CFRP Strand Application on Penobscot Narrows Cable Stayed Bridge

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Rohleder, Jr., P.E., S.E., W. Jay Page 10PROJECT CARBON FIBER SYSTEM COMPONENTS AND DESIGN DETAILSKey components for the <strong>CFRP</strong> strands installed into the Penobscot Narrows Bridge cable stay system are theanchorage sockets and permanent jacking chairs.<strong>CFRP</strong> strands are low in shear strength and subject to brittle fracture when using biting wedges. As such, gripingwedge style anchorages that are conventionally used for steel post-tensioning cannot be used for <strong>CFRP</strong> strands.Alternatively, the carbon strands for this project were bonded in a threaded socket using highly expansive grout (seeFigure 8 and 9). The socket is a long threaded rod with a voided center slightly larger than the diameter of thestrand. The annular space between the inside socket wall and the strand is filled with a cementatious based material,which exhibits a high degrees of expansion (HEM) during curing. As curing takes place expansion of the material isconstrained by the socket wall and the strand. This constraint ultimately produces a confining pressure ofapproximately 11 ksi, locking the socket and strand end together. This is described as approximate since the actualconfining pressure for a specific anchor is dependent on the actual grout thickness. General testing and experiencewith the Bridge Street Bridge 11 has shown that this confining pressure from the HEM is valuable for avoiding creepconcerns as might be found if an epoxy agent had been used to anchor the strand in the socket.FIGURE 8 – General View of CFCC TM Anchor Sleeve with Nut and <strong>Strand</strong>.FIGURE 9 – End View of CFCC TM Anchor Sleeve with HEM retaining the <strong>Strand</strong>.Custom permanent jacking/anchor chairs were fabricated (see Figure 10) for this application. These chairs weredesigned to straddle the existing anchorage elements. Extensive proof load testing was performed in the laboratoryto document the behavior of the chairs under actual field conditions. Force from the carbon fiber strand bears on acenter hole load cell and permanent anchor chair via the threaded socket nut (see Figure 10). The threaded portionof the socket provided plus/minus 4" of adjustment from the anticipated position. The load cells were initiallycalibrated prior to installation. The need for additional calibration throughout the life of the permanent installationwill be evaluated as time progresses. However, a primary value of the load cells will be to compare the relativestrain differences between the carbon fiber and epoxy coated steel strands.
Rohleder, Jr., P.E., S.E., W. Jay Page 11FIGURE 10 – Permanent Anchor Chair with <strong>CFRP</strong> Load Cell and Monitoring Lead Wires.The reference strands from stays 2, 10 and 17 over the western (observatory) pylon were replaced. Final loadslocked into the <strong>CFRP</strong> strands varied from 21.05 kips to 25.27 kips depending on strand location. Table 2 belowcontains a full description of the permanent initial loadings placed on the carbon fiber strands.LocationLength from Jacking ChairBack Face to Pylon Face(Length of <strong>CFRP</strong> <strong>Strand</strong>)(feet)Adjacent Steel <strong>Strand</strong>Force at Time of <strong>CFRP</strong><strong>Strand</strong> Stressing(kips)Actual <strong>CFRP</strong> <strong>Strand</strong>Lock-offStressing Force(kips)Stay 02 Back Span 134 25.20 25.27Stay 02 Main Span 149 22.20 22.02Stay 10 Back Span 303 22.00 21.05Stay 10 Main Span 346 21.10 21.66Stay 17 Back Span 459 22.20 21.66Stay 17 Main Span 526 21.30 20.58TABLE 2 – Carbon Fiber <strong>Strand</strong> Stressing Results.Other than an underestimation of initial length needed to take out slack in the cable as it was threaded intothe stay, and the additional stretch of the cable within the pylon (approximately 3 inches at all locations), theelongations as pre-calculated were within expected tolerance of actual installation values.
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CFRP Strand Application on Penobscot Narrows Cable Stayed Bridge

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