Sign Components

Sign assemblies consist of four components:

• The sign panel on which the message is displayed

• The signpost

• Mounting hardware and fasteners

• The base for the post

Sign Panels. The majority of sign panels in use today are made from sheet alu­minum stock [19]. The thickness of the stock varies depending upon the sign size but is generally not less than 0.16 in (4.0 mm). Plywood is occasionally used by some agencies as the blank material for the reflective sheeting face in areas of frequent van­dalism due to gunshots. Wooden sign blanks deform less from gunshots, are easier to repair, and are not as attractive a target as aluminum sign blanks. Plywood, however, does not weather as well as aluminum and, if the edges are not sealed correctly, has a relatively short life. More important, the plywood is heavier than aluminum, thus requiring a stronger post system and increasing the probability of intrusion into the passenger compartment upon impact. Composites such as fiberglass have also been used as sign blank materials with limited success. Early problems with composites included separation of the material and problems with the reflective sheeting adhering to the sign blank. A relatively new sign blank manufactured from recycled thermo­plastic soft drink bottles is available from Composite Technologies [20]. These sign panels are molded with sealed edges, will not bend like aluminum, offer excellent bonding to adhesive sheeting, are weather and corrosion resistant, and are cost-effective compared with current aluminum pricing.

Sign Posts. The sign support must be strong enough to resist the wind and other loads yet safely give way when struck by a vehicle [21]. The loading conditions for which the support must be designed are illustrated in Fig. 7.4. The required size of a signpost is dependent upon the surface area of the sign it is supporting and the prevailing environ­mental conditions. Each state has a series of tables and/or graphs that specify support post requirements based on prevailing wind and ice loads, sign size, and the height of the sign from the ground. These tables provide the information on the support size, embedment depth, and the support type that is required to withstand the environmental loads. The ability of the sign support to operate safely upon impact is dependent on the sign location, features of the surrounding terrain, and the intended method by which the support will give way. All give-way sign support systems operate by (1) complete or partial fracture of the support post, (2) failure of intentionally weakened (frangible) bolts or splices, and

(3) mechanical release methods. These designs allow the support system to either bend at the base (base-bending) or break away into one or more pieces. Sign support systems that do not give way upon impact are fixed-base supports which must be shielded with an appropriate barrier when placed within the traversable area.

Base-Bending Support Types. A base-bending support (Fig. 7.5) is designed to bend over, lie down, and pass beneath the impacting vehicle. How effectively it performs is dependent upon the type of support and the velocity of impact. These supports tend to perform better at lower-speed impacts, which provide sufficient time for them to func­tion as designed. Impacts at high speeds will frequently result in the support’s partially fracturing or being pulled out of the ground. The performance of base-bending supports is more difficult to predict than that of other support types. Their behavior upon impact is influenced by variations in the depth of embedment, the soil resistance, stiffness of the sign support, mounting height of the sign, and the method of effecting the yielding action. One-piece assemblies are typically either driven directly into the ground or set in drilled holes and backfilled. Instead of a one-piece support, the yielding action is often effected by constructing an anchor system and connecting the sign support to the

anchor assembly. The connection can be by direct splicing or the use of commercially available couplers that are designed to bend (fracturing) or break partially (frangible). The advantage of the two-piece assembly is that the anchor system is often not dam­aged during impact, thereby reducing replacement time. Base-bending supports provide a relatively inexpensive support system that reduces the probability that the sign assem­bly will become a deadly projectile to other traffic, pedestrians, and bicyclists.

Breakaway Support Types. Breakaway sign-support systems (Fig. 7.6) are designed to have the system separate, at or near ground level, into more than one piece upon impact. This is accomplished by complete fracture of the support or by the separation of weak­ened splice parts. Wood is the most common material used for complete fracture designs. Weakened splice parts can be field-assembled splices, commercially available splices, or frangible couplings. Frangible couplings are necked down to provide a reduced cross­section. Frangible couplings can be used for single sign supports but are generally used for

large, multiple-support systems. Breakaway support systems typically work best for high­speed impacts where the vehicle has sufficient energy to both break the support and propel it away or over the vehicle.

Mechanical Release Support Types. Mechanical support types include slip base designs (Fig. 7.7), which have flat plates welded to both the sign support and the anchor piece. Upon impact, the plates slide against each other allowing the connecting bolts to release.