# Experimental and Numerical Analyses of Timber-Steel Footbridges
## 1 Introduction  
Traditional building materials, such as iron, steel, and reinforced concrete, have been preferred over timber from the second half of the nineteenth century onwards. However, in recent decades, in many European countries, the employment of wood in construction--and specifically in the construction of bridges--has been rediscovered. Timber is one of the oldest structural materials and has been used by human beings for thousands of years. The return to the utilisation of this natural building material has beensupported by the development of new wood-based materials and new types of mechanical fasteners, as well as by the introduction of new design methods. Great progress has also been made in improving the durability of timber structures, which represented a key problem in the past, particularly in the field of timber bridge structures. As a natural and renewable building material, timber has excellent ecological attributes. It acts as a carbon sink and has low embodied energy. The energy needed to convert trees into wood, and hence into structural timber, is significantly lower than that required for other structural materials, such as steel and concrete. A simple illustrative example can be found, e.g., in [1], where the energy requirements to produce equivalent beams from glued laminated timber, steel, and reinforced concrete are compared. According to the authors, five times more energy is needed to produce a reinforced concrete beam with dimensions of 400 \(\times\) 250 mm than to produce a glued laminated beam with dimensions of 550 \(\times\) 135 mm. In the case of a 305 \(\times\) 165 mm hot-rolled steel I beam, up to six times more energy is needed. In addition, timber has a very high strength-to-weight ratio. According to [1], for example, softwood of the common strength class C24 or glued laminated timber of class GL24 have about a 1.8 times higher strength-to-weight ratio than the commonly used low-carbon steel. Compared to reinforced concrete, this ratio is up to eight times higher. Due to its low density (400-500 kg/m\({}^{3}\)), softwood can offer lightweight structural solutions, which result in reduced foundation loads and the easier lifting of prefabricated elements during transportation and assembly. Other advantages of using wood for bridge construction include aesthetics, quick and easy assembly, easy shaping and modification of the material, possible recycling of waste material, cost of material (generally lower than the cost of other construction materials, especially in the case of curved geometries), excellent resistance to the salts used in winter road maintenance, etc. [2]. Obviously, there are also several possible disadvantages to using timber as a building material for bridges, e.g., low durability (if the wood is not properly protected from the weather and possible insects), risk of fire (wood is a combustible material), poor shock resistance (for example, due to possible vehicle collision), etc. The basic design requirements for timber bridges according to the Eurocodes are the same as for all building structures, as given in EN 1990 [3]. The partial safety factor method is applied in the design; according to this method, the reliability of a structure is assessed by verifying the reliability conditions related to the respective ultimate and serviceability limit states. Specific requirements for the design of timber structures and bridges are given in EN 1995-1-1 [4] and EN 1995-2 [5]. The natural origin of wood as a building material and its associated increased sensitivity to environmental effects have been reflected in the design values of the material's mechanical properties, which are determined depending on the humidity of the environment in which the structure is located and with regard to the time periods of the applied loads. Currently, timber bridge structures are commonly used in bridge construction in some developed countries of the European Union.