Wooden support posts are available in shaped sizes, as engineered products, and as timber posts. The shaped sizes are described by their nominal dimensions, such as 4 in X 4 in (100 mm X 100 mm). This is their size prior to the surfacing required to provide smooth and straight posts. Their actual size is typically less than the nominal size. A 4-in X 4-in (100-mm X 100-mm) post will therefore have an actual size of 3.5 in X 3.5 in (90 mm X 90 mm). The engineered products are made from laminated or pressure-glued wood and nonwood recycled products. Timber posts are round in shape.

All wooden posts are of breakaway design, with the intended fracture of the post near the base and less than 4 in (100 mm) above the ground. The post features that influence fracture include the size of the post, effective cross-sectional area, embedment depth, type of soil, and the species of wood. The majority of wood post tests have been conducted using grade 2 southern yellow pine posts.

Shaped Wood Posts. The most common size wood post used for single sign installations is the 3.5-in X 3.5-in (90-mm X 90-mm) support. This support should be buried directly in strong soil to a depth of at least 36 in (920 mm). The cross-sectional area of this post is sufficiently small that drilled holes are not needed to provide a weakened section.

A 3.5-in X 6-in (90-mm X 140-mm) post installed in strong soil will provide acceptable performance upon impact without reducing the cross-section. Tests have shown, however, that the 3.5-in X 6-in (90-mm X 140-mm), when installed in loose or

sandy soil, is unacceptable when impacted by a small vehicle. Single wood posts of that size installed in weak soil should be modified with 1.5-in-diameter (40-mm) holes to be crashworthy. The holes should be centered at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Table 7.3 provides the modifications required for acceptable performance of various shaped wood post sizes. The holes in each case are drilled at 4 and 18 in (100 and 460 mm) above the ground line and perpendicular to the roadway centerline.

Typical details for installation of shaped wood posts by direct burial and concrete methods are presented in Fig. 7.12. The 6-in X 6-in (140-mm X 140-mm) post should be set in unreinforced concrete to help ensure that the post fractures upon impact. To make it easier to remove a broken stub, a post can be wrapped with 0.5-in-thick (13-mm) Styrofoam prior to filling with concrete.

90 50 0ІА HOLES

2750

CROSS SECTION 90×90 OR 90X140

STYROFOAM

910

Some states have used larger shaped wooden posts, such as 6 in X 8 in (140 mm X 215 mm), with appropriately sized holes to reduce the cross-section area. These holes provide a weak section that appears acceptable, but the increased mass of these posts and lack of testing result in unpredictable impact performance. Shaped wooden posts larger than those that have been crash-tested should not be used. If larger posts are required, then a multiple-post configuration, slip base design, or other alternatives should be used.

Engineered Wood Posts. A number of relatively new products have been developed for use as sign supports. These include engineered wood product posts made from recycled plastics and wood chips, and laminated veneer lumber posts. The Microllam laminated posts in 8 in X 8 in (200 mm X 200 mm) and in 15 in X 8 in (380 mm X 200 mm) have been accepted for use. These posts, manufactured by the Trus Joist MacMillan Corporation, have a wall thickness of 1.25 in (32 mm) and mitered 45° corners. The post is placed in predrilled holes and backfilled. The posts require four 1-in-diameter (25-mm) holes drilled on the two sides parallel to the direction of travel. Two of the holes are at 3 in (76 mm), and the other two holes are at 18 in (457 mm) above ground height. A saw cut parallel to the ground that connects each set of holes is required.

Timber Poles. The majority of wooden sign-support systems consist of square or rectangular shapes. However, round timber poles, up to 7.5 in (190 mm) in diameter of southern pine, grade 2, have been accepted for use by the FHWA [25, 26]. The acceptable sizes and required holes to provide acceptable breakaway performance are presented in Table 7.4.

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.