## 3 Test Results  
### Failure Mode  
For the layout of all the specimens, as shown in Figure 2, the typical failure modes for the three empty tubular trusses--T-HW, T-HN, and TS-AS--are basically the same, and the bottom chord has different degrees of bending along the span of the truss. For specimen T-HW, distinct concave deformation occurred around the tubular joints 'k' and 'p' at the bottom chord member, and the bending deformation occurred at the bottom chord member. The failure of this specimen resulted from the surface plasticity of the bottom chord member, and the bending of the bottom chord member, as shown in Figure 5. The failure of specimen T-HN was similar to specimen T-HW, while it had a greater degree of the bending deformation of the bottom chord member and surface plastic deformation around the tubular joints 'k' and 'p' at the bottom chord member, as shown in Figure 6. For specimen TS-AS, the concave deformation occurred around the tubular joints 'k' and 'p' at the bottom chord member, and the weld fracture occurred around the tubular joint 'n' along the diagonal brace 'fn', while the local buckling of diagonal brace 'bk' around the tubular joint 'k' was observed. In addition, the bottom chord bent along the truss span. It should be noted that the concave deformation around tubular joints 'k' and 'p' of TS-AS were less than the concave deformation of specimen T-HW and T-HN, while its concave deformation areas of tubular joints 'k' and 'p' were more extensive than that of the T-HW and T-HN specimens. The failure of this specimen resulted from the local buckling of the diagonal brace member, the surface plasticity around the tubular joints at the bottom chord member, and the bending of the bottom chord, as shown in Figure 7. For specimen TS-FC, when reaching the maximum load the test setup could afford, it did not reach failure state and no obvious deformation occurred at each member and joint with slight overall flexure.  
It was shown from the tests that the failure modes of the trusses with hidden welds welded or not are basically the same, which are the bending of the bottom chord and surface plasticity of the bottom chord member. In addition, it is shown from the test that TS-AS failed by the local buckling of a  
Figure 4: Arrangement of displacement transducers and strain gauges. (**a**) Arrangement of the displacement transducers; (**b**) Front view of single element strain gauges; (**c**) Top view of single element strain gauges; (**d**) Bottom view of single element strain gauges; (**e**) Joints of top chord; (**f**) Midspan joints of bottom chord; (**g**) End joints of bottom chord.  
diagonal brace member and the surface plasticity of the bottom chord member because each joint was reinforced with a half outer sleeve on it. The TS-FC specimen did not fail until the test setup reached the maximum load it could impose, which demonstrated that the reinforcement of filling concrete into the top and bottom members made a greater contribution to reinforce the Warren CHS tubular truss than adding a half outer sleeve to each joint. It was shown from the comparison that the failure modes of T-HW and T-HN are different from the two specimens with reinforcement--TS-AS and TS-FC--and the latter two specimens can improve the mechanical behavior and integral stiffness of trusses.  
Figure 5: Failure modes of specimen T-HW.  
Figure 6: Failure modes of specimen T-HN.