Analysis of Displacements and Deformations of a Steel Footbridge

In in-service structural diagnostics, besides the monitoring of changes in physical properties of materials, it is essential to determine the dimensional stability of the structure as a whole. This is particularly important in the case of non-building structures exposed to variable and dynamic loading, such as overhead cranes, bridges, flyovers, telecommunications towers etc. Moreover, precise determination of the extent of deformation of structural elements allows for quick identification of any weak points or damaged areas which need to be further tested and assessed. The object of the analysis carried out in this article is a steel footbridge over the Brda River in Bydgoszcz. The main part of the structure stretching between the river banks comprises nine pin-jointed bridge decks suspended on pylons using steel cable stays. The jointed bridge decks make up the top slab. The structural design of the footbridge is simple and clear. The footbridge is rather susceptible to static loading and dynamic ambient excitations.

The research involved displacement and strain gauge measurements of the footbridge subjected to a test load. The test loading was applied by moving a loaded hand truck along the centre line of the footbridge deck and stopping it at predefined locations on deck segments. Vertical displacements were measured for each load setting at selected specific points of the bridge and the strain of the cable stays was determined on the basis of the measured values of the displacement. Precise surveying technology was applied to measure the vertical displacement, enabling the location of control points with an accuracy of 0.25mm in three dimensions, whereas deformations and strains were determined using strain gauges. This article also includes an account of changes in the geometric features of the footbridge resulting from its long-term use. A simplified static load analysis of the load-bearing system of the footbridge was performed (2D model) with simulated test loads. The loads applied in the FEM model were equivalent to the load values determined through geodetic measurements. The developed model was then used to identify a displacement of control points. A comparison of the measurement results with the results of the numerical analysis revealed inconsistencies, both as regards vertical displacements of footbridge deck segments and cable stay strains. An attempt was made in the article to explain the differences.