Acceptable single-sign-support performance can be achieved with the use of frangible couplings and load concentration couplers (Fig. 7.21). These couplings are either fabricated from die cast aluminum or extruded from an alloy. The couplers are used as inserts that bolt the support post plate to the anchor piece plate. They present a weak point on the sign-support assembly that fractures upon impact. The majority of applications for frangible couplings are for multiple sign supports. These couplings are discussed more fully in Arts. 7.5.2 and 7.8.2.
7.4.1 Considerations in Design of Slip Bases
Failure of slip base designs to release properly can be due to the bolt torque, the gauge or thickness of the keeper plates, or the weight of the support. The following should be adhered to in the design of slip base supports:
• Horizontal and inclined slip bases can be constructed with wide-flange, standard — shape, and round signposts. Multidirectional designs are usually constructed with round signposts to enable the multidirectional rising action of the lift cone.
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TABLE 7.7 Round Sign-Support Sizes for Slip Base Designs Based on Sign Area
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• The post should not weigh more than 45 lb/ft (67 kg/m), and the total weight of the support post, hardware, and sign panel should not be more than 600 lb (270 kg).
• The bolts clamping the top and bottom portions of the slip base together should not be tightened more than the specifications. Overtorquing creates high friction between the slip base elements and may prevent the post from releasing properly. The clamping force must be controlled by installing the bolts with a torque wrench, using torque-limiting nuts, or using designs that are not dependent upon specific torque requirements.
• Washers used with the clamping bolts must be of sufficient strength to prevent the washers from deforming into the plate slots when the bolts are tightened to specification.
• The stub height must be no more than 4 in (100 mm) above ground level at the highest point of the slip plate assembly.
• No bolts from the anchor piece should project into the upper support assembly.
• The choice of the proper sign-support type for slip base designs is dependent upon the wind load, sign panel size, and the criterion that the weight of the sign support and sign not exceed 600 lb (270 kg). As a general rule, the maximum sign area presented in Table 7.7 can be used in selecting the appropriate size of wind sign support. Determining the maximum sign size for areas with high wind loads, or for the selection of post sizes other than round shapes, should be performed with reference to state requirements.
