Numerical parametric analysis of gravity column base-plate connections

Abstract

Gravity column base-plate connections are normally treated as pinned connections in design. Correspondingly, a low rotational stiffness value is specified in the New Zealand steel design standard, NZS 3404. However, previous experimental and numerical studies showed that these pinned connections are significantly stiffer than the recommendation in NZS 3404. Such unexpected rotational stiffness may lead to yielding at the bottom of the column. More importantly, the pre-existing axial compression load could potentially facilitate the occurrence of column axial shortening under yielding, which is difficult and costly to repair. Motivated by the observed discrepancy and the undesired consequences, a numerical parametric study has been conducted to determine the accurate rotational stiffness for gravity column base-plate connections, as well as to investigate the influence on the stiffness of various parameters, including the base plate thickness, anchor rod diameter, axial load ratio, foundation depth and anchor rod pitch distance. The results agree with already published studies and further confirm the fact that the NZS 3404 requirement underestimates the rotational stiffness of gravity column base-plate connections. It has also been found that the axial load ratio has the most prominent effect on the connection behaviour. Results from an ongoing work, which investigates the susceptibility of gravity columns to axial shortening, will also be presented. These more detailed studies are showing that, despite the considerable stiffness and high applied axial compression load, the amount of axial shortening seemed to be acceptable, and demolition of the structure may not be required.