Wood-Concrete-Composite-Technology in Bridge Construction Michael FLACH Professor University of Innsbruck Innsbruck, Austria Michael.flach@uibk.ac.at Caroline D. FRENETTE Engineer Arborescence s.a.r.l. Cantercel, France 1978 Dipl.Ing, Munich, D 1997 Dipl. Ing, Paris, F 1989-98 Manager, ICS-Bois, F 1998-2002 Manager Arborescence, F 2002- Univ. Prof. Innsbruck, A 1993 Engineering, Sherbrooke, Ca 1997 M.A.Sc, UBC, Vancouver, Ca 1997-98 Training, ICS-Bois, France 1998- Engineer, Arborescence, Fr. carofrenette@hotmail.com Summary In wood buildings, the use of concrete composite slabs brings a better sound insulation and a higher fire resistance. For bridges, the wood-concrete technology can improve the load bearing and distribution capacity, and leads to ecologically sustainable constructions. There is however a large field of unexplored phenomena such as the behaviour under temperature variation and the evolution of internal forces due to differential creeping and moisture migration. Recent examples of road bridges in France and Austria show various approaches and pilot projects to introduce and develop the mixed technology in heavy road bridges made of wood and concrete. Wood Bridge, Heavy Traffic Bridge, Pedestrian Bridge, wood-concrete composite, connected system 1. Introduction During the last century, steel and concrete have replaced wood in many constructions. However, the last few years have shown a change of direction. More and always bigger bridges have been built out of wood, and the recent wood-concrete-composite-technologies offers always new possibilies. Regarding pedestrian and bicycle bridges, wood has the advantage of offering a lightweight construction. It is particularly interesting for wide span where it leads to efficient solutions (Fig.1). As for steel bridges, vibrations can be controlled by damper systems without having to add heavy weights. Fig. 1 Project foot-bridge with a span of 120 m, Grenoble / F 2001
Traffic bridges in wood are not frequent, although interesting solutions can be developed, especially for bridges limited to car traffic. Good examples are the wood-bridge of Thalkirchen in Munich and the longest traffic wood bridge in France, built over the Drome river in Crest with a length of 100 meters (Fig.2). Fig. 2 Bridge of Crest / F 2001 2. Heavy traffic bridges in wood For heavy traffic bridges, the application of wood constructions have certain limits. High concentrated loads, such as the standardized 100 kn wheel, bring problems to the secondary and third structural systems. Its limited resistance perpendicular to grain and its relative small shear capacity desadvantage the use of wood for these components. It often leads to inefficient solutions with heavy secondary systems piled up one on top of the each other, creating a clumsy and expensive support where a durable road coating is difficult to obtain. Another limitation for using wood is the connection of the crash carrier. Firstly, it is difficult to transfer the crashloads to a wood construction. And secondly, the crash carriers are, in most countries, standardized to be fastened to a concrete structure. Because of these difficulties, the wood-concrete-composite-technology presents an opportunity for heavy traffic bridges. Two possibilities are feasible: connected wood-concrete complexes and dissociated wood-concrete systems. Both technologies offer interesting solutions and will be presented here. 3. Bridges in wood-concrete-composite The addition of a concrete slab to a wood bridge is particularly advantageous. With its high shear resistance, the slab can efficiently transfer concentrated loads with a good cross distribution. It also take care of brake-forces, crash loads and other horizontal loads. Furthermore, it support effectively the road-coating and the watertight layer, and protects wood against water. Generally, composite constructions make sense when each material is used according to his capacities. Concrete is the cheapest building material to support compression loads, whereas wood is perfect to take over tension forces, replacing steel reinforcement. Wood-concrete compounds are lighter than concrete bridges and offer a better stiffness than lightweight constructions. They represent a good ecological balance regarding savings of energy and reductions of toxic emisson. The first heavy traffic wood-concrete bridge in France was the Sanne bridge / F with a span of 15 m (Fig.3). The composite structure was composed of vertical planks, nailed together, connected to a concrete slab. As a secondary system, it carries the loads across only 4 m. Consequently, the effects of temperature variation are reasonable. The joints between the planks absorb temperature dilation in the longitudinal direction. Fig. 3 Heavy traffic bridge over the Sanne / F 1998
The bridge of Fayette (Fig.4), is a heavy traffic bridge with a span of 28 m. The wood-concretecomposite-deck is made of Glulam ribs and a connected concrete slab carrying loads across 7 m in the transversal direction. Fig. 4 Bridge of Fayette / F 1998 To avoid incompatibilities between the temperature deformations over the 28 m length, wood and concrete were not connected in this direction. Expansion joints could have been a possibility, but it is known to be expensive and technically unfavourable. Since steel and concrete have the same temperature dilation, it was decided to built a steel bottom girder and to connect it to the concrete slab (Fig.5). The result is a wood-concrete-steel composite system where the concrete-steel connection operates in the length direction and the wood-concrete slab in the transversal direction. Fig. 5 Bridge of Fayette Steel bottom girder
Gluelam girders, fixed on the outside of the steel member, support the cantilever trusses of the sidewalk. Once finished, the bridge looks like a wood construction (Fig.6), but it is actually a mixed construction in wood, steel and concrete, working together as a double composite system Fig. 6 Heavy traffic bridge of Fayette / F 2000 4. Limits of wood-concrete compound The composite construction seems to be very advantageous, but they also have limits, mainly caused by the differential behaviour of wood and concrete. Shrinking and swelling are quite similar for both materials, but they are different in time. The temperature dilation factor for concrete is two times higher than for wood! Simulation calculations show that the internal forces due to this difference represent an large part of the total internal forces of wood-concrete-constructions. The influence of these internal forces, as well as the creeping interaction has not yet been explored. Analytical results show that high inner forces are produced in case of temperature changes. These effects may reduce the efficiency of wood-concrete-compound. As far as there is no experimental measurement, the mixed technology should be used with precaution for wide spans. 5. Dissociated wood-concrete structures The wood-concrete-compound technology can be compatible with the differential behaviour of wood and concrete for small bridge length. An 54 m heavy traffic bridge was build in wood across the highway A89 in the area of Correz/F (Fig.7). The building contractor didn t accept a composite structure since no references were available. Two solutions were considered: expansion joints in the concrete slab, and a dissociated woodconcrete structure Fig. 7 Heavy traffic bridge across the A 89 / F 2001 A pure wood structure was not possible because there was no standardised crash carrier agreed on a wood support. Consequently an unconnected mixed construction of wood and concrete structures was adopted.
The primary structure is composed of 5 girders in Glulam spanning continuously over the whole length of the bridge and supported by spatial struts (Fig.8). The decking, made of prefabricated slabs in B50 high performance concrete, lies on Teflon plates fixed on the primary wood structure. Fig. 8 Transversal section of the bridge across the A 89 / F A global Finite Element computer model, integrating the wood and the concrete structure, was necessary to determinate the reactions between both materials. Some of the supports had to be connected in order to attach up-lift loads, others were fixed to transfer horizontal loads from the concrete to the wood structure. Finally the concept of this bridge led to a reliable construction. The experience of this project shows however that it is easier to connect the two structures than to dissociate them properly. Furthermore an unconnected mixed structure does not beneficiate from the principal advantage: the reduction of the statical height! There is actually a need for more scientific investigations and practical experience about the differential behaviour of wood-concrete composite when applied to bridge design. For this project, time depended 4D Finite Element computer simulations calculated the evolution of inner tensions in function of interacting parameters as climatic cycles, hygrometric evolution, temperature and creeping. Measuring instruments, fixed on the bridge, will allow to verify the computer simulations. The first results prove that creeping and relaxation effects will reduce a part of the inner tensions. Based on this experience, the Institute of Steel, Wood and Mixed Technology at the University of Innsbruck will develop pilot projects, such as the future heavy traffic bridge in Kössen / Austria (Fig.9). At the moment, more investigations are necessary to benefit fully from these multimaterial technologies in bridge design. Fig. 9 Model of a heavy traffic bridge near Kössen, Austria New dimensions and possibilities could be explored for wood bridges. For instance, there is already a concept for a 200 m bridge in wood for city buses in St.Saint Gervais / F. This project, which includes a wood-concrete-compound deck, has a primary structure spanning over 130 m. 6. Conclusions The wood-concrete-composite technology offers the possibility of building economic and technically efficient heavy traffic bridges in wood. The differential behaviour between wood and concrete is presently reduced for applications including wide span bridges. Interesting solutions for composite bridges could be developed for the secondary systems of these structures. Scientific research and measurements should investigate the load distribution, the internal forces, and the dynamic behaviour. These results are necessary in order to allow the design of efficient composite bridges in wood, capable to compete steel and concrete bridges for heavy traffic.
7. References [1] Steurer A., Holz/Beton-Verbund im Brückenbau Tragende Verbundkonstruktionen mit Holz, 31 th edition, Weinfelden, Austria, 1999 [2] Schickhofer G., Vorstellung der Murbrücke, St. Georgen/St. Lorenzen Europa- Holzbrücke Wood Bridge Construction Seminar, University of Vienna, Austria, May 1993 [3] Flach M., Frenette C.D., Le retour du bois, Tracés, no128-12, Lausanne, 2002, pp.18-23 [4] Kappeli Th., Energie- und Stoffflussanalyse für Deckensysteme in Holz und Beton, Diplom Thesis, SISH Biel, Switzerland 1993 [5] Kuhlmann U., Schänzlin J., Erweiterung des Anwendungsbereichs von Holz-Beton- Verbunddecken durch Erfassung von Kriechen und Schwinden am Beispiel der Brettstapel- Beton-Verbunddecke, AIF - Research Report, Stuttgart, 2002 [6] Bou Said E., Constribution à la modélisation des effets différés du bois et du béton sous conditions climatiques variables, Application aux structures mixtes Bois-Béton, Ph.D.Thesis, INSA de Lyon, France, 2003