GNGTS 2015 - Atti del 34° Convegno Nazionale

222 GNGTS 2015 S essione 2.3 Structural identification of the Pietratagliata cable-stayed bridge based on ambient vibration testing C. Bedon 1 , A. Morassi 2 1 Department of Engineering and Architecture, University of Trieste, Italy 2 Department of Civil Engineering and Architecture, University of Udine, Italy Introduction. This paper presents the results of an experimental and theoretical investigation on the Pietratagliata cable-stayed bridge (Udine, Italy). Based on ambient vibration and local dynamic experimental results carried out in order to estimate the dynamic characteristics of the lower vibration modes of the bridge and the axial forces on the suspending cables, the structural identification is performed by means of a geometrically detailed, computationally expensive but refined Finite Element (FE) model, being the optimized configuration representative of an increasingly accurate manual tuning and model updating procedure. Based on the rather close agreement between EMA and FE results, the same optimized FE model (FE-OPT, in the following) is then used to investigate the sensitivity of the bridge dynamic properties to damage in the suspending cables. The Pietratagliata bridge. The construction of the bridge was completed in 2007 and the infrastructure was opened to traffic in 2008. The bridge consists of a steel-concrete composite deck simply supported at the ends, a system of double-plane cables supporting the deck, and an inclined steel tower, see Fig. 1a. The total length of the deck is 67 m, while the bridge width is 11.10 m, including two lanes and two lateral footways. The deck structure consists of ‘Predalles’ concrete panels and a reinforced concrete (RC) slab supported by two lateral steel girders and a longitudinal central beam. The lateral longitudinal and transverse girders have double-T cross-section, 1.27 m and 1.20 m height respectively, while for the central longitudinal girder a HEB500 type cross- section is used. The interaction between the RC slab and the upper flange of the longitudinal girders consists of welded steel stud connectors. The bridge deck is supported on a RC pier on the National Route (NR) n.13 side and on a cast-in-place RC foundation block on the Pietratagliata side, see Figs. 1a and 1b. On the NR n.13 side, two unidirectional bearing supports are used to sustain the lateral girders. On the Pietratagliata side, conversely, the lateral girders are restrained by means of spherical hinges. Three groups of forestays on the upstream and downstream side of the bridge provide additional support to the deck. Each group of cables consists of four Dywidag bars which are connected to the main girders by means of special metal devices (see the detail of Fig.1c). Further backstays connect the steel tower to a RC foundation block. The tower consists of two inclined columns, having thin-walled circular cross-section 1.10 m in diameter (thickness 20 mm). The connection between the inclined columns is given by two additional thin-walled tubes, 0.50 m in diameter (thickness 15 mm). Special steel restraints are realized at the base of the steel tower, in order to reproduce the effect of spherical hinges. Dynamic testing programme and methods. Dynamic testing was carried out in 2009. Both ambient vibration testing and local dynamic measurements on the cable system were carried out. Ambient vibration testing. Among the tools currently available for structural investigations, dynamic techniques play an important role for several motivations. Based on the measurement of the response of a given structural system, dynamic techniques allow to identify the main parameters governing the dynamic behavior. Ambient measurements do not interfere with the normal service of the structure, and can also be repeated. Ambient vibration testing methods, moreover, are particularly suitable for flexible systems like suspension or cable-stayed bridges (Caetano and Cunha, 2011; Benedettini and Gentile, 2011; etc.), since the most significant modes of vibration in the low range of frequencies are excited with sufficient energy by the environmental actions only, hence a large number of normal modes can be identified from