Abstract: The performance of an innovative form of steel-encased concrete composite beams was experimentally investigated both in the construction state and in the reliability of serviceability (composite) state. The behavior observed during the tests is very similar to the analytically predicted performance. In the construction state, the deformations of the steel casing acting alone to resist the excentrical applied loading from the slab elements are small. However, a 6m span edge beam would require one central prop to prevent lateral torsional buckling of the steel casing. In the reliability of serviceability (composite) state, the sustained shear force exceed the calculated shear capacity by a considerable margin and the load-deflection curve of the test beams show that they are very ductile. The bond between the concrete and the steel section representing the external reinforcement is fully effective.