Inclined slip base designs, commonly used for single sign supports, cause the sign to rise up upon impact and allow the vehicle to pass beneath the sign. In a multiple-sign- support system, each support is fastened to the other supports by the sign panel and any existing sign panel bracing. When an inclined slip base is used on multiple-support assemblies and only one support is struck, the sign panel stops the impacted support from moving upward. As a result, the slip base can become locked, or the sign panel torn from the other supports, causing intrusion of the panel or support into the vehicle. Inclined slip bases should be used only for multiple-support assemblies when all supports are within 6 ft (1800 mm) of each other. The horizontal slip base (Fig. 7.25) and the frangible coupler (Fig. 7.21) are the most frequently used designs for multiple-support systems. The horizontal slip base design, details of which are shown in Fig. 7.26, operates through separation of the top plate from the anchor plate.
Frangible coupling designs, presented in Fig. 7.27, are designed to effect separation from the anchor plate by fracturing the couplings. Figure 7.27a presents a load concentration design in which the small cross-sectional area, at the necked-down portion of the coupling, breaks at impact. Figure 7.27b presents a frangible aluminum coupling, by Transpo Industries, designed to break upon impact. The couplings are available in
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FIGURE 7.25 Installation of horizontal slip base. |
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FIGURE 7.28 Base with load concentration couplers. |
different sizes, designs, and resistance to fracture. Figure 7.28 presents a frangible coupler application for a large sign support. Notice the low profile of this design. The only portions of the sign assembly above ground level are the frangible couplings, so that the possibility of snagging the vehicle undercarriage is practically eliminated.
Horizontal slip base and coupler designs are intended to safely operate by allowing the vehicle to pass under the sign and support assembly upon impact, as presented in Fig. 7.29. This is accomplished by providing a hinge at least 7 ft (2100 mm) from the bottom anchor plate to allow the support to swing away.
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FIGURE 7.29 Illustration of hinge action for large multiple-support sign. (a) Vehicle passes under. (b) Hinge activates. (c) Vehicle impact. |
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FIGURE 7.30 Common hinge designs used for large multiple supports. (a) Saw-cut support with front plate. (b) Saw-cut support with front plate and rear hinge plate. (c) Saw-cut support with weakened front plate and rear hinge plate. |
