7.28.1 Conduit on Bridges and Median Barriers
The current design standards for bridge rail and median barrier rail are very similar. Consequently, the method for installing conduit for one is applicable to the other. In the past, the most common method for installing bridge conduit was to attach it to the underside of the bridge deck. This required both the installer and the maintenance crews to work outside the bridge rail and underneath the slab by constructing special scaffolding or using expensive trucks with articulating booms. A method has recently been developed that places the conduit and junction boxes within the bridge rail. The conduit may be galvanized rigid, PVC, or high-density polyethylene (HDPE). HDPE is available on large reels in lengths up to 60 ft (1500 m). The junction box is a curb box (Fig. 7.76) and may be a standard galvanized cast iron, or a box with reinforced fiberglass sides and a polymer concrete ring and cover as manufactured by CDR [17]. The CDR box has the advantage of not requiring the cover to be grounded for safety reasons. Details of this wiring method with junction box and luminaire foundation are shown in Fig. 7.77.
This same method is also used for installing conduits for median-mounted lighting systems. In some areas, whenever a median of a divided highway is closed and a median barrier installed, an empty HDPE conduit is installed inside the barrier for future use. The advantage of using the HDPE in this case is that there are no joints required other than at bridge ends. No special skill is required to install the HDPE, so the concrete barrier construction is not delayed. When the conduit is for a future lighting system, no foundations are installed at the time the rail is constructed. A section of the barrier can be removed at a later time when the lighting system is installed to allow the conduit ends to be connected to the junction box placed at each luminaire location. Details of this foundation are shown in Fig. 7.78. Luminaire poles can be
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FIGURE 7.76 Curb box and cover section used for electrical junctions within bridge rail. Conversions: 6 mm = 0.25 in, 13 mm = 0.50 in, 20 mm = 0.75 in, 180 mm = 7 in, 225 mm = 9 in, 250 mm = 10 in, 360 mm = 14 in. |
easily installed on existing barriers without the encased conduit by using a modification that places the conduit in the roadway shoulder below grade and connected to the barrier rail-mounted junction box.
