ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/395356
Pretensioning Strand Profiles: Harped or Straight by Dr. Maher Tadros, eConstruct Pretensioning is a common method of prestressing in the United States. It requires the use and set-up of a prestressing bed, normally in an off-site plant. The strands are tensioned in the bed between stressing heads. When concrete reaches adequate strength, the strands are detensioned, thus transferring the prestressing force from the prestressing bed to the concrete product. A large amount of prestressing gives the midspan of a flexural member a large reserve capacity to resist gravity loads. In order to control flexural stresses at the member ends, where the stresses from the girder weight are less, some of the prestressing effects near the member ends must be reduced. Due to the nature of pretensioning, the tensioned strands must follow a series of straight lines. The two commonly used methods of stress control are harping and debonding. HARPED STRANDS The word “harped” is synonymous with the words “draped” and “deflected.” Commercial hold-down devices may be used to drape the strands. The strands typically pass through rollers at a spacing of 2 by 2 in. In the Northwest, the strand group passes as a bundle through a tube and is then spread out into a 2- by 2-in. spacing pattern at the end of the member. In both types, the hold-down device is anchored to the bed pallet using threaded rods. When the prestress is ready to be transferred, the rods are cut before the rest of the detensioning proceeds. The designer should be aware of the limitations of the production facilities near the bridge location, in relation to the amount and location of the hold-down force. The pretensioning arrangement for the 205-ft-long, 100-in.-deep Alaskan Way Viaduct girders in Seattle, Wash., had 46 straight and 26 harped strands in the bottom of the girder. The harped strands were held down in five bundles consisting of four sets of six strands each and two strands, that were spread out into a 2- by 2-in. spacing at the girder ends. Each girder also had eight temporary top flange strands that helped control the top fiber stresses at time of prestress transfer and member camber making a total of eighty 0.6-in.-diameter strands. In Washington State, it is a common practice to drape about one third of the total number of bottom strands.The top strands also aided in the stability of the long, slender girders during handling and shipping. They were bonded only in the end 10 ft. They were cut after the girders were erected and braced and before the intermediate diaphragms were cast. STRAIGHT STRANDS This process is a simpler one as it does not involve harping. For it to work, however, some of the strands at the members ends may be required to be “debonded,” also known as “shielded” or “blanketed.” Strand debonding is done by enclosing the strands in plastic sleeves. Slit sleeves have been used in the past to allow the sleeves to be placed on the strands after the strands have been tensioned and anchored.