SLIP BASE DESIGNS

Slip base designs for small sign supports consist of two components: (1) the anchor assembly up to the bottom of the slip base, and (2) the sign support, containing the top of the slip base on the lower end and the sign panel on the upper end. Small sign slip bases are categorized as unidirectional or multidirectional.

Slip base designs allow the use of stronger sign supports than can safely be achieved by base-bending or fracture designs. The anchor piece of slip base designs is fixed into a foundation and should remain immovable during an impact. The sign sup­port is connected to the anchor piece with bolts through a plate, which are attached to a similar plate on the anchor piece. The holes in the plates are slotted. When a vehicle impacts the sign support, the top plate, which is attached to the sign support, slides along the bottom plate until the bolts slide free of the slots. Inclined slip base designs, or designs with raised center cones, cause the sign support to move upward to allow the impacting vehicle to pass under the sign without being hit on the windshield by the sign during high-speed impact.

When slip base designs were first used, problems were encountered with assemblies that came apart without an impact. This was due to the wind and ice loads vibrating the assembly and causing the bolts to “walk” out of the slots, as in Fig. 7.17. This problem was solved by using a thin (0.04 to 0.02 in or 1.0 to 0.5 mm) keeper plate to ensure that the bolts remain properly located in the slots. During an impact, the bolts tear through the thin keeper plate as they slide free of the slots.