Davide Roia

Dynamic monitoring of an isolated steel arch bridge during static load test

This paper describes the dynamic monitoring carried out by means of vibration measurements during the standard static load test of a half-through steel arch bridge on the Potenza river. The structural design of the bridge is characterized by some notable aspects: the river crossing is obtained by two coupled steel arches, having a span length around 115 m, and a steel-concrete composite deck sustained by thirty couples of steel hangers; the approaching spans are realized with continuous girders on intermediate supports, having cross section with variable height; the arches are supported on rubber bearing seismic isolators. The vibration measurements were nearly continuously acquired during the loading tests to detect the occurrence of anomalies in the structural behavior. In particular, the global dynamic characteristics of the structure, in terms of modal parameters, were determined using the data set of measurements at specific phases of the load test: first on the unloaded configuration, then on two different loaded configurations and finally after the bridge unloading. Measurements of vibrations due both to the ambient and to the impulse produced by a fully loaded truck passing over a bump, were carried out. The experimental results, in terms of modal parameters of the bridge, are in agreement with the theoretical results obtained with the predictive finite element model developed for design purposes and opportunely modified to account for the real conditions of the bridge during the tests.

Operational modal analysis on a r.c. building for the evaluation of the dynamic changes due to retrofitting

The paper deals with the changes of the modal parameters of a r.c frame building sited in Camerino, Italy. Operational modal analysis were performed before and after the seismic retrofitting of the building achieved with an innovative system that uses external steel towers equipped with dissipative devices. Before retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic behavior of the building including the contribution of non-structural components (e.g. external and internal walls); modal parameters, i.e. natural frequencies and mode shapes determined by means of experimental modal analysis, allowed the calibration of a predictive f.e. model of the building which was crucial for the final design of the retrofitting system. Tests were repeated after the retrofitting works, with the main objective to verify that changes of modal parameters are in agreement with those predicted by the f.e. model. Results and comparisons between the modal parameters obtained before and after the retrofitting works demonstrate that the ambient vibration tests and the data processing techniques used allow the identification of the dynamic behavior of r.c. few-storeys buildings under operating conditions and, then, the identification of dynamic changes due to seismic retrofitting works performed with external steel “dissipative towers”. A model updating procedure is carried out to identify the value of the elastic modulus of the different materials constituting structural and non-structural elements.

Linear and Nonlinear Dynamic Response of Piles in Soft Marine Clay

AbstractThis paper presents the results of free vibration tests carried out at different load levels on a system of three near-shore steel pipe piles vibro-driven into soft marine clay. Piles are arranged in an L-shaped horizontal layout and are free at the head. The instrumentation consists of strain gauges placed at selected levels along the shaft of the loaded pile and accelerometers at the head of the receiver piles. The aim of this experiment is to analyze the dynamic soil–water–pile and pile-to-pile interaction and to investigate the development of nonlinearities at increasing load levels. The results of experimental modal analyses, in terms of natural frequencies and damping ratios of the system, are presented and the complex dynamic behavior of the vibrating soil–water–pile system and the pile-to-pile interaction are discussed. For a better reliability assessment of the system response in the range of linear behavior, the results of free vibration tests at the lowest level of the applied loads are...

Experimental study on instrumented micropiles

In the present work, first steps of an extensive experimental study carried out on two vertical micropiles in alluvial silty soil are presented. One of the vertical micropiles is injected throughout valves a-manchèttes placed along the steel core of the shaft, while the other one is grouted with a unique global injection. Both of them are instrumented with several strain gauges along the shaft and an accelerometer at the head. The protection technique adopted to prevent damages on the strain gauges during micropile installation and high-pressure injections is also discussed. The main objectives of this experimental campaign are the monitoring of the modifications in the dynamic characteristics of the complex soil-micropile system due to execution techniques and construction stages, and the investigation of the dynamic response considering different kind of cyclic and dynamic loading. In particular, the effect of the high-pressure injections on the dynamic response of vertical micropiles is here investigated, by means of series of ambient vibrations and lateral impact loading tests. Experimental data of impact load tests are finally compared with results obtained from an analytical model.

Soil-Foundation Compliance Evidence of the “Chiaravalle Viaduct”

This paper focuses on the contribution of the soil-foundation compliance on the measured dynamic response of the Chiaravalle viaduct subjected to ambient excitations. The viaduct, linking the SS76 with the airport of Ancona (in Central Italy), is 875 m long and is constituted by four kinematic chains separated by expansion joints. The r.c. deck is formed by three V-shaped beams supported by column bent piers characterized by circular cross-sections. The piers foundation is constituted by a group of six concrete piles. Ambient vibration measurements have been performed to evaluate the actual natural frequencies and mode shapes of the bridge and data have been used to calibrate a numerical model that was adopted to design the bridge seismic retrofit. Experimental modal properties are evaluated by means of the Operational Modal Analysis; due to the great length of the viaduct, different configurations of sensors placed on the deck are used to study the overall behavior of the viaduct and the Pre Global Estimation Re-scaling method is adopted to merge mode shapes obtained from each configuration. Furthermore, with reference to the transverse behavior, some tests are performed in correspondence of one pier, measuring accelerations of the foundation cap (both translational and rotational components) and the pier bent, in order to identify contribution to the transverse modal displacement due to the elastic deflection of the pier and the foundation rocking. Test results are compared with those obtained from a 3D Finite Element model of the viaduct, including or not the Soil-Structure Interaction problem. The numerical model on compliant restraints is able to capture the foundation rocking, experimentally detected, and provides modal parameters in good agreement with experimental ones.

Experimental Modal Properties of a Steel Arch Bridge During the Static Load Test

This paper deals with the identification of the dynamic properties of a steel arch bridge located on the No. 77 main road, close to Macerata in central Italy. The main span of the bridge, of about 115 m, is formed by two inclined steel tubular arches that sustain the steel-concrete composite bridge deck by means of couples of hangers. During the static load test, before opening the bridge to traffic, an accelerometer array was installed over the bridge deck to evaluate the dynamic characteristics of the structure under different load conditions, in particular for bridge unloaded (before and after the load test) and for bridge loaded according to different load configurations obtained by positioning trucks over the bridge deck. Measurements of ambient vibrations, and operational modal analysis, were performed to estimate modal properties of the deck, namely natural frequencies and mode shapes. These experimental modal parameters determined at different loading phases were compared with the numerical ones obtained with the structural finite element model of the bridge. The dynamic interaction between bridge and trucks was also investigated by modeling the trucks as dynamic systems, i.e., not only in terms of added mass but also including their vertical and transversal stiffness. The obtained results show a good agreement between theoretical and experimental results, in terms of frequencies and mode shapes, both in the unloaded and loaded configuration; the first results on how different strategies of modelling trucks influence the modal parameters of the bridge are also discussed.

Current state of the dynamic monitoring of the “Chiaravalle viaduct”

This paper describes some notable aspects regarding the dynamic monitoring planned for the Chiaravalle viaduct, linking SS76 and airport of Ancona in Falconara, in Italy. The viaduct is 875 m long and consists of four r.c. kinematic chains separated by joints. The deck is constituted by three beams supported by column bent piers constituted by two circular r.c. columns founded on a continuous footing on six concrete piles. The first purpose of the dynamic monitoring is to determine the experimental natural frequencies and mode shapes of the viaduct before a scheduled seismic rehabilitation. Then, comparison of the first results with those obtained after the retrofit of foundation and piers and finally after the substitution of the deck bearing, will contribute to evaluate efficiency of the executed works. Two different inputs are used for the dynamic tests, ambient and traffic vibrations. An Operational Modal Analysis is carried out starting from the collected data, using the Enhanced Frequency Domain Decomposition technique. Different sensor configurations are used to evaluate the overall behaviour of the viaduct. The Pre Global Estimation Re-scaling method is adopted in order to merge the different parts of mode shapes relevant to each configuration. Results of dynamic tests are interpreted through a 3D Finite Element model of the viaduct. Firstly, a conventional fixed base model is considered, then a model including the Soil-Structure Interaction phenomena is developed in order to improve consistency between numerical and experimental results. The modal parameters obtained by the model including Soil-Structure Interaction are in good agreement with those estimated experimentally.

Dynamic structural control of the “Caffaro Viaduct” by means of vibrational measurements

This paper deals with the dynamic tests carried out with the purpose of controlling the behavior of the recently built Caffaro Viaduct. This test was required to integrate the final load test in view of the importance of the viaduct and because of its structural typology that implies a particular and quite complex dynamic behavior. The structural control is performed by comparing modal parameters obtained experimentally with those predicted by the design structural model, and represents the first step of a future structural health monitoring program that foresees tests performed every two years. The Caffaro Viaduct is a part of the project for modernization and retrofit of the A3 highway (Salerno-Reggio Calabria), near Lauria in south Italy. The longitudinal development of the viaduct is about 400 m; it is formed by two separate steel-concrete composite decks, 12.7 m wide, supported each one by two steel trestles that are connected nearly the top by transverse steel truss beams. The overall dynamic response of the structure is checked by means of three different sensor placements with the aim to highlight three different behaviors: i) the global behavior of both carriageways, ii) the flexural-torsional behavior and iii) the transverse one of the southbound carriageway. The data collected during ambient vibration tests are processed with Stochastic Subspace Identification Covariance Driven in order to estimate the modal parameters. Natural frequencies and mode shapes experimentally determined are compared with the numerical ones obtained with the predictive finite element model developed for design purposes and then with the model slightly modified to account for the real operating conditions of the viaduct, in terms of both stiffness and masses. A refined model updating is in progress to fix a benchmark, necessary for the scheduled structural health monitoring program.