This research presents an experimental and theoretical study investigating the flexural capacity of built-up steel cold-formed I-beams strengthened in the hollow web with different materials. Eight built-up cold-formed steel I-beams were prepared and experimentally tested. As a control specimen, one was not strengthened, steel shear connectors strengthened one without materials, and six specimens were strengthened by filling the hollow web with different materials. The strengthened materials used are wood wastes (Sawdust with epoxy- Sawdust with polyester), lightweight concrete, normal-weight concrete, High-Strength concrete, and polymer mortar. The specimens' method of failure, load at failure, and vertical displacements were recorded. The relationship between vertical load and deflection at the span's midpoint has been graphed to analyze the impact of strengthened materials. Using polymer mortar resulted in the highest capacity, outperforming other materials. Finite element models of the tested beams were established. Good agreements between experimental and numerical models were observed. 84-FE numerical models were established to determine the effect of cover plate thickness on flange width ratios and the height-to-width of the strengthening material. Finally, new equations that calculate strengthened beam capacity were presented.