Force Prediction of Cylindrical Steel Beams on Pinned-pinned Supports Under Axial Load by Vibration Technique

Abstract

Quick monitoring of the axial force of hangers of a suspension or a cable-stayed bridge are of significant importance since large vehicle loads may have affected, if not changed, the axial forces along the life cycle of the bridges. The axial force formulae do not consider changes of support type and shape of hangers with increasing axially tensile or compressive loads. Such changes may, theoretically, have lead to inaccurate axial force predictions. This study focused on predicting the axial force of models with different L/d ratios of solid circular cross section upon pinned-pinned supports using vibration techniques. The lengths of the specimens were 101.2cm and 139.5cm, measured from the centre to centre of the supports, and the diameter of both specimens was 24.9mm. Pinned-pinned type of supports and various axially tensile and compressive loads were applied. The supports were designed and manufactured so as to comply with ideal pinned-pinned conditions by inserting bearings of 10 ton capacity inside the confining system. In every applied load the time domain vibration signals were recorded and the natural frequencies were analyzed by means of the Fourier transformation technique. The experimental results show that the increasing natural frequencies with increasing tensile loads, as well as the decreasing natural frequencies with increasing compressive loads, are in line with the theoretical prediction. The change of rotational support conditions (purely pinned to semi-fixed) at higher axially tensile forces especially for the slender beam (139.5cm) is blamed to cause higher first natural frequency predictions. On the contrary, both beams in compression (101.2cm and 139.5cm) indicate better conformity within 45% of their critical loads (between experiment and theory). Inconsistency of results occurs when hangers start to buckle.