Multiple sign supports are designed to operate correctly when either one or all of the supports within a 7-ft (2100-mm) radius are impacted. When only one support is impacted, the remaining signpost should support the sign and prevent it from penetrating the windshield. The desired impact performance of slip base and frangible coupler designs for large sign supports is depicted in Fig. 7.29. The base releases upon impact and the impacted support rotates up, allowing the vehicle to pass underneath the sign. This requires that the post be cut, at least 7 ft (2100 mm) above the ground, to provide a hinge for rotation.
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TABLE 7.14 Details of Concrete Foundation Design for Large Slip Bases
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As shown in Fig. 7.30 and discussed in Art. 7.5.3, hinges for large sign supports consist of three basic designs: (1) partially cut post with front friction plate, (2) completely cut post with front friction and rear hinge plate, and (3) completely cut post with weakened front plate and rear hinge plate. Proper performance of the hinge requires the correct selection of plate size, bolt size, and torque. Figure 7.51 and Table 7.16 present the design values for friction plates. Figure 7.52 and Table 7.17 present the design values for hinge plates. The bolt torque values for both friction and hinge plates are the same as presented as Table 7.15 for slip bases. Proper sized flat washers should be used under each nut and the head of each bolt.
The hinge systems shown in Fig. 7.30 are all unidirectional designs and should not be used in areas requiring bidirectional breakaway performance. Only the Transpo hinge system shown in Fig. 7.31 offers bidirectional breakaway capability.
