*Joint School of Nanoscience and Nanoengineering of North Carolina A&T State University
This study is limited to the more severe tensile rather than compression loading in composite joints. Composite joints subjected to compression loading are less sensitive to joint geometry and are generally stronger than joints subjected to tensile forces. Members are loaded in the lengthwise orientation according to the direction of the pultrusion to utilize the maximum tensile strength available from the FRP. All edge distances exceed the minimum recommendations relative to bolt diameter. As such, the predicted failure mode is bearing failure, rather than failure by tension or shear out. Bearing failure is caused by the bearing pressure forces from the bolt applied to the hole boundary producing delamination of the composite. Historically, bearing failure has been defined as 4% elongation of the bolt-hole diameter. No appreciable load capacity can be expected after the 4% diameter elongation is met. Any further elongation of the holes only allows the structure to become loose and unstable.
The FRP materials used for this test are fire retardant polyester, compliant with the Cooling Tower Institute (CTI) industry standards. Cross-sectional dimensions of the tubes are 88.9 mm square x 6.4 mm thick (3.5" square x 0.25" thick). Cross-sectional dimensions of the straps are 76.2 mm wide x 9.6 mm thick (3.0" wide x 0.38" thick). Fastener materials are all of S30400-grade stainless steel, Ø12.7 mm-13 (Ø½"-13) UNC. One flat washer is placed below the head of the bolt. One flat washer and one helical locking washer is placed under the nut. All threads are lubricated with a graphite-petrolatum anti-seize compound.
Like many engineering and structural industries, one of the major advancements in today’s construction of cooling tower framing is the use of polymer composites also known as fiberglass reinforced polymer (FRP) square tubes. Pultruded FRP fiberglass structure members are lighter and cheaper than steel and unlike traditional wooden framed towers, they are unaffected by moisture, even after decades of exposure and direct contact with water.
A slight disadvantage of FRP framing is that the composite material is somewhat flexible making the material more susceptible to structural collapse, especially over time in strong wind and storm areas such as coastal plains. Flexing causes movement and bolt friction to the drilled hole in the FRP fiberglass. With back and forth flexing movement, over time the bolt hole size slowly increases and the bigger the hole increases, the more rapid the pace is. This is especially true where contact is made with the bolt’s threading that now acts like a metal file on plastic fiberglass. Vibrations from running motors or other moving mechanical equipment can also cause hole wear in the FRP tubing. Eventually with continuous movement and flexing the cooling tower becomes more prone to a structural collapse.
Cyrco developed, patented, and manufactures a FRP Shear Bushing that fully encloses the bolt which then overcomes the FRP fiberglass bolt thread hole wear caused by continous flexing of the fiberglass members under wind or vibration conditions. In addition to preventing FRP hole wear, Cyrco’s FRP Shear Bushing mate in the middle which stops the operator from over torquing the bolts and prevents ruinous stress cracking to the FRP tubing. The joined mating also prevents under-tightened, loose nuts that often unthread and fall to the ground, as the installer knows when to stop tightening as the nut can not tighten any further. With the use of Cyrco's FRP Shear Bushing the nut and bolt are tightened within an ideal torque range, not over torqued, nor under-threaded. Another advantage of our mating bushings is that the tubular connections act as a pass through guide for the bolts, making for a much faster, rapid assembly.
Results from a series of independent testing performed by the Joint School of Nanoscience and Nanoengeering of North Carolina, outlined that Cyrco’s FRP Shear Bushings clearly outperformed all other commonly used FRP shear bushings. Cyrco’s FRP Shear Bushings created a tower shear service factor of 6.0, which is twice the Cooling Tower Institute (CTI) recommended standard.
In all FRP member framing applications, but especially in high wind geographic locations such as coastal plains where hurricanes and tropical storms are frequent, Cyrco strongly recommends the use of our FRP Shear Bushings. With the use of Cyrco’s FRP Shear Bushings, today’s FRP cooling tower’s framing can now become capable of withstanding a Category 6 hurricane.
With FRP fiberglass composite materials becoming ever so more present in a wide array of
everyday engineering and structural industries, Cyrco's FRP Square Tube Shear Bushings
are not just limited or beneficial to the cooling tower industry.