Determining Geometric Out-of-Plane Imperfections in Steel Tied-Arch Bridges Using Strain Measurements

The structural behavior of steel tied-arch bridges is determined by the presence of a large compressive force. As a consequence, slender steel arches are highly sensitive to in-plane as well as out-of-plane buckling. At present, no specific buckling curves for out-of-plane buckling exist for nonlinear or curved elements in the international codes and calculation methods. This research is mainly concerned with quantifying the geometric imperfections of real arch bridges based on the results of on-site strain measurements. The arches of six bridges have been equipped with strain gauges along their entire length. For the actual determination of the imperfections, a detailed finite-element model has been developed. It is assumed that the actual geometric imperfections are a linear combination of a number of theoretical imperfections. These imperfections are characterized by a theoretical lateral displacement in a single cross section of the arch. The influence of these theoretical imperfections can be calculated using the described finite-element models. The actual imperfections are determined based on the comparison of measured strain values with the strain values based on a linear combination of theoretical imperfections. This method is verified using the results of topographical measurements of one of the studied bridges. Both sets of results are in agreement when it comes to size and shape of the imperfections. The shape of the resulting geometric imperfection is a half-sine wave for most of the studied bridges. However, the most important result is that for all considered bridges, the size of the imperfections is substantially smaller than the one predicted by design codes.