Today the reuse and recycling of steel-concrete composite structures is a topic of growing interest. However, traditional systems do not facilitate easy dismantling, making reuse and recycling hardly possible. Research projects, such as the LWT-FLOOR project at the University of Zagreb, Faculty of Civil Engineering, Croatia, aim to address these challenges. The project aims to develop a novel structural floor system with a demountable shear connection. The built-up steel girders consist of four cold-formed C profiles connected by spot welds to corrugated web and additional shear plates near supports. A demountable connection between the steel and the lightweight aggregate concrete slab is achieved using bolts, allowing ease of disassembly for reuse or recycling at the end of system service life. This paper provides an overview of the project’s background, a summary of experimental findings on the system components and a detailed experimental setup and results regarding six bending tests of 6 m long steel and composite girders. Three tests are related to built-up cold-formed steel girders with different cross-section characteristics while three tests are related to a composite solution with an integrated concrete slab over built-up cold-formed steel girder. The key system performance indicators regarding bending capacity and stiffness, failure modes, deformations in steel and concrete components and concrete slab end slip are investigated in detail. The obtained results show that the failure modes of beams are complex and influenced by the height of the girder cross-section, the thickness of steel elements and consequently characteristics of spot welds. Four specimens (two steel and two composite) show expected results regarding preliminary numerical studies while two specimens (one steel and one composite) failed due to premature failure of spot welds. The obtained results will be base for the calibration of numerical models for future parametric studies.