C-3.53.5 fabrication and erection requirements for fatigue

It is essential that when longitudinal backing bars are left in place, they are continuous or spliced using flush-ground complete-joint-penetration groove welds before attachment to the parts being joined. Otherwise, the transverse nonfused section constitutes a crack-like defect that can lead to premature fatigue failure or even brittle fracture of the built-up member.

Welds that attach left-in-place longitudinal backing to the structural member will affect the fatigue performance of the structural member. Continuous longitudinal fillet welds are stress category B; intermittent fillet welds are stress category E. Longitudinal backing may be attached to the joint by tack welding in the groove, attaching the backing to one member with a fillet weld, or attaching the backing to both members with fillet welds.

The use of weld tabs at transverse butt-joint groove welds enhances weld soundness at the ends of the joint. Subsequent removal of the tabs and grinding of the ends flush with the edge of the member removes discontinuities that may be detrimental to fatigue resistance.

In transversely loaded T-joints subjected to tension, a lack-of-fusion plane between the steel backing and the flange of the T acts as an initial crack-like condition. In groove welds, the root at the backing bar can have discontinuities that can reduce the fatigue resistance of the connection. Removing the backing, back gouging to sound metal, and rewelding eliminates the lack of fusion plane and undesirable discontinuities. Also, attachments with no transition result in abrupt changes in stiffness of the stressed member corresponding to the stress the attachment attracts from the main member. Longer and thicker attachments attract more stress and make the connection of the attachment stiffer. These stiffness changes act as stress concentrations that limit the ability of the main member to resist and aggravate fatigue cracking growth.

The addition of contoured fillet welds at transverse complete-joint-penetration groove welds in T- and corner-joints and at reentrant corners reduces the stress concentration and improves fatigue resistance.

Experimental studies on welded built-up beams demonstrated that if the surface roughness of flame-cut edges was less than 1,000 μin. (25 μm), fatigue cracks would

not develop from the flame-cut edge but from the longitudinal fillet welds connecting the beam flanges to the web (Fisher et al., 1970, 1974). This provides stress category B fatigue resistance without the necessity for grinding flame-cut edges.

Reentrant corners at cuts, copes, and weld access holes provide a stress concentration point that can reduce fatigue resistance. Discontinuities introduced by punching or thermal cutting also reduce fatigue resistance. The stress ranges allowing for a detail with a radius of R ≥ 1 in. (25 mm) require reaming subpunched holes and grinding thermally cut surfaces to bright metal to eliminate the effects of notches on the sur- face of the radius. The limited stress range permitted for details with a radius ≥ a in. (10 mm) allow for these surfaces to be left in the as-produced condition for punched or thermally cut holes. Reaming surfaces with small radii dimensions is difficult.

For Cases 1.4, 1.5, and 3.3 in Table A-3.1, Yam and Cheng (1990) reported that fatigue performance of reentrant corners less than 1 in. and not ground smooth is similar to stress category C when calculated with a stress concentration factor. To be consistent with other cases in this appendix, reentrant corners with radii as small as $\frac{3}{8}$ in. (10 mm) and not ground are assigned stress category $\mathrm{E}^{\prime}$ and do not have to be calculated with a stress concentration factor. Reentrant corners with a radius of at least 1 in. (25 mm) and meeting surface requirements and NDE requirements are associated with stress category C, except for built-up members, where it is stress category D.

For Cases 3.5, 3.6, and 3.7 in Table A-3.1, coverplates and other attachments wider than the flange with welds across the ends are subject to fatigue stress categories E and E', depending on the thickness of the flange. There has been little research on connections with coverplates that are wider than the flange, where the flange is thicker than 0.8 in. (20 mm) and without welds across the ends; therefore, this detail is not permitted, as indicated for Case 3.7. Cover-plated flanges thicker than 0.8 in. (20 mm) are permitted when the ends are welded.

As shown in Case 7.1 in Table A-3.1, base metal subjected to longitudinal loading at details with parallel or transverse welds with no transition radius is subject to stress category C, D, E, or E' fatigue stresses, depending on the length and thickness of the attachment. Attachments with no transition result in abrupt changes in stiffness of the stressed member corresponding to the stress the attachment attracts from the main member. Larger attachments attract more stress and make the attachment connection stiffer. These stiffness changes act as stress concentrations and aggravate fatigue crack growth. The geometric limits of the attachments at the threshold of each category were rewritten in 2022 to clarify their application.

The use of run-off tabs at transverse butt-joint groove welds enhances weld soundness at the ends of the joint. Subsequent removal of the tabs and grinding of the ends flush with the edge of the member removes discontinuities that are detrimental to fatigue resistance.