Experimental and Analytical Behavior of GFRP-Reinforced Concrete Box Girders under Pure Torsion

Abstract

Concrete box girders are significant structural elements in various engineering applications due to higher torsional stiffness and low self-weight. The use of glass fiber–reinforced polymers (GFRPs) as internal bars has been adopted for many structural applications. So far, the torsional behavior of reinforced concrete (RC) box girders reinforced with GFRP bars has not been addressed. Therefore, this paper presents experimental data on the torsional behavior of RC box girders reinforced with GFRP bars and stirrups. Six box girders measuring 4,000 mm in length, 380 mm in height, and 380 mm in width, with a wall thickness of 100 mm, were tested under pure torsional moment over a clear span of 2,000 mm. The test parameters included the type and amount of torsional reinforcement. The test specimens comprised four box girders entirely reinforced with GFRP reinforcement, one box girder with only longitudinal GFRP bars, and one box girder reinforced with steel reinforcement as a reference specimen. The test results indicate that the torsional strength of the steel and GFRP box girders was significantly affected by stirrup capacity; stirrup stiffness did not affect the torsional strength. Increasing the web’s GFRP reinforcement ratio increased the torsional strength and stiffness. An analytical iterative softened membrane model for torsion (SMMT) was modified to predict the entire torsional behavior of the GFRP-reinforced concrete box girders. The model was validated by comparing the analytical results to the experimental results of the four GFRP-reinforced concrete box girders. The comparison indicates that the model was able to predict the cracking and ultimate torsional strength and the corresponding twist with reasonable agreement. In addition, a noniterative Rahal model was modified to predict the ultimate torsional strength of FRP-reinforced concrete members. At last, the experimental ultimate torsional strength was assessed with the corresponding value calculated according to available fiber-reinforced polymer design codes and guides.