High-strength bolted connections in steel structures work in one of two ways. A bearing connection transfers load through the bolt shanks contacting the sides of their holes. A slip-critical connection transfers load through friction between the connected plies, generated by clamping the plies together with tensioned bolts. The second depends entirely on the tension actually present in the bolts, and that tension is not what an installer directly controls. Understanding the gap between what is applied and what results is central to investigating why a bolted joint moved or came apart.
Torque is an indirect and unreliable proxy
Most of the torque applied to a fastener is consumed overcoming friction under the nut face and in the threads, with only a small fraction converted into bolt tension. The proportion depends on thread condition, lubrication, surface finish and cleanliness, all of which vary substantially between bolts and between lots. The result is that a correctly applied torque can produce a wide scatter in actual pretension, which is why structural specifications do not generally accept torque alone as an installation method.
The installation methods and what each controls
Turn-of-nut installation brings the joint to a snug condition and then applies a specified additional rotation, using the bolt's own elongation to set tension — which makes it comparatively insensitive to friction. Direct tension indicators compress in a measurable way at a target tension and give visible evidence afterwards. Tension-control bolts shear off a splined end at a calibrated torque. Calibrated wrench installation depends on periodic verification against a tension measuring device. Each leaves different physical evidence, which matters when reconstructing what was done.
Snug-tight is a defined condition, not an approximation
Turn-of-nut depends on starting from a properly snugged joint with the plies in firm contact. Where the plies are not brought together first — because of poor fit-up, distortion or a gap — part of the specified rotation goes into closing the gap rather than tensioning the bolt, and the resulting pretension falls short. Joints with fit-up problems are therefore also joints where pretension is suspect, and the two issues frequently appear together.
Faying surface condition governs slip resistance
In a slip-critical connection, the friction available depends on the condition of the contact surfaces as much as on the clamping force. Specifications assign slip coefficients to surface conditions, and applying an unqualified paint or coating, or leaving mill scale, oil or debris on a surface intended to be clean, reduces slip resistance regardless of how well the bolts were tensioned. Painted faying surfaces in a slip-critical joint are a recurring finding and are visible on recovered hardware.
What slip looks like afterwards
A joint that slipped leaves evidence: bolts bearing against hole edges with corresponding deformation, scoring or fretting on the faying surfaces, elongated holes, and paint or coating disturbed in a pattern showing the direction and magnitude of movement. Where slip was not intended, this establishes that the friction mechanism was overcome — which redirects the inquiry to pretension, surface condition or the load, rather than to the strength of the bolts themselves.
Verifying the bolts were what they should have been
Head markings identify grade and manufacturer and can be checked directly against the specification. Where markings are absent, ambiguous or inconsistent within a single joint, hardness testing and chemical analysis establish the actual grade. Mixed grades within one connection, or fasteners of a lower grade than specified, are findings that stand on their own. Fastener certification records and lot traceability connect the recovered hardware back to what was purchased and accepted.
Fatigue, relaxation and loss of preload
Pretension is not permanent. Short-term relaxation occurs as surface irregularities bed in, and further loss follows from creep in coatings, from thermal cycling and from vibration. A properly installed joint in a static application generally retains adequate tension; one subjected to cyclic loading, temperature swings or a thick coating under the bolt head may not. Where a joint loosened progressively rather than failing suddenly, this is the mechanism to examine, along with whether the application should have been slip-critical in the first place.
What the inspection record should show
Bolt installation is an inspected operation, and the record should identify the method used, the calibration of any wrench or tension device, the inspector's verification of a sample of joints, and the treatment of any that failed verification. Direct tension indicators leave permanent physical evidence that can be checked years later. A joint with no installation record, in a structure where the method required verification, is a gap that is itself relevant.
What to preserve
All fasteners from the failed joint kept together and identified by position, with any direct tension indicators or washers retained undisturbed. The connected plies with faying surfaces unclean and unaltered. Fasteners from adjacent unfailed joints of the same type as a comparison population. And the fastener certifications, installation procedure, inspection records and any coating specification for the faying surfaces.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.