Embedded column base connections subjected to seismic loads: Strength model

Seismically designed mid- to high-rise steel moment resisting frames in the United States are commonly constructed with the lowest portion of the first story column embedded into a concrete footing to transfer large moments and forces. Despite their prevalence, these Embedded Column Base (ECB) connections have not been experimentally examined until recently, such that strength estimation methods for other types of connections are usually adapted for their design. Based on recent experiments, current approaches for designing ECB connections are examined, and a new strength characterization method is presented. Mechanisms for internal force transfer within the connection are postulated; these include horizontal bearing of the column flange against the concrete, panel zone shear, and vertical bearing of the embedded base plate against the concrete. The proposed approach has two components: (1) a process for moment distribution between the horizontal and vertical bearing mechanisms, and (2) idealized stress distributions for estimation of associated limit states. The proposed method is able to characterize the experimentally observed specimen strengths with good accuracy, such that the average test-predicted ratio is 1.01, with a coefficient of variation 0.06. No significant bias in accuracy of the method is observed with respect to test variables. Limitations of the study are discussed; these arise from its empirical aspects which may not be generally extrapolated to configurations significantly different from the experiments upon which the method is based.