Tensile Strength of Embedded Anchor Groups: Tests and Strength Models

Steel column bases in seismically braced frames and other similar structures must be designed for high uplift or tensile forces. A common detail for this connection involves anchors embedded in the footing with a plate at their lower end, also embedded in the footing. This detail is increasingly prevalent in construction practice because it is exempt from the strength calculations of ACI 318 Appendix D. However, no experimental data or validated design guidelines are available to support the design of this detail. As a consequence, approaches from other similar situations (such as punching shear of slabs) are adapted for this purpose. To address this practical need, this paper presents tension tests on two full-scale specimens featuring this anchorage detail. The main variable examined in the experiments is the embedment depth, such that two depths (12 and 18 in.) are tested. The test specimens exhibit a classic concrete failure cone extending upward from the edges of the embedded base plate. The experimental data provide evidence that the anchorage detail provides an effective means to carry high-tensile loads. The data are evaluated against three strength models, including the ACI 318 Appendix D method, the ACI 318 punching shear equation and the concrete capacity design (CCD) method. It is determined that the ACI 318 Appendix D method is significantly conservative (average test-predicted ratio of 1.34) because it does not consider the beneficial effects of the embedded plate. On the other hand, the punching shear method has an average test-predicted ratio of 0.62. The CCD method shows the most promise, with an average test-predicted ratio of 0.99. Limitations of the study include the small size of the test set and the minimal reinforcement in the specimens.