, the following demands and requirements are identified. The public requirements are considered non-negotiable and must be fulfilled for the proposal to be feasible. In addition, to achieve an attractive bridge proposal, the client’s demands must be met. - Clients demands - Physical and visual relation between Wendelstrand and Landvettersjön - Make the lake accessible for residents as well as visitors - Footbridge design with a high presence of timber - Proposal for additional qualities in the design - Public regulations - The Swedish Transport Administration - Västra Götaland County Administration - Shore protection - Västra Götaland County Administration Following, the contextualisation reveals numerous unspecified design aspects that will affect the bridge design. These design aspects will guide the development of possible solutions in the next design phase. Each aspect will be investigated both separately and combined, with the aim of enabling a large variety of design proposals: - Path and landing points - As the exact landing points of the bridge are not specified by the client, the movement of the bridge path is free to investigate. - Length and inclination of the path - The length of the bridge path is related to the inclination. The inclination is regulated, thus governing the total length of the path. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 21 - Integrated functions - A general expectation for the bridge design includes additional functions in the structure. These are not stated specifically from the client. - Architectural appearance - The client expects a bridge design that relates to the profile of Wendelstrand. No further demand is stated and is therefore part of the design investigation. - Structural concept - A free investigation of the most suitable structural concept for the bridge is allowed. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 22 5 Step II: Conceptual design When the specific site and different criteria are defined, the search for a suitable bridge concept, which is an answer to these, can begin. The search starts broad with many various suggestions and is then narrowed down into a suitable concept. To create a framework for the design proposals a set of general design criteria are formulated. These are based on qualities which are strived for and include contextual limitations and challenges, and clients demands. In addition to this, the designers aim of combining architectural qualities with structural engineering knowledge is considered in the design criteria. As stated in Chapter 2.2 the Conceptual design phase includes three sub-phases: Intuitive, Intentional and Evaluation. The Intuitive phase focus on exploring the possibilities on the site with less consideration of the design criteria. Any subjective opinion is set aside during this design process to allow for a large variety in the generation of the first ideas. Combined with a thorough review of the expected design qualities, three concepts are developed from the initial proposals in the Intentional phase. Each concept is developed to a certain level to enable a thorough comparison of the proposals and to determine the feasibility of the designs. In the last part of the Conceptual design phase, the three concepts are evaluated in relation to the stated design criteria and identified demands. The result is an appropriate solution for a footbridge in Wendelstrand. A set of defined evaluation criteria creates the transition between each design phase. The evaluation criteria are categorised into design criteria and demands, with the purpose of securing fulfilment of the contextual requirements in the design development, and inclusion of expected design qualities. To enable a large variation in the proposals, a reduction in the evaluation criteria is exercised in the transition between the first two phases to discover qualities in other solutions. The evaluation criteria used are illustrated in Figure 5.1. The proposed design will fulfil every stated evaluation criterion before being developed in the Preliminary design phase. Figure 5.1 Illustration of the governing evaluation criteria. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 23 5.1 Design criteria The design criteria are divided into three main categories: spatial qualities, bridge qualities, and structural concept. The different aspects of the categories are explained in the following three subchapters. 5.1.1 Spatial qualities The spatial qualities consider the architectural aspect of the structure and its relation to the surroundings. This is implemented in different aspects, which are stated below. Visual guidance The client requests a visual and physical connection between Lakehouse and Landvettersjön. The design aims to establish a reachable structure with a sense of predictability for the users. In addition, the design should invite visitors from other areas and enable approach from different directions. The design criteria can be met either partly or completely, such as only a visible landing point, path extending from the city square, or a straight bridge path from Lakehouse to Landvettersjön. Relation to scenery As for all built structures, the relation to the scenery is of great importance. The architecture of Wendelstrand follows the same manner and blends in with the surroundings. It is therefore of interest to develop a bridge design with similar characteristics. As the purpose of the bridge is to connect the residential area and the lake, the design of the bridge must relate to the characteristics of both the residential area and the nature around the lake. Architectural appearance Furthermore, the structure should hold certain architectural qualities. This is not necessarily directly related to breath-taking design with large visual impact. On the contrary, such qualities can be found in careful detailing of functions such as water drainage, hand railing or torsional stability. It can also be found in a design that relates to a specific site characteristic. Although this design criterium is rather vague, the assessment is grounded on substantial arguments. The aim is to develop a structure that holds architectural qualities and simultaneously relates to its surroundings. The criteria relation to scenery and architectural appearance therefore determines a level of accepted visual impact. 5.1.2 Bridge qualities The bridge qualities consider the architectural aspect of the design in relation to its form and concern the following aspects. Presence of timber As Wendelstrand will become Northern Europe’s largest residential area in timber, it is expected that the bridge proposal aims for a high presence of timber with an obvious timber structure. Yet, a large amount of timber is not necessarily the most suitable interpretation of this design criterium but rather a careful design of the different structural members with the resulting visual expression as governing criterion. This CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 24 aspect is also related to the design category regarding Structural concept which will be elaborated further in the next subchapter. Additional functions The common function of a bridge is to be part of a traffic flow. In this case, the bridge will be part of a pathway where the landing point is the goal. In addition to the sole purpose of connecting Lakehouse and Landvettersjön, it is of interest to develop a design that adds other qualities to the site. For instance, integrated seating or permanent furniture as an extension of the superstructure, a viewing platform expanded from the bridge path, a pier stretching out over the water, or weather protection underneath the bridge. 5.1.3 Structural concept The third and last category incorporate design criteria into the structural design. Characteristic structure First and foremost, the structural design of a bridge is related to the visual appearance of the bridge. Consequently, the choice of structural concept affects the design criterion of Spatial qualities and Architectural appearance, and vice versa. The intention is to develop a bridge design where both architectural and engineering qualities are considered, with the aim of contributing to the development of timber bridge design. Therefore, a conscious consideration of these aspects is required, which ultimately will result in an enhancement of the qualities rather than compromising. Logic structure The structural concept of the bridge must be logic in the sense of the utilization and purpose of the structural members. All members will contribute to the structural performance of the bridge, which is related to both the structural and architectural aspects of the design. Form follows function, and function follows form. Whichever is governing is determined by the other evaluation criteria. Furthermore, the bridge design aims for a high presence of timber. A structural concept best suited for timber will be developed, as timber will not be chosen for the sole purpose of timber presence. An investigation of the most suitable timber product is necessary. Accessibility As Wendelstrand will lodge residents in the span from children to elderly, the bridge aims to be accessible for everyone. An inclination of 2° or less is stated as the requirement for wheelchair users (Göteborgs Stad Trafikkontoret, 2017), which is challenging to combine with the large height difference. For this reason, the bridge design will aim to meet the inclination requirements to an extent where everyone can utilize the main functions of the bridge. Whether or not the whole bridge design will meet the accessibility requirements depends on how the concept meets the other design criteria stated in this chapter. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 25 5.2 Intuitive phase The Intuitive phase focus on generating a diversity of possible solutions. Figure 5.2 shows the evaluation criteria which are the most emphasised in this phase. Figure 5.2 Emphasised evaluation criteria in the Intuitive phase are highlighted in orange. As neither the starting- or landing points are explicitly stated by the client, an exploration of different movement patterns with corresponding qualities and consequences are explored. Three main aspects are identified as governing for the movement pattern of the bridge. Firstly, the path must connect to Lakehouse, preferably as an extension of the adjacent seating platform. Secondly, it should be possible to access the bridge from the bicycle and pedestrian lane on Boråsvägen, from both directions. As illustrated in Figure 5.3, a parking space is located along the road, which will be used by visitors to Wendelstrand. Third and last the bridge should connect to Landvettersjön. Both a physical and visual connection to the water will be explored. Figure 5.3 illustrates the three landing points to be considered. Figure 5.3 Landing points. Lakehouse, Boråsvägen with parking and Landvettersjön. Depending on the chosen path, the design criteria are fulfilled to different extents. As a result, varying spatial qualities are explored. For instance, to what extent the starting point connects to Lakehouse, whether the bridge is accessible from the road and in CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 26 which direction, or whether the bridge has a physical connection with the water. A selection of possible movement patterns is illustrated in Figure 5.4. Figure 5.4 Variation of movement patterns. The Intuitive design phase aims to explore the possibilities of the site, which is made possible with a large variation of ideas. Based on the exploration of possible patterns in Figure 5.4 different structural bridge concepts are assigned to the different paths. As the idea of a structural concept is formulated, the design is developed with principal sections and connections. The result is twelve different design concepts, illustrated in Figure 5.5. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 27 Figure 5.5 Initial proposals. Summary of the Intuitive phase The design proposals developed in the Intuitive phase are generated through conscious consideration of: - Spatial qualities - Investigate different options of movement pattern when connecting Lakehouse, Boråsvägen and Landvettersjön. - Explore the possibilities and limitations of the site when considering the movement of the bridge. - Structural concept - Explore the possibilities of the movement patterns by applying different structural concepts, both inspired by reference projects and developed from the movement pattern itself. - Explore the possibilities and limitations of the movement patterns when considering the structural concepts. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 28 5.3 Intentional design phase The Intentional phase aims to develop the intuitive proposals and narrow them down to three structural concepts. This is done by considering the aspects of the evaluation criterions highlighted in Figure 5.6. Figure 5.6 Emphasised evaluation criteria in the Intentional phase are highlighted in orange. Focus lies on how the desired qualities can be met for different movement patterns. The study aims to determine whether one pattern is superior in meeting the design criteria, or if different movement patterns meet the desired criteria on an equal level. A variation of spatial qualities is found for different movement patterns, where different paths meet the design criteria on different levels. An exploration of different movement paths results in a large variation between the design proposals in the Intentional phase. Three different movement patterns are chosen for further development. They are illustrated in Figure 5.7 and differs in the following sense: - Accessible ramp - Direct or curved path - Ramp in one or two directions - Bridge reaching out over the water - Bridge reaching into the water Figure 5.7 Three different proposals with three different movement patterns. Next, the proposals from the Intuitive phase are evaluated for the chosen evaluation criteria of the current phase. As the criteria are not prioritised, the proposals are not CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 29 being rated in a matrix. Instead, they are evaluated in relation to each other and to which extent they satisfy the design criteria: - Does the bridge relate to the scenery? - Is the presence of timber high? - Does the design integrate additional functions in the structure? - Does the structural design add an architectural quality? - How is the impact on the surroundings? - Does the design create a physical and visual connection between Lakehouse and Landvettersjön? - Is it possible to comply with the accessibility requirement? Three keywords are formulated to give impact on the development of the design proposals: nature, embracing and sweeping. Nature as in relation to the surroundings, or resemblance of the characteristics of the site. Embracing as a quality in the bridge, which can be achieved in the design of the railings or integrated seating, and implementation of timber presence. Sweeping describes the movement of the bridge, which resembles a natural path. These keywords represent the essence of the design criteria formulated in Chapter Error! Reference source not found.. In the further development of the proposals, characteristics from these keywords are implemented to a different extent to ensure a variation in the proposals. The movement pattern governs the development of the design, where each path is assigned with a suitable structural solution. The proposals are developed to enhance a conceptual idea rather than searching for the one most suitable solution. To enable an evaluation of the proposals in the next phase, divergence is strived for. Reference projects are of great importance to support the design development. Three resulting bridge proposals are presented in the following part of this chapter. Proposal 1 The aim of this concept is to create a path that winds among the trees in two levels, where the delta on the southwest side of Landvettersjön is explored. This area is outside the local plan, but holds a lot of potential and qualities. Figure 5.8 shows a conceptual illustration of the proposal. The essence of this proposal is the relation to the scenery. The structure blends with the trees and with supports resembling tree trunks and a movement resembling a natural path. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 30 Figure 5.8 Bridge proposal 1 seen from the road. The structure consist of paths in two planes; an upper leading the visitor from Lakehouse across the road, and a lower path from the road with an accessible inclination. The upper path lands in an elevated platform above the water, while the lower path stretches out in a pier below the platform, which is seen in Figure 5.9. The two paths are connected with a staircase. Figure 5.9 Perspective of proposal 1. The load-bearing system consists of a curved glulam beam with Gerber hinges. There are inclined supports every 10th meter. The torsional stiffness comes from transverse rigid steel frames. Over the road, the span of 15 m is carried by two larger parallel glulam beams with intermediate diagonal glulam beams, creating a truss. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 31 Proposal 2 The second proposal is a bridge that follows the straight line of sight from the road that go towards the square outside Lakehouse, across Boråsvägen, into the water, landing in a long pier, see Figure 5.10. The large height difference impose the design to include stairs to access the bridge. An accessible ramp is integrated as an extension between the pier and the road. The aim of the design is to create a clear visual and physical connection to Lakehouse, with an characteristic structure that adds an architectural quality. Figure 5.10 Bridge proposal 2 seen from the road. The structural concept of the second proposal’s superstructure is characterised by beams in a sinus pattern. The beams are rigidly connected to create interaction for vertical and horizontal bending. A perspective of the proposal is seen in Figure 5.11. Figure 5.11 Perspective of proposal 2. To emphasise the concept of curved beams, the railing consists of thin wood elements woven and stacked ontop of each other, which in turn adds to the torsional stability of CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 32 the bridge. The superstructure is protected by a bridge deck, but is exposed on the lower side. The same structural concept is applied on the stairs leading down to the pier, as well as the vertical support elements. The visible structure adds an architectural quality and experience for the visitors approaching from the road. Proposal 3 The third proposal aims to establish a clear visual and physical connection to Lakehouse, but does not have a physical connection to the water. Instead a platform is reaching out over Landvettersjön, see Figure 5.12, to create a visual connection. The pathway widens over the water, to give space for seatings on the bridge deck. Figure 5.12 Bridge proposal 3 seen from Lakehouse. The structrual design resembles reeds found along the lake, lifting the structure. The goal is to have slender columns, concentrated at few points, to achieve an airy appearance. A perspective of the bridge is shown in Figure 5.13. Figure 5.13 Perspective of proposal 3. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 33 The structural concept is described as a beam-column system. The main load-bearing capacity is provided by two outer glulam beams with intermediate diagonal glulams beams in a zig-zag pattern. Beams of smaller dimensions are perpendicularly connected to the zig-zag beams. This will not only contribute to the horizontal stiffness, but it will create a characteristic pattern from beneath. Summary Intentional phase The design proposals developed in the Intentional phase focus on exploring the possibilities of the site outside of the local plan and investigate which additional qualities that could be included in the bridge design. The accessibility requirement played a larger role in the design proposals, as well as the intention of including visitors from every direction. The intuitive concepts are developed with emphasis on the following characteristics: - Nature: relation to scenery - Embracing: additional quality in the bridge, implementation of timber - Sweeping: movement of the bridge and relation to scenery 5.4 Evaluation phase In the last phase of the Conceptual design, the bridge proposals are evaluated in relation to the design criteria, contextual requirements, and the client’s demands. The aim is to narrow down the proposals into one, suitable concept. Client evaluation As the footbridge in Wendelstrand is requested by Next Step Group, the three intentional design proposals were pitched in a meeting on March 11, 2021. The presentation was customised to communicate the bridge concepts as a fictive design competition, considering Next Step Group as a client. Our understanding of the project Wendelstrand and analysation of the site were introduced. The three bridge proposals were presented as solutions, with their different possibilities and qualities. Already built reference projects were shown to support the arguments. From the discussion afterwards, feedback from Next Step Group focused on how to involve visitors as well as residents. As Wendelstrand aims to attract visitors from the surrounding area, a bridge structure connected to the water will be included in the overall experience of the area. It is therefore of interest to include visitors approaching the bridge and lake from Boråsvägen as well as from Lakehouse. Physical contact with the water is of greater interest than initially communicated, and the client also emphasised on including additional functions in the bridge design, to offer an experience to the public. The client also wanted suggestions for possible activities in the lake as an extension of the bridge. Moreover, the client focused on how the bridge design is affected by the Swedish Transport Administration, shore protection and private landowners of the areas outside the local plan. The demands from the Swedish Transport Administration are non- negotiable and are therefore complied with as requirements. The extent of the shore protection area was vaguely defined and was therefore not strictly incorporated as an evaluation criterion. From this meeting, it is clarified that there is a small area excepted from the shore protection regulations. A maximum of two support foundations are CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 34 allowed within this area. The impact on the seabed should be limited, where floating solutions should be aimed for. The delta area is outside the local plan of Wendelstrand, owned by private individuals and the municipality. To get permission to build something here is rather difficult to achieve. The client cannot plan for a project relying on agreement from outside parties with unpredictable interests. Based on this discussion, proposal 2 and 3 are found to be the most feasible bridge proposals to be developed, according to the client. These consider the shore protection to some extent and have a clear visual and physical connection between Lakehouse and Landvettersjön. Next Step Group emphasises on the physical connection to the water, as well as the accessibility from the road. Proposal 1 is disregarded because it is located outside the local plan. Summary of the Evaluation phase As a summarize of the client’s meeting it can be concluded that the final bridge proposal must fulfil the following: - Contextual demands: - The proposal must be within the borders of the local plan. - Respect the shore protection regulations and minimise the amount of supports along the shoreline. - Avoid supports in the lake. - Respect the demands of the Swedish Transport Administration. - Design demands: - Strong visual connection to Lakehouse, preferably in a straight line from the city square. Aim to create a sense of predictability for the users. - Establish a physical connection with the water. Not necessarily continuous from Lakehouse to the water. - The bridge design should include a proposal for additional functions. - The whole bridge does not have to be accessible from Lakehouse to the lake, since there already is a planned accessible path close to the planned bridge. However, the water must be accessible from Boråsvägen. - Design ambition: - Relate to scenery, both in aspects of nature and architecture. - Clear presence of timber. - Architectural quality in the structure. - Resemblance of a natural path. The development of the final bridge design is determined by the following aspects regarding spatial and bridge specific qualities: - Limited area for a landing point on the shoreline can complicate the stair connecting the bridge and the ground. - Visual expression of the pier in relation to the bridge structure and surrounding architecture. - Relation between the pier and pedestrians approaching both directions along Boråsvägen. - Possibility for a variation in additional functions on the pier. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 35 6 Step III: Preliminary design The conclusions drawn from the evaluation phase specify the framework for the final design proposal. This is a tool to develop the concept to where it meets every defined evaluation criterion. Physical models are used to investigate the feasibility of the chosen structural concept. The aim of this phase is to develop the final design proposal to an extent where it can be verified and proven in the last design step: Final design. 6.1 Final design proposal The final design proposal is a development of design proposals 2 and 3, where a straight path from Lakehouse ends in a platform, creating visual connection to the lake. A physical connection to the lake is made possible by stairs. Figure 6.1 illustrates the final concept in its context and the following subchapters describes the concept more in detail. Figure 6.1 Conceptual model of the final design. 6.1.1 Overall bridge design The client’s wish for a strong physical and visual connection to the water is recognised by the straight sight line from the residential streets and the small square in front of Lakehouse. The bridge is accessed by stairs in the hillside. The straight path ends in a wide viewing platform, enabling a pause for the pedestrians. Stairs wind down to the ground and lands on the floating pier. A plan view of the final design proposal is seen in Figure 6.2. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 36 Figure 6.2 Plan view of the final design proposal illustrating the sight line from the city square and Lakehouse to the lake. The connection between the bridge and the pier is established by a curved staircase, which ensures a landing point within the specified area along the shoreline. An accessible ramp follows the topography and is then led parallel with Boråsvägen down under the viewing platform and then onto the floating pier. Pedestrians that approach from the other direction can access the pier by stairs down from the walkway. As the client specifically requested, the bridge must offer something more than just a physical connection to the water. A circular floating pier is chosen to be an extension of the accessibility ramp and is proposed to facilitate different possible activities to meet the client’s requests. The pier is inspired by the sculpture The Infinity Bridge by Gjøde & Povlsgaard Arkitekter in Aarhus. The circular pier offers an inner pool, as well as the possibility to anchor floating saunas, restaurant rafts and canoe rental. Additionally, the circular shape relates to the design language of the curved staircase and ensures a smooth transition for visitors approaching from three directions. Visitors can pause at any point on the circular pier, instead of being led on a straight path out in the water. 6.1.2 Structural concept The structural concept is a beam bridge with inclined columns, with a total span of approximately 27 m. The superstructure is built up by two outer straight beams and a horizontal truss action is achieved by actively bent beam in a sinus pattern. It is assumed that the internal stresses from active bending provide more horizontal stiffness to the superstructure than pre-bent beams with the same dimensions and geometrical pattern. The vertical supports are V-shaped in the longitudinal direction. Structurally this will lead to smaller span lengths, and an architecturally visually resemblance of reeds that stretches up along the shoreline. Figure 6.3 shows a side view of the bridge. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 37 Figure 6.3 Side view of the bridge illustrating the V-shaped supports, the circular staircase, and the floating pier. To minimise the total length of the staircase, which has a height to length ratio of 1:2, the staircase winds down to an intermediate platform and is then led down straight to the ground. In addition to this, the straight staircase provides global horizontal stability to the bridge structure. The statical concept is visualised together with the bridge structure in Figure 6.4. Figure 6.4 Concept for horizontal stability of the viewing platform. The floating pier is supported by pontoons, which are anchored to the same abutment as the staircase on land. Spacing between the pontoons and the pier deck allows for sunlight to reach the seabed and ensure healthy biotope conditions in the lake. 6.1.3 Experiment with active bending As the production and assembly method is part of the bridge design the concept of using active bending in the superstructure needs to be tested and verified. The assembly of this structural concept is assumed to be rather complex compared to a simple beam bridge. Instead of performing another literature study, an investigation of physical models is used to gain understanding of active bending. First small and simple conceptual models of actively bent members are built to understand the forces and failure modes better. Then larger and more complex models are built to test the assembly method. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 38 To begin with, small strips of cardboard and paper are bent and attached to each other in different combinations. Figure 6.5 shows four models that actively bends the strips in different ways. The cardboard has much more stiffness than what paper has and is therefore referred to as the stiffer part in the observation. The bridge proposal suggests having two outer straight elements, here referred as beams, and inner bent elements, here referred to as lamellas. Figure 6.5 Experimental models of actively bent cardboard and paper strips. In Figure 6.5a, a stiffer straight beam frame is used with less stiff bent lamellas. The lamellas are connected to each other, layer after layer. Without the frame, the lamellas lay parallel to each other, and active bending is first introduced when the two outer lamellas are stretched out and connected to the stiffer straight beam frame. In Figure 6.5b, same stiffness on the outer beams as well as the inner bent lamellas is used. The curvature of the lamellas is achieved when the lamella is longitudinally pushed together and attached to the straight beam. The adjacent lamella is equally pushed and connected to the neighbouring lamella, until the outer straight beam is attached and force the deformation in the opposite direction. This method results in buckling of the lamellas. As a conclusion, the curved lamellas must have a significantly smaller stiffness than the outer beams. In Figure 6.5c, stiffer transversal beams are used with less stiff bent lamellas. The lamellas are attached together in the same manner as in Figure 6.5a, but instead of being anchored to a stiffer frame, transversal elements push the lamellas apart causing the curvature. The model in Figure 6.5d has used same stiffness on transversal beams and lamellas, in this case cardboard instead of paper. This model generates the best result, where an even curvature is achieved. As a result, the curved elements interact and create a continuous element. The resulting stiffness generated by the interaction of the curved lamellas is sufficient to maintain a desired shape for an external force and resembles the behaviour of a truss. A conclusion from this test is that a combination of a stiffer, outer frame and transverse distancing elements is considered as optimal. It is easier to control the deformation of CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 39 the lamellas with the help of transversal stiffeners. It can also be observed that the curved shape of the lamellas can be achieved in two ways: either induced by a transversal distancing force or by buckling caused by a longitudinal compressing force. Without any transversal or longitudinal force, the lamellas will automatically lay flat against each other. If looking at the boundary conditions instead, a longitudinal force can be represented in fixed connections at the ends of the curvature, while a transversal force can allow for a roller support at one of the ends. A combination of these can of course be done to reduce the residual forces required at the boundary conditions. The correlation between boundary conditions and shape is illustrated in Figure 6.6. Figure 6.6 Observations of different boundary conditions and resulting shape and forces due to active bending. The aim of using actively bent elements is to mimic a truss and thereby create horizontal stability in the superstructure. As seen in Figure 6.7 the bent elements resemble a truss, and forces can diagonally be transported. The inclination of the structs can be altered to achieve different visual appearances. Figure 6.7 Theoretical perception of the structural behaviour of beams in a sinusoidal pattern. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 40 6.2 Model development of structural concept Based on the knowledge gained from the previous small experiment, the next step is to build a concept model in wood to simulate a more realistic assembly method and to test the structural behaviour of the actively bent elements. The assembly method of the conceptual model is illustrated in Figure 6.8 and can also be described as the following: 1. A thicker element is used as outer beam and a thinner element for the curved panels. The elements are predrilled, with a few millimetres offset in the panels. 2. Threaded steel bars are inserted through one of the outer straight beams and fastened with nuts on the outside. 3. A straight panel is mounted on the threaded bars. Deformation of the panel is induced due to the offset of the holes as well as from nuts placed at desired transversal distance on the bars. 4. The rest of the panels are assembled in a similar way, creating the waved truss. 5. Lastly, the other straight beam is assembled, and secured by nuts on the outside. Figure 6.8 Diagram of the assembly method of the concept model. Evaluation of the concept model As can be seen in the second step of the assembly method in Figure 6.8, the outer beam started to deform due to the built-in longitudinal forces as described in the previous subchapter. This deformation got larger for each new panel added. To attach the last straight beam, large amount of external force was required. The predrilling of the holes had a relatively large offset, creating large amplitude of the sinus curves. As a result, the position of the nuts was adjusted thereafter, resulting in unprecise spacing. Buckling CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 41 of the threaded bars could also be observed. Ideally the bars should be in tension, but in this model, they are in compression because no spacers were used. As a conclusion from these observations, compression members should be used to secure the deformation of the panels, allowing the threaded bar to be in pure tension. Secondly, the panels should be mounted on the bars as straight elements with compression spacers at specified positions. Thereafter, as nuts in each end of the threaded bars are screwed tighter, forced compression is induced on the wood elements. With specifically placed compression distancers, the sinusoidal pattern is created. This method will ensure an even deformation and stress distribution in the structure. Regarding the pre-drilling, the position of the holes should be more accurate and measured beforehand, so the right spacing is achieved. The holes in the bent panels can preferably have larger dimension than the dimension of the threaded bars to secure some tolerance during assembly. Concerning the structural behaviour of the structure, it works well in vertical and horizontal bending. The torsional stiffness is however weak. This can be increased by using an extra layer of threaded bars, in addition to an increased height of the wooden elements. On the other hand, interaction of the curved panels is secured by pre- tensioning the superstructure. This phenomenon will increase when the threaded bars work purely in tension, and the deformation is secured by separate compression members. Applied force in horizontal direction proves that the curved panels act like a truss, which distributes the forces in compression and tension to the straight, thicker beams. Continuing from this, the concept model is developed according to the observed results. The superstructure is built in a model in scale 1:10 to verify the new assembly method as well as attempt to increase the torsional stiffness. A model of the complete bridge structure is built in scale 1:20 to verify the production method of the whole bridge structure. Moreover, this model will demonstrate the connection between the superstructure and columns as well as the horizontal stability from the straight staircase. Spatial qualities and the bridge’s context in its surrounding is demonstrated in a landscape model in scale 1:400. These models are presented in the next chapter. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 42 7 Step IV: Final design The final bridge design proposal is a straight bridge deck across the road, V-shaped column supports, a circular staircase, and a circular, floating pier. The bridge deck ends in a viewing platform, with the staircase winding around and underneath the structure. Simultaneously, an accessible ramp from the road is led through the V-shaped platform columns, underneath the staircase. Any visitor will be able to experience the bridge structure from underneath, where every path leading to the floating pier is interplayed with the bridge structure. As a result, the bridge design proposal offers both a transport route, a destination, and an experience. The overall concept together with its context is shown in Figure 7.1 and Figure 7.2. Figure 7.1 Scale model in 1:400 of the proposed design in its context. Figure 7.2 The whole bridge model in scale 1:20. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 43 The structural concept of the bridge deck is developed to decrease the effective height of the deck, while maintaining, or increasing, the horizontal and vertical capacity. Panels of Laminated Veneer Lumber (LVL) are actively bent into a sinusoidal pattern, secured by two outer, parallel straight LVL-beams and pre-tensioned with threaded steel bars into a uniform element. Active bending is applied to achieve the wanted performance of the deck, where built in stresses allow the curved panels to interact in a truss-like behaviour giving both vertical and horizontal stiffness. The superstructure with its actively bent lamella panels can be seen in Figure 7.3. Figure 7.3 The bridge superstructure seen from the intermediate platform. V-shaped column supports decrease the span-lengths of the bridge and as a result reducing the governing forces in the superstructure. The columns go up into the superstructure and the bent panels are spread to give room to the connection. This creates a homogeneous meeting between the bridge and supports and enhances the architectural appearance of the concept. The connection is in the mid height of the superstructure, generating no extra cantilevering point. The integrated meeting between columns and superstructure is shown in Figure 7.4. Requirements on the site concerning clearance from the road and shore protected area determine the chosen landing points. The direction of the bridge is on the other hand determined by the visual connection between the lake and Lakehouse, but also by the challenges of connecting the bridge with a staircase. The design aims to limit the impact on the ground, while simultaneously achieving the required stability of the global bridge structure. The straight staircase is anchored on the shore strip in a perpendicular direction to the superstructure to achieve global horizontal stability. To reduce the total length of the straight staircase it starts at the intermediate platform, which is seen in Figure 7.5. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 44 Figure 7.4 The supports are integrated into the superstructure creating a homogeneous appearance. Figure 7.5 Viewing platform and pathway down to the ground level. The straight staircase also contributes to the horizontal stability. CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 45 7.1 Bridge dimensions A land section of the final bridge design with its correlation to Lakehouse and Landvettersjön is illustrated in Figure 7.6. An overview of the governing dimensions, and clearance is given in Figure 7.7. To ensure water drainage along the bridge, an inclination of 2% towards Lakehouse is applied. Figure 7.6 Side view of the bridge in relation to Lakehouse and Landvettersjön. Figure 7.7 Land section with main dimensions. A cross-section of the bridge is illustrated in Figure 7.8 with labels of the structural elements and governing dimensions of the bridge deck. The specific dimensions of the load-bearing elements are summarised in Table 7.1, while dimensions of the bridge deck components such as floor beams and railing are summarised in Table 7.2. The dimensions of the support columns are presented in Table 7.3. The foundation elements are suggested to be in reinforced concrete but required dimensions are not calculated. A rough estimation of the required capacity of these elements is presented in Table 7.22. CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 46 Figure 7.8 Cross-section of the superstructure and bridge deck with corresponding materials and dimensions. Table 7.1 Dimensions of the load-bearing elements. Element Material Dimension Value [mm] Straight beams LVL, Kerto-S w x h 108 x 600 Curved panels LVL, Kerto-S w x h 54 x 600 Compression spacers CHS d, t 76.1, 8 Prestressing bar Dywidag, 26WR d 26.5 Table 7.2 Dimensions of bridge deck components. Element Material Dimension Value [mm] Transverse floor beams Solid timber w x h, c-c 75 x 90, 600 Longitudinal floor beams w x h, c-c 90 x 45, 600 Plank deck w x h, c-c 195 x 22.5, 210 Railing w x h, c-c 120 x 70, 1200 Solid board Plywood t 9 Bitumen felt YEP 2500 t 2 Table 7.3 Dimensions of support elements. Element Material Dimension Value [mm] Support columns intermediate LVL w x h 216 x 260 Support columns platform w x h 270 x 260 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 47 The design of the staircase is performed as a simply supported beam subjected to vertical load according to the load cases calculated for the superstructure. The structure is built up similarly as the bridge superstructure, with two outer straight beams and actively bent LVL panels. The final dimensions of the staircase structure are presented in Table 7.4. Table 7.4 Dimensions of the load-bearing elements of the staircase. Element Material Dimension Value [mm] Straight, outer beams LVL, Kerto-S w x h 108 x 400 Curved beams LVL, Kerto-S w x h 54 x 400 Compression distancers CHS d, t 76.1, 8 Prestressing bar Dywidag, 18WR d 17.5 7.2 Input data To verify the bridge design with its cross-sections hand calculations and a simplified FE analysis are performed. The overall bridge design is based on European Standards and Swedish Standards. General requirements for timber footbridges are formulated by the Swedish Transport Administration. Overall bridge design concerning the capacity is covered in Bärighetsberäkning av broar and states which European standards that are used to determine the capacity of the structure (Ronnebrant, s2020). Detailed design of the bridge is covered in Krav Brobyggande (Krona, 2019). To ensure a more concise writing, EC5-1 will be used as abbreviation for Eurocode SS-EN 1995-1 etc. The following standards are used for the different calculation aspects: - Design material properties for LVL: EC5-1-1 (SS-EN 1995-1-1:2004) - Bridge specific properties: EC5-2 (SS-EN 1995-2:2004) - Dimensioning with the partial factor method: EC0 (SS-EN 1990:2010) - General loads - Wind load: EC1-1-4 (SS-EN 1991-1-4:2005) - Snow load: EC1-1-3 (SS-EN 1991-1-3:2003) - Bridge specific loads: EC1-2 (SS-EN 1991-2:2003), with corresponding partial factors - Load combinations: EC1-1 (SS-EN 1991-1-1:2005) - Bridge details: The Swedish Transport Administration, Krav Brobyggande (TDOK 2016:0204) 7.2.1 Partial factors The timber bridge is dimensioned according to EC0 where the partial coefficient method is applied. On the load side partial factors are used to consider exposure, load duration, load situations and -combinations. The material related partial factors are taken from EC5. Material properties The characteristic material properties are adjusted with partial factors that considers exposure, load duration, material, and element size according to EC5-1, which gives the design material properties: CHALMERS, Architecture and Civil Engineering, Master’s Thesis ACEX30 48 𝑓ௗ = 𝑘௠௢ௗ 𝑓௞ 𝛾ெ [𝑀𝑃𝑎] 7. 1 The correlation factor kmod considers the load duration and moisture content of the structural material. The bridge is classified as service class 3 according to EC5-1. However, if the main structural members can be sufficiently protected against rain, service class 2 can be applied. In this case, a conservative approach is chosen and thereby service class 3 is used. Self-weight is classified as permanent load while imposed loads such as wind load (EC5-1) and traffic load from pedestrians (EC5-2) are considered short-term loads. According to EC5-1, combination of loads of different duration should be determined for the shortest load duration, which in this case is short- term load duration. For LVL exposed to short-term load duration in service class 3 (EC5-1), the correlation factor is set according to 𝑘௠௢ௗ = 0.7 7. 2 The partial factor 𝛾ெ is determined by the specific material and differs for control of capacity and control of deformation. For LVL, the partial factor according to EC5-1 is: 𝛾ெ = 1.2 7. 3 For LVL with rectangular cross-section and majority of the veneers are oriented in the same direction, the size effect in bending and tension must be considered. For LVL in bending, the reference height is 300 mm. For any element height in bending other than 300 mm, the characteristic bending strength fm.k should be multiplied with the factor kh according to EC5-1. For LVL in tension, the reference length is 3000 mm. For any element length other than 3000 mm, the characteristic value ft.0.k should be multiplied with a factor kl according to EC5-1. The design capacity of the LVL beams and panels are determined for veneers in the same direction. The dimensions determining the size effect factors for LVL in bending and tension, and the resulting factors are summarised in Table 7.5. Table 7.5 LVL size effect. Load action Value Element height in bending hlvl 0.6 [m] Element length in tension L 27.3 [m] Size effect for bending kh.lvl 0.92 Size effect for tension kl.lvl 0.876 Load combinations As the risk of personal injury in the case of structural collapse is considered serious, the bridge is categorized as safety class 3 according to 13 § in Boverket EKS 10 (Boverket, 2016). The partial coefficient considering the safety of a load bearing structure is set according to 14 § EKS 10: 𝛾ௗ = 1.0 7. 4 CHALMERS Architecture and Civil Engineering, Master’s Thesis ACEX30 49 The timber bridge is verified in Ultimate Limit State (ULS) for deformation of the structure, STR: “Internal failure or excessive deformation of the structure or structural members, including footings, piles, basement walls, etc., where the strength of construction materials of the structure governs” (Chapter 6.4.1, EC0). Load combinations in ULS considering the capacity of the structure is calculated according to Equation 6.10a and b in EC0. For unfavourable permanent loads G: 𝐿𝐶ீ = 𝛾ௗ ∙ 𝛾ீ ∙ 𝐺௞ 7. 5 For variable loads Qk.i, the loads are combined as follows: 𝐿𝐶ொ.௜ = 𝛾ௗ ∙ 𝛾ொ ∙ 𝑄௞.௜ 7. 6 For interacting variable loads Qk.i and Qk.j, where Qk.i is the main load, the loads are combined accordingly: 𝐿𝐶ொ.௜௝ = 𝛾ௗ ∙ 𝛾ொ ∙ 𝑄௞.௜ + 𝛾ௗ ∙ 𝛾ொ ∙ 𝜓଴.ଵ.௜ ∙ 𝑄௞.௝ 7. 7 The partial factors for load combinations are found in Table 7.6 according to EC0 and Boverket EKS 10. Relevant load factors are found in Table 7.7, extracted from Table A2.2 in EC0. Table 7.6 Partial factors for load combinations. Type of load Characteristic Frequent Quasi-permanent Permanent load 𝛾ீ 1.2 1.0 1.0 Variable load 𝛾ொ 1.5 1.0 1.0 Risk class 3 𝛾ௗ 1.0 1.0 1.0 Table 7.