# Shear Behavior of Concrete Encased Steel Truss Composite Girders
## 2 Experimental Program
### Test Results  
Figure 7 shows the shear force-deflection relationship of the specimens. As shown in Figure 7a, in the PV specimen with the Pratt truss applied and the vertical stirrup placed, the first flexural-shear cracks occurred at a shear force of 468.7 kN, and web shear cracks were observed at a shear force of 543.6 kN. Even after the web shear cracks appeared, the stiffness decrease in the specimen was insignificant. Meanwhile, at a shear force of 543. 691.6 kN, the specimen underwent failure as horizontal shear cracks occurred along the top chord, as shown in Figure 7(a). In the P specimen without a vertical stirrup, flexural-shear cracks appeared at a shear force of 325.9 kN, whereas web shear cracks were observed at a shear force of 421.5 kN. Subsequently, a rapid decrease in stiffness occurred as horizontal shear cracks propagated along the top chord at a shear force of approximately 500 kN. After that point, ductile behavior was observed, followed by the load gradually decreasing after a maximum shear force of 520.5 kN. Even in the case involving the P specimen, it was discovered that horizontal shear cracks that occurred along the top chord led to the dominant failure mode of the specimen, as shown in Figure 7(b). Figure 7(c) shows the shear force-deflection relationship of the WV specimen with the Warren truss applied and vertical stirrup placed. In the WV specimen, flexural-shear and web shear cracks occurred at shear forces of 355.6 and 569.8 kN, respectively, and the specimen underwent shear failure as the width of the diagonal cracks increased rapidly at a shear force of approximately 659.5 kN, as shown in Figure 7(c). Furthermore, the behavior of W specimen without stirrup was very similar to that of the WV specimen, as shown in Figure 7(d). In particular, although the stirrup was not placed in the W specimen, the failure mode (i.e., diagonal shear failure) and maximum shear force (661.9 kN) were similar to those of the WV specimen in which the stirrup was placed. The cause can be inferred from the behavior of the diagonal web member of the EST shown in Figure 9. As shown in Figure 9, the tensile strain of the diagonal member--measured using a strain gauge during the maximum shear force--was \(1155\ \mu\)e, which was approximately 49% of the yield strain (\(2343\ \mu\)e) derived from the material test. This implies that in the W specimen series, the diagonal web member failed to fully contribute to the shear resistance. Figure 9(b) shows the shear failure of the screw rod observed from the EST node area of the W specimen series after the test had completed. It was assumed that as the diagonal web member lost its shear resistance performance due to the damaged screw rod during the test, the W  
Figure 7: Shear force-deflection response curves of test specimens. specimen series underwent premature failure. If the screw rod is affected by shear failure prior to the yielding of a diagonal member, then the diagonal member loses its anchorage performance, and the tensile force is exerted only by the bond between steel and concrete. In general, the bond strength between steel and concrete is extremely small compared with that between deformed bars and concrete. Therefore, when the anchorage at the truss node fails, the shear resistance performance of the diagonal member decreases rapidly, which may result in the brittle fracture of the member. Hence, it was assumed that because of the failure of the screw rod during the test, the stirrup placed in the WV specimen did not effectively contribute to the shear resistance. Figure 8: Crack patterns of test specimens at failure. Figure 9: Shear resistance mechanism of web members. Table 3 summarizes the failure modes and maximum shear forces of each specimen. In the P specimen series applied with the Pratt truss, the dominant failure modes of the specimens were the horizontal shear cracks that occurred along the top chord.