Online automatic identification of modal parameters of a bridge using the p-LSCF method

A concrete arch bridge over the Douro River, at the City of Porto in Portugal, is being monitored by twelve accelerometers since September 2007. The present paper describes in detail the methodology used to perform the on-line automatic identification of the modal parameters using the poly-Least Squares Complex Frequency Domain method (p-LSCF). The results obtained with this algorithm are compared with the ones obtained with the previously implemented Covariance driven Stochastic Subspace Identification method using one-year database. 22 IOMAC'09 – 3 International Operational Modal Analysis Conference reinforced concrete arch, 1.50 m thick (Fig. 1). The arch spans 280 m between abutments and rises 25 m until the crown. In the 70 m central segment, arch and deck join to define a box girder 6 m deep. The arch has constant thickness and its width increases linearly from 10 m in the central span up to 20 m at the springs (Adão da Fonseca and Millanes Mato 2005). Owing to the high stiffness of the deck in relation to the slenderness of the arch, the structure behaves as a beam bridge defined between abutments and with intermediate elastic supports 35 m apart (distance between the concrete elements that provide the connection between deck and arch). Figure 1 : “Infante D. Henrique” bridge view from upstream (Porto at the right side and Gaia at the left). The dynamic monitoring system installed in the “Infante D. Henrique” bridge is essentially composed by 12 force balance accelerometers (Kinemetrics, Episensor), 2 digitizers (Kinemetrics, Q330) and an internet router, which are placed inside the deck box girder and distributed along the bridge according to the scheme presented at Fig. 2. A more detailed description of the hardware is presented in reference Magalhães, Cunha et al. (2008). The bridge is roughly symmetric and the previously performed ambient vibration test proved that the mode shapes are approximately symmetric. Therefore, as the number of available sensors was twelve, it was decided to instrument just one half of the bridge instead of smearing the sensors along the whole bridge, in order to obtain a good spatial characterization of as many modes as possible. Consequently, those accelerometers were distributed along four sections between the mid-span and the abutment at the Porto bank. Three sensors equip each section: one to measure lateral acceleration and two for vertical accelerations at the downstream and upstream sides (the ambient test showed the existence of torsion modes in the analysed frequency range). The data produced by the digitizers is received at FEUP, as ASCII files containing the acceleration time series with a predefined sampling rate and length. For the monitoring of this bridge, a sampling frequency of 50 Hz and a record length of 30 minutes were selected. These files are then processed by software developed in MatLab at the Laboratory of Vibration and Structural Monitoring (www.fe.up.pt/vibest) of FEUP. The software, called DynaMo (standing for DYNAmic MOnitoring), includes the online execution of the following tasks: − creation of a database with the original data (sampled at 50 Hz) that can be later used to test alternative processing methodologies; − pre-processing of data to eliminate the offset and to reduce the sampling frequency from 50 to 12.5 Hz (the first 12 modes are below 5 Hz); − processing of data for automatic identification of modal parameters using three different identification algorithms: FDD, SSI-COV and p-LSCF; − creation of a database with the results of the processing. The database with all the results can then be consulted using a graphical user interface (DynaMo Viewer) that comprehends tools to create several types of plots for a given time interval, like the ones presented at section 4. The dynamic monitoring system is complemented by a static monitoring system that includes strain gages, clinometers and temperature sensors. The data of the temperature sensors is of particular interest for the development of numerical models to eliminate the effect of the environmental temperature on the modal parameters.