**4. Test Results**

Table 3 shows the summary of test results, including maximum vertical loads, displacement, connection rotation, and failure mode.


Since the pushdown force is applied to the middle column, the vertical displacement gradually increases. In simple connections such as top and seat angle where the connection possesses a limited capacity to develop the full plastic moment of connected beams, the specimen rotates at both ends following a major deflection below the removed column. The normalized force versus normalized rotation for all tested simple connections is shown in Figure 4. Figure 4 indicates that at the initial pushdown stage, there is no substantial loading resistance. The average *F*/*F<sup>p</sup>* is around 0.2 at the normalized rotation of 5, until developing axial tensile force in the connected beam as a result of large deflection, indicating the beginning of catenary action. Besides, the shear fracture of bolts causes several jumps in the curve in which the connections experience major localized bearing deformations in the vicinity of bolt holes. Overall, the previous literature indicated that axial tensile force has a major contribution toward progressive collapse resistance in flexible double-span assemblies in which the catenary mechanism takes place before connection component rupture or shear fracture of bolts. Also, the important feature of progressive collapse performance of flexible connections is that although double-span assemblies experienced large deformation, the connected beam remains in the elastic region and connection components experience large plastic strain.

**Figure 4.** Normalized pushdown resistance against normalized rotation of flexible connections.

The normalized force versus normalized rotation for all tested semi-rigid connections is shown in Figure 5. Figure 5 indicates that semi-rigid connections have higher initial stiffness as a result of high flexural capacity. Nevertheless, after normalized rotation of 5, the stiffness experiences a decrease in most of the specimens as a result of limited capacity connections' components, i.e., top and seat angles. Overall, the previous literature indicates that connections in this category mainly fail due to bolt thread stripping and fracture or fracture at the web angles. Besides, connection failure is categorized in two phases, in which, at the first phase, the connection resists vertical pushdown force through flexural action, and after large plastic rotation, the connections go into the catenary mechanism, indicating phase 2.

F / Fp

**Figure 5.** Normalized pushdown resistance against normalized rotation of semi-rigid connections.

The normalized force versus normalized rotation for all tested fully rigid connections is shown in Figure 6. This category possesses the highest stiffness, where in the normalized rotation of around 2, almost all specimens develop the full plastic moment of connected beams. In addition, Figure 6 shows that by considering unique configuration and large *R<sup>i</sup>* , SidePlate has the highest stiffness and ultimate strength. Generally, the previous literature indicated that ductility demand for traditional rigid connections in the case of column removal is mainly controlled by the column shear panel zone, while in the SidePlate connection, it is controlled by a connected beam. Also, in fully rigid connections, the fracture at beam flange or shear plate are the main reasons for failure mode without premature weld or connection's component failure.

**Figure 6.** Normalized pushdown resistance against normalized rotation of fully rigid connections.

Yield mechanisms and failure modes are the factors that control both the resistance and ductility or rotational capacity of the connection. Failure modes and yield mechanisms are related but are inherently different. Failure modes cause fracture, loss of deformation capacity, or significant loss of resistance. Yield mechanisms induce inelastic deformation and result in dissipation of energy and changes in stiffness without inducing fracture or excessive loss of resistance. In fully rigid connections, the ratio of rotational capacity of connection to rotational capacity of beam ( <sup>ϕ</sup> ϕ*p* ) is around 12, where the failure mode is controlled by fracture at shear plate and beam flange. For some specimens in this category, such as I-W, weld fracture causes significantly less ductility, energy dissipation, and plastic rotational capacity. The ratio of <sup>ϕ</sup> ϕ*p* is around 16 and 13 for semi-rigid and flexible connections, respectively. The failure modes in semi-rigid connections are governed by local buckling and deterioration caused by the large inelastic deformation web cleats and bolts. So, it can be concluded that semi-rigid connections provide large ductility and a ductile failure mode compared to fully rigid connections.
