Operational modal analysis of a historic GRP structure

The Aberfeldy Footbridge was the world’s first major advanced composite footbridge. Constructed from glass-fibre reinforced plastic (GFRP) in 1991, the lightweight cable-stayed footbridge provided the client with a low-cost access route which needed minimal site equipment for erection. Since opening to the public, the resin used within the structure has degraded exposing the glass fibres and there have been issues with the connections between various components of the bridge. Following damage to the structure in 1997 by the crossing of a light vehicle, the bridge deck was strengthened with bonded GFRP panels. It has been reported that there has been a gradual change in dynamic properties. Between 1995 and 2000 the damping of the first vertical mode is reported to have reduced by 52% with a reduction in natural frequency of 2%. Additional testing carried out in 2011 suggests that the natural frequency of the first vertical mode had reduced by a further 5%. This paper presents a new analysis of data from previous dynamic tests carried out by other researchers in 2011 and 2013, alongside data from tests carried out by the authors in 2019. A variety of time and frequency domain operational modal analysis techniques are used to quantify the uncertainty within the results. The reported changes in dynamic properties over the past 14 years are compared to the known degradation and repair work carried out on the structure. These results indicate that the previously reported changes in dynamic properties may be due to weakly non-linear behaviour of the structure, close modes and non-damage induced changes in dynamic behaviour. This illuminates the information which can be gained from in-depth dynamic analysis of civil structures, the challenges of damage detection and the need to quantify the range of dynamic behaviour a structure may exhibit.