Unidirectional slip bases for small sign supports consist of inclined slip bases, as shown in Fig. 7.18. The upper support piece is made from rolled-steel shapes, standard pipe, or structural tube. The base of the support assembly is inserted into a concrete footing to prevent movement of the anchor assembly.
The upward thrust obtained from the inclined slip base design is important to the proper action of a single-support sign system. The upward thrust causes the sign panel and support to rise and rotate when vehicle impact separates the mechanism. The sign panel and support stay together as a unit, which passes up and over the vehicle and lands behind it. This action is obtained only when the support is impacted from one direction. An impact from the opposite direction actually pulls the sign support downward, causing the support and sign panel to rotate toward the vehicle. Inclined slip bases should not be used where impact from more than one direction is expected. Horizontal slip bases will separate when impacted from the front or the rear but will not provide the uplift capability obtained from inclined-base designs. A typical design for an inclined slip base is provided as Fig. 7.19.
Horizontal slip bases, discussed in Art. 7.5.2, are not recommended for single sign supports. When impact can be expected from more than one direction, a multidirectional slip base design should be used.
