Discussion on High-Strength Bolt Pretension Upper Limits and Practical Mechanical Behavior

Engineer Chen Cheng-ping published an article examining engineering inquiries regarding whether high-strength bolt pretension should have an upper limit. Domestic steel design and construction specifications establish that the required minimum pretension is 70% of tensile strength (0.7Tub), but provide no maximum ceiling. Similarly, RCSC specifications state that bolts exceeding specified pretensions need not be rejected. During tightening, bolts experience combined tension and torsion; once installation is complete, torsional stresses dissipate, leaving lower operational stress. Consequently, as long as bolts do not undergo obvious plastic deformation or fracture, joints perform as intended. Practically, pretension should not exceed 95% of the bolt's proof load, damaged or excessively tightened bolts must be replaced, and yielded bolts should not be reused.

Key takeaways

• Codes specify a minimum installation pretension of 70% tensile strength to secure slip resistance and joint performance, without imposing an upper cap.
• RCSC specifications state that exceeding specified pretension does not warrant rejection, as torsion is released upon completion.
• Threads typically yield after pretensioning; practical guidelines suggest bolt pretension should not exceed 95% of proof load, and used bolts should not be reused.
• Safety assessments should focus on observable plastic deformation, fracture, or relaxation rather than solely checking if pretension exceeds minimum values.