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Experiences from Timber Bridge Inspections in Sweden – Examples of Influence of Moisture

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Experiences from Timber Bridge Inspections in Sweden – Examples of Influence of Moisture Experiences from timber bridge inspections in Sweden – examples of influence of moisture Anna Pousette, Per-Anders Fjellström SPSveriges Tekniska Forskningsinstitut SP Rapport 2016:45 Experiences from timber bridge inspections in Sweden – examples of influence of moisture Anna Pousette, Per-Anders Fjellström 3 Abstract This report contains a compilation of experiences that SP Technical Research Institute of Sweden has obtained from inspections of timber bridges for bridge owners. The report has been prepared within the project DuraTB – Durable Timber Bridges. Most damages recorded at the inspections of timber bridges are connected to high moisture levels, such as cupping of deck plates, problems with edge beams, anchor plates and asphalt surfaces and rot. In this report the values of moisture content are the values measured at the time of inspection. Details of timber bridges are described, the focus is on timber railings and stress- laminated decks, because of the importance of good wood protection by design in these areas. Several examples are described with type of bridge and location, materials and dimensions, moisture contents and rot propagation and probable cause of the damage. Key words: timber bridge, stress-laminated decks, moisture content, bridge inspection SP Sveriges Tekniska Forskningsinstitut SP Technical Research Institute of Sweden SP Rapport 2016:45 ISBN 978-91-88349-49-1 ISSN 0284-5172 Borås 2016 © SP Sveriges Tekniska Forskningsinstitut AB 4 Contents 1 Introduction 6 1.1 Timber bridge inspections in Sweden 6 1.2 Measured moisture contents 7 2 Summary of inspection results 8 2.1 Beam bridges 8 2.2 Stress-laminated deck plates 9 2.3 Stress-laminated box-beam and T-beam bridges 10 2.4 Trusses, arch bridges, suspended bridges and cable styed bridges 11 2.5 Comparison with earlier inspection project in Sweden 11 2.6 Comparison with bridges in Norway 12 3 Details – influence of moisture 14 3.1 Example 1 – Railing post 14 3.2 Example 2 – Railing post 16 3.3 Example 3 – Railing panel 19 3.4 Example 4 – Railing panel 21 3.5 Example 5 – Stressed laminated timber deck 23 3.6 Example 6 – Stressed laminated timber deck 25 3.7 Example 7 – Stressed laminated timber deck 27 3.8 Example 8 – Stressed laminated timber deck 30 3.9 Example 9 – Stressed laminated timber deck 33 3.10 Example 10 – Stressed laminated timber deck 36 3.11 Example 11 – Stressed laminated timber deck 39 4 References 41 Appendix A. Moisture measurements on timber truss bridges from Martinsons 42 © SP Sveriges Tekniska Forskningsinstitut AB 5 Preface This report has been prepared within the project DuraTB – Durable Timber Bridges. The objectives of the project are to develop sustainable timber bridges by developing guidelines for moisture design methods and new and improved structural designs and bridge details with respect to durability and maintenance aspects. The project DuraTB is a Wood Wisdom-net project with participants from Norway, Finland, Sweden and USA. Project manager is Kjell Arne Malo, NTNU, Norway. The project has been funded under the WW-net+ Research Programme by the ERA-NET Plus Scheme of the Seventh Framework Programme (FP7) of the European Commission. The Swedish participation has been funded by Vinnova and industry partners Martinsons Träbroar, Moelven Töreboda, Limträteknik and Trafikverket. This report contains a compilation of experiences that SP Technical Research Institute of Sweden has obtained from inspections of timber bridges for bridge owners during several years. The report is part of WP2 “Performance based service life design of timber bridges”, managed by Sven Thelandersson, Lund University, and is a result from task 2.1 “Collection of field data from existing instrumented bridges” that Anna Pousette, SP Technical Research Institute of Sweden, is responsible for. © SP Sveriges Tekniska Forskningsinstitut AB 6 1 Introduction Improved guidelines to avoid the risk of moisture damages is one of the goals of the project Durable Timber Bridges. Data concerning actual service life performance of existing timber bridges can be used as a basis to evaluate today’s bridges and to develop new designs and new design tools. Field data regarding moisture content and temperature in timber bridges together with information about the bridges and the climate of the sites can be used to calibrate calculation models of moisture distribution in timber. To validate tools for service life design it is likewise useful to have field data regarding moisture content and temperature, but also information concerning actual service life performance from inspections. Any damages that has occurred in existing bridges are valuable. This report describes some results from timber bridge inspections in Sweden, carried out under the ordinary bridge management (1). It includes a compilation of bridge damages at the time of inspections. Some examples with measurements and assessments of damages are included. They show moisture content levels measured at different areas in bridges together with descriptions of where and in which extent a rot development has been observed. Inspections of some Swedish timber bridges were also made in an earlier project in 2004 (2). The bridges were selected to include different bridge types, ages and geographical locations. No serious damages, which should affect the bearing capacity of the bridges over the next 10 years, were found. However, there were some details and moisture levels which in the longer term should be taken care of. The inspected bridges had several damages due to lack of detailed workmanship or unsuitable materials, which have then been improved in later bridges. Some comparisons with the earlier study are made in chapter 2.5. Inspections and evaluations of timber bridges have of course also been carried out in other countries. There are many timber bridges built in Norway over the last 25 years (3). They have used somewhat different materials, treatments and constructions compared to Sweden but it may still be interesting to compare the experiences, see chapter 2.6. The timber bridge manufacturer Martinsons has built many timber truss bridges for pedestrians/cyclists. They made a follow-up of some truss bridges in Sweden in 2009, when the bridges were 8-12 years old. Moisture was measured during one day, see Appendix A. 1.1 Timber bridge inspections in Sweden During 2011 to 2015 SP Technical Research Institute of Sweden inspected about 145 timber bridges built after 1990. The inspections were made on behalf of the Swedish Transport Administration, local authorities or engineering consultants. The bridges were geographically spread all over Sweden, from Lund in the south to Kiruna in the north, but most were located in the northern parts of Sweden. The bridges presented in this report is not a random selection from all timber bridges in Sweden. It contains only a minor part of the results from bridges that have been inspected by SP, mainly as part of the planned bridge management. In chapter 3 Details, the focus is on timber railings and stress-laminated decks, because of the importance of good wood protection by design in these areas. © SP Sveriges Tekniska Forskningsinstitut AB 7 The inspected bridges were built during 1990-2015 and included 27 beam bridges (plus 12 older beam bridges); 66 stress-laminated deck bridges; 10 stress-laminated box-beam bridges; 9 stress-laminated T-beam bridges; 30 bridges of other types like trusses, suspended and cable-stayed bridges, etc. This report describes the type of damages at the time of inspection, but not the Condition Class. The Swedish Transport Administration has four classes 0-3 for the condition of a bridge, see table 1.1, and the requirement is a main inspection of bridges every six years. Table 1.1. Condition classes of bridges in Sweden (4) Condition class Definition 0 Malfunction beyond 10 years 1 Malfunction within 10 years 2 Malfunction within 3 years 3 Malfunction at the time of inspection The documented deficiencies in this report required measures ranging from immediately to within ten years. 1.2 Measured moisture contents In this report the values of moisture content are the values measured at the time of inspection. Moisture content values reported as high moisture values in this report are over 20% MC. Serious moisture levels in wood are above saturation point, which for spruce and pine is 25-30 %. Long periods with MC above the critical value of 25 % can cause problems with rot in the wood. Continuous measurements or measurements repeated often can give more accurate data, since measurements taken for example directly after a rainy day might not be representative if the wood has the possibility to later dry out. But data from the inspections can still provide valuable information if you also take into account the structure and location of the bridge. And MC from different depths in the wood can give information if the measured moisture is only temporary on the surface after rain or is spread into the whole wood element and probably will not dry out so quickly. The moisture content in wood vary naturally with the seasonal variation of humidity of the air. The cause of the measured high moisture contents are typically condensation, rain or running water, and not high relative humidity of the air, but the reason for the high MC can sometimes be difficult to determine when bridge inspections are infrequent. To really understand the reasons for the high MC, measurements should be carried out more frequently. Also inspections on rainy days can provide more information and show how water spreads on the bridge. For bridges crossing over rivers the underside of decks can be influenced by evaporated water from river that can condensate. The distance from water can in these cases be an important factor for the MC at the bottom part of a bridge deck.
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