David Grilli
Published: 2015
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This dissertation investigates the seismic response of Embedded Column Base (ECB) connections and anchor groups, and the rotational stiffness characteristics of exposed column base connections. The research presented is intended to address various unresolved issues on the seismic performance of column base connections. ECB connections are common components in mid- to high- rise steel moment frames, used to transfer base moments and forces into the footing. Despite their prevalence, published research on ECB connections is sparse, and a strength characterization method is yet to be established. Additionally, column base connections are often designed for uplift, requiring use of an anchoring system. Provisions in ACI 318-Appendix D generally govern such systems. However, an alternative detail has become increasingly common, wherein anchors are embedded in the footing with a plate at their lower end, also embedded in the footing. This configuration is subject to less cumbersome design requirements, though no experimental data or validated design guidelines are available to support the design of this detail.Three studies are presented, investigating (1) seismic response of EBC connections, (2) tensile capacity of anchor groups, and (3) rotational stiffness of exposed base connections. The first study includes five full-scale tests on ECB connections subjected to cyclic lateral deformations in the presence of an axial (tensile or compressive) load. Test variables for ECB connections tests include embedment depth, axial load, and column size. The connections demonstrated excellent deformation capacity, and based on test observations, it is determined that ECB connections resist applied moment, axial force, and shear through a combination of the following mechanisms: (1) horizontal bearing stresses acting on the column flanges (2) vertical bearing stresses acting on the embedded base plate at the bottom of the column, and (3) panel shear. In addition, it was determined that although designed as rotationally fixed, the specimens have some flexibility which must be considered in simulation and design. Based on observations, a method is developed to predict the strength of and ECB connection, for the purposes of developing a design model. The method shows good agreement with test data (1.01 test-to-predicted ratio), however, careful application of the method is recommended due to its reliance on empirical factors calibrated to a limited data sample. The second study involves two full-scale tests on anchors grouped to an embedded plate, and analyzes the effect of plate embedment depth (12 and 18 inches) on concrete breakout strength. The results are compared to three strength models, including ACI 318-Appendix D method, the ACI 318 punching shear equation, and the Concrete Capacity Design (CCD) method. It is determined that The CCD method shows the most promise, with an average test-predicted ratio of 0.99, while the ACI 318-Appendix D method and ACI 318 punching shear method appeared conservative and unconservative, respectively. Limitations of the study include the small size of the test set, and the absence of reinforcement in the specimens. The third study was conducted to address observations of significant rotational flexibility in exposed base connections. A method is presented to predict the rotational stiffness of an exposed base connection, validated by experimental data. The method is particularly accurate for conditions where the moment to axial load ratio is large, whereas it overestimates the stiffness of connections where this ratio is low. A detailed analysis and discussion of limitations of each study is provided.