In traditional terms, a double tee is a load-bearing structure that resembles two T-beams connected to each other side by side. The strong bond between flange and the two webs creates a structure that is capable of withstanding high loads despite a long span. Concrete precast double tees have been a construction industry standard for more than 50 years. FRP double tees have the “look and function” of conventional concrete precast double tees, yet are significantly lighter and provide lower installation costs. An FRP double tee weighs 15 to 22 psf compared with a concrete tee which weighs 120 to 130 psf.
In 2006, Hamilton County bridge engineers were looking for material options with a long life span for a short span bridge. Just 22 ft. long, the two-lane crossing was expanded to 62 ft. wide to accommodate trucks and future road-widening projects. Eight Mile’s specifications called for L/800 load deflection, deck depth of 5.5 in., panel depth of 22 in., and a panel weight of 26 psf. The vehicle superstructure had to meet American Association of State Highway and Transportation Officials (AASHTO) HS 20 loading [alternative military truck loading].
Previous FRP vehicle bridge projects were constructed with an FRP deck fastened to traditional steel or concrete beams. For Eight Mile, Composite Advantage developed a drop-in-place integral system that eliminated the joints between deck and beams. The longitudinal beams and deck were molded to form one part. This gave the panels the look and function of concrete precast double tees. The ability to prefabricate multiple structural elements reduced manufacturing and installation costs. As part of new product development, the FRP double tee was tested to static loads and fatigue of 2 million cycles.
Eight Mile was the first bridge to receive the structurally successful FRP double tee. Installation took one day. Load testing with fully loaded trucks was conducted after installation and again, two years later. In addition to being a proving ground, the project yielded some lessons learned. In a vehicle bridge setting, the composite double tee didn’t provide quite enough cost savings to supplant the conventional concrete deck and beam configuration. Full highway truck loading requirements limited FRP span lengths. Although a technical success, the market fit was not optimal.
In October 2019, CA engineers visited Eight Mile Bridge to perform a visual inspection. The evaluation focused primarily underneath the bridge at joints, panel-to-panel and panel-to-abutment areas. Engineers found no evidence of water and salt leakage on the FRP deck’s surface or the supporting structure. Panels remained well connected and double tee stiffeners looked like new.
The FRP double tee was used in several pedestrian bridges following the installation of Eight Mile. These projects’ lower load requirements allowed FRP panels to reach spans up to 50 ft. long. The lightweight prefabricated construction allowed for fast installation. The integrated structure and color options provided the added advantage of improved aesthetics for parks and trails like this Loveland, Ohio example. FRP triple tees provided a solution to the project’s depth / elevation restrictions. For sites needing longer spans, FRP truss bridges like the ET Techtonics product line offer a robust alternative.
Realizing that pedestrian loading ratings at spans up to 50 ft., was a sweet spot for FRP double tees, some designers and owners started looking at the product for rail platform ramps. The Americans With Disabilities Act (ADA) dictated an inclined ramp with a maximum length of 30 ft., followed by a landing area of five ft. or eight ft. in length. Widths are typically six ft. to 10 ft. Lightweight FRP tee panels up to 30 ft. long made installation quick. Superstructure requirements were reduced with lower dead load; allowing smaller support beams and walls. Boston’s Bellingham Square—a multi-modal transit site in a densely populated environment—is a perfect example of how the switchback ramp proved to be the right market fit.
The departments of Transportation and transit agencies grapple with maintenance issues caused by de-icing chemicals. Deteriorating concrete can shorten the life of a rail station’s loading platform to just 15 years. Surface spalling poses additional safety hazards for passengers. These factors have primed a new niche space for FRP double tee panels. A typical rail platform design uses concrete piers at spacings of 16 ft. to 50 ft. Sitting on the piers, the platform surface is either concrete precast double tees or steel stringers with concrete slab decking. While FRP decking can provide corrosion resistance, there are multiple structural elements involved. FRP double tees provide a single structural element between the piers that can be quickly installed between train schedules. The product works well for rehabilitation and new construction. At less than 20 percent the weight of reinforced concrete, the reduced dead load allows for fewer piers at wider spacing. The combination of corrosion resistance, light weight and fast installation are benefits that owners and designers find valuable. This is an application that no one considered at the start of the FRP double tee journey. Today, the FRP product is becoming the baseline for rail platforms in many locations. To read more about this product and its application in the rail industry click here. We also recently held a webinar and answered attendees’ questions about the product so be sure to check that out.
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