From a conceptual design to a structural solution and erection of a timber bridge – a case study Master of Science Thesis in the Master’s Programme Structural Engineering and Building Performance Design MATTIA TOSI Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2010 Master’s Thesis 2010:78 MASTER’S THESIS 2010:78 From a conceptual design to a structural solution and erection of a timber bridge – a case study Master of Science Thesis in the Master’s Programme in Structural Engineering and Building Performance Design MATTIA TOSI Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2010 From a conceptual design to a structural solution and erection of a timber bridge – a case study Master of Science Thesis in the Master’s Programme in Structural Engineering and Building Performance Design MATTIA TOSI © MATTIA TOSI, 2010 Examensarbete / Institutionen för bygg- och miljöteknik, Chalmers tekniska högskola 2010:78 Department of Civil and Environmental Engineering Structural Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Rendering of the Fish Belly Bridge in Borås Department of Civil and Environmental Engineering, Göteborg, Sweden 2010 I From a conceptual design to a structural solution and erection of a timber bridge – a case study Master of Science Thesis in the Master’s Programme in Structural Engineering and Building Performance Design MATTIA TOSI Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures Chalmers University of Technology ABSTRACT In recent years, timber as construction material for bridges and footbridges has had a great development both in urban contexts and in natural environments since it allows to perform lightweight, economic and aesthetically pleasing as well as durable and easy to implement structures. Improvement and refinement of manufacturing processes of the "artificial" wood based products such as glued laminated timber (glulam) and Laminated Veneer Lumber (LVL), considerable progress over the last twenty years in the production of more and more efficient connecting devices, in making marketing of effective and safe products for wood protection and in the evolution of building systems have contributed decisively to this new trend in the structural field. This Thesis, inspired by the Timber Bridge Competition that took place at Chalmers University of Technology in February 2010, analyzes the whole design process of the 22 m long glulam timber footbridge to be built in Borås, which has been the winner project. All the different steps that have followed during the development of the project are particularly analyzed: from the first ideas sketched on paper during the conceptual design phase, through the analysis of the two models in scale 1:20 built in wood and tested to failure during the Competition, up to the precise definition of all the constructive details and structural solutions needed to actually build the footbridge and ensure it a good durability and its effective construction. The Thesis has been articulated as follows: in the beginning, along with an attempt to clarify the role that the bridge has always played in the European culture, the theoretical basis of the conceptual design process have been described. A wide overview of the most important examples of timber bridges built in history (from Julius Caesar to the present day) made to clarify the background that has laid the introductions for the current developments of this type of structures and an analysis of the advantages and disadvantages associated with the use of timber in bridges conclude the introductive section. In the second part the description and the analysis of the most interesting project among the five ones participating to the Competition and of its structural characteristics are carried on. In the third section the analysis focuses only on the winning project and its gradual translation from the idea to an actually feasible structural solution on the basis of building technologies, features and details to ensure durability and on the verifications in ULS and SLS based on EC5. In the fourth part the SCP, the Maintenance Plan and the Building Booklet are presented in order to describe how to build and to maintain the footbridge. Key words: timber footbridge, conceptual design, durability, glulam timber, LVL timber products, constructive details, building site. II III From a conceptual design to a structural solution and erection of a timber bridge – a case study Master of Science Thesis in the Master’s Programme in Structural Engineering and Building Performance Design MATTIA TOSI Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures Chalmers University of Technology ABSTRACT Negli ultimi anni il legno come materiale da costruzione per ponti e passerelle pedonali ha avuto un grande sviluppo sia in ambito urbano che in contesti naturali considerato che esso permette di realizzare strutture leggere, economiche, esteticamente accattivanti oltre che durature e di semplice realizzazione. A questa nuova tendenza in campo strutturale hanno contribuito in maniera determinante il miglioramento e il perfezionamento dei processi produttivi dei materiali “artificiali” a base di legno quali il legno lamellare e il Laminated Veneer Lumber (LVL) e i notevoli progressi compiuti negli ultimi venti anni nella produzione di elementi di connessione sempre più efficienti, nella messa in commercio di prodotti per la protezione del legno efficaci e sicuri e nella evoluzione dei sistemi costruttivi. La Tesi, prendendo spunto dalla Timber Bridge Competition che si è tenuta presso la Chalmers University of Technology nel Febbraio 2010, analizza nella sua interezza il processo di progettazione della passerella pedonale in legno lamellare di 22 m da realizzare a Boras che è risultata vincitrice. Vengono in particolare analizzati tutti i diversi step che si sono susseguiti durante lo sviluppo del progetto: dalle prime idee schizzate sul foglio di carta durante la fase di conceptual design, passando attraverso l’analisi dei due modelli in scala 1:20 costruiti in legno e testati a rottura durante la Competition, fino ad arrivare alla definizione precisa di tutti i dettagli costruttivi e le soluzioni strutturali necessari per poter costruire realmente il ponte e garantirne una buona durabilità nel tempo. La Tesi è stata articolata nel modo seguente: nella parte iniziale, oltre a cercare di chiarire il significato che il ponte ha sempre avuto nella cultura europea, sono state spiegate le basi teoriche del processo di conceptual design. Una panoramica dei più importanti esempi di ponti in legno realizzati nella storia (da Giulio Cesare a giorni nostri) fatta per chiarire il background che ha posto le premesse per gli sviluppi attuali di questo tipo di strutture e un’analisi dei vantaggi e svantaggi legati all’impiego dei ponti in legno concludono la parte introduttiva. Nella seconda parte si procede alla descrizione e all’analisi delle caratteristiche strutturali e del comportamento statico del progetto più interessante tra i cinque altri partecipanti alla Competition. Nella terza parte infine l’analisi si concentra unicamente sul progetto vincitore e sulla sua graduale traduzione da idea a soluzione strutturale effettivamente realizzabile sulla base di tecnologie costruttive, dettagli per garantire funzionalità e durabilità e le verifiche agli Stati Limite Ultimi e Stati Limite di Servizio sulla base dell’EC5. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 1 Contents ABSTRACT I ABSTRACT III CONTENTS 1 PREFACE 5 Notations 6 1 GENERAL INTRODUCTION 9 1.1 Problem description 9 1.1.1 Background 9 1.1.2 Aims of the thesis and limitations 9 1.1.3 Outlines 10 1.2 About the bridge 11 1.3 Conceptual design 13 2 ASPECTS OF TIMBER BRIDGES 19 2.1 Advantages of timber bridges 19 2.2 General problems connected to timber bridges 22 2.2.1 Manufacturing issues 22 2.2.2 Durability issues 23 3 HISTORICAL EVOLUTION OF TIMBER BRIDGES: STRUCTURE, AESTHETICS, CONSTRUCTION 25 3.1 Caesar’s Bridge over Rhine 25 3.2 Trajan’s bridge over Danubius 30 3.3 Palladio’s bridges (bridge over Cismon river and first invention) 32 3.3.1 Bridge over the Cismon river 34 3.3.2 The first invention 36 3.4 Bridge over Rhine in Schaffhausen by Hans Ulrich Grubenmann 37 3.5 American timber bridges 41 3.6 Accademia Bridge in Venice by Eugenio Miozzi 44 3.7 Essing Bridge by Richard Dietrich 48 3.8 Traversina footbridge by Jurg Conzett 51 4 ANALYSIS OF ONE OF THE BRIDGE PROJECTS TAKING PART IN THE COMPETITION AT CHALMERS UNIVERSITY, GOTHENBURG 56 4.1 The Competition at Chalmers University of Technology 56 4.2 Analysis of the “Reverse Arch Bridge” 59 4.2.1 First model 59 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 2 4.2.2 Second model 62 4.3 Second team proposals 64 4.3.1 First model 64 4.3.2 Second model 66 4.4 Third team proposals 67 4.4.1 First model 67 4.4.2 Second model 67 4.5 Fourth team proposals 68 4.5.1 First model 68 4.5.2 Second model 69 4.6 Fifth team proposal 70 4.6.1 First model 70 4.6.2 Second model 71 5 ANALYSIS AND DEVELOPMENTS OF THE WINNER PROJECT (FISH BELLY BRIDGE) 72 5.1 Analysis of the design process of the models participating in the Competition 72 5.2 Materials composing the bridge 80 5.2.1 Glued laminated timber beams 81 5.2.2 LVL KertoQ 85 5.3 Description of the static behaviour of Fish Belly Bridge 87 5.4 Structural details 93 5.4.1 Connection between the two chords of the lenticular beams 94 5.4.2 Connection between parapet and lenticular beams 97 5.4.3 Connection between deck and lenticular beams 99 5.4.4 Connection between vertical stud and upper chord 100 5.4.5 Connection between vertical stud and lower chord 101 5.5 Maintenance and durability of the bridge 101 5.6 Renders of the bridge 109 6 VERIFICATIONS OF THE FISH BELLY BRIDGE BASED ON EUROCODES 112 6.1 Design values for the strength of materials 112 6.2 Loads acting on the bridge and loads combinations 113 6.3 Load cases 115 6.4 Stress verifications in the ULS 119 6.4.1 Compression and tension parallel to the grain 120 6.4.2 Bending 121 6.4.3 Combined bending and axial tension 122 6.4.4 Combined bending and axial compression 122 6.4.5 Stud subjected to compression 123 6.4.6 Beam subjected to combined bending and compression 123 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 3 6.5 Horizontal loads 124 6.5.1 Tranverse beam subjected to compression 124 6.6 Design in the SLS 124 6.6.1 Deflection of the lenticular beams 124 6.6.2 Vibrations 126 7 CONSTRUCTION AND MAINTENANCE OF THE FISH BELLY BRIDGE 127 7.1 Safety and Coordination Plan (SCP) 128 7.1.1 Arrangement of the building area 129 7.1.2 Trench works 129 7.1.3 Construction of the abutments in reinforced concrete 130 7.1.4 Positioning of the footbridge complete with parapets and bracing system in the factory on the lorry with mobile crane 131 7.1.5 Transport of the footbridge on site 132 7.1.6 Launching with mobile crane and fixing of the footbridge to the supports 133 7.1.7 Waterproofing and asphalting 135 7.1.8 Dismantling and closing down of the building site 136 7.2 Maintenance Plan and Building Booklet 137 8 FINAL REMARKS 142 8.1 Conclusions 142 8.2 Limitations 142 8.3 Future developments 142 9 REFERENCES 144 APPENDIX A: Diagrams of internal actions found with SAP 2000® 148 APPENDIX B: Calculation of the connection between the vertical stud and the upper chord 155 APPENDIX C: Calculation of the resultant wind force 158 APPENDIX D: Calculation of the fundamental frequency of the bridge 159 APPENDIX E: Sheets concerning the main working sub-phases 162 APPENDIX F: Technical drawings 167 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 4 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 5 Preface This Master Thesis work has been developed from January 2101 to June 2010 at the Division of Structural Engineering, Department of Civil and Environmental Engineering at Chalmers University of Technology in Göteborg, Sweden and then finished at the University of Trento, Italy during September and October 2010. First of all I like to thank Prof. Maurizio Costantini, Professor of Building Production at the Faculty of Engineering, University of Trento, and Prof. Maurizio Piazza, Professor of Timber Engineering at the Faculty of Engineering, University of Trento for accepting to be my supervising professors for this Thesis. My heartfelt thanks then dutifully goes to my supervisor Eng. Roberto Crocetti, Professor at the Chalmers University of Technology, who also played the role of co- supervising professor, for having proposed me the theme of this Thesis, for giving me a patient and constant help and also for having supported me in difficult times and giving me the opportunity to do such a work and experiences that otherwise I would have not be able to do in Italy. This Thesis has been the opportunity to face in person the design of a wooden structure and has greatly enriched my cultural background, not to mention the benefits of having developed the work in English and having worked closely with students in Sweden: I have felt pride and a "Chalmerist” in effect. I must also thank Prof. Karl-Gunnar Olsson who, though not officially involved in this Thesis, and despite being constantly busy, has always given me his time and desire to recommend me interesting cues to be added in my work and kindness to lend me books of his personal library to be included in my bibliography. It should not be forgotten that the software PointSketch used in Chapter 4 is one of his creations. I would express my gratitude to Prof. Robert Kliger, examiner of this thesis at Chalmers University of Technology, for his interest in me. I would also like to thank Edwin Ogbeide, my opponent, for his help and his comments on my Thesis, Georgi Nedkov Nedev and Simon Johansson for the beautiful experience lived together and their collaboration, and Thomas Kruglowa, researcher and teaching assistant in the course of Timber Engineering at the Chalmers University, for his kindness. My special thanks and infinite gratitude goes then dutifully to my parents, who have always supported and listened to me throughout my university career, but particularly during my six not always easy “Swedish” months. Verona, October 2010 Mattia Tosi CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 6 Notations Roman upper case letters A Cross-sectional area eC Exposure coefficient tC Thermal coefficient 05,0E Fifth percentile value of modulus of elasticity meanE ,0 Mean value of modulus of elasticity RkaxF , Characteristic axial withdrawal capacity of the fastener RkvF, Characteristic load-carrying capacity per shear plane per fastener dwF , Wind horizontal force meanG Mean value of shear modulus J Moment of inertia M Total mass of bridge dM Design bending moment RkyM , Characteristic fastener yield moment N Axial force W Section modulus dX Design value of a strength property kX Characteristic value of a strength property Roman lower case letters 1,verta Vertical acceleration from one person crossing the bridge nverta , Vertical acceleration from several people crossing the bridge d Fastener diameter dcf ,0, Design compressive strength along the grain dcf ,90, Design compressive strength perpendicular to the grain kcf ,0, Characteristic compressive strength along the grain kcf ,90, Characteristic compressive strength perpendicular to the grain dmf , Design bending strength kmf , Characteristic bending strength dtf ,0, Design tensile strength along the grain dtf ,90, Design tensile strength perpendicular to the grain ktf ,0, Characteristic tensile strength along the grain ktf ,90, Characteristic tensile strength perpendicular to the grain dvf , Design shear strength kvf , Characteristic shear strength CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 7 vertf Fundamental natural frequency of vertical vibrations kg Characteristic value of uniformly distributed load (selfweight) modk Modification factor for duration of load and moisture content zck , Instability factor critk Factor used for lateral buckling defk Deformation factor mk Factor considering redistribution of bending stresses in a cross-section vertk Coefficient l Length m Mass per unit length n Number of pedestrians efn Effective number of fasteners in line parallel to the grain dq Design value of uniformly distributed load snowq Characteristic value of uniformly distributed load (snow) vkq Characteristic value of uniformly distributed load (live load) ks Characteristic value of snow on the ground it Timber thickness depth finu Final deformation instu Instantaneous deformation Ginstu , Instantaneous deformation for a permanent action G Qinstu , Instantaneous deformation for a variable action Q finnetu , Net final deflection Greek lower case letters cβ Straightness factor asphaltγ Weight per unit volume of asphalt Gγ Partial safety factor for permanent loads hGL24γ Weight per unit volume of glulam timber GL24h KertoQγ Weight per unit volume of KertoQ Mγ Partial factor for material properties Qγ Partial safety factor for variable loads solidγ Weight per unit volume of solid timber ζ Modal damping ratio yrel,λ Relative slenderness ratio corresponding to bending about the y-axis zrel,λ Relative slenderness ratio corresponding to bending about the z-axis zλ Slenderness ratio corresponding to bending about the z-axis iµ Shape coefficient CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 8 kρ Characteristic density dc ,0,σ Design compressive stress along the grain dm,σ Design bending stress dt ,0,σ Design tensile stress along the grain χ Shear factor 0ψ Factor for combination value of a variable action 2ψ Factor for quasi-permanent value of a variable action CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 9 1 General introduction 1.1 Problem description 1.1.1 Background In recent years, timber as construction material for bridges and footbridges has had a great development both in urban contexts and in natural environments since it allows to perform lightweight, economic and aesthetically pleasing as well as durable and easy to implement structures. Improvement and refinement of the manufacturing processes of the "artificial" wood based products such as glued laminated timber (glulam) and Laminated Veneer Lumber (LVL), considerable progress over the last twenty years in the production of more and more efficient connecting devices, in making marketing of effective and safe products for wood protection and in the evolution of building systems have decisively contributed to this new trend in the structural field. The theme of this Thesis has been the analysis of the entire process of designing a footbridge in order to understand in detail the dynamics and everything that happens throughout the iter: in fact, one usually focuses on some specific aspects, such as structural calculations. In this way, the Author believes he has made an interesting overview of the complexity of the work of a designer, treating both the architectural and structural aspects and not forgetting the most practical one, that is the effective execution of the project. 1.1.2 Aims of the thesis and limitations The theme of this Thesis is a complete analysis of the design process of a wooden footbridge. From the embryonic stage, that is the moment when the designer decides, starting from the needs perceived by the client and the environmental conditions of the site and other types of constraints, such as the static scheme and the structural principle to adopt, through the development of technological solutions and constructive details in order to translate into reality what he thought, to go then for checking the strength of the various elements on the basis of the Eurocodes and then finishing with the analysis of the strategies, machineries and phases by which actually build the bridge. It is extremely interesting to analyze the process in its entirety to understand what and how many variables are involved at each step of the design of the structure, which obviously go to significantly influence the following stages, dramatically changing the decisions that have to be taken. Obviously, since due to the vast scope of the investigation it has been necessary in some cases, however, to simplify or streamline the analysis in order not to lose sight of the ultimate goal of the work that aims precisely to provide a comprehensive and as much exhaustive as possible overview of the entire process. Since the design of the structure has been quite empirical (the competition provided the building of scale models without any preliminary calculation) so the definition of the static model to be used has had a simplified management and in-depth analysis on CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 10 the possible alternatives have been not conducted. Finally, after having seen that the model built to scale had a satisfactory response, it has been decided to go further. Concerning the definition of the loads acting on the footbridge, being the structure located between a building and the bank of the canal, the actions of any emergency or cleaning vehicle transiting on the deck have been disregarded, considering in the end only the actions of wind, snow and crowd (as provided by the Eurocodes). To conclude in the chapter dealing with the installation and maintenance of the bridge the regulations have been complied with as regards the substance but from the formal point of view they have been adapted to the purposes and the need for discussion of the topic: in particular, the sheets concerning the use of all machineries and equipment used in the different working phases have been left out and the Maintenance Plan has been partially merged with the Building Booklet. 1.1.3 Outlines This Thesis work is essentially divided into four parts. The first part is from Chapter 1 to Chapter 3 and serves to clarify and present the discipline in which we will work, to introduce properly discussion of the subsequent topics, or the wooden bridge designed by the Author. Chapter 1 is the very incipit of the work: in addition to a general description of the problem underlying the thesis, it describes what is the symbolic meaning and social value that the bridge has generally in the collective imagination and its importance to levels not only practical but also philosophical. The second paragraph presents a concise treatment of logical and mental processes that govern the conceptual design phase of any building. In Chapter 2 the focus shifts to the technological aspects related to the use of wooden bridges: the benefits that they bring to those of reinforced concrete and steel and at the same time the related problems are analyzed, both from the point of view of the manufacturing and of the durability (element to be designed carefully in the timber structures field). Chapter 3 presents instead a historical analysis of the evolution of wooden bridges over the centuries: from the first of which we have a complete description, that is Julius Caesar's bridge over the Rhine, up to present days underlining the constants and differences and focusing on structural, constructive and aesthetic aspects. The second part (Chapters 4 and 5) enter the substance of the main topic of the thesis, namely the design of a footbridge made of glulam timber. Since the beginning of the process has been the Bridge Competition that took place at Chalmers University in Goteborg, in Chapter 4, the course of the competition and the most interesting results produced by the participants have been illustrated and then the discussion has focused, in Chapter 5, only on the winning project, developed by the Author in co-operation with two other students. Initially it is described the whole design process undertaken to define the shape of the bridge and then the two structural solutions for the bridge are analyzed, in the form of scale models, based on the strengths and weaknesses found during the load tests. It continues with a description of the peculiarities and technologies used to produce the materials that make up the structure (glulam timber, KertoQ®), the analysis of static behavior of the footbridge conducted with the help of SAP2000® software (finite element analysis) and then it concludes the chapter with CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 11 the definition of all main structural connections and analysis of solutions used to ensure the longest possible service life to the bridge. The third part, consisting of Chapter 6, includes verifications of the bridge based on the Eurocodes, in particular 5. Design values for the strentgth of the materials, loads acting on the bridge, load cases, stress verifications in the ULS and the design in the SLS are analyzed. All calculations have been performed by hand, basing on the results obtained from FE analysis developed by SAP2000®. The fourth and last part, or Chapter 7, finally, is the most practical on the bridge: it deals with its construction and maintenance. In fact, based on the Italian law, the PSC have been developed (Plan and Coordination of Safety) which defines the steps necessary to prepare the building site and to assemble the bridge, the risks and safety measures to be taken to grant the health of workers, and the building area layouts that graphically represent the building site and its organization. Finally the Maintenance Plan and the Building Booklet have been prepared: they are the documents to be used during maintenance to ensure an adequately long service life to the construction and to define security measures to be taken to carry out maintenance operations without risks to workers’ health. 1.2 About the bridge The bridge, one of the most ancient archetype of the architecture, suggests two ways of movement: that of the river flowing beneath and that of the road running above: two movements which form a cross, two movements that become possible (though the one seems to prevent the other) through the rising of the road ribbon that shirks the binding condition of the crossing, the bridge replaces the ford, it is a partial and ephemeral victory of the man on the obstacle of water. The victory becomes lasting when the road has changed into a bridge: it has jumped over the obstacle, has joined the two opposite “banks” into one thing; a sight continuity that passes over the fracture and makes it forgotten. Using an intense poetic image Giorgio De Chirico attaches the Romans’ passion for the arch to the desire of infinite that hits the man observing the sky vault: "A Rome le sens du présage est quelque chose de plus vaste. Une sensation de grandeur infinie et lointaine, la même sensation que le constructeur romain fixa dans le sentiment de l'arcade reflet du spasme d'infini que la couche célestielle produit quelquefois sur l'homme. Souvent le présage y était terrible come l'hurlement d'un dieu qui meurt. Des nuages noirs devaient s'approcher jusque sur les tours de la ville". Through the bridge the river reveals its mobile nature, its flowing towards a constant direction. When a man looks out and sees the stream, his sight is something like dragged away, his imagination supposes the presence of a spring and of a mouth far away. So, he feels steady in the middle, in the very heart of what moves, of what continuously changes. The water hit by his eyes is after a moment no longer the same. So the bridge is the archetype of the fight between being and becoming. The bridge has an anthropomorphal matrix because our body can be a bridge any time, like when we jump across a little creek included between the pair of compasses formed by our legs, CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 12 it is changed into a bridge when two hands shaking each other create a symbolic union between two living beings. A godness with arched body symbolized in ancient Egypt the sky vault. The terminology of the bridge expresses its anthropomorphal matrix; the bridge has its shoulders, its brain, and its back. Every bridge has its parapet that enables us to lean out on the hollow, a wall with the right height to keep us back, that needs and gratifies the contact with our body. The symbolic fascination of the bridge had seduced even Goethe, who in his Maerchen in 1795 tells the story of a snake finally transformed into a wonderful bridge. “Do honour the memory of the snake, said the man, you owe him the life , tanks to him the two banks are now one village full of life.” Goethe saw in the snake mainly the sign of friendship and of dialogue, but the image of the “transparent and bright snake” transformed into a bridge , like the one Palladio imagined for Rialto, contains in itself , besides the mystic architectural union between Palladio and Bernini, also the intuition of a deep and mutual analogy. Bridge and snake share the linearity, the dynamism, the repeated structure, the double movement in two directions, the making one thing out of two.. In the tale the snake winds the remote in one circle, thus recalling the ouroburos, symbol of eternity. But in this Goethe sees something else, an allegory of a happy conjunction that the man can make true with love, confidence and friendship. The most clever and ingenious scientist of morphology, D'Arcy Wentworth Thompson, tells how a Swiss engineer became aware of the analogies between the bones’ structure and a giant crane he was designing. Prof. Culmann of Zurich learnt from his friend, the anatomy scientist, that the bone fibers arrangement is no more no less that the diagram of the isostatic lines, directions of compression and tension of the loaded structure: shortly, nature makes the bone stronger exactly in the mechanically necessary way. In the arm of a crane the internal side on which the loaded end protrudes is the compressed part, the external one is the tense part: the compressed isostatic lines, starting from the loaded surface, join together in the direction of the resulting pressure, until they form a narrow beam that runs along the compressed side of the arm., while the tense isostatic lines running along the opposite side of the arm, spread in the end crossing the compressed lines at right angle. The head of thigh-bone (femur) is a little bit more complicated in its shape and a little less symmetric of the schematic crane of Culmann [...]. but we have no difficulty to see that in a few words the mechanical disposition of the structure meets exactly the theoric diagram of the crane. Taking into account of the different situations , we can draw the same disposition in any other bone bearing a load and subject to flexion. [...].Observing animals in captivity, we see that bone structure much differs from free wild animals. We can learn many things from the examination of the birds’ bones, where the small bones corresponding to those of the hand have a heavy task to comply, since they support the long feathers of the wings and form the stiff rod of the final part of wings. The simple tubular structure, good for the long and thin bones of the arm, is no longer sufficient, because here a higher stiffness is requested. No other anatomy part is more beautiful that the metacarpal bones of the vulture. The engineer can see here a perfect Warren scaffolding like that of the beams of aircrafts. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 13 The analogy between the Warren beam and the vulture’s metacarpal bones introduces the question of the truss spatial structures and the great nature scientist emphasizes the complex relationship with the sinews and muscles system. He compares the skeleton of a horse with the Firth of Forth bridge, one of the masterpieces of the 17th century engineering. In a horse or an ox it is obvious that the two piers of the bridge, i.e. the fore and rear legs, do not bear (as in the a.m. bridge ) independent loads, but the entire system makes up one structure. The calculation is a little more complicated, but we can simplify the problem if we consider the skeleton as two shelves (fore and rear legs) and later consider that they are not independent but connected to each other in one system of forces and in one building design. The book of D'Arcy Thompson was published in 1917 and influenced in some way the discussion that prepared the terrain to the architectural rationalism developed by the historical avantgardes in the years around the first world war. It is asserted by Robert Maxwell in 1978, when he presented the works of Robert Venturi: “the book “On Growth of Form” shows that the forms of natural living structures are in harmony with a series of building principles and of geometric proportionality. Nature seems to act as if its forms were the result of a time saving economy and need of functionality. When these principles are applied to engineering structures like bridges, one discovers that they behave the same way and produce similar or almost similar forms. This discovery seemed to suggest that the general design principles were common to all natural or artificial constructions and that in them there were nothing specifically human, hence merely human conventions become useless and would be rather obstacles to a natural process that can attain to perfection if inspired by the laws of evolution”. 1.3 Conceptual design Underlying the construction of any structure there is always a perceived need by someone (and the term someone can be referred both to an individual and a community of individuals) and thus the spontaneous request for a solution that satisfies it was born. The process of constructing a building or infrastructure, as it can be a bridge, is the result of the close interaction between a large number of figures involved with various functions and at different stages of the design in order to be able to translate into practice what initially was perceived as a need. The design of the structure is divided into phases in which the various professionals give their contribution relating to their capacities and the task they are requested to perform. To understand the extent, the methods of implementation and the organization in carrying out any building the project life cycle should be described; it is a scheme of the typical stages in a process that follows any project to be transformed from a simple sketch on a sheet of paper to a real object: a design phase is followed by a construction phase which then ends to make room for the service life of the building where the structure, after being completed, is ready to be used by those who have expressed the need to make this happen. At each stage there are of course some inputs to elaborate and some objectives to achieve in order to get to the end of this phase with certain results then have to be used in subsequent processing. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 14 As it is possible to see from Figure 1.1 the design phase is subdivided into three subcategories: conceptual design, preliminary design and detailed design. This thesis only deepen the discussion about the conceptual design, referring to other texts for discussion of other topics. Figure 1.1 The project life cycle Conceptual design is the starting point for the implementation of any project: it can be defined as the founding moment of the entire design process, as during this phase something completely new is going to be created on to satisfy the needs perceived by those who requested the project. The ideas that emerge during the analytical and preparation work lead to the implementation of different solutions that try to satisfy as fully as possible the needs (which are the real booster of the whole design process). The options processed (usually three, but even more are possible) are compared with each other trying to find the one which optimizes the factors involved and which could be the best answer to all requirements, so that in the conclusion of this stage a final conceptual solution that will be later further developed is chosen. It is important to emphasize that this phase is extremely important to the economy of the entire process of design and construction of the structure, because in this moment the foundations are laid and major decisions are taken that will have great influence on later stages and on the entire work . The very first step to take is that of analyzing and identifying in a precise and complete way the needs which give rise to the request of a new structure: this will lay the foundations for an effective design that will be even much better the more clearly the needs are understood. In this way the design requirements are also focused, so that all the professionals involved can understand what their work and what their operational role will be. During the identification of the needs it is essential to understand what the real needs are because very often clients who express this state of necessity are not experts and therefore they are not able to put questions in technical terms. Here, therefore, the analysis of the needs makes the designers able to understand the difference between perceived problems and real need, or understand clearly what are the client’s needs rather than what he thinks he needs (Kroll, 2001). During the needs analysis it is also important to recognize and avoid referring to bias in the definition of the need itself: the designer must be able to ensure that potentially suitable alternatives are not excluded “a priori” because of the influence of solutions already taken or ideas related to the everyday practice. The best way to produce viable solutions is to succeed in focusing only on current issues without referencing to ideas already used: doing this way there will be the certainty that the final result will be of higher quality and we will be sure to have produced something truly innovative. At the end of this stage there will be a sufficient number of design requirements. To be able to get a good assortment of solutions to satisfy the needs it is helpful to see possible solutions as a black box with inputs and outputs: the solution will be acceptable if it enables the designer to connect any requirement “ingoing” with one of CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 15 the “outgoing” project objectives. The task of the designer will be to understand and clearly define all the elements required to make this black box work (Kroll, 2001). Once a number of alternatives have been worked out they must be compared with the constraints of the real world. Imagining the set of all potential solutions as a virtual space of infinite dimensions it gets restricted by some boundaries that reduce their potential viability (see Figure 1.2). The constraints are such if their violation makes the final product not suitable to fulfil the initial needs. Figure 1.2 Schematic of a solution space bounded by constraints. Task of need analysis is to maximize the size of the solution space (Kroll, 2001) The list of these constraints must obviously include all the elements that really act as such in order to better focus attention solely on practical solutions; attention must then be paid to the artificial constraints that might reduce and limit the space of action for no reason. The constraints (that can be regulations, specific requests of the client, design requirements for previous similar situations, experience, knowledge) can be classified into explicit or implicit. The former are the easiest to identify because they are contained into the initial design task or directly derived from it, they must be approved by the client in case of their change. The implicit ones are more complex because they are not known a priori before the beginning of the design process and may be the result of initial calculations, analysis of the project site or simply as a product of the needs analysis. An intuitive way to find out what the constraints are is to classify them according to a list of five broad categories that allow the designer to have clear the boundary situation avoiding accidentally omitting some elements and to carry on the understanding of project tasks. These five categories, as it is also possible to see in Figure 1.3, are: performance, value, size, safety and special (Kroll, 2001). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 16 Figure 1.3 The five general categories of constraints (Kroll, 2001) Once clearly defined the size of the space of potential solutions, the degree of freedom granted by the boundary conditions and all design requirements which have arisen from the need analysis (which must be chosen with care to avoid an inappropriate choice to go affecting the proposed alternatives), it is necessary to identify the key parameters that become the sine qua non criterion to say whether a solution can take the next step or not. Like all other steps of conceptual design also the definition of key parameters is extremely important and needs attention: for example, it is absolutely useless act schematic or repetitive because each project has its own peculiarities and characteristics and there are no ready-made solutions and it is extremely dangerous the endeavor to adapt solutions already used in similar cases because this attitude can lead to problems of considerable magnitude below the design process. In order to point out as much precisely as possible all the key parameters it could be useful to bear clear in mind which are the requirements and which the objectives the project is going to face. The parameter could be a factor, an issue, an information or a concept but it has not to be a dimension or a physical property in order to avoid misunderstandings that can compromise the proper conduct of the operation. A set of well chosen parameters needs knowledge, experience, mental flexibility and also good qualities of inventiveness (Goral, 2007). The conceptual design process concludes with a final conceptual solution which will be detailed later in the preliminary design phase. It is worth underlining how the various project phases of a structure are not among them watertight and how it is always possible to "go back" to change and better define the design requirements but the goal of conceptual design is to define precisely at the best all basic information to be able to avoid problems, misunderstandings or conflicts of roles during later stages. An effective scheme (Figure 1.4) was developed by Engström and Lierud (2006) based on the theories of Knoll to exemplify the process of choosing a suitable alternative during conceptual design phase and it seems appropriate to bring it: CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 17 Figure 1.4 Five-step methodology to choose the best alternative (Goral, 2007) Until now there has been talking about conceptual design in terms of generic structures but it can be useful to propose a reading that gives the general lines in the case of designing a footbridge. Being facilities for pedestrian traffic only this type of bridges have more freedom than the road and railway bridges: their path is not simply to connect point A with point B in the shortest possible time and with less waste of resources but rather to highlight and promote territorial development around them; they have then the opportunity to free themselves from the bond to be a simple straight line and can become free spatial forms that offer to those who pass through experiences that transcends the simple moving across an obstacle. Their full usability and their human scale size obviously make them objects of direct experimentation by pedestrians who stop on it, look out and if possible sit on it: ultimately they live it. The design of a footbridge becomes then more complex than that of other bridges due to this great variability of factors that go to determine what form and structure it should have. In particular concerning the needs of the customers, who are the users, designers must take into account and pay attention to: � control of vibrations during the service life of the structure in relation to the kind of users � adequate sizing of the carriageway in relation to the expected traffic flows in order to allow easy passage even in case of overcrowding, not to mention that many times too narrow pathway produces a feeling of insecurity CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 18 � possibility of including benches or resting places where environmental conditions allow it and are deserving � particular forms (for example spirals) or too small dimensions of the staircases can cause difficulties to elderly, disabled or people with physical impairments � inclination of the ramps must, wherever possible, permit the transit of wheelchairs; if the boundary conditions do not permit it the placement of a lift must be taken in consideration � access as easy and comfortable as possible at the entrances of the bridge to ensure in all conditions; especially in case of very narrow access areas viable alternatives should be considered including linking to adjacent buildings � if the footbridge is placed in a densely populated environment and it passes through residential building areas it should be taken into account the fact that it is not pleasant to feel observed, for both residents and passersby, and adjusting accordingly plant, height or shape of the structure. Also regarding the choice of materials and construction technology to build a footbridge, the variety is very wide; since the loads are usually modest and limited deformation takes place it is possible for the designer to experiment with a variety of different materials such as steel, concrete, timber, masonry, aluminum alloy and plastic composites even. This way you can effectively investigate new forms of expression and new structural solutions, being also easier to convince the client due to the usually small size of the structures. This freedom of choice of materials must still not obscure that the national or European regulations have to be fulfilled however, that the relationship with the urban context in which the bridge will be put has to be protected and that the materials chosen have to be suitable for the kind of users that will transit on it. As already highlighted footbridges are structures that have the ability to become social condensers, symbols of a real intention to renew and in this way carriers of values and meanings. Understanding this valence is essential and therefore to obtain a final result that is fully satisfactory, and indeed goes beyond the expectations of the client (also referred to a community), it is desirable architects and engineers to work together in the implementation of the project to give both aesthetic and structural aspects. The design work should be carried on since the early steps so that the two contributions come together organically and harmoniously, avoiding harmful and unnecessary adjustments during the work. It often happens that only engineers design the bridge, implementing the structure with an eye only on cost and structural efficiency, and then the architect is left with the task of trying to give formal contents to a standard solution. There is also the case in which the project is initially developed by architects who, based solely on aesthetic considerations, develop solutions structurally unacceptable then engineers must try to fulfill the minimum requirements: in this way it often leads to a radical change of the form and an increase in costs, making it useless de facto the whole process of conceptual design (FIB, 2005). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 19 2 Aspects of timber bridges 2.1 Advantages of timber bridges Bridges have always played an extremely important role in the lives of human beings. Every bridge that is built brings human beings into contact with one another, and this – over and above its purely technical character – makes the bridge both a symbol and part of our culture. As structures, bridges are not only a part of human history, but also a measure of the planning expertise and technical skill we have attained (Wittfoht, 1984). The age of wood spans human history and it was probably the first material used by humans to construct a bridge. The stone, iron, and bronze ages were only interims in human progress, but wood - a renewable resource - has always been at hand. One reason why wood has remained indispensable for constructing bridges for thousands of years is that it is natural: as a building material, wood is abundant, versatile, and easily obtainable. Without it, civilization as we know it would have been impossible. The first bridges were made of simple, unhewn tree-trunks. To this days, the basic types of load-bearing structures – namely the beam and the arch – have not changed since human beings first attempted to make them. Until about the 1970’s not great changes occurred in the use of the material despite a great number of different structural solutions: wood has never been changed in its nature but was adapted in the form that was necessary from time to time. It is only with the relatively recent developments of new wooden materials with large dimensions, a high load-bearing capacity and improved connections that wood has again attained novel and independent status in the engineering field of bridge construction. This has resulted in new footbridges and road bridges, characterized by original, high-quality design and impressive dimensions. In most countries where timber bridges have been always used, an evolution, divided in three phases, can be observed: the situation in Finland may serve as an example. The first generation timber bridges owned by the Finnish Road Administration were built before the 1970's, made of logs and sawn timber, without any preservative treatment. In the middle of the 70's, the second generation, the glulam bridges took over. Now the third generation, in Finland represented by the wood-concrete composite bridge, is developing. Many of the first generation timber bridges were not properly designed and some were not even treated with chemical preservatives. In many other European countries, the second generation is almost completely lacking (NTC, 1997). A period of more than half a century of bridge construction was dominated by concrete and steel: the situation with the discarding of older functionally obsolete or structurally deficient timber bridges, and replacing them with concrete or steel bridges, has lead to a decline of timber as bridge construction material. Moreover, people often expect only half the service life for timber bridges compared to concrete and steel bridges, even though it is a well-known fact that concrete and steel bridges have often required substantial repairs after a short time of use. There are many examples of very old and well-kept timber bridges: in Switzerland, for example, several covered timber bridges with superstructures that date back a couple of hundred years ago are still in use. In spite of the fact that nearly 50% of the timber bridges in the United States are assessed as structurally deficient, states and counties continue to build bridges out of wood as it has numerous characteristics that makes it a desirable material for transportation structures (NTC, 1997). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 20 Until recent years, the worldwide fall out of favor of timber was characteristic: it was the result of the fact that older timber bridge designs do not fulfill the requirements for modern roadways. Poor detailing on many of the structures also combined with low durability timber, resulted in the perception of timber bridges being short term solutions to bridging problems. This perception is still held by many engineers practicing today. Concrete in particular was perceived as providing long term maintenance-free structures and remarkable advances in steel technology provided opportunities for longer span structures with increased load capacities. The limited knowledge and experience of timber of most design engineers has lead to a generally negative attitude towards the use of timber for bridges. The result was stagnation in the advancement of technologies which would enhance the use of wood in transport infrastructure projects. Timber bridges were delegated to a solution for small pedestrian bridges and low load capacity vehicle structures on the likes of golf courses and private rural farm properties. A gradual revival of timber bridge building in Europe started during the 1980's. A circumstance leading to the current increase in the use of wood as the construction material for bridges is some loss of image of the materials competing with wood, particularly the increasing awareness that concrete is by far not the everlasting material it was expected to be. New timber bridge designs are also more competitive than the old ones. The third generation timber bridges, represented, for example, by stress-laminated bridges, are providing better load distribution and a solid base for the moisture barrier and the wearing surface. The competitiveness of timber bridges depends on costs for construction and maintenance as well as expected service life, in comparison with other materials. In many cases, aesthetical factors are also important for the choice of construction material. Ultimately, the timber strength is based on the balance between function, cost- effectiveness, technology and aesthetic considerations. In Finland, a study of construction costs of timber bridges compared to concrete bridges, pre-stressed, precast concrete bridges and steel bridges has been carried out. This study shows that the cheapest alternative for the shortest bridges is concrete bridges cast in situ, but timber bridges provide the cheapest alternative for bridges with a span of about 14 meters up to the maximum span for timber bridges (NTC, 1997). Two Swedish studies which have been also carried out report on attitudes and reasons for decisions taken when building bridges: these studies show that the cost often is a decisive factor. The timber bridge is preferred because it is the cheaper alternative compared to concrete or steel bridges. In addition, timber bridges usually offer aesthetical advantages along with short construction time and a simple construction process (NTC, 1997). Depending on its special qualities, timber is totally different from other construction materials and it offers a potentially low-cost alternative to materials such as steel and concrete. The mechanical performance of timber is closely related to the natural origin of the material and to the functions that this material has in nature: it in fact has the duty to support the foliage, acting as a cantilevering structure. The morphology of the cells of wood and their conformation ensure high resistance values with low dead loads. The cellular organization of wood is the foundation of a strong anisotropy of mechanical properties and this leads to a marked difference in values of strength and CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 21 stiffness depending on the direction of the grain: timber is in fact more stiff for stresses oriented along the direction of the fibers and, conversely, is much less efficient for ones orthogonal to the direction of the fibers (especially in tension). In the case of stresses parallel to the grain, timber has a high structural efficiency compared to other construction materials. One possible criterion for defining this efficiency is the ratio between a resistance parameter f of the material (for example to compression) and its density ρ: the resulting value is very similar to that of steel and it is about five times higher than of reinforced concrete. These values show that it is possible therefore, with wooden elements, considerably lighten the structure, with many advantages, not least in the seismic design (Piazza, 2007). Equally important as structural parameter is the ratio between the modulus of elasticity E and the parameter f, which takes values equal to one third of those of reinforced concrete and equal to those of steel. Even if timber life is not particularly long, it may be improved if one takes appropriate measures: in fact building with timber always requires comparing tight with the issue of a constant exposure to weather conditions to which the structure is subjected, due to the natural tendency of material to biological degradation. This trend must be prevented or at least delayed as long as long is the lifespan required for the considered element and it must be considered that bridge protection is also crucial to maintenance costs: in fact bridges designed with exposed supporting structures, e.g. trough bridges, show more damage and higher costs than ones designed with good wood protection by design. Although timber, among construction materials, is the most sensitive to degradation, it is at the same time the one which can provide a significant durability to the structure, as long as it is properly protected: a recent research indicates that timber bridges may be more durable than those constructed from other materials, particularly in cold climates where salts and other deicing agents are frequently used (NTC, 1997). It is therefore clear that it needs a proper planning and execution of the work so that it can fulfill not only the aesthetic, architectural, structural, economic and functional requirements, but also those related to durability and to the eventual efficient and effective maintenance of the whole structure. In conclusion, wood protection is crucial to the function and life of a timber bridge, and must be given the same attention as the strength. Accordingly, protecting the wood against deterioration in the first place relies on limiting the moisture content through the type of construction: the timber components that are exposed to the weather must be designed so that the water drains away as quickly as possible and just as important as a fast water run-off is the rapid drying out of the water absorbed by the wood through effective ventilation of the respective component. Where it is not possible to limit the moisture content or to avoid a quick drying out of water, resistant species of wood can ensure a durable construction: it is important to choose a species of wood that is resistant to the particular action and a wood-based product class to suit the respective application. In terms of detailing, protection for the wood begins with cutting relieving grooves in logs or squared logs to avoid uncontrolled splitting; and fissures are always an entry point for insects or water. Glued laminated timber and wood-based products are less at risk because they have a lower tendency to split. To ensure that the effects of shrinkage and swelling do not cause any damage, small material cross-sections and small surfaces are preferred for components exposed to the weather in particular. Only in places where neither the detailing nor a resistant species of wood can be used to provide protection is it necessary to use chemicals. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 22 In conclusion, the advantages in using timber as a bridge construction material can be summarized as follows: � it is a renewable resource and it suits the cycle in nature and should, in a good way, manage a life cycle analysis and environment declaration compared to other bridge construction materials � due to timber low dead weight, timber bridges are very light and could be almost totally be erected in the factory and lifted in place with minimal workforce, so that the time to completely erect the whole structure is much shorter � a timber bridge that has been built and maintained in the proper right way, should not have a substantially shorter service life or higher maintenance costs than other bridge alternatives. � it is naturally resistant to the effects of deicing agents (so timber bridges can be particularly suitable for cold climates) � often timber is the cheapest alternative all in all on a first-cost analysis and shows great advantages when life cycle costs are compared � timber bridges allow easy assistance for maintenance and repair problems � timber bridges present a natural and aesthetically pleasing appearance and due to their non-traditional applications lead to a wide range of construction practices and design concepts unique to timber alone. 2.2 General problems connected to timber bridges 2.2.1 Manufacturing issues Up until the recent past, the chances of building wooden structures were limited by the fact that it was necessary to adapt to the possibilities offered by the size of the elements found in nature in the form of logs and sawn timber: ultimately the maximum lengths reachable were about 20 m and cross-sections seldom exceeded 150x450mm. Nowadays, thanks to technological innovations introduced in the field of timber constructions, if one needs large items it is possible to create composite elements consisting of multiple parts joined together by adhesive (just think about glulam timber). In this way two advantages will then be obtained: firstly, structural elements are produced with sizes and shapes that would be difficult to find in nature, and secondly building materials with easily verifiable physical properties are available. Timber in fact, being a natural material, has a number of defects, which affect its reliability, but splitting it into smaller pieces and rearranging them, one can minimize or distribute these "non-compliances" in the new material. The so-called wood-based products (glulam timber, plywood, OSB, LVL and others) are made of veneers, flakes, chips and fibers (Polastri, 2006). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 23 During the production of these materials great attention should be paid to all the different stages of the process, from the choice and quality of raw materials, through continuous monitoring of subsequent phases to finally arrive at the finished product. In the factory it is also necessary to carefully control internal temperature and humidity since timber tends naturally to reach balance with hygrothermal conditions of the environment in which it is located. To prevent such phenomena it is advisable to thoroughly dry the wood before processing. A recurring problem in the field of timber constructions is connecting together different components to obtain complex structures: hence it becomes crucial in properly designing the connections so you can get both a structural solidarity between the various elements and an appropriate management of the dimensional changes that timber undergoes as a result of climate variations. Connections must then: � Make the joints between elements as easy as possible � Create linear paths for the involved forces in order to prevent building of unexpected stresses � Ensure that forces are always possibly directed in a direction parallel to the grain � Respect minimum spacing required by law The design of the supports needs much attention because they have the task of transferring the forces to the bearing structures or foundations. They, theoretically speaking, should allow translations and movements of timber elements due to the variation of moisture content, in reality, they must remain as close as possible to the static model developed in the design stage. In case of steel-to-timber connections (the most frequent ones) it is necessary to check the strength of both metal fasteners and timber elements, in particular with regard to normal and shear stresses in the most unfavorable combination (Polastri, 2006). Not to forget the checks on stresses perpendicular to the grain. Very important are finally the checks on glued connections that are increasingly being used in the field of timber constructions: this kind of adhesives tends to create a union as if the structural material were still intact. 2.2.2 Durability issues A very important aspect of the design of timber structures concerns the durability of the timber elements, which naturally tend to deteriorate, particularly if exposed to direct contact with weather agents. The variation of moisture content within the timber causes dimensional variations and in the long run leads to a loss of mechanical strength. Timber is also subject to attacks by microorganisms such as fungi and molds that threaten its integrity. For these reasons, it is necessary to plan, beside the use of protective coatings and elements of "active protection", a scheduled maintenance in order to achieve a service life as long as possible. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 24 If timber is in direct contact with water (in liquid form or in the air) it swells: the important thing is to allow this increase in volume with appropriate planning arrangements (otherwise you get the risk of developing dangerous internal actions) and above all to enable water to evaporate quickly. The critical areas which need to be focused on are the connections in which the structural discontinuity and the different behaviour of the materials can produce cracks, which gradually get deeper due to further water introduction and to the potential growth of microorganisms harmful for timber. Also the sun radiations have negative effects on timber structures because electromagnetic waves building them affect lignine, an organic binder, which being soluble, is then washed away by water in its various forms. Particularly dangerous for glulam timber are finally the frequent moisture variations for the so-called risk of “glue line delamination”: delamination takes place between the lamellas and so the strength of the material decreases esponentially, especially concerning the shear resistance (Polastri, 2006). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 25 3 Historical evolution of timber bridges: structure, aesthetics, construction This Chapter will analyze the most important wooden bridges built during various historical periods in order to be able to understand the changes that have occurred and the constants that these structures have maintained over time. The works that will be described have structural features and qualities that have permitted them to emerge and to become models and examples to follow in the field of timber constructions. It is very important to understand the development of technologies and of structural solutions that have occurred over the centuries to get a better comprehension the of background for this type of wooden structures and in order to succeed, in the design stage, in managing the creative process, being aware of what has been done before us by others. The bridges analyzed cover a span of more than 2000 years and include the Caesar’s and Trajan’s bridge, the bridges of Palladio, Hans Ulrich Grubenmann’s masterpiece, North American timber bridges, the bridge over the Canal Grande in Venice by Miozzi (one of first glulam timber bridges), and two contemporary works extremely innovative due to technologies and static systems adopted: the Essing bridge by Dietrich and the Traversina footbridge by Conzett 3.1 Caesar’s Bridge over Rhine In ancient Rome the bridge had always had a particular importance since it had meanings that were at the same time both symbolic and functional. Suffice it to think that bridge Sublicio, the first bridge in Rome, was made entirely of wood and its connections were completely devoid of metal fasteners, and for this was rebuilt and restored several times, always using the same technique and numerous public ceremonies took place on it (Maggi, 2002). Culture of ancient Romans placed man and his skills at the center of the universe and therefore in this sense the tremendous efforts made by Roman engineers in creating bold civil engineering works including roads, bridges and aqueducts can be fully understood. Managing to achieve these constructions allowed to assert the superiority of human intellect against the power and the limitations imposed by the forces of nature. The bridge in particular was considered the most important of the infrastructures as it could overcome natural obstacles such as rivers, which with their length defined and imposed borders often impassable. Much more than the stone ones, perceived as definitive and stable, timber bridges (especially those built during military operations) have left deep marks in the collective memory and have been subjected to detailed descriptions on their structure. They were indeed held in high esteem due to their complex construction because, besides having to be able to combat and subdue the natural elements, they also should be constructed rapidly and quickly destroyed or rendered unusable in case of need (Maggi, 2002). Particular importance in history has played the Caesar's bridge over the Rhine built in 55 BC during the war against the Germans: his detailed description, made by Caesar himself, in fact constitutes the first evidence of a construction manual applied to wooden bridges and formed from the Renaissance onwards the starting point for the CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 26 study and design of any wooden bridge, not to mention also that the difficulties related to the width and depth of the Rhine had urged the adoption of unconventional technical solutions (Maggi, 2002). The bridge solution was also, in Caesar’s opinion, certainly more worthy than a bridge on boats. Description given by Caesar in his De Bello Gallico (Book IV, XVII-XVIII), necessarily brief, reveals a project in which the static and functional roles of the different elements of the construction are clear: the integrated ensemble pier- foundation, the additional stiffeners, primary and secondary structures of the deck, not to forget the works of passive defense. Caesar, indicating that it was completed in ten days, accurately describes the work of engineering but in the text there are no few ambiguities and uncertainties, especially related to the meaning of some technical terms. Just these black spots have generated great interest and produced a series of constructive assumptions by many scholars and architects on the possible shape of the bridge. Based on the original text we can deduce the general structure of the work: piers were made by wooden stakes driven into the riverbed through mechanical devices specifically designed or simply manually operated by workers on board vessels. Stakes were set not vertically but with a certain inclination: in the direction of the current upstream and in the opposite direction downstream. Each pier consisted of two piles and a transversal beam, that engaged the pier located on the opposite side, was inserted in the room between them. The transverse structure that was thus created resembled so much like the cover of a roof and worked in traction rather than in compression (Maggi, 2002). To complete the structure additional piles at an angle were placed downstream close to the piers and groups of piles were placed upstream in order to protect the piers against the impact of material transported by the stream. The deck consisted of planks nailed on top of longitudinal beams that had a span equal to the distance between two non-consecutive piers. The greater uncertainty in the Latin text turns around the term "fibulae" used to indicate the means of union between the piles making up the piers and the transversal beams and generally called the connection to the top of two pieces of wood and many interpretations of this term were different. The first representation ever of Caesar’s bridge over the Rhine, and for this the most valuable, has been the effort to create a graphic model, as shown in figure 3.1, by an anonymous designer based on the translation from Latin into ancient Italian by Pier Candido Decembrio but it is so rough that does not allow a real evaluation of the chosen solution (Maggi, 2002). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 27 Figure 3.1 The first known representation of Caesar’s bridge by an anonymous drawer (Maggi, 2002). Fra Giocondo in the sixteenth century suggested two oblique elements on each side between the transversal beam and the pier (Figure 3.2) that it has not a clearly understandable utility considering that in Caesar’s text there is no mention of such a solution. Moreover two diagonals partially immersed in water are then added, which is quite doubtful effectiveness. Figure 3.2 The detail of a pier of Caesar’s bridge according to Fra Giocondo (Maggi, 2002). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 28 Leon Battista Alberti in his version of Caesar's bridge (Figure 3.3) represented probably hemp ropes which tightly tied the beam with the pier: this solution seems unlikely, given the low efficiency of ropes in the presence of water. Figure 3.3 The solution proposed by Leon Battista Alberti in the XVI century (Maggi, 2002). The most reliable proposal (Figure 3.4) from the structural point of view is undoubtedly that of Palladio which draws a wood bracket with notches: doing this way it was possible to eliminate holes and ropes that are not suitable to a structure placed in water, also considering the significant forces, but mainly it fastens the two piles together and keeps a constant distance at the same time; it also allowed the structure to be stabilized with increasing vertical loads. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 29 Figure 3.4 The Palladio’s hypothesis of Caesar’s bridge (Puppi, 1999). The reconstruction of Sir John Soane, the famous English architect, is also noteworthy and it can be seen in Figure 3.5, who in addition to place a parapet that it is not mentioned in the Latin text, performed the connection between the transversal beams and the piers through metallic elements and set elements between the two piles forming a pier in order to space them. His solution provided also internal diagonals placed next to the piers, very similar to what has been done by Fra Giocondo (Maggi, 2002). Figure 3.5 The impressive reconstruction of the Caesar’s bridge prepared by Soane for his Lecture Diagrams (Maggi, 2002). CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 30 3.2 Trajan’s bridge over Danubius The Trajan’s bridge over the Danube at Drobeta in Dacia Inferior still remains one of the biggest and most daring feats of engineering ever built by the Romans. After a prolonged and violent military campaign in 101 AD Emperor Trajan succeeded in defeating and subduing the Dacian tribes commanded by General Decebal. The region of Dacia, geographical classification which covered a wide territory of central Europe corresponding to the current areas of central Romania and Moldova, was thus transformed into a Roman province and many settlers from all over the empire moved there in order to pacify it permanently, introducing the Roman’s language, customs and traditions. In the peaceful period immediately following these events the bridge at Drobeta was built (103-105 AD), strongly backed by Trajan in order to be able to move quickly and safely troops across the Danube and create a reliable supply route, without resorting to risky temporary bridges of boats and above whatever meteorological condition (crossing the river was extremely dangerous with the winter frosts). The responsible figure for achieving this noticeable building was Apollodorus of Damascus, the official architect of the Emperor, author of, among other things, the Trajan’s Column, who designed an extremely complex structure in the incredible reduced period of one year, choosing the location of the bridge just near Drobeta, given the presence of a natural ford which gave a narrowing of the riverbed (800 meters) and at the same time an outcropping of land: the bridge reached the considerable length of 1135 meters and for more a thousand years was the longest bridge ever built. Its most innovative feature was specifically that the arches were not made of single wood trunks, but they were polygonal that is formed by a series of pieces with relatively small dimensions: in this way the entire process of construction became easier and faster. The quality and accuracy of the extant sources is quite low and rough concerning the description of the bridge, but fortunately the sculptures from the Column of Trajan (Figure 3.6), in the scene where the victory of Trajan over the Dacians is celebrated, show that it was made up of twenty timber arches resting on twenty masonry piers. At each edge were two fortified towers that controlled the passage. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 31 Figure 3.6 The scene from the Trajan’s Column: on the background one can see the wooden bridge built in Dacia (Maggi, 2002). Information that can be derived about the kinds of carpentry joints is very limited but from the sculptures may be understood as the arches had an average span of about 30 meters each and they were made of six curved timber beams parallel one to each other, which in turn consisted of three arches concentric mutually interconnected and simultaneously joined with the upper deck. The connection between the arches and the horizontal part was performed by some inclined element positioned in the radial direction and whose length variable depending on their location. On the piers, in the meeting point between two adjacent arches, there were triangular structures to balance the horizontal pushes provided by the arches (Paolillo, 1999). There is no certainty either on the types of connections used to build timber structures but it seems likely that on the top of the studs, where they met the longitudinal beams, there were nailed joints; nodes where the arrows were going to engage on the arches were rather more complex, as in the same place there was the presence of transversal beams that served to stiffen the six arches and most likely the connection was performed through the combined use of nails and metal strips. Each arch was also segmented into different straight bars and the connection between segments occurred at the studs: the structural continuity was ensured through the use of structural carpentry joints (tenon and mortise) and with the addition of nails passing through both the stud and the two overlapping bars (Paolillo, 1999). The deck was 12 meters wide and consisted of the main longitudinal beams on the top of those the secondary warping of beams was placed, positioned in the transversal direction with a spacing of about 80 cm. A simple nailed plank rested on it. Longitudinal beams behaved as simple supported beams since they were placed in the room between one stud and the other and connections were performed similarly to what has been achieved between the different segments of an arch and the corresponding studs: also the edges of the beams presented notches in order to perform a tenon-mortise joint and then there were nails passing through to integrate the two parts with the corresponding arrow. The parapet was modular, stiffened by CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 32 diagonals, connected by ropes and nails to the transversal beams that supported the deck. Concerning the piers, they were clearly oversized in relation to the relatively low weight of the timber structures above them: this is because Trajan probably had wanted the arches made of timber in order to use the bridge as quickly as possible due to the ongoing military conflict, referring perhaps later the final realization of the stone arches, but never realized. They were realized using the usual Roman technology: the core was composed of stone fragments and rounded river stones held together by mortar (a typical opus caementicium) and the outer sides were covered with bricks (opus latericium) at the bottom and with stone plates on the higher level. A possible reconstruction of the bridge was proposed by Rondelet in his Treatise (edited in 1832) in the section on wooden bridges, as shown in Figure 3.7. Figure 3.7 The Rondelet’s solution concerning the Trajan’s bridge (Maggi, 2002). Regarding the building aspects of the bridge and the solutions adopted in the construction site, after underwater inspections it has found out that formworks for piers were made up of oak trunks and they formed a double layer sealed by the same method used for boats, that is with pitch. The room between the two formworks was drained from the water using buckets and filled with pressed clay and other materials. Each pier was based on a palisade of oak posts whose ends were covered with mortar and formed the base for piers themselves. (Paolillo, 1999). The issue regarding the installation of the timber arches was rather different, which was implemented for a real process of prefabrication: the wooden arches already formed and grafted with the studs, after being assembled on the ground, were put on boats and brought near the piers, on the top of which were installed cranes that provided to hoist structural components and place them in the correct position. 3.3 Palladio’s bridges (bridge over Cismon river and first invention) Although during the Renaissance timber bridges were considered to be less “noble” than the stone ones, nevertheless Palladio was a smart designer as he thought they had the advantage of being nice and cheap. Nice since timber texture was elegant, and cheap because timber had a lower price, both for the material itself and for the CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 33 setting up. Moreover this kind of works were lighter and more flexible than the stone ones. Palladio’s timber bridges have a great importance due to the fact that they give us the idea both of his static perceptions and the terrific success they have had, not only at the time of their setting up , but also and above all in the iconography and in the later literature, thus becoming a model in the 18th century England. The architect in the third of his Four Books of Architecture, not only analyses and hypothesizes the aspect of the Caesar’s Bridge over the Rhine, but also proposes the projects for five timber bridges. Apart from the Bassano Bridge, there are four others, only one of which has been actually erected (the one over the Cismon), while the remaining three ones are just “inventions” in order to be able to build bridges “without posts in the river”. According to these solutions every element is strictly essential and functional, and the structure is simple and minimal, in a way that even the smallest piece can’t be taken away without making the whole bridge to collapse. Unlike the masonry bridges, there are no decorations and the bearing system coincides precisely with the visible architecture. The models carried out by the Author are really similar to the modern truss beams, with a much more lower height of the beam: these bridges behave like a typical truss work because they are made up of relatively small size webs, simply tensed or compressed, of hinges and with point loads on the nodes. Moreover Palladio drew very clear structures showing the kind of timber joints to be performed and he not only indicated the whole span of the bridge , but also the ratio between the height and the width of the beams, i.e. 4:3 (Funis, 2000). One of Palladio’s most important achievements, besides the fundamental one concerning the extraordinary distance between the banks, has been the constructive simplification of the bridge structure and the innovations related to the building site organization. These improvements have been obtained making it easier to assembly the webs thanks to the offsite timber being cut following a list of pieces, the lifting in order (absolutely rational) in the right altitude, the easy positioning on the beams (which working also as spacers, determined a constant spacing between the elements), and speed and precision in performing the connections through the “arpesi”, rejecting the traditional complex carpentry joints in use at that time. The transversals had predrilled holes in the edges in order to make the arpesi come through and the first of them, measured and drilled with precision, acted as a ruler for the next ones, so ensuring the whole structure a high quality assembly and a geometrical regularity and thus avoiding any instability risks (Tampone, 2000). Therefore the arpesi and the particular building site organization have made it possible to achieve an improvement in the structure prefabrication, already inherent in timber construction. At the same time the process contrived and the devices in use to carry it out, did significantly cut both building time and costs . It may seem hard to carry on an aesthetic analysis of the proposed structures , due to their extreme essentiality, but it is indeed their main feature which allows a better understanding of the modernity and actuality of their designer, who presents his bridges not as arid technical handworks but as architecture pieces with their own dignity and with aesthetic, functional and structural features. Palladian bridges are pure structure, immediately intelligible. Their beauty derives from the expressiveness of the shape compared to the assigned function, the slenderness of the elements and the proportions between the parts and the transparency of the warping. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 34 3.3.1 Bridge over the Cismon river Palladio, quite similar to those roof structures he designed for many of his villas (Villa Emo above all), proposed in the bridge over the Cismon a genuine timber truss working as it is possible to see in Figure 3.8. Figure 3.8 The Bridge over the Cismon river (Puppi, 1999). In this bridge the carriageway on the lower part and the variable loads are applied, due to construction, on the lower nodes and the global behaviour presents a great static and functional clearness and furthermore it allows an excellent load distribution. The architect, without intermediate supports in the riverbed, designed a very daring bridge of 36 meters span, size really hard to be spanned by any other kind of timber structure. Which is why the issue arose of devising a system membering, in which the elements, despite being much more shorter than the distance to span, were assembled in order to form a single beam (Tampone, 2000). All the webs work exclusively in tension or compression, while only the longitudinal beams and the rafters, supporting the deck, work in bending, transferring the loads to the bearing structure through the junctions. The bridge ultimately works as a simple beam, whose elements are simply tensed or compressed, depending on their position: the upper and the inclined ones work in compression, whereas the lower and the vertical ones in tension. The bridge was composed of two big truss works, with a height varying from 1/7 to 1/12 of their length , placed at a certain distance between them. The pins were joined on the bottom, in correspondence with the vertical studs, to tough transversals. Then rafters were put on and finally the carriageway was placed upon these elements so the two truss beams acted also as parapet. From the plan inside the Four Books it can be seen that the deck is 4,37 m wide. Each truss beam was divided into six equal bays by vertical studs, the colonnelli, resting on the bottom chord and anchored to it through metal fasteners, the arpesi. In order to close the triangular section there were the upper chord and the inclined pins. Inclined pins, the longer ones, disposition is very important in a composite structure since in the event they join the colonnello in the lower part they work in tension, while if they join in the upper part, as in the Palladian bridges, they work in CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 35 compression. The drawback in this case is that the compressed pins are the longest, that is the inclined ones, and it is thus necessary to oversize them in order to prevent buckling (Funis, 2000). Single elements were connected between them, as already stated, with metal fasteners: the arpesi (see Figure 3.9). Palladio invented these fasteners as special devices strictly functional in his bridge structure: they were expressly created to the composability of the bridge and they had the advantage of being able to be manufactured elsewhere and then assembled directly on site during construction. Figure 3.9 Detail of the connection between a colonnello, the longitudinal and transversal beam by an arpese (Funis, 2000). The use of arpesi is particularly important because it reveals which were, according to Palladio, the limits of wood: in joints subjected to tension he put auxiliary metal implants, which were used to fill defects or limitations of timber. Even if timber resists well to tension, the difficulty was to create a T-shaped link, which resisted well to tension: this is the joint between the colonnelli and the transversals, which simultaneously serves as a graft also between the longitudinal beams and the transversals. The difficulty of achieving the joints between the different elements and their limited resistance are therefore, according to Palladio, the limits of timber and the reasons why, in these areas of 'weakness', he considered appropriate for metallic accessories that they had good resistance to tension. The arpesi were so artfully placed in the connection among the colonnelli, the longitudinal beams and the transversals, in order to hold together both colonnelli with longitudinal beams, and to suspend the carriageway to the bearing structure. Analyzing the original print a further construction detail will be also noticed : a line under the longitudinal beams that is nothing but the prospect of the rafters that support the floor system. Indeed, they do not follow the inclination of the bridge, but they are flat. The lower part of the truss work does not coincide with the carriageway. The truss structure and the deck are kept in this distanced position by the inclusion of some wedges that, placed between the longitudinal beams and the transversals, are held together to the whole structure by the arpesi. They are of increasing thickness from the banks towards the center, in order to always fill the gap between the arch and the rope (Tampone, 1999). Palladio thus distinguished the bearing structure, i.e. the two trusses, from the borne one, the carriageway, which was suspended from the bearing structure. Through this device the deck, for the convenience of passers-by, was plan, without thereby CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 36 compromising the efficiency of the whole structure, which, for structural reasons, could maintain the arched bottom. About the organization of the construction site of the bridge over the Cismone, following the description (but not complete in this regard) provided by Palladio (who, after having divided the width of the river in six equal parts, laid the tranversals) it may be assumed that the structure was assembled on a rib or through the creation of five temporary vertical supports in the riverbed; The other hypothesis, less plausible however, is that the entire structure was built on the ground and then placed at a height using ropes and trestles. The structure of the bridge over the Cismone, as shown on the Four Books, has anyway a weakness, that is, being deprived of a bracing. This needed to make the structure behaving as a box and also resistant to the force of the wind, could be added later, perhaps even as a coverage, both for the convenience of pedestrians, and to provide protection to the deck and structural elements. 3.3.2 The first invention Unlike the Cismon Bridge, this project has never been carried out; however this invention is important for the role it has played, along with the other two, in the history of trusses and due to the originality of constructive solutions. As to the bridge over Cismon, the truss beams have a similar structural organization and they are quite high in order to fill the gap without big inflections, and burnishings are defined by the same nomenclature (see Figure 3.10). Figure 3.10 The first invention, or the bridge with a variable section (Puppi, 1999). The structure is composed of eight bays, the bottom of the bridge is formed by juxtaposed beams, each sticking out in relation to the more central one. These beams are arranged in such a way that the bridge at the bottom, from support to the mid- length, has a variable section decreasing by four beams up to one. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2010:78 37 According to some theorists this bridge was not leaning on the banks but clamped to them. According to this hypothesis the variable section of the lower beam is not therefore representing a case. Were it a wedged beam, it would be then appropriate to thicken the beams near the joints where the bending moment is higher. Most likely the variable section of Palladio’s first invention has to be sought in constructive motives rather than in an equoresistant section. This is an application of construction techniques for gradual steps. In this technique, the construction, which is carried out through ropes and stands, starts from the longitudinal beams of the shorter length, the most external ones. These, already linked to colonnelli, are put in place and retained cantilevering by mean of fixed cables. The construction proceeds by putting in place the beams a little longer, also already connected to the colonnelli, and kept cantilevering through another system of ropes. This new cantilevering system is then connected to the previous one by the inclined and upper beams. In this way the structure proceed gradually from the two sides towards the middle. The last beam to be put is the central one, which is composed of one piece. At the time that the two parts join together in the centerline, through the last beam, the bridge does not need cables or stands anymore and it behaves as a single unit, supported at both ends. The construction of the first invention was then carried out moving one bay of the bridge after another, and provides indications of the more general case in the construction of infrastructure, which occurs when the bed of the river is impassable, and in this the invention is placed as an alternative to that on Cismon where precisely it is supposed to be possible to put an intermediate temporary support. 3.4 Bridge over Rhine in Schaffhausen by Hans Ulrich Grubenmann There were many works of Hans Ulrich Grubenmann that, due to their effectiveness and their daring, made his name known, not only within the borders of the small Appenzell canton where he was born, but even in Switzerland and later, thanks to the travel reports of many foreign scholars attracted by the enthusiastic descriptions of his work, throughout Europe. But why Grubenmann will be remembered are undoubtedly his ingeniously designed bridges of considerable length, the most famous of them are certainly the bridge over the Rhine at Schaffhausen and the Wettingen bridge on the Limmat. Their extraordinary singularity unfortunately did not allow them to be saved: the French troops destroyed them, along with 8 other of his bridges, in 1799 during the war against Austria to secure the retreat. So now there are only two Grubenmann’s bridges: the Kubel and Tobel bridges over Umasch near Herisau (Killer, 1985). Fortunately, however, the sketches of plants and sections in the travel notebooks of many travelers, first of all, the English architect John Soane, allow to keep their memory alive and give us the opportunity to study them and still admire their beauty. The wonderful works of Grubenmann were based on a synthesis of knowledge, experience and personal research, embracing a unique insight and an acute sense of observation, honed by experience and supported by a vast construction expertise, as result of a secular tradition. He was in fact the descendant of a family of great carpenters and then consolidated tradition in working with wood combined with direct experience in building roofs for c