## 3 Design Code Provisions and Previous Test Results
The cross-sectional shape of the beam comprised rectangular and T-beams, and the area of the flange was designed to be larger so that the compressive force was not smaller than the tensile force by the tensile reinforcement due to the horizontal shear force of the interface. Detailed information is available in the references [24; 25; 26]. The total of 17 specimens comprised 13 specimens with T-type interface and 4 specimens with C-type interface, while 11 specimens with T-type interface showed sliding shear failure along the horizontal interface. Only six specimens (ITB-ST, SF-291-B, SF-6.82-C1, SF-6.82-C2, SF-4.55-C1, SF-4.55-C2) exhibited flexural compression failure after yielding of longitudinal reinforcements and developing the nominal flexural strength for the composite section. The crack patterns and failure modes are presented in Figure 4 which shows the sliding shear failure of the interface and flexural compression failure of the composite T-shaped beam. The flexural cracks first occurred at the bottom of the loading point for all specimens. After more flexural cracks developed, horizontal shear cracks on the interface appeared for specimens with T-type interface. Finally, web diagonal cracks were observed for all specimens both with T-type interface and with C-type interface. No horizontal crack along the interface appeared in six specimens until the flexural compression failure. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Stage**} & \multirow{2}{*}{**Spelines**} & \multicolumn{3}{c}{**Interface**} & \multirow{2}{*}{**d**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} & \multicolumn{3}{c}{**frame**} \\ \cline{3-14}  & & \multirow{2}{*}{**Location**} & & \multirow{2}{*}{**Surface**} & \multicolumn{1}{c}{**Width**} & \multirow{2}{*}{**(mm)**} & \multicolumn{1}{c}{**PC**} & **CPF** & **Reiner** & \multicolumn{1}{c}{**\(\mathbf{\theta_{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{   }}}}}}}}}}}}}\) & **GrGr** & **GrGr** & **GrGrGr** & **GrGrGrGr** & **GrGrGrGr** & **GrGrGrGrGrGr** & **  
Figure 5 shows the load vesus deflection relationships for specimens in the Phase 1 to Phase 3 studies. In Figure 5, applied loads were normalized as divided by the nominal flexural strength (2M\({}_{\text{n}}\)/a in Table 1), and the vertical displacements were also divided by the span length of each specimen due to comparison under the same conditions. Load ratios greater than 1.0 are observed in the specimens leading to flexural compression failure without horizontal shear crack along the interface. Most of the specimens with T-type interface that failed in horizontal shear of the interface showed a maximum load of 0.6 to 0.75 times the nominal flexural strength. These results indicate that the composite concrete beam with T-type interface did not show flexural behavior for the given composite section. From these results, it can be suggested that the composite beam having an T-type interface did not develop the horizontal shear strength specified by the design provisions. Figure 4: Crack patterns and failure modes. Figure 6 compares horizontal shear stresses obtained by Equation (1) for 17 specimens in the Phase 1 to 3 studies, comparing the horizontal shear stresses of push-off tests [9,10] and beam tests [5]. The 13 composite beam specimens with T-type interface showed maximum horizontal shear stress similar to or smaller than the nominal horizontal shear strength calculated by the shear friction design method of PCI or ACI. Especially, seven specimens with T-type interface showed lower horizontal shear strength than the nominal horizontal shear strength of ACI. Therefore, it is debatable whether the horizontal strength equation of the current design code can provide adequate safety.