Flexural strength and stiffness of block-out connections for steel columns

In many steel buildings, the columns are attached to the foundation through a block-out in the slab-on-grade that is later filled with unreinforced concrete. Engineers typically neglect the block-out concrete in design, effectively treating block-out connections as exposed connections with pinned behavior. Quantifying the flexural strength and stiffness of block-out connections is helpful for determining moment demands on foundations and may lead to more economical connections at the base of steel moment frames. Eight experimental specimens (two-thirds scale) were subjected to lateral loads to investigate the effects of column size, block-out thickness, and load orientation on connection flexural strength and stiffness. The observed flexural strengths were 1.4-2.7 times greater than those calculated neglecting the block-out concrete, because the block-out concrete effectively thickened and expanded the column base plate. A simple method was developed that predicted the flexural strength of the block-out connections to within 10 percent. The effective flexural stiffness at the base of the columns that were tested could be reasonably estimated using a model that combines the theory of beams on elastic foundations with a base rotational spring.