BASES

Breakaway luminaire poles are designed to yield at their base attachment to the foun­dation. There are numerous types of bases currently in service. Some of these are designed for breakaway operation and others are not designed to yield. The nonyielding types have application where vehicle speeds are low and the danger from a falling pole is greater than the hazard of hitting the rigidly mounted pole. A description of the most common base types follows. Not all of these bases are crashworthy.

Direct Burial Base. The direct burial base allows the pole to be directly embedded in the soil. It is the most economical, since it eliminates the need for a foundation. It is the common type of base for wood and is used frequently with concrete and fiberglass reinforced plastic (FRP) poles. FRP poles are the only direct burial poles currently approved for breakaway use. The other types are normally limited to low-speed facili­ties or should be located out of the recoverable area.

Flange Base. Most steel and aluminum poles are fitted with a plate or flange at the base of the pole. With steel poles, this usually involves welding a steel plate to the bottom of the pole. With aluminum poles, a cast-aluminum shoe base is usually fitted to the bottom of the pole. The use of a flange base implies that the flange is to be fas­tened directly to the anchor bolts embedded in the foundation or to some type of breakaway device. When a flange is in direct contact with the concrete, some method needs to be employed that will allow water to flow out and not be trapped in the base of the pole. Trapped water can cause premature failure of the pole due to corrosion on the inside. Flange base designs without breakaway features are not crashworthy and should be restricted to where the hazard from a falling pole is greater than the hazard of impacting the rigidly mounted pole. A flange base is illustrated in Fig. 7.73.

Cast-Aluminum Transformer Base (T-base). T-bases may be steel or cast alu­minum and were originally devised to house the transformer. The T-base (Fig. 7.70) proved unacceptable for storage of the ballast because of moisture and insect damage to the electrical components. However, the cast base proved to have safety advantages, since it yielded and broke apart upon impact. The ballast is rarely stored in the base anymore, but the T-base is still frequently installed because it serves as an electrical junction box and because of its breakaway characteristics.

Frangible Couplings. A number of manufacturers have developed cast and extruded aluminum frangible couplings. The typical coupling (Fig. 7.74) is a short connector attached to the foundation on the bottom and the flange of the pole on the top. Upon impact, the coupling fractures, separating the pole from the foundation. The proper performance of frangible couplings requires proper matching of coupling and pole. Stiffer poles work best with frangible couplings, since the stiffness of the pole results in impact forces remaining in the direction of impact (shear). Flexible poles, such as aluminum poles, bend upon impact, resulting in translation of some of the impact force to vertical forces. This places the couplings in compression and tension, forces the couplings are specifically designed to resist. Frangible couplings often need to be enclosed in skirts to keep dirt and water from entering the conduit and to keep rodents from eating the wire insulation.

Slip Base. Luminaire support slip bases are designed to resist wind and vibration loads while safely releasing upon impact from any direction. A typical base consists of two triangular plates, one welded to the support pole and the other welded to the foun­dation attachment. The plates accommodate three anchor bolts and are slotted to allow release upon impact. If installed correctly, the foundation part of the slip base will be

• Any bolts used to anchor the foundation piece to the foundation must be lower than the plane of the slip base.

• The upper surface of the foundation piece must be no more than 4 in (100 mm) above the surface of the surrounding terrain.

• A keeper plate, 0.05 to 0.03 in (1.3 to 0.76 mm), must be placed between the sur­faces of the slip base to prevent the device from slipping apart in response to wind loads.

• Washers of sufficient strength to prevent deformations into the vee slots must be used between the plates and on the top and bottom.

• The bolts must be torqued to the specified level.

A four-bolt slip base (Fig. 7.75) is also available. Developed by Valmont Industries, it provides added structural resistance to environmental loads and is used extensively by some western states.

Shear Base. The design concept for the shear base is to load the rivets or welds that secure the base to a foundation plate. When struck by a vehicle, the rivets or welds are sequentially sheared and the support breaks away. Typical designs for shear bases are thin-walled stainless steel bases and a family of cast-aluminum bases (not T-bases).

Other. There are other breakaway methods that relate to specific materials used for the pole. These include a fiberglass-reinforced plastic pole with an anchor base that will break above a cast-aluminum base, and several schemes approved for use with aluminum poles.