International Telemetering Conference Proceedings / October 17-20, 1994 / Town & Country Hotel and Conference Center, San Diego, California / The XL-L Two-axis Pedestal is a novel adaptation of an existing design using a cross elevation over elevation axis configuration. This design affords full hemispherical coverage without gimbal lock (keyholes). In addition, the system provides continuous coverage without the necessity of rotary joints and sliprings. The design is ideally suited for shipboard systems, but is equally advantageous for any tracking mission where the target can approach a zenith gimbal lock or keyhole.

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Overheight vehicle impact to bridge decks is a major problem in the transportation networks in the United States. An important factor that causes this problem is inadequate vertical clearance of bridges. Using steel pedestals to elevate bridge decks is an efficient and cost-effective solution for this problem. So far, steel pedestals have been used in the low seismic regions of the United States and therefore, their design has been based on providing enough strength to carry vertical loads and the lateral behavior of bridges elevated with pedestals have not been a major concern. But even in low seismic zones the seismic hazard should not be completely ignored. Also there might be some bridges in medium or high seismic regions that need to be elevated because of the lack of enough vertical clearance and using steel pedestals can be considered as an option for elevating those bridges. To address the mentioned needs, this dissertation proposes a framework to determine the structural reliability of bridges elevated with steel pedestals by developing probabilistic capacity and demand models for the slab-on-girder bridges subjected to lateral loads. This study first compares the behavior of previously tested pedestals with the behavior of elastomeric bearings in low seismic regions using statistical tests. Then, to provide a general framework, which can be applied to all bridges that are elevated with steel pedestals, this dissertation develops probabilistic capacity and demand models for steel pedestals considering all the aleatory and epistemic uncertainties of the problem. Using the developed probabilistic models along with the available models for other components of bridges, seismic fragility curves for elevated bridges are obtained and used to determine the structural reliability. Finally, this study uses the developed framework in a decision analysis that helps the engineering community and decision makers to check if the installation of steel pedestals on a specific bridge has financial justification or not. Results show that for a typical two-span slab-on-girder bridge, the use of steel pedestals has financial justification only in low seismic regions and if the societal benefits of elevating the bridge can at least cover the installation cost of pedestals.

Many bridges are damaged by collisions from over-height vehicles resulting in significant impact to the transportation network. To reduce the likelihood of impact from over-height vehicles, steel pedestals have been used as a cost-effective, efficient means to increase bridge clearance heights. However, these steel pedestals installed on more than 50 bridges in Georgia have been designed with no consideration of seismic loads and may behave in a similar fashion to high-type steel bearings. Past earthquakes have revealed the susceptibility of high-type bearings to damage, resulting in the collapse of several bridges. Although Georgia is located in a low-to-moderate region of seismicity, earthquake design loads for steel pedestals should not be ignored. In this study, the potential vulnerabilities of steel pedestals having limited strength and deformation capacity and lack of adequate connection details for anchor bolts is assessed experimentally and analytically. Full-scale reversed cyclic quasi-static experimental tests are conducted on a 40' bridge specimen rehabilitated with 19" and 33" steel pedestals to determine the modes of deformation and mechanisms that can lead to modes of failure. The inelastic force-deformation hysteretic behavior of the steel pedestals obtained from experimental test results is used to calibrate an analytical bridge model developed in OpenSees. The analytical bridge model is idealized based on a multi-span continuous bridge in Georgia that has been rehabilitated with steel pedestals. The analytical bridge model is subjected to a suite of ground motions to evaluate the performance of the steel pedestals and the overall bridge system. Recommendations are made to the Georgia Department of Transportation (GDOT) for the design and construction of steel pedestals. The results of this research are useful for Georgia and other states in low-to-moderate seismic zones considering the use of steel pedestals to elevate bridges and therefore reduce the likelihood of over-height vehicle collisions.

The Georgia Department of Transportation has been installing steel pedestals on bridges, ranging in height up to 33½” (0.85m) to increase the vertical clearance of many multi-span simply-supported and multi-span continuous bridges in Georgia. But there is a concern about the performance of these steel pedestals as they are designed without seismic consideration and may perform poorly compared to high-type steel “rocker” bearings, which were found to be unstable supports in previous earthquakes. This research models a candidate bridge using experimental data that captures the force-displacement hysteretic behavior of the steel pedestals. The results show how these steel pedestals behave when subjected to a range of ground motions. Nonlinear time history analysis is conducted using SAP 2000 software on a three-dimensional model of the candidate bridge. In addition, parametric studies of various critical parameters that can affect the seismic performance of the bridge are investigated, such as 1) varying the mass of the structure, 2) varying the stiffness of the deck joint, 3) varying column heights, and 4) seismic retrofitting using cable restrainers. The results show that these pedestals should not be used in regions of high seismicity, and in regions of low seismicity, it is likely that they need to be retrofitted. They can, although, be used safely in regions of low seismicity. In addition, it was shown that the mass of a superstructure and height of the columns significantly affect the behavior of these steel pedestals, and should be given a careful consideration before usage. It was also shown that the stiffness of the expansion joints does not significantly affect the displacement of the steel pedestals and the forces transmitted to them. However, if the expansion joints are too stiff compared to the adjacent bridge components, then the forces transferred during pounding of superstructure is increased significantly.