Slip base designs for multiple sign supports are usually of horizontal design as shown in Figs. 7.23 and 7.26. Horizontal slip bases, when used in multiple-sign-support systems, operate satisfactorily when impacted from only one direction. Horizontal slip bases should not, therefore, be used for multiple sign supports where there is a high probability of impacts from more than one direction. In Fig. 7.26, the keeper plate prevents the bolts from “walking” out of the assembly as a result of wind vibration (Art. 7.4). The washers should separate the upper and lower slip plates by at least И in (3 mm), but not more than И in (6 mm), to prevent mating of the surfaces and possible binding due to friction. Proper size washers must also be used under the nut and bolt head to prevent the washers from deforming into the slots of the slip plates and binding the mechanism.
A typical concrete foundation detail is shown in Fig. 7.50, and specifications for the anchor piece of slip base designs are presented in Table 7.14. Notice that the foundation design includes eight reinforcing bars spaced around the anchor piece. This is a typical installation which is effective in maintaining the integrity of the foundation
a BARS V
DRILLED SHAFT (CONCRETE)
FIGURE 7.50 Horizontal base and concrete foundation detail.
under vibrations resulting from environmental loads. State specifications should be consulted to determine if local requirements deviate from details shown in Fig. 7.50.
Proper functioning of the slip base requires correct selection of bolt size and torque. Table 7.15 gives typical design specifications for large roadside sign slip bases and concrete foundations.
An omnidirectional triangular base, mounted in standard soil with three 2V2-in X 21/2-in (64-mm X 64-mm) perforated steel tube supports located within a 7-ft (2100-mm) radius, has been approved for use [49].
