Column base connections are critical to the performance of steel structures, serving as the interface between the steel superstructure and concrete foundation. Historically under-researched, these connections have received significant attention over the past two decades, leading to major advances in understanding their strength, stiffness, seismic behavior, and simulation. This paper provides a synthesis of these developments, focusing on exposed and embedded base connections across a range of loading conditions and structural configurations. Key contributions include improved strength models, detailed assessments of seismic performance, and the introduction of modeling tools now included in the 3rd Edition of AISC Design Guide 1. The paper discusses elastic and inelastic behavior, base connection-frame interaction, and the emerging use of base connections as yielding elements in seismic design. Advances in simulation of base connections, including rotational spring and line-element-based models, enable more accurate structural simulations. Despite this progress, notable gaps remain, including the behavior of braced frame connections, foundation-soil interaction, and performance-based design for repairability and resilience. This work aims to consolidate recent findings, inform ongoing research, and guide future design practices toward a more integrated treatment of base connections in steel structures.
Design of members subjected to combined forces is an iterative process in which the designer selects a trial section and checks the selection for compliance with the appropriate provisions of AISC Specification for Structural Steel Buildings Chapter H. This process is repeated until a desired section is identified. The introduction of AISC Steel Construction Manual Table 6-1 has made the calculations involved in checking different trial sections considerably easier and faster. However, there is still no comprehensive resource for the designer to select an initial trial section for various scenarios of members subjected to combined forces which leads to an optimal final selection quickly. This paper presents a method and aids for more rational and efficient selection of trial sections for W-shapes subject to combined compression and bending about one or two principal axes, tension and bending about one or two principal axes, or biaxial bending. Unlike some previous methods, there are no limitations on the values of unbraced and effective lengths (e.g., requiring that L-b = L-cy). The design aids are developed for all W-shapes in Grade 50 steel using both LRFD and ASD methods. The method presented converges quickly, reducing the need for multiple repeated calculations before a final W-shape is selected. Example problems are also included.
An evaluation of the AISC Specification Section F13.1 provisions for the reduction in strength of beams and girders with holes in the tension flange is performed. Twelve simply supported rectangular hollow section (RHS)-to-RHS splice connections with variations in RHS shape, number of bolts, and bolting arrangement are tested under monotonic four-point bending, and net-section rupture (in the tension flange) is achieved. The results demonstrate that, for RHS members, the current AISC Specification Section F13.1 provisions are vastly conservative. The current results are combined with the previous data from 138 tests on wide-flange girders with holes in the tension flange, and a new nominal flexural strength (M-n) equation is developed using net section properties. The new equation is shown to increase the accuracy of M-n predictions and reduce scatter in the predicted results. A reliability analysis is performed to demonstrate that the new approach provides acceptable levels of reliability when compared to the target reliability index of 4.0 specified in Chapter B of the Specification commentary.
Braced frames traditionally have been analyzed and designed as trusses with all joints modeled as pins, such that only the braces provide lateral force resistance. However, frames with gusset plate connections create a rigid joint zone between frame beams and columns, effectively resulting in moment frame behavior, particularly at larger drift angles when the braces have yielded or buckled. Described herein is a rationale for multistory braced-moment frames that includes the frame's moment resistance to lateral displacements when subjected to story-drift angles where the lateral resistance of the frame comprises both brace and moment frame action.
Shallow-depth composite floor systems aim to improve building economy, speed, and efficiency. Currently, an array of proprietary systems is available that comprises asymmetric steel sections (I-beam or trapezoidal box) with a relatively wide bottom flange. This wide bottom flange facilitates the placement of decking or precast/prefabricated panels. These systems are fast to assemble compared to cast-in-place flat slab construction. However, extensive fabrication is typically required for the current steel sections. As a result, this paper presents a design methodology for shallow-depth systems utilizing conventional asymmetric steel I-beams (AIBs) that can be hot-rolled or built-up. The design methodology covers the limit states critical for construction in the noncomposite state and in-service as a composite system. The composite design presented is a lower-bound approach with no mechanical connectors. The horizontal shear transfer is through steel-concrete bond and friction, resulting in a partial composite design. This design methodology was developed through an experimental testing program on shallow-depth floor systems. The testing reduced the uncertainty of several design parameters and validated certain assumptions. Three main phases of testing were (1) an initial large-scale concept test, (2) nine composite beam tests, and (3) a full-scale system validation test. The critical findings of each phase are provided. Finally, design aids are presented to equip future engineers with resources to efficiently design shallow-depth floor systems.
