Commentary — Chapter K Additional requirements for HSS and box-section connections

Chapter K includes the following major changes and additions in this edition of the Specification:

(1) Section K1.2 now includes separate equations for the local yielding and punching shear effective widths.
(2) Equations for the chord stress interaction parameter have been revised and collected in Section K1.3.
(3) Additional limit states for rectangular HSS moment connections have been added to Table K4.2.
(4) In Table K4.2, the maximum stress in the sidewall for cross-connections due to in-plane moment is limited to 0.8Fy.

Chapter K addresses the strength of connections to hollow structural sections (HSS) and box sections of uniform wall thickness, where seam welds between box-section elements are complete-joint-penetration (CJP) groove welds in the connection region. The provisions are based on failure modes that have been reported in international research on HSS, much of which has been sponsored and synthesized by CIDECT (International Committee for the Development and Study of Tubular Construction) since the 1960s. This work has also received critical review by the International Institute of Welding (IIW) Subcommission XV-E on “Tubular Structures.” The HSS connection design recommendations are generally in accord with the design recommendations by this Subcommission (IIW, 1989). Some minor modifications to the IIW recommended provisions for some limit states have been made by the adoption of the formulations for the same limit states elsewhere in this Specification. These IIW connection design recommendations have also been implemented and supplemented in later design guides by CIDECT (Wardenier et al., 1991; Packer et al., 1992), in the design guide by the Canadian Institute of Steel Construction (Packer and Henderson, 1997), and in CEN (2005a). Parts of these IIW design recommendations are also incorporated in AWS D1.1/D1.1M, Structural Welding Code—Steel (2020). A large amount of research data generated by CIDECT research programs up to the mid-1980s is summarized in CIDECT Monograph No. 6 (Giddings and Wardenier, 1986). Further information on CIDECT publications and reports can be obtained from their website: www.cidect.com.

Chapter K does not prohibit using joints that fall outside the listed limits of applicability; however, this Specification and Commentary do not provide connection capacities or guidance when doing so. A rational approach to their design is left to the designer. This Commentary gives some insight into the failure modes that should be considered. However, some of the discussions presented later concerning which limit states need to be checked, which can be eliminated, and when they can be eliminated, may or may not apply when outside the limits of applicability. There is also one notable failure mode (local buckling of

the chord face) that has been eliminated from consideration in both the Specification and Commentary due to the fact that, in tests, it did not control the connection strength when staying within the limits. All potential failure modes should be investigated by the designer when working outside the limits of applicability listed in Chapter K.

When inelastic finite element analysis is used, peak strains in the thick shell $(T \times T \times T)$ elements should not exceed $0.02 / T$ at the nominal capacity, where $T$ is the thickness in inches.

The connection capacities calculated in Chapter K are based on strength limit states only. There is no connection deformation limit state considered in these provisions. Subcommission XV-E of IIW, in their most recent design recommendations (IIW, 2012), have now adopted a limit of $0.03 D$ for round HSS, where $D$ is the HSS diameter, and $0.03 B$ for rectangular HSS, where $B$ is the HSS chord width, as the maximum acceptable connection displacement, perpendicular to the main member face at the ultimate load capacity. This limit state equates to approximately $1 \%$ of connection deformation at service loads.

While the majority of Chapter K is in agreement with the previous IIW design recommendations (IIW, 1989), it was determined that adopting a connection deformation limit state for HSS would not be consistent with this Specification, which does not include deformation limit states for connections; however, designers should be aware of the potential for relatively large connection deformations in certain HSS joint configurations. In order to meet the new deformation limit state, IIW and the International Organization for Standardization (ISO) have made some modifications to the range of validity of T, Y, X, and K gap connections and to the calculations of connection strengths, including changes to the strength reduction based on the chord or main member stress function, $Q_{f}$ . The change in the chord stress function is particularly noticeable in high tension areas of main members where no chord stress reduction is necessary when using strength checks only. $Q_{f}$ currently is 1.0 for main members in tension. Where connection deformations would be a concern due to serviceability or stability, the IIW (2012) or CIDECT (Wardenier et al., 2008; Packer et al., 2009) recommendations could be used.

The scopes of Sections K2 and K3 note that the centerlines of the branch member(s) and the chord members must lie in a single plane. For other configurations, such as multi-planar connections, connections with partially or fully flattened branch member ends, double-chord connections, connections with a branch member that is offset so that its centerline does not intersect with the centerline of the chord, or connections with round branch members joined to a square or rectangular chord member, the provisions of IIW (1989), CIDECT (Wardenier et al., 1991; Packer et al., 1992), CISC (Packer and Henderson, 1997; Marshall, 1992; AWS, 2015), or other verified design guidance or tests can be used.

To be consistent with the requirements of Chapter K, box-section members require CJP groove seam welds in the connection region so that each of the member’s faces acts as a single element and is able to develop the full capacity of that element for all viable failure modes depending on the type of connection, geometric parameters, and loading. This constraint guarantees that box-section connections behave in a manner similar to HSS member connections with the same applicable failure modes. The length of the connection region along each member is determined based on the maximum extent of influence of all possible failure modes for the connection. These failure modes are described by Wardenier (1982) for both rectangular HSS truss connections and rectangular HSS moment connections. A

conservative distance equal to the width of the member away from the face of the intersecting member in the connection can be used to define the connection region.

A 50% reduction is applied to the available strength where the connection occurs at a distance less than $l_{\text {end }}$ from an unreinforced end of the chord. Cap plates attached to the ends of round and rectangular HSS members contribute to stiffening the end of the member. If a cap plate is sufficiently welded on all sides, a transverse load applied near the end of the member can be conservatively treated as if it were applied to a continuous member with load applied far from the end of the member. Therefore, there is no minimum end distance requirement in the case of a cap plate. The cap plate will allow the HSS member to develop either the strength of the connected face (plastification or shear yielding) or the strength of the sidewalls (yielding or crippling).

C-KCommentary — Chapter K Additional requirements for HSS and box-section connections