Measurements of ambient vibrations for a cable-stayed bridge including the soil-foundation system

This paper deals with the dynamic characterization of a cable-stayed steel-concrete composite bridge located on the Saline river, close to Pescara in central Italy, carried out during the static proof load test. The bridge is 189 m long and is composed by 4 inclined steel pipe pylons that sustain a steel-concrete composite deck by means of 40 stay cables. During the proof test, an extensive experimental campaign is carried out to check the overall structural performance before the bridge openings to traffic. In detail, the main girder deflections, the axial loads on strands, the pylons displacements, and the bearings displacements were monitored through conventional and non-conventional techniques, such as the laser scanner technique. In addition, an accelerometer array is installed during the proof test to evaluate the dynamic characteristics of the structure in its loaded and unloaded conditions through ambient vibration measurements. Sensors are positioned on the deck, on two pylons and on the foundation system, in order to characterize the whole structural dynamic behavior, including the contribution of the soil-foundation compliance. With reference to the unloaded bridge, the latter issue is analyzed in this paper, focusing on the different restraint condition exerted by the soil-foundation system of the pylons, because of the different embedment of the reinforced concrete basements. It is shown that the soil-foundation compliance has greater effects on higher modes for the bridge at hand and that possible non-symmetries due to the different foundation degree of restraint can be suitably captured through ambient vibration tests. Data from experimental tests are crucial not only to check consistency between the design and the real structure before the opening to traffic, but also to calibrate refined finite element models to be used for the structural health monitoring of the bridge. In this sense, a reliable evaluation of the contribution of the soil-foundation system is mandatory, considering that the bridge, having foundations located in the riverbed, is prone to the scour hazard.