Effects of semi-rigid joint modelling on structural and material efficiency of steel grid floors and ribbed domes

The study presents the moment–rotation (M–φ) characteristics of semi-rigid I-profile joints made using header plates and flush end-plates. The analyzed joints were applied as representative connections in a steel grid system. The M–φ characteristics were determined using the Frye–Morris polynomial model for header plate joints and the Richard–Abbott formulation adapted by Chen and Kishi for flush end-plate joints, with joint resistance and initial stiffness established using analytical procedures. The approximate M–φ relationships were described using a three-segment model, enabling a simplified yet realistic representation of joint behaviour in engineering calculations. The resulting curves were adjusted to the calculated design moment resistance. These models were implemented in Autodesk Robot Structural Analysis Professional (ARSAP) and used in the global analysis of two structural types: a steel beam floor system and a ribbed dome. The study aimed to assess the impact of semi-rigid joints on the utilization of structural members compared to typical pinned and rigid joint models. The comparison included internal force distribution, member utilization under ultimate limit state conditions, serviceability performance, and steel section selection. For the floor structure, replacing pinned joints with flush end-plate connections reduced beam utilization by 11.5%. In the case of the ribbed dome, the demand-to-capacity ratio for the parallels was reduced by an average of 9.9%. These results indicate that accounting for joint semi-rigidity can significantly improve material efficiency and structural safety in design practice.