Carbon Fibers Reinforced Polymers (CFRP) are widely utilized to strengthening various structural elements in order to sustain higher loads
or to restore the strength of the deteriorated elements. This paper aims to investigate the behavior of hollow steel beams reinforced by unidirectional
carbon fibers composite laminates applied in longitudinal and transversal directions. Experimental and numerical investigations were conducted in this
study. Six specimens of rectangular hollow sectional (RHS) were subjected to a four-point loading test and divided into two groups. Each group of three
specimens was tested in different positions ―Mx and My‖. Each group included one reference beam and two, specimens strengthened with CFRP
laminates in longitudinal and transversal directions. Ultimate loads and deflection were determined through the tests. Numerical analysis was performed
using a finite element program in order to determine the failure load of beams. Results of finite element model were verified with their experimental
counterparts, it is found in a good agreement. As per experimental results, using the CFRP strengthening system effectively improved the strength and
the ductility of strengthened beams. Also, strengthened beams with longitudinal laminates achieved a higher improvement compared to strengthened
beams with transversal laminates