An alarming problem of leaking dampers has emerged in recent years in some California bridges, such as the west span of the San Francisco–Oakland Bay Bridge and the Vincent Thomas Bridge crossing the Los Angeles Harbor. This reports presents the results of an experimental testing program that was undertaken to develop and validate a simple and reliable way to monitor the force output of viscous fluid dampers in bridges.
The research includes indoor and outdoor experiments on two medium-size (250 kips at 42 in/sec piston velocity, ±8.0 in. stroke capacity) and two large-size (450 kips at 85 in/sec piston velocity, ±19.0 in. stroke capacity) viscous fluid dampers at the Earthquake Simulator Laboratory of the Pacific Earthquake
Engineering Research (PEER) Center at the Richmond Field Station, University of California, Berkeley. The former two dampers are identical to the fluid dampers installed in the 91/5 over-crossing in Orange Country, CA, while the latter two are identical to the Type-A dampers that were installed at the west span of the San Francisco–Oakland Bay Bridge.
In Chapter 2 we discuss the concept of measuring the force output of a damper using strain gauges attached on the damper housing or on the damper
extender. To prove the concept, we present the results of a series of indoor experiments that were conducted at the Earthquake Simulator Laboratory and included tests with a wide range of combinations of displacements and velocities. The damper force obtained from the strain gauges was in very good agreement with the damper force measured directly with a load cell. The force obtained from the strain gauges in-situ can be compared to the theoretical force output of a damper which is a function of velocity, F (v) = Cava where the velocity v can be measured using a transducer. If the difference between the measured force and the theoretical force exceeds a threshold, the damper needs to be checked for damage. The tests showed that the theoretical force function (which is obtained at design velocities) is not valid at low velocities. Consequently, dampers to be health monitored on a bridge need to be tested before installation not only at design velocities, but also at the range of low velocities that the bridge is expected to experience under service conditions.
In Chapter 3 we describe a sequence of experiments that we reconducted outdoors. After the concept of using strain gauges to estimate the damper force was proven in the indoor experiments, a 450-kip damper was mounted on a surplus steel test frame outside the laboratory, located F v( ) Cava = iv very near the San Francisco Bay (approximately 0.3 mile) and exposed to wind coming in from the bay. The purpose of this series of experiments was to investigate possible changes in the force output of the damper over time under service loading conditions, where the frequency and amplitude of the displacement cycles are very small. The damper was cycled repeatedly for an average of ten hours a day for approximately 3.5 months. In addition, the damper was again instrumented with position transducers as well as strain gauges, to examine the effect of environmental conditions on these sensors. The data was transmitted to a remote location inside the laboratory.
Finally, in Chapter 4 we describe a portable data acquisition system with very flexible capabilities that can be used to collect and transmit data from a damper on a bridge to a remote location. The system which is based on commercially available components manufactured by Opto 22 uses a wired Ethernet network interface and/or a wireless LAN interface to transmit/receive data. The device is highly scalable to accommodate a number of channels with a variety of different sensor types. There is large flexibility on sampling rates, and the device can be equipped with a buffer memory to save data before an event is triggered. The components of the system are contained within a heavy-duty plastic, hermetically sealed enclosure box. Special sealed fittings can be installed, through which cables can enter the enclosure.
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