Zhixiang Zha

To address the issues of poor fire and corrosion resistance in traditional steel structure reinforcement methods, the construction constraints of cast-in-place partially encased steel–concrete composite beams (PEC beams), and their difficulty in application for the rapid retrofitting of existing structures, this paper proposes a novel fully dry-assembled prefabricated partially encased steel–concrete composite beam (PPEC beam). This structure reliably connects prefabricated concrete beams to an H-shaped steel beam using bolts, achieving an efficient, dry, and cast-in-place-free reinforcement method. To investigate its flexural mechanical behaviour and failure mechanism, one cast-in-place PEC control specimen and three PPEC specimens with different connection and assembly configurations were designed. Four-point static bending tests were conducted to systematically analyse the failure modes, load–deflection curves, cross-sectional strain distributions, and ultimate load-bearing capacity. The results indicate that the PPEC beams exhibit comparable flexural capacity to the cast-in-place PEC beam, demonstrating excellent overall composite action. The flange-bolted connection achieves stronger steel–concrete composite action, with elastic stiffness essentially identical to that of the cast-in-place specimen. The web connection provides weaker restraint against local buckling of the steel flange, resulting in slightly lower elastic stiffness. The fully prefabricated assembled specimen shows significantly better plastic deformation capacity than the segmentally assembled specimen. The ultimate loads of all specimens exceed the code-predicted values. The proposed PPEC beam, with its convenient construction and reliable mechanical performance, is suitable for the reinforcement, retrofitting and replacement of simply supported secondary steel beams, working platform beams, and walkway beams.

Objectives: The local buckling stress and ultimate strength of an H-section beam cannot be accurately predicted based on the conventional method. This study aims to develop calculation methods of high accuracies for both elastic local buckling stress and ultimate strength.Methods: A simplified theoretical method for the elastic flange local buckling is presented, in which rotational restraints and torsional restraints provided by the web were considered.Results: It is validated that the buckling stress of single flange obtained by theoretical analyses agrees well with the results of finite element analyses when the local buckling is controlled by the flange local buckling. Based on theoretical analyses, important parameters are determined which are considered to evaluate the buckling stress.Conclusions: Through parametric studies, the local buckling stress of the H-section beams covering all cases of local buckling types (flange buckling, web buckling and coupled buckling) are suggested. A design formula is proposed, which outperforms conventional methods in predicting the normalized ultimate strength.