CompositesWorld
Issue link: https://cw.epubxp.com/i/944580
MARCH 2018 46 CompositesWorld FOCUS ON DESIGN vertical), 2) a minimum bending moment of 3,276 kip-inches, and 3) a minimum bending stiffness of 2.76 x 10 9 lbs-in 2 . Given these parameters, and the 40.76-kip-ft impact load force, House was able to design the strength and stiffness of the pilings, using Kenway's in-house design spreadsheet. e extensive spreadsheet, developed over many years by Harbor Technologies, is based on "lots of historical test data," says House: "We've worked with the University of Maine's Advanced Structures and Composites Center [ASCC, Orono, ME, US] to do full-scale failure testing of our parts, using ASCC's large press, to test a variety of lami- nates and wall thick- nesses." Each project is customized, using the design spreadsheet. ere are no off-the- shelf pilings. For the Manahawkin project, the NJDOT specified a 16-inch (400-mm) outside diameter, but, says House, "we were able to optimize the piling wall thickness with our spreadsheet calculations." House explains that a pile that is, for example, 300 mm in diameter with a wall thickness of 25 mm (1 inch) has the same strength and stiffness as a 400-mm-diameter pile with a wall thickness of 12.5 mm (0.5 inch). is is because of the geometric properties of the tubular pile; the larger diameter translates to a stiffer part: "It depends on the distance between the centroid, or neutral, axis, to the wall — the larger that distance is, the stiffer the pile becomes," he states. Translated to the composite design, the 300-mm pile might use 45 kg of material, whereas the 400-mm pile requires only 32 kg of material because of the thinner wall, yet it achieves the same strength and stiffness properties. "So, I've used less material, for less cost and achieved a larger, more useful shape," adds House. He stresses, however, that this principle can be extended only so far. A 750-mm diameter pile with a wall thick- ness of 1.5 mm might appear on paper to be stiff enough, but "that very thin wall obviously wouldn't withstand the driving loads and would buckle." House's spreadsheet calculations indicated that, given the water depth and soil conditions, a 400-mm-diameter pile with a wall thickness of 9 mm (0.375 inch) and a length of 18.5m (~60 ft) would meet the NJDOT requirements. And, says House, the driving, or installation, loads actually proved pivotal to the design: "Piles are typically driven with a vibratory hammer or impact hammer, both of which generate significant axial loads on the pile. A thinner wall could have met the structural requirements of the fender, but we needed the 9-mm wall to stand up to the driving." Heavy knitted quadraxial fabrics, ranging from 3,390-6,800 g/m 2 , used to fabri- cate the piles (more on that below) ensured axial strength was adequate to stand up to the driving forces. NJDOT also specified that when in place, the pilings would be filled with concrete, which works to composites' advantage in this application. When a hollow tube fails, it buckles and collapses, but concrete performs well in compression, to prevent buckling, explains House: "We have extensive test data that show that concrete-filled piles actually exhibit greater deflection, essen- tially two times more than a hollow pile, before failure." Adds Read this article online | short.compositesworld.com/MBBFender Read CW's previous story on composite pilings, including those made by Harbor Technologies, online | short.compositesworld.com/OtWFPiles Transition from old to new The new bridge span piers with composite fender system are on the left; the older truss span piers, protected by wooden fenders, are on the right. Source | Schiavone Construction Cascading savings based on materials selection The comparatively low weight of the composite piles made it possible to drive them into the sea bed with a relatively small excavator, which clamped to the top of each pile — a source of cost savings on the project. Source | Schiavone Construction