The precursors of timber truss bridges

Structural typologies of ancient timber constructions have offered to modern bridges suggestions of undoubted importance that were transposed, filtered and brought to basic static and geometric forms from new materials techniques and technologies. The proposals of Palladio for bridge girders had offered reticular schemes unexceptionable under the static and geometric profile: their essentiality, however, was incompatible with a material - solid wood - with relatively limited elastic modulus and with connections typologies achievable at that time, therefore, with scarce overall stiffness that gave rise to structures of large deformation and little reliability. Wood girders then diverged more and more from the schemes of Palladio, strongly increasing (and often disorderly), the number of rafters of reticular scheme in a constant attempt to couple the work of the beam to the work of the arc. Only after 1830 in the United States appeared the first timber trestle railway bridges in which the stiffening elements, longitudinal and diagonal rafters, had substituted the arc, assuming the main load-bearing functions. The paper describes the various stages of the evolution of trusses, patents, typologies, models, etc.