Step 3. The ConXL moment connection is a true biaxial moment connection; thus, the committee determined that columns must be sufficiently strong to permit simultaneous development of flexural hinging in all beams framing to a column, not just beams along a single plane. This biaxial column-beam moment evaluation is more conservative than current AISC Seismic Provisions requirements that consider plastic hinging of beams in a single plane only, even though columns supporting moment frames in orthogonal directions are possible with other connections using built-up box sections or other built-up column sections. In calculating the ConXL biaxial column-beam moment ratio, it is permitted to take the actual yield strength of the column material in lieu of the specified minimum yield stress, , and to consider the full composite behavior of the column for axial load and flexural action (interstory drift analysis). The default formula for column strength provided in the design procedure assumes that equal strength beams are present on all faces of the connection. When some beams framing to a column are stronger than others, it is permitted to use basic principles of structural mechanics to compute the actual required flexural strength.
The design procedure also considers the critical beam strength as it relates to the column strength at locations just above the beam top flange and just below the beam bottom flange, where flexural demand on the columns are greatest. Flexural demand on the column within the panel zone is less than at these locations.
Step 5. The available tensile strength for the bolts used in the ConXL connection is specified as the minimum bolt pretension load. The purpose of assigning the minimum pretension load as the available bolt tensile strength is to prevent overcoming of bolt pretension, at least up to the bolt loading subjected by the probable maximum moment. The minimum bolt pretension load is 102 kips (450 kN). Bolts are checked for tension only because the frictional force developed by the bolt pretension will resist beam shear (see Steps 6 and 7).
Step 6. Beam shear is resisted by the friction developed between the collar flanges and the collar corners. The collar flanges are clamped against the collar corner assemblies and column when the collar bolts are pretensioned. This pretension clamping force creates friction between the machined surfaces of the collar flanges and collar corners. The machined surfaces are classified as a Class B surface (unpainted blast-cleaned steel surfaces). From AISC Specification Section J3.9, the design frictional resistance per bolt is:
(Spec. Eq. J3-4)
| where |
|---|
| Du = 1.13 |
| Tb = 102 kips (450 kN) |
| hf = 1.0 |
| ns = 1 |
| μ = 0.50 |
| φ = 0.85 for oversized bolt holes |
| φRn = 0.85(0.50)(1.13)(1.0)(102 kips)(1.0) |
| = 49.0 kips/bolt (220 kN/bolt) |
There are 16 bolts per beam end providing a total of 784 kips (3,500 kN) of frictional resistance against shear. This frictional force is significantly greater than any beam shear developed by an allowable beam.
Steps 8 and 9. The available length of weld for the collar web extension and collar corner assemblies allow for minimum-sized fillet welds to resist beam shear.
Step 11. The collar corner assemblies provide additional strength to the column walls to resist panel zone shear. Without taking into consideration the contribution of the concrete fill, the column section along with the collar corner assemblies should provide sufficient strength for anticipated panel zone shear; this should be confirmed during the design procedure calculations.
Step 12. The ConXL moment connection is a biaxial connection. Strong-column/ weak-beam requirements specified by the AISC Seismic Provisions were formulated considering the typical planar framing prevalent in moment-frame construction following the 1994 Northridge earthquake. Because the ConXL connection is primarily used in intersecting moment frames, with biaxial behavior an inherent part of the design, the CPRP felt that it was imperative to require that columns have sufficient strength to develop expected simultaneous flexural hinging in beams framing into all column faces. The biaxial calculation considers all moment beams attached to the column.