Tannery
In structural engineering, a tannery girder or articulated girder is a girder over several supports that is subdivided with joints so that it is statically determined .
It is named after the civil engineer Heinrich Gottfried Gerber , who first described that (statically indeterminate) continuous beams are statically determined by the inclusion of joints and can thus be calculated more easily. He is therefore considered to be the inventor of the hinge bracket. He first used such girders in bridges in 1867 , for which the names Gerber girder bridges and Gerber girders for the suspended middle part ( hanging girders ) became established.
Gerber girder bridges are the most common case of cantilever bridges in practice (which also occur without suspension girders). In English, both girder bridges and cantilever bridges are indiscriminately referred to as cantilever bridges .
history
Simple wooden cantilever bridges with a central part placed on top have been known for a long time, which allowed spans greater than the length of the existing trunks.
In the 19th century, however, the question arose as to how wrought iron girder bridges could be calculated statically correctly and without major problems. Even before Gerber there were a few suggestions, but he had the basic idea, for which he received the Bavarian patent for beam carriers with exposed support points in 1866 . In 1867 he built the first bridge based on this principle over the Regnitz near Bamberg and then a Main bridge in Haßfurt . With his construction he was able to extend the span (in Haßfurt to about 38 meters). The High Bridge (1876) over the Kentucky River in the USA is considered to be the first large, wide-span tanner girder bridge.
Gerber carriers were mainly for large truss - rail bridges used that are not considered because of their wingspan and higher compared to road traffic loads of trains suspension bridges could be executed. Only a few Gerber girder bridges were built in the 20th century. This was due not least to the two collapses of the Québec Bridge during the construction phase, which caused many responsible persons to lose confidence in this construction. Only in America, where the construction method was supported by the steel industry, several large tanner girder bridges were built. Significant structures outside of North America that emerged after the Québec Bridge are the Story Bridge , which opened in Brisbane , Australia, in 1935, and the Howrah Bridge, which opened in 1943, in India .
The Gerber girder bridges were mainly replaced by prestressed concrete - girder bridges , which are easier, faster and cheaper to build.
bridges
The most common design of bridges consists of two river piers and a beam with two joints above the central opening. The two outer parts of the beam are designed as cantilever beams , to which the middle part is attached as a suspension beam . The load of the suspension beam is compensated in many cases by the dead weight of the cantilever on the section between the river pillar and the bank, so that no tensile forces occur on the abutments. Otherwise the tensile forces have to be absorbed by anchors in the abutments or by anchor piers ; the section of the cantilever between the river pillar and the bank or anchor pillar is then also referred to as the anchor girder .
The construction of such a bridge begins with the cantilever girders, which are erected from the abutments towards the middle of the river. The first parts are pre-assembled on the bank and placed on an auxiliary pillar between the bank and the first pillar. From there, the cantilevered girder is assembled up to the river pillar and beyond in cantilever construction . The second cantilever is created in the same way. At the end, the suspension beam is used, which is usually delivered on a barge on the water and is lifted into its final position with winches attached to the cantilever beams.
Purlins
The tanner girder is occasionally still used in building construction , especially as a rafter purlin in flat roof constructions on unsafe ground where subsidence is to be expected. The main advantage here compared to a continuous purlin is that there is no need for the relatively complex, rigid beam joints . The joint only has to transmit transverse forces and can therefore be designed with web plates at a comparatively low cost.
technical description
In order to obtain a statically determined girder in spite of several supports, the Gerber girder is provided with moment joints so that no forces can be transmitted between the individual parts that arise from the bending of the parts.
The carrier is statically determined if the following rules are followed:
- The number of joints is one less than the number of fields .
- A maximum of two joints may be placed in the middle fields, and a maximum of one in the end fields.
- Central fields adjoining central fields with two joints may have a maximum of one joint.
- End panels adjacent to central panels with two joints must not have a joint.
The joints are expediently arranged approximately where, in the case of a continuous beam, the moment line has a zero crossing in the dead weight load case .
The advantage of the static determinacy is that the structure is insensitive to constrained loads, such as B. subsidence or temperature stress. In addition, the internal forces can be calculated more easily with a statically determined load-bearing system.
The static determinacy has a disadvantageous effect on the removal of longitudinal forces that occur in bridges such. B. caused by traffic loads . A further disadvantage are the complex joints that require later maintenance , as well as the greater deformations and lower bearing reserves compared to a continuous beam . The cantilevered parts of a Gerber beam above the middle supports allow better utilization of the cross-section than single-span beams .
See also
Web links
Individual evidence
- ↑ Gustav Lang: On the development history of the tensioning works of the building industry . N. Kymmel, Riga 1890, p. 153–158 ( limited preview in Google Book search).
- ↑ Description of the patent on December 6, 1866 awarded to the engineer Gerber on beam supports with exposed support points. In: Journal of the Bavarian Architects and Engineers Association , 1870, p. 25 (p. 29 in digitized version)
- ↑ Figures on page VI of the patent
- ^ Walter Pelikan: On the 125th birthday of Heinrich Gerber . In: Der Stahlbau , 26th year, issue 11, November 1957, p. 317.
- ↑ Karl Haber Cold: The new bridge or at Schönbrunn. : Allgemeine Bauzeitung , year 1900, p. 78 (online at ANNO ).
- ↑ Konstantin Meskouris, Erwin Hake: Statics of the rod support structures: Introduction to structural engineering . 2nd Edition. Springer, Berlin Heidelberg 2009, ISBN 978-3-540-88992-2 , p. 78 ( limited preview in Google Book search).
- ↑ R. Harte, K. Meskouris: Structures 1: Theory and calculation methods statically determined rod structures . Springer, 1999, ISBN 3-540-66402-5 , pp. 135–138 ( limited preview in Google Book search).