Modal identification of a GFRP-concrete hybrid footbridge prototype: Experimental tests and analytical and numerical simulations

The use of fibre reinforced polymer (FRP) structural elements, including glass fibre reinforced polymer (GFRP) pultruded profiles, has increased in the last few decades. The high potential of these materials for civil engineering structural applications, particularly for footbridges, stems from their high strength, low self-weight and good durability. However, when designing GFRP structures, the high deformability and the susceptibility to instability phenomena seldom allow the full exploitation of the GFRP material. In order to overcome these limitations, several hybrid structural systems have been proposed, namely GFRP-concrete hybrid systems. In this context, the evaluation of the dynamic characteristics of GFRP-concrete hybrid structural systems is very important, especially for footbridge applications, whose design is generally governed by pedestrian comfort criteria. This paper first presents modal identification experimental tests on a 6.0 m long and 2.0 m wide GFRP-concrete hybrid footbridge prototype, made of two I-shaped GFRP main girders and a thin steel fibre reinforced self-compacting concrete (SFRSCC) deck. The tests consisted of applying an excitation to the deck with an impact hammer and measuring both the applied excitation and the structure's response. The natural modes of vibration, frequencies and damping were determined using (i) a conventional input-output modal identification technique, based on the Rational Fraction Polynomial method, and (ii) a simple output-only method directly based on the Fast Fourier Transform (FFT) of the response (mode frequencies only). In the second part of the paper, experimental data are compared with analytical and numerical simulations in order to assess the accuracy of such design tools in predicting the dynamic behaviour of GFRP-SFRSCC hybrid structures, in terms of mode shapes and frequencies.