Steel U-Channels

The steel U-channel support is the most common type of single sign support used in the United States [19]. The steel U-channel is a unidirectional support available in different sizes and stiffnesses from a variety of manufacturers. The most popular steel U-channel sizes are 2, 2.5, 3, and 4 lb/ft (3, 3.7, 4.5, and 6.0 kg/m) (weight is prior to making the fastening holes). The channel is constructed with %-in (9.5-mm) holes on 1-in (25.4-mm) centers to eliminate the need for drilling to mount the sign panel. The posts are available with baked alkyd resin, with gloss enamel paint, or hot-dipped galvanized to inhibit corrosion. The stiffness of U-channel posts is a function of the material from which they are made, and the method by which they are shaped. The literature refers to billet steel or rail steel as the material from which U-channel is constructed. Rail steel is old railroad track—which has a high carbon content—that has been rerolled into the U-channel shape. The high carbon content results in a steel that is strong but relatively brittle. Billet steel is newly formulated steel. The most common grade of billet steel is A36, which is a relatively low-carbon “mild” steel, but billet steel can be manufactured with sufficient carbon to equal or exceed the strength characteristics of rail steel. For years the FHWA required “rerolled rail steel” instead of billet steel, since such a speci­fication helped ensure a high carbon content. High-carbon billet steel U-channel posts are available from manufacturers, but most state specifications still refer to “rail steel.”

Crash tests indicate that, under some test conditions, high-carbon steel U-channel sign posts perform differently from those made of steel having a lower carbon content. The reason for this performance difference is that high-carbon steel posts, because of their low fracture toughness, break under high-speed impacts [22]. Lower-carbon steel posts bend and shape themselves to the front of the vehicle, thereby forming a tether­ing hook. Billet steel posts that have carbon content similar to that found in rail steel posts match the performance of the rail steel posts when crash-tested.

Table 7.2 presents the maximum allowable sign area for one-piece rail steel U-channel installations. The table is for a maximum allowable pressure resulting from a 70-mi/h (113-km/h) wind velocity. State guidelines should be followed for the expected wind veloc­ities for different regions of the state and to obtain support sizes for other wind velocities.

Base-Bending Installation. One-piece base-bending U-channel post installations are usually obtained by driving the post directly into the ground with a sledge hammer, manual post driver, or air-operated post driver. Drive caps should be used to protect the top end of the post while it is being driven into the ground. U-channel posts should not be encased in concrete unless a breakaway design is used. Typical embedment depth is 3 ft (920 mm), and for ease in removing damaged posts, the driven depth should generally not exceed 3.5 ft (1070 mm). The patented RIB-BAK design of a U-channel has a ribbed back and flange. This design provides extra strength, a flush back-to-back sign-mounting surface, and a ridge for mounting channel locking clips. An alternative to the direct burial system is the V-loc anchor from Foresight Industries. The V-loc anchor is currently the only alternative method of anchoring an unspliced U-channel post that has been found acceptable by FHWA. This anchor system uses locking inserts to hold the U-channel securely into the V-shaped anchor piece. Upon impact, the post will bend at the ground line and may pull completely out of the V-loc anchor.

Breakaway Installations. The repair and performance of large U-channel posts can be eased by using a breakaway design. Breakaway design in U-channel installations is obtained by developing splices at ground level. The splice consists of attaching the signpost to an anchor piece that is embedded in the soil or a concrete foundation. Splice designs include the Eze-Erect by Franklin Steel, the Minute-Man by Marion

TRAFFIC

FIGURE 7.8 Breakaway devices for U-channel posts. (a) Eze-Erect splice joint. (b) Details of Eze-Erect splice. (c) Minute-Man coupler. Dimensions shown as mm; 100 mm = 4 in.

Steel, and various lap designs [23]. The intent of the splice designs is for the splice to fail upon impact. The commercially available splices are designed so that the signpost remains attached to the embedded anchor piece and passes beneath the impacting vehicle. This is accomplished by designing the splice device to partially fracture or to completely fracture a frangible coupling. To prevent vehicle snagging, the anchor piece should not extend more than 4 in (100 mm) above the ground. Two commercially avail­able breakaway designs are presented in Fig. 7.8. The Minute-Man consists of a frangible coupling with a backup plate, to hold the anchor and sign-support pieces together. The Minute-Man coupler makes the U-channel a multidirectional support system.

The generic splice (Fig. 7.9) does not require special hardware [24]. It is acceptable for use on 4-lb/ft (6-kg/m) U-channel, or less, installed in strong soil. The generic splice consists of an overlap of 6 in (150 mm) and uses two %s-in (8-mm) bolts spaced 4 in (100 mm) center to center. Spacers, %; in (8 mm) thick, are used to separate the U — channel signpost and the anchor piece. The spacer must be strong enough to transfer the load between the webs of the signpost and the anchor piece. The signpost should be mounted behind the stub.

The anchor piece of all breakaway devices should be the same size as the signpost and must not extend more than 4 in (100 mm) above the ground. A splice configuration, as in Fig. 7.10, does not provide protection for the anchor and increases the probability of snagging or of the sign’s entering the passenger compartment. Breakaway devices improve the safety characteristics of the post and generally reduce maintenance costs. They should always be used when the sign support is placed in concrete areas. If the sign can be impacted from different directions, then a breakaway device similar to that shown in Fig. 7.8 should be used. Splicing the signpost to the anchor piece with bolts, with or without the splice breakaway device of Fig. 7.8a, does not make the U-channel support a multidirectional sign support.

Mounting Concerns. The U-channel post is approved for use in strong soils when impacted from a frontal direction. Installing the support in weak soils or in locations where

BASE POST

FIGURE 7.9 Details of generic splice configuration. Dimensions shown as mm. Conversions: 8 mm = %5 in, 25 mm = 1 in, 100 mm = 4 in, 150 mm = 6 in, 950 mm = 37 in.

it can be impacted from more than one direction requires more than direct burial to make the U-channel perform as required. If the U-channel is installed in weak soil, an anchor plate, similar to that shown in Fig. 7.11, can be used to hold the sign in its proper position and to help ensure proper operation upon impact. In addition, the generic splice can allow the signpost to separate from the base. The possible consequences of this separation, and the trajectory of the sign assembly, should be considered prior to use of the generic splice.