Overhead sign structures - gantry

Abstract

The standard sign gantries at the highways in the Netherlands have different geometrical and structural characteristics than sign gantries in other countries. The sign gantries have in total four oblique columns instead of two straight columns and have a triangular shaped spatial truss beam. The design loads that are currently taken into account include the effects of the self weight, natural wind and settlements. This research focuses on the structural response to vehicle induced wind loads.

The vehicle induced wind loads cause a vibration of the truss beam mainly in its first horizontal mode. The vibration of the beam can be modelled by discretizing a simply supported Euler-Bernoulli beam with rotational springs at the supports that take into account the rotational stiffness of the columns. From literature the vehicle induced wind load is characterized as a pulse load, which is applied to the discrete beam model. The mass, stiffness and damping matrices are used to compute the structural response numerically in the time domain using MATLAB. This numerical calculation model is verified and fitted to the full scale measurements. The measurements were performed with multiple video cameras that were focussed on specific details of the structure and the vehicles that pass the structure. It was found that it is possible to approximate the amplitude of vibration in time using a single pulse load for trailer trucks and trucks.

The stresses in the structure are calculated by applying a deformation that was caused by vehicle induced wind loads to a calculation model with bar elements in MatrixFrame. These stresses are compared with the cut off limit for fatigue detail classes. Based on the preliminary measurements and the calculation model it can be concluded that no fatigue damage is caused by vehicle induced wind loads for the newer series of structures (2012). The older series (2005) could encounter fatigue damage, but it depends on the span length of the beam.