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This proprietary connection (Seismic Structural Design Associates, Inc. US Patent Numbers 5,680,738 issued 28 October 1997 and 6,237,303 issued 29 May 2001) is shown schematically in Figure 1. It is similar to the popular field welded–field bolted beam-to-column moment frame connection, shown in the AISC LRFD and ASD steel design manuals, that has become known as the "pre-Northridge" connection. Based upon surveys of seismic connection damage, modes of fracture, reviews of historic tests, and recent ATC-24 protocol tests in the SAC/FEMA research program, it was concluded by SEAOC (1996 Blue Book Commentary) that the pre-Northridge connection is fundamentally flawed, and should not be used in the construction of new seismic moment frames. Subsequent finite element analyses and strain gage data from ATC-24 tests by Seismic Structural Design Associates, Inc. (SSDA) of this pre-Northridge connection have shown large stress and strain gradients horizontally across and vertically through the beam flanges and welds at the face of the column. These large flange stress and strain gradients are caused by a very large component of the seismic shear, typically 50% or more, that is carried by the beam flanges. It is these flange shear forces that produce a prying moment in the welds and flanges at the face of the column and at the weld access holes that led to the weld and flange fractures and to the column flange divot modes of connection fracture.

 

However, by (1) separating the beam flanges from the beam web in the region of the connection and (2) welding the beam web to the column flange, the force, stress and strain distributions in this field welded-field bolted connection are changed dramatically in the following ways:

The beam web resists the entire beam vertical shear and its share of the beam moment whereas the flanges resist only moment. Moreover, the SlottedWebTM connection is kinematic so that the connection force distributions are statically determinate and are independent of interstory drift. Full scale low cycle fatigue tests have shown that this kinematic attribute more than triples the fatigue life over non-slotted connections that subject the beam flange/welds to large beam seismic shear forces.

Since most W sections have a flange to beam modulus ratio of 0.65 < Zflg /Z < 0.75, both the beam web slots and the beam web-to-column flange weldment force the beam web to resist its portion of the total beam moment.

Fig 1Proprietary SlottedWebTM Connection

The beam web separation from the beam flange reduces the large stress and strain gradients across and through the beam flanges and at the weld access hole by permitting the flanges to flex out of plane, which reduces the beam flange/weld prying moment on the column flange. Typically, the elastic stress and strain concentration factors (SCFs) of the pre-Northridge connection, caused by severe local beam flange and/or column flange distortions, are reduced from 4.0 to 5.0 in non-slotted beams down to 1.2 to 1.4, which dramatically reduces the beam flange prying moment and the accumulated plastic strain and ductility demand under cyclic loading. These attributes enhance and extend this connection's fatigue life to more than triple the fatigue life of non-slotted moment frame connections.

By allowing the beam flanges to flex out-of-plane, the kinematic slotted connection eliminates the limitations on panel zone strength relative to beam strength. These limitations are required for non-slotted unreinforced welded flange connections to provide a balance between yielding of the panel zone and other connection elements to prevent excessive panel zone deformations that result in the kinking of the column flanges that increases the fracture toughness demands on the joint weldments.

The lateral-torsional mode of beam buckling that is characteristic of non-slotted beams is eliminated. The separation of the beam flanges and beam web allow the flanges and web to buckle independently and concurrently, which eliminates the twisting mode of buckling and its associated torsional beam flange/weld stresses. Elimination of this buckling mode is particularly important when seismic moment frames that are located on the perimeter of the building support the exterior cladding of the building. Even a small amount of twisting of these beams will be reflected as significant visible distortion of the exterior cladding.

The separation of the beam web and the flange results in biaxial rather than triaxial stress and strain states in the region of the connection, which increases its fatigue life. Moreover, residual weldment stresses are significantly reduced because the slots also provide a long structural separation between the vertical web and horizontal flange weldments.