Inelastic Flexural Behaviour and Ductility Characteristics of Steel-Encased Composite Beams Incorporating Normal and High-Strength Concrete Under Out-of-Plane Actions: Nonlinear Finite Element Investigation

This study presents a rigorous investigation into the inelastic flexural response of steel-encased composite reinforced concrete beams incorporating out-of-plane segmental configurations fabricated from normal strength concrete and high-strength concrete. A total of thirteen beam specimens were organized into two series of geometrically and mechanically identical configurations, differentiated solely by concrete compressive strength grade, thereby isolating the influence of this material variable on the observed failure mechanisms and load-deformation characteristics. Parametric results indicated that the introduction of out-of-plane segmental geometry induced a progressive and monotonic deterioration in ultimate load-carrying capacity, cracking load beam, and yielding resistance, with the magnitude of degradation correlating directly with the number of segmental joints. While failure of straight beam specimens was uniquely in pure flexure with peak midspan deflections, segmented configurations demonstrated a coupled shear-torsional failure mechanism with oblique deformation paths traversing along the interior perpendicular axes of the segmental interfaces; an indication of stress redistribution mustered by geometric discontinuities introduced at the joint locations. While HSC specimens exhibited higher initial flexural stiffness than their NSC counterparts on account of initial load-deflection relationship, this advantage did not extend to post-cracking response due to the inherent brittleness associated with HSC causing an abrupt drop in strength after cracking occurs for the non-composite behaviour configuration and ultimate load capacity alone is not sufficient for assessing structural adequacy. The presence of continuous internal steel section was the most significant parameter considering all configurations under investigation yielding simultaneous ultimate resistance increases close to 200 kN in NSC comparative beams, yet with large displacements ductility and energy dissipation capacity enhancements. More importantly, the surrounding steel element has changed the nature of structural functionality: no longer are segmental connection zones regions of stress concentration and early failure, but areas of cohesive and relatively uniform stress transfer, returning global structural continuity over multiple joints.