The development of transversely stress-laminated timber arch bridges for pedestrian and minor vehicle use

Over the last fifty years, the Forestry Commission has developed various low cost bridge designs for its large Estate. These bridges are used for timber lorries and pedestrian access.This thesis describes many of these designs and leads on to the development of stress laminated timber as a modern solution, utilising readily available materials to build sustainable bridges. Timber is the only carbon neutral construction material and its use is, therefore,very appropriate in these times of perceived environmental damage. It also happens to be the product of the Forestry Commission. Recently, the major emphasis of land use on the Estate has moved from production of timber to Public access,for the intended benefit of the nation's health. This has exponentially increased the requirement for low cost footbridges with desirable aesthetic qualities. Timber is the obvious choice but the product available from home grown sources is fast-growing and, consequently,of low structural quality. The development has therefore concentrated on structural methods which could utilise this timber. Mechanical stress lamination had been pioneered in a few locations around the developed world since 1980, so the Author formed a group of consultants and researchers,called Innovative Timber Engineering in the Countryside, to bring these solutions into UK practice. In the UK, species of timber and climatic conditions in which the structures would be used varied greatly from international practice, so further development was required to assimilate existing solutions and devise new ones. This led to considering shallow arch construction for longer spans, instead of cellular or 'T' beam construction used elsewhere. These constructions either trapped damp air or required LVL which has to be imported into the UK. Over a three year period, since Autumn 2002, a series of laboratory and field tests were carried out on a variety of stress laminated timber arch bridges in order to develop an understanding of the structural mechanisms involved. Test bridges included a 6m span,a 15m span and four 2.1m spans,all built under laboratory conditions and statically load tested.A 20m span was specifically built, in the forest, to be statically and dynamically load tested while a number of other commercially built bridges were dynamically tested, both with and without a bonded bitumen topping. The programme of tests highlighted factors which needed further investigation, so a further four laboratory test bridges were built and loaded to investigate the transfer of load and load capacity under varying conditions. The test bridges all had spans of 6m, with different arch rises. A series of loads was applied at varying lateral tensions and measurements were taken of deflections and lateral spread.The purpose of these tests was to confirm past findings and extend the understanding of the manner in which the load is transmitted through these shallow arch structures. All of the test load effects on bridges showed good correlation with elastic analysis and indicated an optimum span-to-rise ratio for maximum stiffness. Further, there were a number of other factors e.g. lateral bar tension, which could not be integrated into an analytical model, so an integrated approach was required. For this purpose, a parametric study of a series of arches was undertaken. This study considered a range of different spans, rises and structural arch depths and related these factors to stiffness. To integrate the other factors, a generic semi empirical model was used to model all relating factors to the arch stiffness. The stress-laminated arch bridges have very good load carrying capacity when compared to structures built from heavier alternative materials. They have a span to structural depth limit of approximately I to 100 because more slender structures could resonate under pedestrian loading. The bridges are low cost, sustainable, carbon neutral solutions,using home grown material to produce aesthetically pleasing bridges for an increasing recreational market. The work is a very good foundation for future development and forms a basis for a UK code of practice in mechanical stress lamination.