What is the difference between a bearing connection and a slip-critical bolted connection?
A bearing connection transfers load through the bolt shanks contacting the sides of their holes, while a slip-critical connection transfers load through friction between the connected plies, generated by clamping the plies together with tensioned bolts. These are the two ways high-strength bolted connections in steel structures work. A slip-critical connection depends entirely on the tension actually present in its bolts, and that bolt tension is not what an installer directly controls. Understanding the gap between what an installer applies and the bolt tension that results is central to investigating why a bolted joint moved or came apart.
Why is torque an unreliable measure of bolt pretension?
Torque is an indirect and unreliable proxy for bolt pretension because 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 of applied torque that becomes bolt tension depends on thread condition, lubrication, surface finish and cleanliness, all of which vary substantially between bolts and between lots. As a result, a correctly applied torque can produce a wide scatter in actual bolt pretension, which is why structural specifications do not generally accept torque alone as an installation method.
What are the bolt installation methods, and what does each one control?
Turn-of-nut installation controls a specified rotation of the nut, direct tension indicators compress in a measurable way at a target tension, tension-control bolts shear off a splined end at a calibrated torque, and calibrated wrench installation depends on periodic verification against a tension measuring device. 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 turn-of-nut installation comparatively insensitive to friction. Direct tension indicators give visible evidence afterward. Each of these bolt installation methods leaves different physical evidence, which matters when reconstructing what was done on a bolted joint.
Why does the snug-tight condition matter in turn-of-nut bolt installation?
The snug-tight condition matters because turn-of-nut installation depends on starting from a properly snugged joint with the plies in firm contact; snug-tight is a defined condition, not an approximation. Where the plies are not brought together first, because of poor fit-up, distortion or a gap, part of the specified turn-of-nut rotation goes into closing the gap rather than tensioning the bolt, and the resulting pretension falls short. Bolted joints with fit-up problems are therefore also joints where bolt pretension is suspect, and the two issues frequently appear together.
How does faying surface condition affect the slip resistance of a bolted joint?
Faying surface condition governs slip resistance: in a slip-critical bolted connection, the friction available depends on the condition of the faying (contact) surfaces as much as on the clamping force from the bolts. Specifications assign slip coefficients to faying surface conditions, and applying an unqualified paint or coating, or leaving mill scale, oil or debris on a faying 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 evidence does a bolted joint that slipped leave behind?
A bolted 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, that evidence establishes that the friction mechanism of the bolted joint was overcome, which redirects the inquiry to bolt pretension, faying surface condition or the load, rather than to the strength of the bolts themselves.
How can you verify that the bolts in a failed joint were what was specified?
Bolt head markings identify grade and manufacturer and can be checked directly against the specification, and where head markings are absent, ambiguous or inconsistent within a single joint, hardness testing and chemical analysis establish the actual grade of the bolts. 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.
Can a bolted joint lose its pretension over time?
Yes, bolt pretension is not permanent. Short-term relaxation occurs as surface irregularities bed in, and further loss of preload follows from creep in coatings, from thermal cycling and from vibration. A properly installed bolted joint in a static application generally retains adequate tension; a bolted joint subjected to cyclic loading, temperature swings or a thick coating under the bolt head may not. Where a bolted joint loosened progressively rather than failing suddenly, loss of preload is the mechanism to examine, along with whether the application should have been slip-critical in the first place.
What should the bolt installation inspection record show?
The bolt installation inspection record should identify the installation method used, the calibration of any wrench or tension device, the inspector’s verification of a sample of joints, and the treatment of any joints that failed verification, because bolt installation is an inspected operation. Direct tension indicators leave permanent physical evidence that can be checked years later. A bolted joint with no installation record, in a structure where the installation method required verification, is a gap that is itself relevant.
What should be preserved after a bolted joint fails?
After a bolted joint fails, all fasteners from the failed joint should be kept together and identified by position, with any direct tension indicators or washers retained undisturbed. The connected plies should be kept with their faying surfaces unclean and unaltered. Fasteners from adjacent unfailed joints of the same type should be preserved as a comparison population. The fastener certifications, installation procedure, inspection records and any coating specification for the faying surfaces should also be preserved.