Civil Engineering Reference
In-Depth Information
A number of long-term bridge performance studies have been undertaken on heavily instrumented bridges. In many of
these studies, the behavior of the bridges over long periods of study has been described using modal parameters (i.e., modal
frequency, modal damping, and/or mode shapes). Modal analysis is well suited for these studies because modal parameters
represent the global characteristics of the structure and, in theory, should remain fixed for an invariant system. However, the
subtle variations in a structural system's behavior due to environment and time can be efficiently described using modal
parameters. Modal properties extracted are also used to calibrate the finite element models of the bridges instrumented.
Updated finite element models are essential tools that are used by engineers to estimate the health of a structure after extreme
loading events such as strong ground motions. Some examples of long-term bridge performance studies that adopted modal
parameters are the long-term structural monitoring systems on the Tamar Bridge in the UK [
2
] and the Ting Kau Bridge in
Hong Kong [
3
]. The Tamar Bridge is a large-scale suspension bridge that is instrumented with three comprehensive
structural monitoring systems. Principle component analysis and mathematical models have been applied to study
relationships between modal frequencies and changes in the environment and traffic. The Ting Kau Bridge is a cable-
stayed bridge located in Hong Kong and is permanently instrumented with 67 channels of acceleration data and 83 channels
of temperature data. Support vector machine techniques have been used to quantify the relationship between modal
frequency and temperature.
In this paper, long-term modal analysis is performed on the Alfred Zampa Memorial Bridge (also termed the New
Carquinez Suspension Bridge). This bridge has previously been instrumented with a permanent wireless monitoring system
to record the behavior of the bridge under its normal operating loads. A total of 124 sensing channels have been installed in
the bridge including accelerometers to measure deck and tower accelerations, strain-gages to measure deck flexural
responses, and potentiometers to measure deck displacements at the towers. In addition, temperature and wind loads are
assessed using thermometers, wind vanes and anemometers. In this study, the accelerometers installed along the bridge deck
are primarily used to estimate the modal properties of the bridge using output-only system identification methods.
Autonomously extracted modal parameters are analyzed to identify trends in modal parameters due to changes in tempera-
ture and time of the day (which will correlate to traffic loading). The paper begins with a description of the New Carquinez
Suspension Bridge long-term monitoring study followed by a detailed description of the system identification methods used
to extract modal parameters. Next, the results of the study are presented with the paper concluding with a summary of the
study's findings.
8.2 New Carquinez Bridge Long-Term Monitoring Study
TheNewCarquinezSuspensionBridge(Fig.
8.1
) is a major suspension bridge built in 2003 over the Carquinez Straights
and connects Vallejo with Crockett, CA. The bridge carries four lanes of westbound I-80 traffic. The bridge is constructed
from a steel orthotropic box girder supported by two major suspension cables and two reinforced concrete towers (referred
to as the south and north towers). The total length of the bridge is over 1,000 m (3,500 ft) and the main span between
towers is over 700 m (2,300 ft). A long-term wireless structural monitoring system was installed starting in the summer of
2010 with the system in continuous operation now for over 2 years [
4
,
5
]. The wireless monitoring system installed is
based on the use of a low-cost wireless sensor node developed at the University of Michigan termed
Narada
[
6
].
Narada
is
a wireless data acquisition system specially designed for monitoring civil infrastructure systems where low power
consumption (i.e.
,
rechargeable battery operated), high data resolution (i.e., 16-bits or higher), and long communication
ranges (i.e., 500 m or longer) are all required system capabilities. On the New Carquinez Suspension Bridge, a total of 33
Narada
nodes each capable of collecting up to 4 independent channels of data were installed over a 2 year period with
various sensing transducers interfaced: 23 tri-axial accelerometers (to measure deck and tower accelerations), 3 string
potentialmeters (to measure longitudinal movement between the deck and the towers), 33 battery voltage detectors (to
monitor the battery level on each
Narada
), 9 thermistors (to measure the ambient and girder temperatures), 2 wind vanes,
2anemometersand6stringgages(tomeasuredeckbendingstrains). A total of 124 sensing channels have been deployed
on the bridge as summarized in Fig.
8.1
.The
Narada
nodes (Fig.
8.2a
) are all powered by rechargeable batteries that are
continuously charged by solar panels installed on the top deck of the bridge (Fig.
8.2b
).
The monitoring system is supported by a custom-designed cyber environment that seamlessly integrates the wireless
sensors with the Internet where data can be stored in remote database systems and processed using powerful analytical tools.
At the bridge, the wireless monitoring system is divided into three sub-networks: one sub-network includes all of the
wireless sensors on the girder centered near the north tower (sub-network #1), another sub-network includes all of
the wireless sensors on the girder centered near the south tower (sub-network #2), and the third sub-network included the
nodes on the top of the two towers (sub-network #3). Each sub-network is managed by a Linux server implemented on an
