Bollman carrier
The Bollman carrier ( English Bollman Truss ) is an after its inventor Wendel Bollman designated bridge girder , the 1852 patent was. The girder was one of the first all- iron structures that could be successfully used in the construction of railway bridges . It was used almost exclusively in the USA and was no longer in use by the mid-1870s.
history
The construction of the Baltimore and Ohio Railroad , the first longer railroad in America , called for cost-effective and lasting bridges. Wooden bridges were unsuitable because they were at risk of being destroyed by fire . Stone bridges were more permanent, but in the case of wider rivers they required piers in the river bed, which obstructed the outflow during flooding , and the route had to be laid at a minimum height above the water so that the arches of the bridge could be built sufficiently high according to the required span.
Latrobe , the chief engineer of the Baltimore and Ohio Railroad , initially used wooden bridges in which only some nodes were made of cast iron and some tension members were made of wrought iron. The load-bearing structure was only partially a real lattice girder , in which the forces were passed on from field to field, rather the greater part of the forces was diverted with diagonal struts directly to the support points of the bridge. This functional structure was a simple further development of trussed or spanned beams, but had the disadvantage that the supports not only had to absorb purely vertical forces, but also a proportion of horizontal forces. They therefore had to be built from heavy masonry .
With these bridges Latrobe was probably assisted by Wendel Bollman, of German descent, who was a learned carpenter and who later took over the construction of the Latrobe completely, so that Latrobe could take care of the further construction of the railway to the west. Bollman developed the construction of Latrobe further by completely doing without the wood in the structure.
The first bridge with a Bollman girder made entirely of iron was built in 1850 near Savage , Maryland on the Washington - Baltimore railway over the Little Patuxent River . It replaced a wooden bridge and had a span of 23 meters. Together with the bridge near Bladensburg built in 1851 , these were the first structures with the new girder. In the same year Bollman replaced the 38 meter long field of the bridge over the Potomac River at Harpers Ferry , which carried the tracks of the branch line to Winchester , which already branched off over the river from those of the main line to St. Louis .
The patent for the new carrier was granted to Bollman in January 1852, so the carrier is named after him, although it must be assumed that essential parts of the construction were not developed by Bollman himself. The girder was never used in Europe, but became more widespread in America because the Baltimore and Ohio Railroad immediately started to replace their wooden bridges with Bollman girders. Only after 1854, when Bollman went into business for himself and founded his own bridge construction company W. Bollman and Company , was the construction used by other railways.
Although operations in Baltimore were suspended during the Civil War , Bollman directed war damage repairs to the strategically important bridge over the Potomac River at Harpers Ferry. The destroyed fields were replaced by Bollman carriers, some even twice. After the war, Bollman's company resumed operations under the name Patapsco Bridge and Iron Works . In addition to the bridges for the USA, they also built a few examples for export to Mexico, Cuba and Chile.
In the mid-1870s, the Bollman girder was no longer used for new bridges. There were several reasons for the move away from the design. The most important of these are likely to have been the increasing traffic loads and the loss of confidence in the sufficient load-bearing capacity of railway bridges with cast iron parts. On the other hand, there were also advances in the calculation of supporting structures and a greater variety of rolled iron profiles was available on the market, so that simpler, more material-saving designs were possible.
Today, a single bridge with a Bollman girder is still standing in Savage, Maryland. It is called the Bollman Bridge and has been added to the list of Historic Civil Engineering Landmarks as a technical monument .
technology
The Bollman girder is not a truss , but a guyed girder. Similar to an under-tensioned beam, the top chord of the girder is provided with posts on the underside, which are connected to the ends of the top chord with tension rods . Cross connections are attached to the lower ends of the posts, which prevent the posts from tipping sideways out of the structural level and at the same time also support the roadway. In addition, diagonal struts are attached between the posts, which make the carrier more rigid.
In contrast to a lattice girder, the forces in the Bollman girder are not passed on from field to field, but, similar to a cable-stayed bridge, are transferred from individual points directly to the supports. The horizontal tensile forces acting on the supports are absorbed by the upper flange of the construction, which is only subjected to pressure and could therefore be made of cast iron . The tension rods are made of wrought iron.
The girder was only suitable for lengths of up to 45 m, above which the uneven thermal expansion of the tension members of different lengths acting at one point became too noticeable.
Web links
- Robert M. Vogel: The Engineering Contributions of Wendel Bollman. In: Smithsonian Institution United States National Museum Bulletin. Smithsonian Press, accessed January 19, 2014 .
- Patent US8624 : Construction of Bridges. Published January 6, 1852 , inventor: Wendel Bollman.
Individual evidence
- ^ Karl Gotsch: Bollman Truss. Retrieved December 20, 2013 .
- ^ JB Calvert: The Bollman Truss. Retrieved December 20, 2013 .