This research presents an experimental and analytical investigation of built-up cold-formed steel beams (CFS), hollow in web, prestressed using steel tendons under bending. Fourteen built-up cold-formed steel I-beams were tested and experimentally evaluated using four-point bending tests. One specimen was left untreated and served as a control for five specimens strengthened using steel prestressing tendons in various shapes. One specimen strengthened by normal-weight concrete served as a control for five additional specimens that were strengthened by filling the hollow web with normal-weight concrete and employing prestressing tendons in various types. Two specimens were tested under a more significant force than the others. The relationship between vertical load and deflection at a structure's midpoint and the effect of strengthened tendons has been investigated. As a result, the bending capacity of the beam was significantly improved, and local instability was delayed under applied loading. All strengthened specimens exhibited enhanced load–midspan deflection behavior compared to the control specimen. These results highlight the effectiveness of the proposed system and its potential contribution to advancing lightweight and efficient structural design solutions. A numerical model that takes into account material and geometric nonlinearity was created for finite element modeling (FEM) [1]. As a result, the effects of concrete strength and tendon force were considered during the analysis. The results also showed a slight impact from the concrete compressive strength. Bending moment capacities are expected from North American Standards AISI-S100–16 [2], which indicates that this code's design method is unconservative compared to experimental test results.