Nine full scale R.C. thin-walled beams were tested to achieve small thickness, self compacting concrete with maximum aggregate size of 10mm was used. Apart from the control specimen, all other beams were internally encased by steel sheath of 0.8mm thickness. Four point loading system was considered with a/d ratio equal to 1.50. The research aimed at explorers the effect of using of different types of filling materials and employing external sheathing compared with internal reinforcement to find out suitable technique to be utilized in ordinary construction specially with small a/d ratio. Selecting of the most suitable shape and detailing of reinforcement, specially in shear domain, was targeted. Vertical deformation, strain at different points of concrete section and steel bars and crack propagations were measured and recorded. To conduct an extensive parametric study, nonlinear three-dimensional finite element analysis has been carried out. The computational model has been verified against experimental testing. The numerical work aimed at providing more insight about the behavior and figuring out more parametric study.