# A Study on the Seismic Performance of Steel H-Column and T-Beam-Bolted Joints
## 4 Parametric Analysis
### Parameter Analysis of High-Strength Bolt Preload  
Figure 26 depicts the stress program of the joint when it attains the ultimate bearing capacity under the hysteretic loading mode with different high-strength bolt preloading forces. As the preloading force increases from 180 kN to 270 kN, the yield surface of the joint shows no significant change. The stress on the wall of the square steel-pipe column increases to some extent, yet it has almost no impact on the overall stress of the joint. The joint exhibited identical failure modes to the BASE joint, characterized by trough plate buckling and beam weld fracture.  
Figure 27 presents the hysteresis curves of nodes under different high-strength bolt preloading forces. The hysteresis curves of each node are fusiform. Under a small pretightening force, the pinching effect of the node is pronounced, suggesting that the degree of bolt slip increases when the pre-tightening force is small. The change in the preloading of high-strength bolts has no impact on the ultimate bearing capacity of the joints.  
Figure 26: Preload joints stress of different high-strength bolts, (**a**) JD-YJL-0.5P, (**b**) JD-YJL-1.5P.  
Figure 25: Energy dissipation curves of nodes with different column wall thicknesses. (**a**) Energy-consuming capacity comparison, (**b**) equivalent viscous damping coefficient comparison.  
Skeleton curves of joints with different bolt preloading forces are shown in Figure 28, and Figure 29 shows the energy-dissipation curve of the nodes. As the displacement increases gradually, the equivalent viscous damping coefficient of each node shows an upward trend. With the increase in the bolt preloading, the equivalent viscous damping coefficient of the node first increases and then decreases, yet this pattern is not very distinct. The energy consumption of the joint reaches its maximum when the bolt preloading is 270 kN. In conclusion, the pre-tightening force of the high-strength bolt has a negligible impact on the energy consumption of the node.  
When the preload is increased from 180 kN (0.8 P) to 270 kN (1.2 P), the maximum stress at the edge of the bolt hole of the column wall is increased from 285 MPa to 312 MPa (increase by 9.5%). The maximum stress on the web plate of T-part 1 only increased from 345 MPa to 358 MPa (3.8% increase) (Figure 26). These increases are much smaller than the yield strength of the material (355 MPa for Q355B steel), indicating that changes in preloading force have a limited effect on the overall stress distribution. When the preload increases from 180 kN (BYJL-1) to 270 kN (BYJL-5), the yield load of the node increases from 372.1 kN to 379.5 kN (an increase of 1.9%), and the peak load increases from 436.8 kN to 442.3 kN (an increase of 1.3%) (Table 3, Figure 28). These increases are much smaller than the material strength dispersion (Q355B steel yield strength is 355 MPa), confirming the conclusion that the preload has 'no material effect' on the joint performance.  
The non-monotonic trend of the equivalent viscous damping coefficient is mainly due to the coupling effect of the interface slip and friction effect: at the initial stage (axial pressure ratio \(\leq\) 0.4), the increase in positive pressure on the contact surface strengthens the friction energy dissipation, and the equivalent damping increases. When the axial compression ratio is greater than 0.4, the combined action of bolt preload and axial compression leads to closer contact surface, reduced slip, and a slower or even lower friction energy consumption increase (Figure 25b). When the preloading force is low (such as 180 kN), the bolt slip is significant, and the friction energy consumption is dominant. When the preload is too high (e.g., 270 kN), the joint stiffness increases to inhibit the slip, and the energy dissipation becomes dependent on the plastic deformation of the material, resulting in the equivalent damping first rising and then decreasing (Figure 29b).