In this study, a post-tensioning method using externally unbonded steel rods was applied to pre-damaged reinforced concrete beams for flexural strengthening. Nine simply-supported beams, three reference beams and six post-tensioned beams, were subjected to three-point bending. The design parameters observed in this study were the amount of tension reinforcements (3-D19, 4-D19, and 2-D22 + 2-D25; “D” indicates the nominal diameter of the rebar) and the diameters of the external rod (φ22 mm and φ28 mm). A V-shaped profile with a deviator at the bottom of the mid-span was applied to the pre-damaged beams, and a post-tensioning force was added to overcome the low load resistance and deflection already incurred in the pre-loading state. The post-tensioning force caused by tightening the nuts at the anchorage corresponded to a strain of 2000 με in the external rods; this value was approximately equal to the strain caused by torque that two adults can apply conveniently. The post-tensioning system increased the load-carrying capacity and flexural stiffness by approximately 40–112% and 28–73%, respectively, when compared with the control beams. However, the external rods did not yield in the post-tensioned beam with larger steel reinforcements and external steel rods. The external rod with the larger diameter increased the flexural strength more effectively.

This study aims to investigate the joint strength of longitudinal plate-to-high-strength steel circular hollow-section X-type joints under plate axial load. The material properties of high-strength steel with nominal yield strengths of 460, 650, 900, and 1100 MPa were used for parametric analysis. The variables for analysis were ratios of chord diameter to thickness, plate width to chord diameter, and utilization. To determine the capacity of connections, the joint strengths using a deformation limit and a strength limit were considered and compared with American Institute of Steel Construction (AISC), Eurocode 3, and ISO 14346. The joint strength determined by the ultimate deformation limit is approximately equal to the joint strength determined by the strength limit state at the yield strength of 460 MPa. The difference between both the joint strengths, however, becomes higher with increasing yield strength. The design equations estimate the joint strength based on the ultimate deformation limit approximately until the limitation of the nominal yield strength in each design code. As the nominal yield strength increases, the joint strengths are overestimated. In using high-strength steel in circular hollow-section X-type joints, the reduction factors of 0.75 and 0.62 for AISC and ISO 14346 are suggested for the nominal yield strengths of 900 and 1100 MPa, respectively. In Eurocode 3, the reduction factor of 0.67 is also suggested for a yield strength of 1100 MPa.

The current design codes i.e. AISC 360-16, CSA-S16-19, EC-04 etc. provide empirical relationships to estimate the capacity of shear connectors which were developed based on pushout tests of headed studs and channels connectors in exposed type sections. Therefore, these equations may result inaccurate predictions for strength of connectors in infilled-type sections. This study presents a detailed experimental study investigating the performance of angle connectors in composite sections. The testing program consisted two series of pushout tests. A total of 36 specimens were tested, considering the influence of several important parameters i.e. the length (L_c), height (h_c), and web-thickness (t_w) of angle connector, length of the weld (w) and the direction of shear connector (forward/backward) etc. The tests results demonstrated that with increasing connector height h_s, and thickness, the maximum load P_max and slip δu were increased. The connector direction didn't change much the load-slip behavior. The prediction accuracy of the existing shear capacity models was evaluated by comparing the predictions with experimental results. The current equations were noticed to be highly inconsistent in predicting the shear capacity of angle connectors, especially in case of infilled type sections. When the entire length of connector was taken as the effective length, the models overestimated the shear capacity. While in case when the welding length was taken as effective length in calculations, the models underestimated the shear strength of angle connectors.