Seismic Response of Block-Out Column-Base Plate Connections under Axial Compression and Flexure

Exposed column-base connections are commonly overtopped with concrete to provide an even surface with the surrounding floor slab. To facilitate construction, this concrete is poured, after construction of the floor slab, into a blocked out region of the slab in which the column is placed. The result is a shallowly embedded connection that has significantly higher lateral strength and stiffness with respect to the exposed column-base plate connection without the embedment. This has implications for design and, more importantly, seismic performance assessment. Three large-scale experiments are presented to examine the response of such connections. The connections were subjected to combinations of axial compression and cyclic lateral deformation representative of seismic loading. Significant increases in both strength and stiffness (relative to exposed base plate connections) are noted, arising from the block-out concrete resisting the uplift of the base plate. Stable and ductile hysteretic response (similar to exposed column-base plate connections) is also noted. Failure modes include concrete breakout above the base plate and fracture of the anchors. Results from this study are synthesized with results from previous studies on similar connections to propose a strength model, and to evaluate previously proposed models for stiffness.