Fixed-Points Method in Robert Maillart’s Analysis of Rigid Frame Bridges

This paper explains Robert Maillart’s analysis approach – using the fixed-points method – for the Weissensteinstrasse Overpass that he designed in 1938. A brief history and a technical explanation of this method are also presented as background information. The structure of this reinforced-concrete bridge consists of two identical plane rigid frames, each possesses five degrees of static indeterminacy. This bridge was one of the few continuous bridges designed by Maillart. Its long-forgotten analysis method, the fixed-points method, is a geometrical method for the elastic analysis of continuous beams and frames, which addresses the static indeterminacy by means of geometric constraint of fixed points on bending moment diagrams. This method was invented by Christian Otto Mohr for analyzing continuous beams, which was based on the work of Carl Culmann, and then refined and extended by Karl Wilhelm Ritter for the analysis of frames. Ernst Suter then generalized and analytically formulated this method for complex cases. In view of the historical context, this method is of special significance for the history of graphic statics. The history and core techniques of this method, including fixed points, crossing-lines, distribution factor, and virtual lateral constraint, are all outlined. The techniques are illustrated with Maillart’s analysis of Weissensteinstrasse Overpass.