Experimental Performance of Block-Out Connections at the Base of Steel Moment Frames

Abstract

In a block-out connection, a steel column is connected to a footing through an opening (block-out) in a slab-on-grade that is later filled with unreinforced concrete. In design, engineers generally neglect any beneficial effects of the block-out concrete. The objective of this study is to experimentally quantify the effect of block-out concrete on the lower-bound flexural strength and stiffness of block-out connections at the base of steel moment frames. Eight specimens were tested with varying column shape, block-out depth, and baseplate/anchor rod designs. The block-out concrete increased the connection flexural strengths by 69%–91% for block-out depths that were 1.2–1.5 times the column depth. A strength model proposed in previous work was found to reasonably predict the flexural strength, accounting for the effects of block-out concrete. With regard to stiffness, specimens with block-out concrete depths at least 1.15 times the column depth could be accurately modeled as fixed at the top-of-footing elevation.