Field load testing and analysis of a new FRP composite tub girder bridge with a concrete deck

Abstract

In December of 2020, a first-of-its-kind FRP tub girder bridge with a composite concrete deck was constructed in Hampden, Maine, USA (the Grist Mill Bridge). Prior to the design of the Grist Mill Bridge, the carbon-glass-hybrid, corrosion-free FRP girder and the mechanical girder-deck shear connectors were developed and experimentally assessed for strength and fatigue resistance by researchers at the University of Maine. The subsequent construction of the Grist Mill Bridge presented a unique opportunity to assess in-situ performance of this novel technology. Following an overview of the development of the girder system and the design methodology employed for the Grist Mill Bridge, this paper details instrumentation and field live-load testing conducted immediately prior to the bridge’s opening. Four overloaded dump trucks with a total weight exceeding 1100 kN were simultaneously positioned at critical locations on the structure to assess girder flexural response. The bridge was instrumented with 24 semi-wireless strain transducers to give a thorough picture of girder flexural response and live load distribution. Despite being designed assuming simply-supported conditions, the field data indicated partial rotational restraint at both abutments, and probable reasons for this are discussed. Measured strains were used to develop field-based flexural rating factors in accordance with the AASHTO Manual for Bridge Evaluation, which indicated overall conservatism in the design as appropriate for the first deployment of a new technology. High-fidelity 3D FE models of the structure were developed, and model predictions compared reasonably well with results of the live load tests. The field data and FE model results are compared with conventional girder-line analysis prescribed by AASHTO, and implications for future designs are discussed along with ongoing research to further advance the FRP girder system.