The following guidelines should be followed for fastening sign panels to supports:

• Bolts smaller than %s in (8 mm) should not be used to fasten sign panels to the support. The bolts must be long enough to provide for bolt extension beyond the fastening nut.

• Carriage bolts, or hex bolts with washers between the hex head and sign face, should be used to reduce the possibility that the sign might separate from the support upon impact. Flat washers and lock washers should be used at the nut end of the bolt.

• The bolts must be tightened sufficiently to prevent loosening, but not so tight as to distort the sign face.

LOCK WASHER HEX NUT

RECTANQULAR HEAD BOLT

Regardless of the type of support that is being used, there are three general rules that must be followed: (1) the top of the sign should be 9 ft (2750 mm) above ground level to reduce the possibility of intrusion into the passenger compartment upon impact, (2) the retaining bolts must be snug but not so tight as to distort the sign face, and (3) the bolts must be of the proper size and length to prevent the sign blank from separating from the support.

Fastening to a U-Channel. Signs are normally mounted on the front face of the U-channel. This is the widest face of the U-channel. Signs can also be mounted on the narrow face, as in back-to-back sign installations, for example, but the decreased surface contact with the sign can result in damage to the sign face due to environ­mental loads. Side mounting on a U-channel requires channel brackets, and brackets for mounting street name signs to the top of the post are available. Typical sign blank installations for a U-channel are presented in Fig. 7.38.

Fastening to Wood Supports. Sign installation on wood supports requires a bolt completely through the post and fastened with a nut. Lag bolts are not recommended for fastening sign blanks to the posts. Fastening with lag bolts is unpredictable upon impact because of the size of the bore hole, possible postsplitting, the presence of knots, and variable characteristics of the wood.

Fastening to Square Tubes. Sign installations on square tubes can be accomplished by %s-in (8-mm) bolts and nuts or by the use of rivets. Figure 7.39 presents typical sign panel fastening details for shaped wooden and square-tube sign supports.

Fastening to Round Steel Shapes. Signs should not be fastened directly to standard steel pipe or to light standards. The contact area between the back of the sign and the support due to the round support shape is too small to withstand the wind and other environmental loads. A number of different fastening methods can be used, including B-clamps, U-bolts, and stainless steel band clamps. Figure 7.40 presents the configura­tion of the B and stainless steel band clamps. The dimensions shown for the B-clamp are typical and vary slightly by manufacturer. The B-clamp is also available in other sizes than shown in Fig. 7.40.

FLAT WASHER

ND LOCK WASHER

50 MIN

FIGURE 7.38 Typical sign blank fastening to U-channel. Dimensions shown as mm: 50 mm = 1 in.

U-bolts can also be used to fasten signs to round supports. The U-bolt is attached to the sign by the use of Z-bar aluminum channel or pieces of U-channel. The U-bolt is purchased with an anchor chair to grip the post. Fastening details for U-bolts with aluminum Z-bar are presented in Fig. 7.41 and with aluminum channel or U-channel in Fig. 7.42.

Fastening to Structural Steel Shapes. Fastening signs to S – or W-shaped beam posts, often used with slip base designs, should be accomplished by using a stiffener. Clamps are used to fasten the stiffeners to the support, eliminating the need to drill into the support itself. Fastening details using aluminum Z-bar as the stiffener are presented in

Fig. 7.43. Figures 7.44 and 7.45 present fastening details for commercially available

aluminum stiffeners specifically designed for mounting signs to beam supports.

Slip base designs for single sign supports provide the opportunity to use stronger sign supports than would be possible without the slip base design. The purpose of the slip base is to provide a separation plane between the sign support and the anchor system. The two pieces are fastened together with bolts that must be properly tightened, or

torqued. If the bolts are not torqued enough, they will be loosened by vibration from environmental loads, causing the sign assembly to separate. If the bolts are torqued too much, the friction between the base of the signpost and the anchor piece will be too large to permit proper separation upon impact. A 20 to 28 gauge metal “keeper plate” should be inserted between the faces of the top and bottom slip bases to prevent the bolts from migrating out of the assembly (Art. 7.4).

There are three basic types of slip base designs for single sign supports. The hori­zontal slip base design (Fig. 7.25) will operate correctly when impacted from either the front or the back. Horizontal slip base designs do not provide the lift capability available from inclined or multidirectional designs. Horizontal slip bases when used as single sign supports therefore do not function as well upon impact as the other slip base designs.

The inclined slip base (Fig. 7.18) is the recommended type of slip base for single sign supports when impact can be expected from only one direction. Its performance upon impact is designed to cause the upper sign support and sign panel to raise up, thus allowing the vehicle to pass completely under the support assembly. The anchor piece of the inclined slip base must be installed so that approaching vehicles encounter the lower edge before the high edge.

The multidirectional slip base is fastened together with three bolts and has a lift cone fastened to the bottom plate. The sign support is tubular with a maximum size of 5 in (127 mm) diameter.

All of the slip base designs require a firm foundation for proper operation. Concrete foundations should be used for all slip bases, since direct burial may result

in base movement and improper release of the slip base. To prevent snagging of the vehicle undercarriage, no part of the anchor piece and its attached slip base may extend more than 4 in (100 mm) above ground level. Horizontal and inclined slip base designs can be constructed with wide-flange, standard-shape, and round sign supports. The concrete footing sizes for wide-flange and standard-shape signposts should be constructed to state specifications. The concrete foundation and anchor stub sizes listed in Table 7.11 are appropriate for round signposts with slip base designs.

Torque Requirements. Specifications for bolt tightness must be followed so that the sign assembly (1) remains intact under normal environmental loadings and (2) separates correctly upon impact. The specifications can be given in a number of ways, such as residual tension, clamping force, or torque. Torque refers to the amount of force used in tightening the nut to the bolt. The result of the nut-to-bolt tightening places the bolt in tension and exerts the clamping force. Measuring the torque is the most convenient method of obtaining a specified tightness. Providing a specified torque, however, does not guarantee a certain clamping force. Irregularities in the threads, heavy coating deposits on galvanized parts, or irregularities on the mating surfaces of the nut and plate faces result in friction forces. These friction forces cause an increase in torque to move the nut without a resulting increase in clamping pressure. To help ensure that the torque speci­fication provides the required clamping force, the nut should be tested for thread irregu­larities by being threaded on the bolt by hand. Also, the proper size flat washers should be used beneath the nut and the head of the bolt. The proper size bolts and recommended torque requirements are provided in Tables 7.12 and 7.13. The tables show requirements for round sign supports by diameter and for other support shapes by unit weight or mass. In U. S. Customary units, S6 X 12.5 designates a standard shape with a depth of 6 in and a unit weight of 12.5 lb/ft; in SI units, S150 X 18.6 member is a standard shape with a depth of 150 mm and a mass of 18.6 kg/m.

Slip Base Orientation. The proper operation of slip bases is also dependent upon proper assembly and correct orientation to the expected direction of impact. The parts,

and orientation to the primary direction of travel, for the three types of slip bases are presented in Figs. 7.19, 7.20, and 7.26. The horizontal slip base is generally not used for single sign supports. Where impact from more than one direction is expected, the multidirectional slip base provides better performance because of its design for lift upon impact.

Guidelines for Slip Base Installation. The following guidelines should be followed for slip base installation:

• Slip base installations require a firm foundation to operate properly upon impact. Slip base installations should always include a concrete base and never be directly buried or drilled and backfilled.

• Use proper size bolts for the slip base fastening. Bolts that are too small may not be able to be sufficiently tightened and may fail under environmental loads. Bolts that are too large may become stuck in the release slots upon impact and prevent proper separation.

• Use proper size and strength washers. The washers beneath the nut and head sur­faces should be sufficiently strong to withstand the torque requirements without deforming into the release slots of the base.

• Three washers should be used on each bolt—one each beneath the nut and bolt head, and one between the upper and lower faces of the slip base. The purpose of the washer between the two slip faces is to prevent binding upon impact. All galva­nizing runs or beads should be removed from both the upper and lower faces in the washer areas.

• The nut should be run by hand down the bolt to find thread irregularities that will provide inaccurate torque readings.

• Torque each base bolt to the required specifications.

• Remember that the top portion of the slip base must be attached to the anchor piece. Therefore it is recommended that the anchor piece be installed so that its highest portion extends no more than 3.5 in (90 mm) above the ground. This will help ensure that the addition of the top plate will not result in a height that can snag the undercarriage of an impacting vehicle. The installation shown in Fig. 7.37 is improper and can snag the undercarriage of an impacting vehicle.

• The bolts must be sufficiently long that they can extend approximately % in (10 mm) beyond the nut after complete assembly.

• Do not install an inclined slip base where impact from more than one direction is expected.