How To Improve A Timber Bridge

Transcription

1 Development of timber bridges in the Nordic countries Aasheim, Erik 1 ABSTRACT As reported at IWEC '96 in New Orleans, USA the Nordic Timber Bridge Program started in The main results from the first phase of the program were reported, and a sketch for further development was introduced. The main objective in phase two was still the same; to increase the competitiveness of timber bridges compared with other materials like steel and concrete. The program is a co-operation with participants from Denmark, Finland, Norway and Sweden. Timber industries, road/bridge authorities and research institutions are working together. An increasing interest for timber bridges has been registered in the Nordic countries during the program. Based on the Nordic Timber Bridge Program and other national and international projects, several hundred timber bridges have been erected. Many of these bridges are pedestrian bridges, but the number of new bridges for heavy traffic is increasing rapidly. In Sweden more than 200 timber bridges have been built. About half of them are stress-laminated decks. One of the more spectacular examples from Sweden is a cable-stayed pedestrian bridge with a span of 90 m. In Norway a 5-span glulam truss bridge with a total length of 180 m was built. In Finland several timber-concrete composite bridges including glulam beams and concrete deck have been developed and erected. A Finish timber bridge with a total length of 182 m, with glulam king-post trusses in the three middle spans of 42 m, was opened in October In Denmark most of the new timber bridges are pedestrian bridges, but bridges for heavy traffic are during planning. In this paper some of the results from the phase two of the Nordic Timber Bridge Program will be reported, and some of the new Nordic timber bridges will be presented. The following projects were included in phase two: Environmental declarations Inspection programs Protection by design Optimum chemical protection Long span timber bridges Timber/concrete composite bridges Fatigue of joints for timber bridges In addition to this, several information activities were carried out in the different Nordic countries. The phase two of the Nordic Timber Bridge Program was finished in INTRODUCTION As reported at International Wood Engineering Conference (IWEC 96) in New Orleans, USA the Nordic Timber Bridge Program started in In the paper (Aasheim 1996) the main results from the first phase of the program were presented, and a sketch for a phase two was introduced. In this paper the preliminary results from phase two are summarised. OBJECTIVES The main objective of the program is to increase the competitiveness of timber bridges compared with bridges based on other materials like steel and concrete. The major goals for phase two were: to show that timber bridges are durable and environment-friendly. 1 Research Engineer, Norwegian Institute of Wood Technology, P.O. Box 113 Blindern, N-0314 Oslo, Norway

2 to develop solutions to increase the use of timber bridges. to disseminate knowledge about timber bridges to designers, users and buyers. The phase two of the program was composed of seven projects. In addition to these technical parts, several information activities have been and are still going on in the Nordic countries. Figure 1 Evenstad bridge, Norway, length 180 m ORGANISATION AND FINANCING The Nordic Timber Bridge Program is based on close co-operation between road authorities, timber industries and research institutions in Denmark, Finland, Norway and Sweden. It is organised as a partnership with a "Steering Group" consisting of two representatives from each of the four countries, one of them from the national authorities and the other from the timber industry. The total budget for phase one and phase two of the Nordic Timber Bridge Program was NOK (about USD ) allocated from the following sources: 50 % from the timber industry and road authorities 30 % from the Nordic Industrial Fund (Nordic Wood) 20 % national research funds The phase two of the program was finished in RESULTS FROM THE PROJECTS The program phase two was composed of seven technical projects. In addition to this, several information activities were important parts of the program. In the following clauses some results from each of the projects will be briefly presented. Environmental declarations Two timber bridges have been analysed and a Life Cycle Assessment (LCA) has been performed for each of the two bridges. One of the bridges was a stress-laminated deck for heavy traffic; the other one was a pedestrian bridge with glulam beams and a solid timber plank deck. The aim of the project was to study how these two bridges affect the environment, and to analyse how the bridges might be improved environmentally. A so-called LCA-index was calculated for each bridge based on three different assessment methods: EPS Environmental Priority Strategies ET Environment Theme method ECO Eco-scarcity method The following effects were studied and analysed. GWP Global Warming Potential

3 AP Acidification Potential EP Eutrofication Potential POCP Photochemical Ozone Creation Potential The results show that the choice of wood included paint/stain/preservatives is the dominant factor, but steel details and transport are also very important factors regarding LCA for timber bridges. Figure 2 Lusbäcken bridge, Sweden, stressed-box Inspection programs The main idea with this project was to give guidelines to the road authorities regarding inspection and maintenance of timber bridges. The report (Henriksen 1999) was printed as a guide for bridge inspectors and shows: How the inspection of timber bridges should be carried out Which tools can be used Which parts of the construction that are mostly exposed to damages. Sketches and photos are used to show typical damages. Protection by design By wood protection by design the aim is to design the structures in such a way that the wood will not be moistened and will not be exposed to direct sunlight. In this project guidelines were presented (Henriksen and Frank 1999) and the following points were emphasised: Water should be led away from the construction Upturning surfaces should be inclined All edges should be rounded Water traps in the construction should be avoided Free end surfaces should be covered or sealed appropriately The heart side of boards and planks should be turned up/outward Fastenings through upturning surfaces should be avoided Fibre destruction should be avoided Joints between construction members should be tight in order to avoid dirt traps Results from long term measurements of temperature, moisture content and forces in steel rods in stress-laminated decks were presented in separate reports. Figure 3 Roskilde pedestrian bridge, Denmark

4 Optimum chemical protection The main purpose of this project was to perform field tests with glulam pieces treated with different chemical systems. Due to different rules regarding the use of impregnation chemicals in the Nordic countries, the test program was divided into two main parts: Optimum creosote pressure impregnation Combinations pressure impregnation/surface treatment Three series with creosote treated test pieces have been produced, with the amount of creosote varying from 90 to 140 kg/m 3 sapwood. This is done by using different pressure, time, temperature, etc. The test pieces are exposed to outdoor climate and are inspected regularly. Special attention will be paid to the leaching of creosote. 30 test series with different combinations of pressure impregnation of the lamellae and surface treatment of the glulam pieces have been produced. The following systems have been used for the lamella impregnation: ScanImp KF Kemwood ACQ 1900 CCA class AB Tanalith E Untreated After gluing, the glulam pieces have been treated with the following alternatives: Royal-oil Ultrawood with CCA Ultrawood with paint Creosote impregnation No treatment The test pieces are exposed to outdoor climate and are inspected regularly. The moisture content and the temperature are monitored to give a rot index. The project will continue for at least three years. Figure 4 Fønhus bridge, Norway Long span timber bridges Timber bridges based on traditional static systems have evident limitations regarding span and length. The main purpose of this project was to give guidelines to designers about possibilities and limitations for longer spans with emphasis to fatigue and vibrations. The project was composed of the following actions: Theoretical models for the reaction of the bridge to wind actions. Measurements of dynamic response on existing bridges. Design rules and structural solutions.

5 Three Swedish timber bridges were used for case studies. The results from these bridges together with other studies were reported. Figure 5 Järna footbridge, Sweden, cable-stayed Timber/concrete composite bridges The first timber/concrete composite bridge in the Nordic countries was erected in Hämeenlinna in Finland in After that these type of bridge were the main topic in the Finish part of the Nordic Timber Bridge Program. The main goals were: To develop further the stud-type connectors so that they can be used to connect prefabricated concrete or wood elements to glulam beams. To develop analysing methods for various connector types and a design procedure for composite bridges considering the Eurocode 5 design method. To acquire relevant feedback information from the executed timber/concrete composite bridges. Experimental research on shear connectors was carried out in the laboratory. Field tests were performed to verify the stiffness of the completed bridges and experiences regarding formwork scaffolding have been studied. The degree of composite action was studied by measuring the deflection of girders and the slip between the deck and the beams. The results were reported in many detailed reports, as shows in the list of references in this paper. Figure 6 Tirva bridge, Finland, wood-concrete Fatigue of joints for timber bridges In the phase two of the Nordic Timber Bridge Program, the fatigue testing was done on larger test pieces. The cross sections of the glulam test pieces were 140 x 200 mm, and two slotted in steel plates with 12 steel dowels were used. The diameter of the dowels was 12 mm and the length was 120 mm. This means that the length of the dowel was a bit shorter than the width of the glulam test piece (140 mm). The reason for this was to reduce the bending moments in the dowels. Static tests were performed, and based on this 50 % and 75 % load levels were tested dynamically. The experimental data indicate that the provisions in Eurocode 5 overestimate the fatigue strength of dowel type joints at relatively high load levels. For smaller loads or load ranges sufficient data is not available to draw any final conclusions, but it seems as if the

6 slope of the design curve should be reconsidered. Furthermore, the use of the stress range as load parameter is most likely insufficient for modelling of the fatigue strength, as also the stress ratio must be taken into account. Figure 7 Vihantasalmi bridge, Finland, length 182 m CONCLUSION An increasing interest for timber bridges has been registered in the Nordic countries during the program. Based on the Nordic Timber Bridge Program and other national and international projects, several hundred timber bridges have been erected. Many of these bridges are pedestrian bridges, but the number of new bridges for heavy traffic is increasing rapidly. In Sweden more than 200 timber bridges have been built. About half of them are stress-laminated decks. One of the more spectacular examples from Sweden is a cable-stayed pedestrian bridge with a span of 90 m. In Norway a 5-span glulam truss bridge with a total length of 180 m was built. In Finland several timber-concrete composite bridges including glulam beams and concrete deck have been developed and erected. A Finish timber bridge with a total length of 182 m, with glulam king-post trusses in the three middle spans of 42 m, was opened in October In Denmark most of the new timber bridges are pedestrian bridges, but bridges for heavy traffic are during planning. A new phase three of the Nordic Timber Bridge Program was started in June 1999 and will be finished in July Further research regarding fatigue properties of joints and regarding design of stress-laminated decks are going on. In phase three only three countries are participating; Finland, Norway and Sweden. CONTACT PERSONS For further information from the projects of the Nordic Timber Bridge Program, please contact one of the national contact persons: Martin Gustafsson Keld Henriksen Aarne Jutila Erik Aasheim martin.gustafsson@tratek.se keld.henriksen@teknologisk.dk aarne.jutila@hut.fi erik.aasheim@treteknisk.no REFERENCES Aasheim, E., 1996, Nordic Timber Bridge Program - an Overview, International Wood Engineering Conference, New Orleans Henriksen, K.H., 1999, Inspection of Timber Bridges, Danish Technological Institute Henriksen, K.H. and Frank, F.C., 1999, Guidelines for Wood Protection by Design, Danish Technological Institute The following references are reports from the phase one of the Nordic Timber Bridge Program They are published by Nordic Timber Council, Drottning Kristinas väg 71, Stockholm, Sweden: Bakke, K. and Solli, K.H., Market Survey Gustafsson, M., Lindberg, G. and Haakana, P., Competitiveness of Timber Bridges

7 Jutila, A., Rules concerning the design of Nordic timber bridges Mäipuro, R., Tommola, J., Salokangas, L. and Jutila, A., Wood-Concrete Composite Bridges Tommola, J., Jutila, A., Rautakorpi, H. and Wistbacka, J., Design of Wooden Arch Bridges Pousette, A. and Marklund, K.-A., Stress-laminated Decks Solli, K.H., Timber Truss Bridges Ellingsrud, O. and Ljørring, J., Joints for Timber Bridges Gustafsson, M., Ström, T. and Nilsson, O., Railings for Timber Bridges Pousette, A., Wearing Surfaces for Timber Bridges Holm, L., Prefabricated Foundations for Pedestrian Timber Bridges Henriksen, K., Guidelines for Wood Protection by Design and Chemical Wood Protection The following references are reports from the phase two of the Nordic Timber Bridge Program They are published by Nordic Timber Council, Drottning Kristinas väg 71, Stockholm, Sweden: Tommola, J., Salokangas, L. and Jutila, A., Wood-Concrete Composite Bridges - Tests on Shear Connectors Mäkipuro, R. and Jutila, A., Wood-Concrete Composite Bridges Analysing Methods of a Composite Girder Containing Wood Salokangas, L. and Jutila, A., Wood-Concrete Composite Bridges Follow-up Tests of the Uusisalmi Bridge Noponen, S. and Jutila, A., Wood-Concrete Composite Bridges Formwork and Falsework Construction Noponen, S, and Jutila, A., Example Drawings for Timber Bridges Wood-Concrete Composite Bridges Savolainen, A., Rautakorpi, H. and Jutila, A., Example Drawings for Timber Bridges Arch Bridges for Pedestrian Traffic Torkkeli, M., Rautakorpi, H. and Jutila, A., Example Drawings for Timber Bridges Arch Bridges for Road Traffic Mäkipuro, R. and Jutila, A., Example Drawings for Timber Bridges Prototype Models of Timber Bridges with Prefabricated Deck Units Malo, K.A., Fatigue Tests on Dowel Joints in Timber Structures Clorius, C.O., Pedersen, M.U., Hoffmeyer, P. and Damkilde, L., Fatigue Evans, F.G. and Solli, K.H., Optimum Chemical Protection Pousette, A., Life Cycle Assessment (LCA) of two timber bridges in Sweden Pousette, A., Cable Stayed Timber Bridges Daerga, P.A. and Fjellström, P.A., Monitoring of Timber Bridges Daerga, P.A. and Fjellström, P.A., Field Measurements of the Lusbäcken Timber Bridge