Torsional Analysis of Multicell Concrete Box Girders Strengthened with CFRP Using a Modified Softened Truss Model

Deterioration and aging of concrete structures are not the only reasons why critical members, such as beams, need to be strengthened. Other possible reasons include upgrading of design standards, compensation for faulty design or construction, changing usage of the structure, and exposure to unpredicted loads, such as impacts or excessive earthquakes. The current study proposes a new analytical method for predicting the torsional capacity and behavior of RC multicell box girders strengthened with carbon-fiber-reinforced polymer ( CFRP) sheets. The conventional softened truss model is modified to consider the influence of CFRP strengthening on the torsional response as an external reinforcement of multicell box girders. The proposed method involves solving the concrete torsional problem by combining the equilibrium conditions, compatibility conditions, and constitutive laws of materials, taking into account the confinement of concrete with CFRP sheets. A specific algorithm was developed to predict the torsional behavior of RC multicell box girders with and without CFRP strengthening. Verification of this method was achieved using extensive comparisons between analytically predicted behavior curves and experimentally obtained ones. The experimental work comprised a series of four torsion tests for single-and triple-cell box girders. Good agreement between the results was obtained, confirming the feasibility of the newly proposed method. Based on the outcome of the comparisons, it was revealed that the only currently available, recommended torsion design method was generally conservative, suggesting that further refinement is essential to make it a suitable design method for torsional strengthening using CFRP sheets. (C) 2014 American Society of Civil Engineers.