C-3.43.4 bolts and threaded parts

Bolts, bolted joints, and threaded parts can be subjected to cyclic forces in shear, ten- sion, or some combination of the two. This section addresses the fatigue resistance of bolts and threaded parts subject to cyclic tension and the fatigue resistance of connected parts subjected to cyclic shear. This section does not address the combined

effect of cyclic shear and tension on the fatigue resistance of bolts, bolted joints, and threaded parts. The fatigue resistance of bolts used in joints subjected to cyclic shear will not govern the fatigue life of conditions considered and permitted (pretensioned joints) in Appendix 3.

The fatigue resistance of connected parts in mechanically fastened joints subjected to cyclic tension is not addressed in Appendix 3. The fatigue resistance of connected parts in mechanically fastened joints subjected to cyclic tension will generally not govern the fatigue life of conditions commonly encountered in structural steel build- ings. Unusual conditions may be encountered for which the fatigue resistance of the connected parts in mechanically fastened joints subjected to cyclic tension will govern the fatigue life.

There are many examples where fatigue cracking is the consequence of out-of-plane deformations. This is referred to as displacement-induced fatigue cracking. Elimination of displacement-induced fatigue cracking is largely a matter of good detailing, which is a difficult thing to quantify.

Satisfying the requirements for force-induced fatigue design does not eliminate the need to examine the possibility of distortion-induced fatigue cracking.

Bolted Connections Subjected to Cyclic Shear Forces. This Specification does not address the use of nonpretensioned fasteners for joints subjected to cyclic shear forces.

The fatigue strength of a bolted shear splice is directly influenced by the type of load transfer in the connection. This load transfer can be completely by friction (slipcritical) at the interface of the connected parts, completely by bearing of the bolts against the connected material, or by some combination of these two mechanisms. In the case where the load transfer is by friction, fretting of the connected parts occurs, particularly on the faying surfaces near the extremities of the joint. Cracks are initiated and grow in this region, which means that cracking takes place ahead of the first (or last) bolt hole in a line, and the crack progresses through the gross cross section of the component. If the connection is not designed as slip-critical, the load should be assumed to be transferred entirely by shear in the fasteners and an equilibrating bearing force in the connected parts. The local tensile stress in the region of the connected part adjacent to the hole is high, and this is now the location where fatigue cracks can start and grow through the net cross section of the connected part. Both types of fatigue crack behavior have been observed in laboratory tests and, in a few cases, both types have been observed within the same test. It is worth noting that there is no history of fatigue failure of high-strength bolts themselves in shear splices; only the connected material is susceptible to fatigue cracking.

Bolted Connections Subjected to Cyclic Tensile Forces. The RCSC Specification for Structural Joints Using High-Strength Bolts (RCSC, 2022), hereafter referred to as the RCSC Specification, requires that bolted joints subjected to tensile fatigue be pretensioned. In pretensioned joints, deformation of the connected parts resulting from the applied tension introduces prying action, the magnitude of which is not completely predictable (Kulak et al., 1987). The effect of prying is not limited to a change in the average axial tension on the bolt but also includes bending in

the threaded area under the nut. Because of the uncertainties in calculating prying effects, definitive provisions for the allowable stress range for bolts subjected to applied axial tension are not included in this Specification. To limit the uncertainties regarding prying action on the fatigue of pretensioned bolts in details that introduce prying, the allowable stress range provided in Table A-3.1 is appropriate for extended cyclic loading only if the prying induced by the applied load is small.

The tensile stress range of bolts that are pretensioned to the requirements of Table J3.1 or J3.1M can be conservatively approximated as 20% of the absolute value of the applied cyclic axial load and moment from dead, live, and other loads. AISC Design Guide 17, High Strength Bolts: A Primer for Structural Engineers (Kulak, 2002), states that the final bolt force is the initial pretension force plus a component of the externally applied load that depends on the relative area of the bolts and the area of the connected material in compression. Test results show that this approach is a good predictor and that the increase in bolt pretension can be expected to be on the order of not more than about 5 to 10%, which affirms that the 20% rule is a conservative upper bound. The approximated stress range is compared with the allowable and threshold stress range.

The prying force, $q_{r}$, determined in Part 9 of the AISC Steel Construction Manual (AISC, 2017) is based on a model that considers only equilibrium, and does not produce an estimate of the prying effect adequate for determination of fatigue resistance. See A Fatigue Primer for Structural Engineers (Fisher et al., 1998) and Chapter 7 of AISC Design Guide 17 (Kulak, 2002), for more information.

Fatigue provisions in Appendix 3 and in the RCSC Specification are applied differ-ently, but produce similar results. Some key differences are as follows:

• (a) Appendix 3 is a stress range applied using a bolt net tension area, where RCSC Specification Table 5.3 is a maximum stress applied based upon the cross-sectional area determined from the nominal diameter.
• (b) Appendix 3 is applied by determining a maximum allowable stress range and a stress range threshold regardless of the bolt material, where RCSC Specification Table 5.3 is applied by determining a maximum bolt stress, which does depend on the bolt material; therefore, the stresses obtained from Appendix 3 should be compared to the tensile stress range including prying, while the stresses obtained from RCSC Specification Table 5.3 should be compared to the total applied tensile stress including prying. The total bolt force may be estimated as recommended by Kulak et al. (1987).

The fatigue provisions in Appendix 3, not those in the RCSC Specification, should be used when this Specification must be satisfied.

Threaded Parts Subjected to Cyclic Tensile Forces. In addition to addressing high-strength bolted joints, which must be pretensioned when subject to tensile fatigue, Appendix 3 also addresses threaded parts subject to tensile fatigue that may not be pretensioned. Examples of such conditions include threaded anchor rods, threaded rod hangers, and threaded rod bracing. Common bolts, which cannot be effectively pretensioned, are not addressed in the RCSC Specification. Common bolts (ASTM A307) are approved for use under this Specification and may be subject to tensile

fatigue. However, bolted joints with only steel within the grip subject to tensile fatigue should employ high-strength bolts and therefore should be pretensioned.

The fatigue resistance of threaded parts subjected to tension is predictable in the ab- sence of pretension and prying action because the stress range is directly and solely related to the applied tension. In pretensioned joints, much of the applied tension reduces the clamping force between the plies and only a small portion (permitted to be conservatively taken as 20% of the applied load) is reflected in the stress range. In non- pretensioned joints, the stress range includes the entire combined effect of the applied tension and prying action. Consequently, the fatigue resistance of nonpretensioned threaded parts subjected to cyclic tensile forces is greatly reduced. High stress ranges combined with an unfavorable stress category, G, may result in impractical designs.