# Experimental and Numerical Analyses of Timber-Steel Footbridges
## 1 Introduction
These are mainly the Nordic countries [6; 7], Germany [8; 9], and Italy [10; 11], but also, for example, Switzerland [12] and the USA [13]. In these countries, timber footbridges and small road bridges form an inherent feature of the built environment. Statistical data on the Nordic countries (e.g., in [14; 15]) suggest that around 5% of bridges in Finland are made of wood, and this figure is even higher in Sweden and Norway, where around 10% and 20% of all new bridges, respectively, are built using wood. The building material used in these structures is mostly glued laminated timber, which is produced by processing round softwood. Bridges made of this natural material undeniably increase the touristic appeal and attractiveness of the urban built environment and have also gradually been gaining popularity in other countries, for example, in the neighbouring Czech Republic [16], Poland [17; 18], and Austria [19; 20]. It is reasonable to believe that Slovakia, which has a robust forestry sector, has enormous potential in this area. Nevertheless, in our country, the return to the use of wood as a building material has been a slow process. Its wider use in the constructions of transport infrastructure has so far been hindered by the fundamental shortcomings of wood as a building material, especially its low quality and insufficient protection, which are the causes of the short lifespans of timber structures. These shortcomings are gradually being eliminated thanks to the increasing quality of wood processing and the introduction of modern high-quality insulating and protective materials. Another obstacle is the number of manufactures of glued laminated timber, which is still low; yet another is a certain fear or reluctance of designers to undertake building projects using timber, which is probably related to the above-mentioned reasons. Decision-makers, designers, and construction companies must be more familiar with the advantages of glued laminated timber. The development of technical manuals and catalogue sheets of basic types of small bridges and footbridges and their details could be of some help. However, to develop these technical documents, it is necessary to carry out extensive analyses of materials, beam samples, and static systems. Achieving this goal requires an optimal combination of experimental research and the latest theoretical and numerical tools, alongside a greater commitment of the state and investors as well as manufacturers of timber structures. Another factor which may aid in broadening the use of wood in load-bearing structures in buildings and in the construction of transport infrastructure is that it can be successfully combined with other building materials, such as concrete [21] or steel [22]. Combined timber-steel bridges represent a very effective type of superstructure in contemporary road and footbridges, especially in natural areas, but also in urban environments. Wood, as a warm natural material, is usually used for the main support system, defining the architectural feature of the bridge object, while steel is utilised for the construction of fasteners and for elements providing the overall stiffness of the superstructure or those exposed to local concentrated forces. A superstructure consisting of a pair of glued laminated timber girders connected at the bottom by steel cross-girders, usually made of hot-rolled I-shaped cross-sections, is a relatively common type of such a combined bridge structure. This article deals with these types of footbridges in terms of verifying their behaviour. Such bridges were built without the knowledge of the real stiffness of the connection between the steel cross-girder and the timber main girder. The main goal of our research was to observe and verify the factors influencing the stability of main girders and the real behaviour of these types of footbridges.