Deflection Theory

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The deflection theory is a static assumption that is mainly used in bridge construction . It was developed by Joseph Melan and used for the first time in the construction of the Manhattan Bridge planned by Leon S. Moisseiff . a. during the construction of the George Washington Bridge by the Swiss engineer Othmar Ammann .

The deflection theory assumes that floating structures with a large dead weight have a self-stabilizing effect. The bridges to the early 20th century built possessed numerous diagonal bracing , which have now been omitted. The deflection theory is based on the second order theory, which is based on very small rotations of the structural parts. These small twists are achieved by high dead weights. It takes into account that the bending stresses on the stiffening girders of a suspension bridge are greatly reduced by the cable deformation.

In the early 20th century, Melan recalculated existing suspension bridges and proved that they were over-dimensioned. With regard to suspension bridges, the deflection theory was further developed in the 1920s and 1930s by numerous, primarily American engineers, and simplified for practical application, u. a. by Stepan Tymoschenko , Moisseiff, Frederick Lienhard, David B. Steinman and Kurt Klöppel . This extended deflection theory attempted to describe the deformation of suspension bridges under static lateral wind loads . After that, a significant part of the wind loads is absorbed by the relatively stiff suspension cables and diverted into the pylons and anchor blocks via the hangers . Accordingly, a relatively small slack in the suspension cables and a flat deck girder with little wind resistance would be advantageous. This ultimately led to the slender construction of the roadway girders as solid steel wall girders , which were cheaper to build and quicker to assemble than the previously common lattice girders , which was popularly accepted during the Great Depression .

Insufficient results

With the collapse of the four-month-old Tacoma Narrows Bridge built with a particularly slim girder in 1940 at 68 km / h wind speed , it became apparent that the deflection theory only dealt with static wind loads, while the dynamic interaction between wind and bridges was not yet understood at the time has been. After the vibrating girder of this bridge tore apart, the vibration behavior of bridges in the wind was investigated using models in the wind tunnel .

literature

  • Karl-Eugen Kurrer: History of Structural Analysis in Google Books , accessed on April 23, 2009
  • Joseph Melan: Theory of the iron arch bridges and the suspension bridges , In: The bridge construction . Handbook of Engineering, Volume II, Section 4, Iron Arch Bridges and Suspension Bridges (Eds. T. Schäfer and E. Sonne), 2nd edition, Engelmann, Leipzig 1888
  • Leon S. Moisseiff, Frederick Lienhard: Suspension Bridges Under the Action of Lateral Forces . American Society of Civil Engineers Transactions, Vol. 98 (1933).