This study presents a critical assessment of the reliability of current base plate connections in steel special moment resisting frames (SMRFs). Using a probabilistic outlook, this research evaluates the reliability of exposed column base (ECB) connections in SMRFs designed based on the current seismic design provisions; it suggests (and implements) a statistical approach to compute resistance factors for three modes of failure (concrete bearing, base plate yielding at tensile interface, and anchor bolt fracture) of ECB connections to achieve a target reliability index, β, of 4.5. Since ECB connections are limited to short buildings, therefore, this study is conducted on two-story and four-story SMRFs which are analyzed using a suite of 120 ground motions originating from strike-slip and reverse faults. ECB connections for the two-story building are designed to simulate pinned connection, while the bases of the four-story building represent moment connections. Detailed methodology for calculating the β of ECB connections is presented considering the three limit states in a moment–axial load interaction curve. Results indicate that the implementation of current seismic provisions results in β ~ 3.3 for non-moment resisting ECB connections for all tried combinations of resistance factors. For moment resisting ECB connections, however, only the designs based on a resistance factor for concrete bearing failure mode less than the current 0.65 result in an acceptable reliability factor of β > 4.5.

Methods for the seismic design of base connections in steel moment frames are well-developed and routinely utilized by practicing engineers. However, design loads for these connections are not verified by rigorous analysis. This knowledge gap is addressed through nonlinear time history simulations using design-level seismic excitation that interrogate demands in column base connections in 2-, 4-, 8-, and 12-story steel moment frames, featuring base connections that reflect current U.S. practice. The results indicate that: (1) for exposed base plate connections, lower bound (rather than peak) estimates of axial compression are suitable for design because higher axial forces increase connection strength by delaying base plate uplift; (2) even when designed as pinned (as in low-rise frames), base connections carry significant moment, which can be estimated only through accurate representation of base flexibility; and (3) the failure of embedded base connections is controlled by moment, which may be estimated either through overstrength or capacity-based calculations.

This paper investigates the behavior of a ductile detail of exposed base plates. This detail consists of a base plate anchored to the concrete foundation through bolts extended to a steel chair configuration. The intention is to concentrate plastic strains mainly in the extended region of the anchor bolts, forcing the other connection components to remain elastic. The scientific background of this research consists of a series of sophisticated nonlinear finite element models subjected to a cyclic load protocol in the presence of an axial compressive force. The models were validated against an experimental test reported in the literature. Forces within the connection components, stresses, strain distributions, and deformation modes were examined. A total of sixteen three-dimensional nonlinear models were created using the ABAQUS simulation platform. The models were separated into two groups: the first consists of models with dimensions similar to the specimens tested in recent experimental programs reported in the past, while the second group simulates connections representative of mid-rise industrial frames. Building on the insights gained from the simulations of the first group, a methodology is proposed to design these column base connections. This suggested methodology is validated with the second group of simulations. Results indicate that the studied configuration detail presents some advantages compared with the traditional detail presented in Design Guide 1. For instance, plastic strains are developed almost exclusively in the anchor rods, and no damage is expected at the remaining components. Another essential characteristic is the exposed stretch length, with which it is possible to achieve a target design rotation without significant strain concentrations in the anchor rods. This characteristic facilitates post-earthquake inspections and repairs, and damage is virtually eliminated in the first story.