CFRP Strand Application on Penobscot Narrows Cable Stayed Bridge

Rohleder, Jr., P.E., S.E., W. Jay Page 8The University of Maine contributed to the project with lab testing and calibration of load cells and fiber opticsensors that are measuring forces on the <strong>CFRP</strong> strands. They also proof tested the permanent anchor chairs andtemporary stressing chairs. They will also provide long term data collection of forces, strains and temperatures onthese innovative <strong>CFRP</strong> strands. The collected data will be analyzed by the design team to compare results withexpected structural performance criteria for the <strong>CFRP</strong> and relative to the adjacent high-strength steel strands. Thecomparative performance criteria will include data such as initial prestressing elongations, force losses, and anchorset values along with long term expected design strand prestressing forces related to time dependent properties andthermal effects experienced by the bridge.BACKGROUND OF PREVIOUS CFCC USE IN VEHICULAR BRIDGESCurrently, advanced fiber reinforced polymer (FRP) materials find worldwide application in theconstruction of small and large structures 1-5 such as beams and bridges. However, prestressed concrete bridgesconstructed using CFCC reinforced polymer (<strong>CFRP</strong>) tendons are few in number 1-6 . The FRP is a linearly elasticmaterial which may not provide a ductile failure mode like steel, however a structural member that is prestressedand reinforced with CFCC can be designed to fail in a similar ductile manner as under-reinforced concrete members.The results of early research investigations 6-10 conducted in the Center for Innovative Materials Research atLawrence Technological University, Southfield, Michigan have shown that internally bonded <strong>CFRP</strong> tendons incombination with externally unbonded <strong>CFRP</strong> tendons can lead to reasonably ductile simply supported 6,7 andcontinuous 9,14 prestressed concrete bridge systems. These results formulated the basis for design of the BridgeStreet Bridge 11, located in Southfield, Michigan. The Bridge Street Bridge is the most recent <strong>CFRP</strong> prestressedconcrete bridge in the USA and the first to use CFRLeadline TM tendons, CFCC TM strands, and <strong>CFRP</strong> NEFMAC TM1The Bridge Street Bridge served as a resource during development of the Penobscot Narrows Bridge <strong>CFRP</strong>strand installation methods, details and material specifications.PROJECT <strong>CFRP</strong> MATERIAL QUALITITES AND CHARACTERISTICSThe specific <strong>CFRP</strong> material used for the Penobscot Narrows Bridge project is CFCC TM strand that isproduced by the Tokyo Rope Mfg. Co., Ltd. It comes in a variety of sizes, from 0.20" diameter single wire to 37individual helically wound wires with a total nominal diameter of 1.57". The actual CFCC TM strand used for thePenobscot Narrows Bridge cable stay test project is a 0.60" diameter, 7 wire strand, with 6 wires helically woundaround a center king wire. There is also a protective coating along the outside of the strand to shield the fibers fromnormal handling and exposure. The properties from the certification tests of the production strands used for thisproject are shown in Table 1. Note that the tensile rigidity for CFCC as included in Table 1 is defined as the slopeof a load-strain curve derived from the tensile tests. The CFCC tensile rigidity is the average value of results fromthe 20 test pieces from the load-strain curve obtained in accordance with a tensile test at 20% and 60% of theguaranteed tensile capacity.While the <strong>CFRP</strong> strand is similar in size to the steel strand, there are some significant material property differences.Due to the smaller effective cross sectional area and modulus of the <strong>CFRP</strong> strand, the elongations are approximately1.73 times that of the steel strand used for this project. The stress-stain relationship of the <strong>CFRP</strong> strand is also linearup to ultimate loading. Thus, there is no ductility within the strand. Compared to steel the <strong>CFRP</strong> strand isapproximately 5 times lighter, which makes handling and installation easier. The shear resistance for the carbonfiber reinforced polymer cable material is negligible; therefore precautions need to be taken to avoid applying anytransverse forces on the cable.Since there are no anchorages within the pylons, the friction between the strands and the cradle sleeves isan important design parameter. The actual friction coefficient was measured in the laboratory during thepreliminary design phase for the steel strand and before installation for the CFCC strand. The friction coefficientmeasured for the steel strand is 0.5 while the CFCC strand value is 0.3. While this decrease in friction for thereplaced strands do not affect the behavior of the Penobscot Narrows Bridge, this property would have to beconsidered for a stay completely constructed using <strong>CFRP</strong> strands or any other prestressing applications whichexperience friction resistance.