Semi-parabolic beam
A semi-parabolic girder is a lattice girder with a straight, horizontal belt for the roadway and a polygonally curved belt according to a parabola , the ends of which, however, do not touch, but are connected to the two ends of a vertical post. When viewed from the side, the beams have the shape of an arc segment cut off at both ends.
description
In the most common form of the semi-parabolic girder, the framework and the curved upper chord are arranged above the horizontal lower chord for the roadway. The posts are usually so high that there is enough space between the roadway and the cross connection of the top chords between the posts for the clearance profile of the railroad or road traffic. Half-parabolic girders were also designed as fish-belly girders with largely the same construction principles, but different compressive and tensile loads. Instead of a belt shaped like a parabola, circular and polygonal belts were also used.
history
The semi-parabolic girder emerged from the endeavor to make the initially usual lattice girders with parallel belts in a more material-saving and thus more cost-effective manner. For this purpose, the knowledge of Paulis was used to adapt the height of the framework to the size of the bending moments . The resulting curved upper chord also gave the bridge a more aesthetic look. In contrast to the Pauli girder, the upper chord was not connected to the lower chord in order to avoid the problems associated with the large gusset plates and to achieve a more favorable stiffening at the ends of the truss. The term semi-parabolic girder was invented and used for this truss shape (although the shape of the girder has nothing to do with a halved parabola).
The Culemborg (then Kuilenburg ) railway bridge over the Lek , built in 1868, was the first large bridge with semi-parabolic truss girders. Their wingspan of 154.4 m remained unmatched for a long time. It was also the first bridge made of steel ( river iron ); Semi-parabolic beams were also made later from the wrought iron , which was still preferred for a long time .
Individual evidence
- ↑ Half -parabolic beam . In: Otto Lueger (Ed.): Lexicon of the entire technology and its auxiliary sciences . Vol. 4, Stuttgart, Leipzig 1906, pp. 757-758. On Zeno.org
- ^ Hans Pottgießer: Railway bridges from two centuries . Birkhäuser Verlag, Basel Boston Stuttgart 1985, ISBN 3-7643-1677-2 , p. 181
- ↑ The first large semi-parabolic beams . Summary of Pottgießer's work on Springer.com