GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 2.3 225 values were calculated based on the identification procedure described in (Brincker et al., 2011), that is by working on the inverse Fourier transform of the fully or SDOF auto-spectral density functions. Dynamic estimation of the axial force on the cables. Further local ambient vibration measurements were carried out on all the supporting cables, with the goal of estimating the axial force acting on them. Dynamic tests were performed by collecting the transverse time-history acceleration of each cable on the vertical plane, at measurement points located at the thirds of each cable length, approximately. Time series of 1200 s were recorded in each experiment. All the cable natural frequencies were identified by computing the auto-spectrum of the acquired acceleration signals. Time series were low-pass filtered and decimated before computing the auto-spectrum via the modified periodgram method (Welch, 1967), for a resulting frequency resolution of about 5/100 Hz. The first six cable frequencies were identified by computing the auto–spectrum of the acquired acceleration signals. Each cable was modelled as a uniform pinned-pinned Euler– Bernoulli straight beam, having (known) mass, density and bending stiffness, subjected to an unknown positive axial force. The axial force on each cable was estimated by means of a variational method. In particular, the optimal value of the axial force was determined so as to minimize the difference between a selected number of theoretical and experimental frequency values. FE modelling approach an solving technique. A geometrically refined FE-model of the Pietratagliata Bridge was implemented by means of the ABAQUS/Standard computer package (Simulia). Geometry and materials. Careful consideration was given to the geometrical description of the bridge components (e.g., deck, pylon, cables and pier), as well as to their reciprocal interaction, since of primary importance for the accuracy of the predicted mode shapes and frequencies of the bridge as a full structural system. 4-node stress/ displacement shell elements (S4R type) were used for the description of the bridge deck (82,000 elements) and the steel tower (29,000 elements). A free meshing technique was used, with average size of these elements equal to 0.15 m and 0.08 m for the deck and the pylon, respectively. Based on the techincal drawings of the bridge, the nominal thickness was then assigned to these shell elements. In the case of the deck, the structural interaction between the concrete slab and the longitudinal girders (e.g., where steel stud connectors are used) was described by means of tie constraints able to account for a rigid connection between the corresponding DOFs, along the bridge length. Beam elements (B31 type) with nominal geometrical properties were used for the double-L shaped metal bracings. Their self-weigth was described in the form of additional lumped masses at the beams ends. Further lumped masses distributed on the concrete slab of the deck were also used to take into account the self-weight of the asphalt layer and the lateral footways. The steel cables consisted in beam elements (B31 type) with nominal cross-sectional area (63.5 mm in diameter) and overall length derived from technical drawings. Lumped masses representative of half the self-weight of the cables were applied at the ends of each beam element. The cables were then connected to the steel tower and to the deck respectively by means of join connectors able to restrain possible relative displacements between the interested nodes. Careful consideration was paid to the geometrical description of the metal supports and devices (Fig. 2b, details A and B), so that local deformations and improper effects could be avoided. The so described deck and pylon were then properly restrained. In the case of the pylon (Fig. 2b, detail C), the metal devices at its base consisted in two inclined steel plates (80 mm in thickness) opportunely constrained, so that the typical base support could behave as a spherical hinge with respect to a local reference system. An analogous modelling approach was used for the description of the deck restraints of the longitudinal lateral girders on the RC abutment on the Pietratagliata side, see detail D in Fig. 2b. The RC pier on the NR n.13 side was finally modelled by means of 3D solid finite elements. Mesh size refinement required by the geometrical features of the pier