Analysis of FRP strengthened deep RC members using the STM and the FEM approaches

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Civil and Environmental Engineering


Riyad S. Aboutaha


Reinforced concrete, Strut-and-tie, Fiber-reinforced polymers, Highway bridges

Subject Categories

Civil and Environmental Engineering | Engineering | Structural Engineering


According to a Federal Highway Administration report (FHWA, 1998), more than 30% of highway bridges in the United State are identified as substandard due to an increase of service load, inadequate maintenance, and environmental deterioration. The demand for strengthening of deteriorated civil infrastructure highlights the need for developing effective means for infrastructure rehabilitation. Fiber reinforced polymer (FRP) composites are the best answer for infrastructure rehabilitation. Current design guides offer sound procedures for analysis of FRP strengthened slender members, however, they fall short of accurately predicting the strength of FRP strengthened deep members.

The Strut-and-Tie Method (STM) and the Finite Element Method (FEM) are very powerful analysis tools for the analysis of FRP strengthened deep concrete members. A practical analysis and design process of FRP retrofitted deep concrete members using the STM is proposed in this research. A total of 22 experimental deep beams with or without externally bonded FRP composites were investigated and analyzed using the proposed STM and the FEM. In addition to suggesting the STM approach for the analysis of FRP retrofitted deep structural concrete, seven effective factor models accounting for reduction of strength in cracked concrete were also investigated to find the optimal effective factor.

According to the analysis of experimental tests using the proposed STM and FEM approaches, it has been shown that the proposed STM approach is a valuable analysis tool for FRP strengthened deep concrete members. The STM approach based on the effective factor that depends on strut angle provides the best agreement with the experimental test results. Evidently the FEM is also a powerful tool for the structure analysis of reinforced concrete having nonlinearity and brittle failure properties. However, the Finite Element Analysis of reinforced concrete with or without FRP reinforcing is sensitive to two modeling factors, the shear transfer coefficient and the convergence tolerance value. Based on the results of finite element analysis (FEA) of experimental test beams, an optimal modeling factor for a shear transfer coefficient of 0.25 and a convergence tolerance value of 0.2 are recommended.


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