Test study on simply supported built-up beams (plate on top of a channel section) with two different strengthening approaches using carbon fiber reinforced polymer (CFRP) fabrics by external bonding namely flange strengthened and modified cross section strengthened has been conducted. The study specimens simulated the flange restrained C-channel sections in lateral bracing of steel bridge superstructure and steel storage structures. A total of seven different strengthening configurations were made based on the two strengthening approaches. The results indicate that the flange strengthening approach is not an efficient method of retrofitting open cross section specimens with longer length compared to the modified cross section strengthening method with infilled core which significantly improved the flexural strength. In addition, the results indicate that with an increase in a number of confinement wrap using bidirectional CFRP fabrics the wrinkling can be prevented. The results also indicate that depending on the magnitude of camber imperfection (initial bent about major axis) different failure modes of CFRP can be observed. © 2017 Elsevier Ltd