Lehman and Roeder 2012
This research investigates and develops design procedures for simple and economical connections of circular CFT piers or columns to reinforced concrete foundations, pile caps and wide cap beams (as used in bridge construction). Nineteen large-scale specimens were tested. The behavior of monolithic and recessed connections was evaluated. The effect of variation in yield stress of steel, diameter of the tube, D/t ratio, and embedment depth on connection performance was also investigated. The effect of variations in load history on connection performance was evaluated. Design procedures and equations for the connection are proposed.
Experimental Study, Results, and Discussion
Nineteen circular CFT column to footing connections were constructed to be similar in their geometry to a full-scale building column or a half-scale bridge pier. The columns were cantilevers anchored into a footing. Eighteen specimens were 6 ft. in length, with a 20 in. circular cross section, and 0.25 in. tube thickness. One specimen was 9.33 ft. in length, with a 30 in. circular cross section, and 0.252 in. tube thickness. Low shrinkage self-consolidating concrete was used and the concrete strength for the columns ranged from 7.83 ksi to 11.89 ksi. Steel tubes had yield strengths ranging from 49 ksi to 75 ksi.
Two types of embedded connection details were tested. The first connection type is a monolithic connection, where the column is placed prior to pouring the foundation. The second type of connection is a recessed connection where the column is placed in a recess of an already poured foundation and grouted into place. The central component for both connections is a flange (annular ring) at bottom of the column. No shear connectors, dowels or internal reinforcement are placed in the tube or penetrate into the adjacent reinforced concrete component. Tests were conducted with the column under constant axial load. A hydraulic actuator at the top of the column applied the cyclic lateral load. The tests evaluated a series of parameters including the type of connection, embedment depth, steel strength, type of tube, axial stress ratio, and tube geometry.
Experimental results of the new connection for circular CFT columns with an annular ring welded to the base of the tube and embedded directly into the foundation were evaluated. The test results indicate that the proposed annular ring embedded connection is effective and practical. Specimens with longer embedment depth are capable of achieving large drifts without degradation of the system and minimal damage to the footing.
Design Formulation
A design procedure for the CFT column-to-foundation connection was developed, specifically providing recommendations on 1) dimensioning of the annular ring; 2) Determination of the required embedment depth of the tube into the foundation; and 3) Determination of the required depth of concrete below the tube to prevent punching failure.
Reference
Lehman, D. E., and Roeder, C. W. (2012). “Foundation connections for circular concrete-filled tubes.” Journal of Constructional Steel Research, 78, 212–225. doi:10.1016/j.jcsr.2012.07.001