Bridge Bearings – a Historical Survey
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Bridge bearings – A Historical Survey Volker Wetzk (From the German by Barthold Pelzer, Berlin) In: Dunkeld, M. et al. (Ed.): Proceedings of the 2nd International Congress on Construction History, Queens´ College, Cambridge University, March 29-April 2, 2006, vol.3, pp.3333-3355. ISBN 0-7017-0205-2 Abstract The support of early bridge constructions was an unproblematic affair. The arches and pillars of the ancient stone bridges amalgamated thus forming a homogeneous unity. In the case of timber constructions on the other hand neither the material nor the widths spanned caused substantial problems and therefore they did not require any particular solutions. With the introduction of iron, reinforced concrete and finally prestressed concrete as building materials new dimensions in bridge building were achieved. Special bearing technologies had to be employed to allow for movements due to temperature differences and impact of heavy traffic. In the course of time engineers have developed a wide variety of bridge bearings to comply with both the requirements of the design and with those of the structure. The following paper portraying the development of bridge bearings evaluates historical and contemporary research on a systematic basis. It follows the chronological evolution of bearings, commencing with wooden bearings, then deals with plane and rotating bearings made of iron and steel, and finally the employment of plastic for deformation bearings used nowadays. Only the evolution of the major forms of bearings will be presented. INTRODUCTION In the course of the past two centuries bridge bearings evolved as parts of immense sophistication and complexity in modern bridges. The ever-increasing spans went hand in hand with the rapid growth of the loads to be carried. For reasons of safety it became essential to implement those bearings in the structures arrived at theoretically by structural calculations before. The first bridge bearings were rather simple objects. In the wake of the railway boom of the nineteenth century, however, they evolved as an independent strand of supporting technology with individual construction characteristics. Products of mechanical engineering such as bolts, pins and rollers were introduced into the realm of bridge construction. “Structures were previously only considered to be rigid and immovable bodies. Now they come to be seen as machines capable of movement.”(Lorenz 1990, p.1). Steel bearings developed at this stage were to be virtually the only form far into the twentieth century. It was the development of new materials which finally led to a generational change in bearing technology. After a brief discussion of the term bridge bearing and a description of the pre-industrial beginnings the following will argue how in the nineteenth century the necessity for bearings of a new type arose, which could relate to the physical properties of iron structures under changing temperatures. The sophistication of new technologies to realize advanced bearing conditions will be exemplified with bearings for girder bridges. THE TERM BRIDGE BEARING The word bearing has different technical meanings. The following is based upon a definition of bearing as given in a reference work for bearing technology and which in its overall tendency is equivalent to our contemporary concept of a bearing. “Bearings are structural elements, which are arranged between parts of the structure to perform support conditions arrived at by structural calculations. “ (Eggert, Kauschke 1996, p.3). Bridge bearings accordingly are structural elements performing those functions in bridges. The limits of this functional definition, however, have to be expanded once a historical perspective is assumed. Whereas structural analysis in our contemporary understanding evolved in the last third of the nineteenth century, bridge bearings can be discerned much earlier. EARLY WOODEN BEARINGS Bridge bearings according to this definition can already be found in early timber bridges. Simple wooden laths (small sleepers) prevented the timber beams of the load-bearing structure from rotting. The beams serving as a base could be exchanged if required. (fig.1) But these beams at the base not only allowed the load to be spread evenly, they also enabled the deflection of the load-bearing structure without edge pressure occurring between bottom chord and masonry substructure. Furthermore the elasticity of wooden bearings absorbed some of the vibrations caused by traffic on the bridge thus less impact being transmitted onto the structure. This welcome property was still being used in the early cast-iron girder railroad bridges that were extremely sensitive to sudden impact. But trains crossing the bridge transmitted increasing weight onto the wooden bearings. These forces could be spread by employing cast-iron plates. In the case of these early iron bridges expansion of the material caused by changing temperatures could be neglected due to their small span. The problem was nevertheless already recognized. PROBLEMS WITH IRON The Cause: Temperature As early as in the seventeenth century it was known that materials reacted to changes in temperature with changing volumes. This was, however, of no consequence to the then existing bridges. For those made of wood tended to be short. If they were composed of more than one beam, changing lengths could be accommodated at the joints. In the case of timber these expansions were rather the result of varying degrees of humidity in the environment than in those of temperature. Bridges composed of stone arches on the other hand were – then as now - not prone to sudden changes in temperature because the mass of the material was very slow to react to temperature changes. Rather than following the short term gradients of day and night, the temperatures of the structure were more significantly related to the oscillations between summer and winter. Cracks and fissures appearing in the course of the cold would close again under warmer conditions. Louis Vicat (1786—1861) began a pioneering work by scientifically examining the periodical appearance and disappearance of cracks depending on temperature variations on his arched bridge in Souillac (finished in 1824). Although these movements would cause changes in the thrust of the abutment, he did not assume this to seriously threaten the stability of the structure. (Müller 1860, pp.215-6) As soon as iron became the material for building bridges critical awareness increased. Firstly there was concern about the influence of changing temperature on the strength of the material. Secondly engineers were seriously considering the degree of movement caused by temperature changes and how this would affect the overall stability of the system. These suspicions were not at all unfounded as was demonstrated by early damages occurring in those few iron bridges already constructed. In the case of the Buildwas Brigde crossing the River Severn (erected in 1796) the iron arches were constantly suffering from temperature changes (James 1988, p.159). And the Pont des Arts in Paris, finished in 1803, had complete blocks pushed out by iron rods expanding (Rondelet 1833, p.470). The engineers grew wary. When John Rennie the Elder (1761—1821) had his Southwark Bridge erected between 1813 and 1819, the middle one of the three cast-iron arches spanning 73 metres and the other two 64 metres, he did not take direct precautions to counteract temperature related deformations. His son, however, rigorously observed the effects of temperature changes while the structure was being built. An increase in 25 Kelvin caused a 31 millimetre rise of the arch (Müller 1860, pp.216). The attempt to wedge the complete load-bearing structure between the buttresses to prevent temperature movements caused the abutment masonry to break which had to be rebuilt (James 1988, p.177-8). Later the periodical movements of the arches would repeatedly produce fissures in both the road and the footpath surfaces. Rennie, however, did apparently not envisage any threats to the safety of both the arches and the abutments. Or did he? The Solution: Movability What we can observe is that shortly after the completion of Southwark Bridge he was searching for a different solution with a bridge crossing the River Aire in Leeds on the length of but 24 metres. Each of his two proposals from July 1820 – a suspension bridge and a bowstring bridge – was planned, so “that the expansion and contraction are wholly independent of any action on the abutments [...] which are intended to support the perpendicular weight of the superstructure only.“ The freedom of movement of the structural elements was to be ensured by “moveable sectors“(Rennie, p. 227). What was actually on Rennie’s mind in mentioning these? Could he have meant sliding bearings? If so, he cannot, however, count as their inventor. In August of that very year the engineering entrepreneur Ralph Dodd (c. 1756–1822) completed a bridge spanning only 9 metres crossing the River Chelmer near Springfield presented to the experts as “the most beautiful ever erected in this kingdom, or probably any other“ (F.M. 1820, p. 236). And it was certainly worth the attention of engineers: Wrought iron bowstring girders were solely resting on tubular cast-iron pillars rammed into the river banks, both being quite new technologies. And certainly another innovation were the “grooves in the top of those iron columns, on which the whole bridge has room to contract and expand, so necessary in this climate, from the various changes of the atmosphere from heat to cold, as the other iron bridges have suffered materially from this want of precaution“(F.M., p. 236). Apparently Chelmer Bridge represents the first instance of sliding bearings having been planned as well as executed. Unfortunately the superstructure was under-designed and soon required additional propping. Finally it had to be rebuilt (James 1980, p.70). Despite all its shortcomings it pointed in new directions for bridge building in particularly with regard to measures coping with temperature changes relayed as loads onto the structure.