Civil Engineering Reference
In-Depth Information
Chapter 8
Long-Term Modal Analysis of the New Carquinez Long-Span
Suspension Bridge
Y. Zhang and J.P. Lynch
Abstract
Recent acceleration in the development of sensing and data acquisition technologies has led to the dramatic
reduction in the cost of monitoring solutions while functionality has improved. These trends have made instrumentation of
large, complex infrastructure systems for long-term monitoring possible. For many structures, long-term monitoring
provides a massive data set upon which the system's modal properties can be empirically identified. Modal properties
have been previously used for model updating and to better understand the performance of the instrumented structure. In
this study, a low-cost wireless sensing platform has been used to permanently instrument a long-span bridge for long-term
monitoring of structural behavior under normal operational loading profiles including exposure to wind, temperature and
traffic loads. The Alfred Zampa Memorial Bridge (also termed the New Carquinez Suspension Bridge) located in Vallejo,
California was instrumented to monitor bridge accelerations, strains, and displacements. In addition, the wireless
monitoring system was used to indirectly monitor thermalandwindloadsbymeasuringtemperatureandwind
characteristics (i.e.,direction and speed) respectively. The study's primary focus is on the autonomous extraction of
modal characteristics of the bridge using the output-only stochastic subspace identification (SSI) method. Using extracted
modal frequencies, relationships are presented between the modal characteristics of the bridge and environmental
parameters such as temperature and traffic activity.
Keywords
Structural monitoring Automated modal analysis Environmental loading Subspace system identification
Wireless sensors
8.1
Introduction
Long-term monitoring of large-scale civil infrastructure systems is desirable because direct observation of structural
responses in normal and extreme load environments can advance the field's understanding of structural behavior. For this
reason, many instrumentation projects are underway in the United States. The most successful long-term monitoring project
is the California Strong Ground Motion Instrumentation Program (CSMIP) administered by the State of California's
Department of Conservation [
1
]. This program has installed a state-wide array of accelerometers to monitor ground motions
and structural responses during earthquakes. In California, many of the long-span bridges including the Alfred Memorial
Bridge has a dense array of force balanced accelerometers installed by CSMIP to monitor the bridge during moderate and
strong ground motions. While many of these instrumentation programs have been motivated by a desire to understand how
bridges behave under extreme loads, recent interest in long-term monitoring has been further motivated by a desire to assess
bridge health conditions from monitoring data. This interest in structural health monitoring (SHM) has been triggered by the
dramatic reductions in cost of traditional sensing solutions while non-traditional sensing technologies (e.g.
,
wireless sensors)
have matured to now offer monitoring conveniences not previously available.
