Road Science Tutorial
Static and Dynamic Tests
A truck load test was carried out at the bridge in September of 2009, which included both static and dynamic tests. The researchers observed:
The GFRP bars withstood normal handling at the precast yard and placement without any major problems. In addition, the light weight of the bars made them easy to carry and easier to place. The precast panels were also lighter and easier to transport to the bridge.
The panels were lifted at the precast yard and transported to the bridge using straps, employing a four-point lift using two different lifting configurations, one at the precast yard and one at the bridge. From strain measurements, it was found that the flexural design method used is very conservative; no cracks larger than hairline cracks were observed during lifting.
The relative deflections between the bridge deck and the west diaphragm were measured during the static tests. The magnitude of the relative deflections was found to be very small and shows that the bridge deck and the girders have good composite action.
“From the tests carried out for the precast concrete bridge deck panels reinforced with GFRP bars, it is clear that this is a viable construction method,” the authors report. The bridge was opened to traffic on October 2, 2009. Long-term monitoring of the bridge is continuing, and a second static and dynamic truck load test series is planned for the future.
Vermont Explores Jointless Bridges
In general, due to the problems inherent in bridge joints – such as joint deterioration due to superstructure segment movement, and their propensity to let chloride-laden meltwater drip onto bridge substructures, thus encouraging rebar corrosion – so-called “jointless” bridge superstructure designs have gained favor with state DOTs in recent years.
Jointless bridges – also called integral abutment bridges – have a superstructure that is cast integrally with the substructure, eliminating costly expansion joints and bearings, according to Chad Allen, geotechnical engineer for the Vermont Agency of Transportation (VTrans), Montpelier, Vt.
VTrans had used jointless bridge designs since the late 1970s, but in 1999, the agency formed an Integral Abutment Committee (IAC) to codify a measured, analytical and multidisciplinary approach to jointless bridge design and construction, Allen says.
VTrans has constructed several jointless bridges in the past decade, finding the structures more advantageous than conventional abutment bridges. Advantages of jointless bridges can include, according to Allen:
Reduced environmental impacts. Abutments farther from the stream banks minimize the effects on stream water, and a longer superstructure allows more room below for wildlife passages.
Lower construction costs. Placement of abutments farther away from the stream often eliminates the need for cofferdam construction.
A more rapid construction schedule. With integral abutment bridges, fewer piles need to be driven.
Elimination of costly future repairs, which can affect users. “Without the need for expansion joints and bearings,” Allen says, “costly, complicated and time-sensitive maintenance activities are eliminated.”
Nonetheless, VTrans engineers often have struggled with how best to approach the design of jointless bridges, because no quantitative data are available, and the American Association of State Highway and Transportation Officials (AASHTO) offers no specific guidelines for integral abutment design, he says.
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