PSI - Issue 44

Marco Gallo et al. / Procedia Structural Integrity 44 (2023) 618–625

619

2

Marco Gallo et al./ Structural Integrity Procedia 00 (2022) 000–000

were different from those of today in terms of defining both vertical and seismic loads. In particular, seismic actions were either neglected or considered in the calculation with a generally smaller magnitude than what they are done today. This article presents the seismic vulnerability check of an existing viaduct by applying the principles of current national and international technical regulations. Seismic vulnerability of bridges can be expressed, in a force-based approach, as the ratio between the value of the demand stresses and the resistance of the piers, which are the seismic resistant elements. In this work, we focus on the phenomenon of soil-structure interaction and on its effect on: a) the dynamic behavior of the infrastructure, b) on the seismic vulnerability assessment and, finally, c) on the amount of interventions needed for structural retrofitting. Finally, the study of the viaduct is enriched by some considerations regarding the type of seismic analysis conducted and the slenderness of the piers. The analyzed bridge is a typical viaduct recurrent in 1960s-70s in Italy. In fact, it was designed in 1973 as a viaduct with simply supported pre-stressed concrete beams and reinforced concrete piles. Different systems are detectable along the viaduct in terms of piles and deck beams, however this work is focused on the 37.8 meters long span. The deck slabs are made by reinforced concrete precast elements ("dalles") and completion concrete castings made on site. The decks are completed by pre stressed concrete longitudinal and transverse beams. The pre-stressed longitudinal beams are 2.30 meters high and they present Gerber saddles at the support areas. Regarding the piers, these consist in most cases of a pair of rectangular columns (with cross section 1.50 m x 0.70 m) supporting a reinforced concrete pulvinus (inverted T-shaped), founded on a reinforced concrete plinth resting on two foundation piles of length 26.50 m and diameter 1.50 m. The typical cross-section of the bridge is shown in Figure 1. For more information about the structure see Gallo et al. 2022.

Fig. 1. Cross-section of the bridge.

2. Seismic analysis input data 2.1. Dynamic identification of the bridge

The identification of the modal parameters of the structure was conducted relying on the measurement of its dynamic response under operating conditions. That was useful to the estimation of natural frequencies, damping ratio and modal shapes. Specifically, the structure’s measured dynamic response has been elaborated using reliable methods available in scientific literature, such as Frequency Domain Decomposition (Brincker et al. 2001) and Covariance Driven Stochastic Subspace Identification (Peeters & De Roeck, 2001). The analysis of the data and the results comparison, outcome of various methods, have allowed the reliable identification of the fundamental vibration modes of the structure. Table 5 shows the values of natural frequencies and damping ratios for the first two of them. The detected first two modes (frequencies equal to 2.45 and 2.67 Hz) are global ones as the membrane behaviour of the slab has resulted effective: they correspond to transversal and longitudinal global translational motions. In the band between 3.5Hz and 3.7Hz several dominant frequencies were detected in the vertical degree of freedom.

Table 1. Natural frequencies and damping ratios Mode f [Hz] Med. value f [Hz] Standard Dev.

ξ [%] Med. Value

ξ [%] Standard Dev.