Each year numerous injuries and fatalities occur as a result of vehicles hitting light poles adjacent to U.S. highways. Until recently the widely used transformer base has been considered "breakaway". The 1975 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals has tightened design standards by calling for a improvement in the performance of the breakaway feature. Satisfactory dynamic performance is indicated when the maximum change in momentum for a standard 1020-kg (2250-lb) vehicle, or its equivalent--striking a breakaway support at speeds from 32 to 97 km/h (20 to 60 mph)--does not exceed 4895 nt,s (1100 lb's). A "preferable" maximum change of momentum of 3337 nt's (750 lb's) is also stated in the above specifications as a goal for the future. The Pole-Safe breakaway support coupling (longitudinally grooved) is a frangible coupling designed for use on light poles so that they will break away under impact within these criteria. The coupling (Figure 1) is made from die-cast aluminum, roughly cylindrical in shape with a 2.54-cm (1-in) 8UNC threaded hole at one end and a 2.54-cm (1-in) 8UNC 304 stainless steel stud protruding from the other end. The coupling thus provides a breakaway spacer between an existing light pole and its foundation. It fits over the anchor bolt, and the stud protrudes through the pole base plate. It eliminates fatigue problems in tension and bending loads that would be present with circumferentially notched couplings, shear pins, notched bolts, and so forth. Longitudinal notches provide high stress concentration for rapid fracture at relatively low impact force levels. Ratio of service-load capacity to failure load is higher than existing designs due to unique geometry. Ratios up to 6:1 are possible. Highest possible control of fracture mode ensures repeat impact performance for any design impact load. /Author/

Media Info

  • Media Type: Print
  • Features: Figures; Photos;
  • Pagination: pp 26-28
  • Monograph Title: Roadside safety appurtenances
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00196617
  • Record Type: Publication
  • ISBN: 0309028248
  • Report/Paper Numbers: HS-026 748
  • Files: TRIS, TRB
  • Created Date: Aug 28 1979 12:00AM