Analysis and Extraction of Temperature Effects on Natural Frequencies of a Footbridge based on Continuous Dynamic Monitoring

The development of efficient vibration based structural health monitoring systems requires distinguishing between abnormal changes in modal parameters caused by structural damage and normal changes due to varying environmental conditions. In this context, this paper is focused on the analysis and extraction of effects of temperature oscillations on natural frequencies of a footbridge, where a long-term dynamic monitoring system was installed by the Laboratory of Vibrations and Monitoring of the Faculty of Engineering of University of Porto. With that purpose, firstly, the temperature influence on the natural frequencies is reported, and correlations between measured temperatures and estimated natural frequencies are analyzed. Then, the Principal Component Analysis (PCA) and the Novelty Detection method are applied to identified natural frequencies: PCA effectively eliminates environmental influence; Novelty analysis on the residual error of PCA predicted model is used as a statistical indication of damage. The proposed procedure is illustrated using continuous dynamic data collected from the footbridge during more than one year. 56 IOMAC'09 – 3 International Operational Modal Analysis Conference 2 DESCRIPTION OF THE BRIDGE AND DYNAMIC MONITORING SYSTEM The new “Pedro e Inês” footbridge over Mondego River is located in the centre of the City Park of Coimbra, recently developed along the two banks of the river and opened to public in April 2007. This new infrastructure, conceived to become a landmark for the city and to contribute to the quality of a new leisure area, was designed by Adão da Fonseca (Adão da Fonseca et al 2005), leading a team from AFAssociados, in collaboration with Cecil Balmond, leading the architectural team from Ove Arup. The bridge has a total length of 275m and is formed by a parabolic central arch with a span of 110m and two half lateral arches, in steel, supporting with total continuity a composite steel concrete deck (Fig.1). The anti-symmetry of both arch and deck cross-sections along the longitudinal axis of the bridge is a unique feature of this bridge, leading to the creation of a central square with 8m×8m at mid-span. (a) Bridge plan and elevation, deployment of accelerometers and sections (S1-S3) with temperature sensors (b) Sections (S1-S3) and temperature sensors Figure 1. Deployment of accelerometers and temperature sensors Numerical and experimental studies, developed by the Laboratory of Vibration and Monitoring from FEUP, showed that this slender footbridge is prone to excessive vibrations caused by groups or streams of pedestrians. Therefore, six groups of tuned mass dampers (TMDs) were installed (Caetano, Cunha et al 2008). Aiming the permanent characterization of vibration levels after construction, the footbridge was also instrumented with a dynamic monitoring system, formed by signal acquisition, data communication and signal processing modules. The signal acquisition system comprises six uniaxial piezoelectric accelerometers installed in correspondence with the location of TMDs (Fig. 1). Five of them measure vertical accelerations (AV1-AV3, AV5-AV6), whereas another one measures lateral vibrations at mid-span (AT4). All sensors are mounted inside the metallic deck and wired to the corresponding signal conditioners and digital computer incorporating an analogue to digital converter and a UPS system, located in one of the concrete abutments of the structure. An automatic signal acquisition toolkit was developed in LabVIEW environment to record the acceleration signals and generate setup files every 20 minutes. The data communication system sends permanently the most recent collected data to a computer located at FEUP using an ADSL line (Moutinho et al 2008). The signal processing system is a toolkit developed in LabVIEW. It automatically searches the latest data transmitted from the bridge in 30.5 0m 64.00 m 55.00 m 55.00 m 64.00 m 6.00 m AV1 AV2 AV3 AV5 AV6