Technical analysis of bolt tension physics, elastic behavior, and the critical mechanical distinction between applied effort and actual clamping force.
The Physics of Bolt Tension
A high-strength structural bolt functions mechanically as a stiff spring. When a nut is rotated, the bolt shank stretches elastically along its longitudinal axis. The internal resistance to this elongation is what generates the clamping force required to hold steel plies in firm contact or to resist slip through friction.
A primary inspector realization is that most applied torque is lost to friction, commonly the majority. This friction occurs at the thread interface and between the nut and washer face. Because factors like surface finish, moisture, and factory lubrication levels vary, relying on a fixed torque value to determine bolt tension without site-specific calibration is mathematically flawed.
RCSC Section 8.2; AISC 360 Chapter J; ASTM F3125.
If a bolt reaches a specific torque but does not show the physical displacement markers required for its method, the inspector should assume high friction is producing a false reading. The bolt may feel tight to a wrench but fail to provide the design clamping force.
Substituting standard torque charts for site-verified tension measurements. These charts do not account for thread condition or the impact of environmental weathering on site-delivered lots.
Structural integrity relies on minimum axial bolt tension. Inspection verification must confirm the installation method has actually produced this tension through objective markers.
Embedment Loss and Relaxation
Shortly after a bolt is tightened, a phenomenon known as bolt relaxation or embedment loss occurs. This is the result of microscopic surface high points flattening under intense pressure and the threads of the bolt and nut slightly yielding into one another. This causes a slight decrease in the initial installed tension.
The RCSC Specification compensates for this certainty by requiring all pretensioned fasteners to hit at least five percent above the minimum required tension during testing. This margin ensures that the joint remains above the required design threshold even after initial settlement. If the steel has thick coatings or uneven surfaces, the loss can be more pronounced.
RCSC Section 7.1; RCSC Commentary Appendix.
In multi-bolt joints, the tightening of one fastener will often relax its neighbor as the steel plies compress further. This requires the installation crew to perform a secondary snugging and tensioning pass to ensure uniform clamping across the entire group.
Tightening bolts in a single pass and moving on. Without a follow-up pass to confirm the first-tightened bolts haven't relaxed as the joint pulled together, the first bolts in the sequence will often be under-tensioned.