Corner-supported modular steel sway frames (CMSFs) with rotary inter-modular connections (IMCs) differed from traditional frames regarding their column discontinuities, beam groupings, and unique intra- and intermodular connections, necessitating the investigation into their compressive performance to guarantee their safe and reliable application. This study investigated the compressive behavior of CMSFs with rotary IMCs using experimental tests, numerical modeling, and theoretical analysis. Three compression tests were conducted on sub-assembled CMSFs, considering varying floor and ceiling beam stiffnesses. The results showed that all frames experienced lateral sway, with upper columns at lower regions undergoing inward or outward elastic and plastic local buckling. RS1 (RS2) demonstrated 12 % (3 %) higher strength than RS3, and stiffness increased by 2 % for RS1 compared to RS3. Pre-and post-ultimate ductility of RS3 was 3 % (13 %) and 20 % (37 %) greater than RS1 (RS2), indicating that increased rigidity with thicker beams enhanced strength and stiffness but resulted in reduced CMSFs' ductility. A finite element model (FEM) was generated, and its accuracy was verified using experimental load-shortening and failure outcomes, revealing an average prediction error of 0.3 %, 9.1 %, and 8.5 % for compressive resistance, stiffness, and ductility index, respectively. Based on validated FEMs, a parametric study was conducted on 77 CMSFs to investigate the effects of varying beam and column sizes, lengths, beam gaps, and connecting plate thicknesses on compressive resistance, stiffness, and pre-and post-ultimate ductilities. Increasing column and beam sizes from 150 to 200 mm and thicknesses from 6 to 8 mm enhanced strength and stiffness by up to 123 % (55 %) and 46 % (10 %), with pre-and post-ultimate ductility growing by 16 % (113 %) and 15 % (19 %). However, lengthening them from 0.6 to 1.2 and 3 m decreased CMSFs' strength (stiffness) by up to 37 % (5 %) and 65 % (71 %), with no IMC failure. The sub-assembled CMSFs' buckling load was evaluated using theoretical models, considering members' stiffnesses and rotary IMC as pinned and semirigid. The average theory-to-FEM buckling load for pinned and semi-rigid IMC was 0.70 and 0.96, indicating that both models were conservative. However, considering IMC's rotational stiffness provided less scattering and a more realistic depiction of the CMSFs' buckling behavior than the pinned model. The study's findings and the accuracy of theoretical buckling models ensured they could conservatively design CMSFs under compressive loadings while considering their uniquenesses.
