Influence of Bolt Pretension on Prying Force and Connection Behavior

Figure 9 shows the comparisons of (a) distributions of bolt forces; (b) M — 6r curves; and (c) initial connection stiffness among four connection models considering different levels of pretension forces in bolts. These models are A1np, FE5, A1, and FE6 for which initial pretension forces (Co) are considered to be 0%, 20%, 40%, and 60% of the ultimate strength of bolt, respectively, and all the other properties are same among them. From Figure 9(a), it can be observed that the bolt tensile forces starting from different levels of pre-tensioning force get closer among them with increasing of bending tension force, and finally those reach at the same point. Thus, prying force near ultimate level of loading is not affected by the magnitude of pretension force. At the higher level of loading, contact force due to pre-tensioning of bolts is entirely neutralized, and only prying force is retained.

Fig. 10. Influence of bolt diameter on prying force and moment-rotation behavior.

From Figure 9(c), it can be observed that initial connection stiffness is largely increased by 85% for increasing pretension force from nil to snug-tightened level (20% of ultimate strength of bolt). Further increment of pretension forces in bolts is less effective in enhancing initial connection stiffness. When pretension force for bolt is increased from C0 = 20% to 40 % and 60 %, initial connection stiffness is increased by 10% and 14%, respectively, with reference to the value at C0 = 20%. However, the ultimate moment capacity remains almost the same among the four cases (see Figure 9b).

Influence of Connection Parameters on Prying Force and M — 0r Behavior