Design of Waterline Pile Cap Footings for Bridges Using Large Diameter GFRP Reinforcing Bars

This research project funded by Florida Department of Transportation (FDOT) provided the opportunity to characterize #11 (36-mm diameter) GFRP bars that are not currently considered in ASTM specifications, but recently added to the FDOT FY2024-25 section 932 material specification. Accordingly, and based on the experimentally-obtained properties of these large-diameter bars, a waterline pile cap footing was selected and designed. As a result, a comprehensive design philosophy and workflow is proposed for GFRP reinforced concrete (GFRP-RC) bridge footings. The case-study waterline pile cap footing is located in the Florida coastal zone and was originally designed in 2017 based on AASHTO LRFD Bridge Design Specifications, 7th edition (2014) using conventional carbon steel reinforcement. The main objectives of this study are to: (1) redesign the foundation with GFRP bars while keeping the cap geometry unchanged; (2) analytically validate the Ultimate Limit State (ULS) and Service Limit State (SLS) designs with the AASHTO LRFD Bridge Design Guide Specifications for GFRP (2018) using a commercial FEM software; (3) compare the amount of #11 GFRP reinforcement with that of carbon steel bars in the as-built drawings of the original steel-RC design; and, (4) discuss the challenges in detailing, including the anchorage length of GFRP #11 bars and minimum bent bar sizes to meet both SLS and ULS requirements. The foundation was designed using both the properties defined by the FDOT FY2024-25 Specifications and those obtained from experimental tests, highlighting that the design with GFRP bars is more restrictive at the SLS and in detailing compared to steel-RC.