A UTILITY'S DEVELOPMENT AND USE OF PRESTRESSED CONCRETE POLES

Abstract

The Virginia Electric and Power Company (Vepco) first looked at prestressed concrete in 1963 during the construction of the Chesapeake Bay BridgeTunnel. The trestle part of this crossing is made of 54-in. O.D. post-tensioned concrete cylinder piles supporting prestressed concrete roadway sections. Subsequently, Vepco had a 5.1 mile, 115 kv transmission line crossing of Albemarle Sound which was designed and built with rectangular base single circuit steel towers supported by two 54-in. O.D. post-tensioned concrete cylinder piles topped with a precast concrete H-cap. In 1965, upon examination of possible alternatives, a three mile transmission line crossing of Currituck Sound was designed on prestressed concrete poles. Several cross sections for the prestressed concrete poles were considered during the preliminary design work: tapered rectangular, tapered I, and tapered cross. Because the I-section offered the combined advantages of much lower weight and savings in erection costs, it was selected even though the forming cost was greater than for the other shapes. The pole was designed, based on weather bureau records, to withstand an ultimate load of a 165 MPH wind. The ruling span was established at 1500 ft. The crossarms, which are also concrete, were designed for a dead load of conductor weight plus 1⁄2-in, radial ice. The pole, designed by Vepco, was built and load tested by Bayshore Concrete Products Corporation of Cape Charles, Virginia. Twelve of these poles were erected across Currituck Sound, alongside the new Wright Memorial Bridge concrete structure built at the same time to serve North Carolina's Nags Head and Kitty Hawk coastal areas. Next, a tapered square, statically cast, prestressed concrete pole was selected for use. This particular shape required a 140-ft. form that had a constant taper and was fabricated in 10-ft. sections to permit the manufacture of poles of various Iengths and cross sections. This form was not a self-stressing form; it had to be used in a regular stressing bed.