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The swing bridge, or Schaukelbrücke, in the Parc an der Ilm in Weimar, Germany is suggested to be the oldest suspension bridge in Germany, with a construction year of 1833. The structure is very flexible and shakes and rattles noticeably as pedestrians or cyclists cross. The back stays and eye-bar members making up the suspension straps on this structure provide excellent opportunities to use phone-based acceleration measurements and simple analytical models to determine internal forces. In fact, this exercise was conducted entirely with a phone, without the use of any other measurement tools.
The backstay length was measured using the Measure app to be 11'-4" long, or 3,454 mm.
The width and thickness were measured within the Scaniverse app after performing a 3D scan. The width and thickness were verified with physical measurements around 2.0 cm and 4.0 cm, with significant variation along the length, given the manufacturing of the time (wrought iron puddling).
The backstay was easily set in free vibration, with the phone supported by the connection in the middle of the stay. The natural frequency was measured with a clearly defined peak at 7.54 Hz using the Vibration Analysis app.
We can compare the tension in the rod (17.5 kN) to the reported [1] working strength of a single eye bar (58 kN). The stress in the back stay (21.3 MPa) can be compared to the tensile strength of wrought iron (234-372 MPa) as a means of identifying the additional capacity of the structure. We can certainly spend more time refining our measurements and comparison values, but we have done a lot of good work in very little time by taking advantage of the tools available in a mobile phone!
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