Civil Engineering Reference
In-Depth Information
Fig. 7.4
Port mann bridge
7.5 Data Analysis
Data analysis is a core functionality of the BCSIMS system. Acknowledging the high cost and risk associated with
continuous data-streaming, founding philosophies of the system is to process data at the site and transmit results in real
time only; sending full data only as the connection/bandwidth is available.
As shown in Fig.
7.1
, the data analysis takes place at a variety of stages and locations. Data is analyzed at the structure site
by the SIMS2 module and at the central site by the SIMS3 PC. Post processing of data includes statistics, ground motion
analysis, modal analysis, drift and hysteresis analysis, damage detection, finite element analysis, and finite element model
updating. All of these results are stored in the global database either directly by SIMS3, or by the process of first storing in
the local database, and synchronizing back to the global database by SIMS2 module.
Statistics are generated at the structure site on all of the data channelsby the SIMS2 module. It is capable of calculating the
mean, RMS, max, min, peak, standard deviation, skewness, and kurtosis. Such statistical values help to better understand the
structural behavior under different loading conditions, such as seasonal temperature change, daily traffic loads on bridge, etc.
The raw ground motion data is processed and stored in the COSMOS Strong Motion Data Format [
4
]. This well-known data
format creates three sets of files. Volume 1 files contain the raw data converted to physical units; these are typically referred to as
the 'uncorrected files'. Volume 2 files contain the products of the processed raw time history data fromVolume 1. This includes
correction for instrument response and digital filtering. The velocity and displacement are also obtained using numerical
integration. Volume 3 files contain all of the spectral products, including the response spectra (absolute acceleration, relative
velocity, and relative displacement). Also included is the Fourier amplitude spectrum.
The SIMS2 module performs modal analysis on the acceleration time histories by means of the time domain Stochastic
Subspace Identification technique as implemented in the ARTeMIS Extractor software [
5
]. The output of the analysis
provides the identified natural frequencies, mode shapes, and damping ratio for a given dataset. These results are stored in
the local/global database and can be viewed on a 3D model of the structure through the web interface. The frequencies are
posted on a Control Chart and can be tracked against time, depending on the frequency of scheduled measurements. The
identified modal properties are also used in the model updating process.
Functionality for drift analysis is setup in the SIMS2 module; the user specifies drift pairs and the system computes the
drift from the integrated displacement values. The displacement values are calculated by double integration of the narrow-
band filtered acceleration data. The filter corner frequencies of each drift analysis depend on the first predominant frequency
of the part of the structure for which the drift is calculated. The peak displacement values calculated during an event are
stored in the database. Any drift value exceeding predefined threshold value will indicate a possible damage in the structure.
