Impact of Inter-Modular Connections on Progressive Compressive Behavior of Prefabricated Column-Supported Volumetric Modular Steel Frames

This study investigates the progressive compressive behavior of modular interior frames with rotary-type module-to-module inter-modular (M2M) connections under sequential column failure. A novel two-stage testing protocol was applied, compressing the left upper column to failure, followed by the right, to simulate realistic loading progression in prefabricated column-supported volumetric modular steel structures. Detailed refined finite-element models (FEMs) were developed and validated against experimental results, accurately capturing local and global responses with an average prediction error of 2–10% for strength and stiffness. An extensive parametric study involving varying frame configurations evaluated the influence of frame member geometric properties, connection details, and column/beam gap interaction on progressive collapse behavior. The results demonstrated that upper columns govern failure through elastic–plastic buckling near M2M joints while other members/connections remain elastic/unyielded. Increasing column cross section and thickness significantly enhanced strength and stiffness, while longer columns and prior damage reduced capacity, particularly during right-column loading. Conventional steel design codes overestimated column strength, with mean Pu,FEM/Pu,code ratios below unity and high scatter (Coefficient of variation ~0.25–0.27), highlighting the inadequacy of isolated member-based design equations for modular assemblies. The findings emphasize the need for frame-based stability approaches that account for M2M joint semi-rigidity, sway sensitivity, and sequential failure effects to ensure the reliable design of modular steel frames under progressive compressive loads.