PSI - Issue 84

Gian Felice Giaccu et al. / Procedia Structural Integrity 84 (2026) 991–998

992

The benefits of steel-concrete systems - particularly reducing required steel section sizes without compromising structural performance - have contributed significantly to their popularity in bridge applications. This efficiency is largely enabled by shear connectors welded to the top flange of the steel beam, which ensure composite action with the concrete slab and promote effective force redistribution across the cross-section. Despite these advantages, the design of composite beams remains challenging due to three key factors: (i) the distinct structural response under positive and negative bending moments, as highlighted by Reissner 1946; and Adekola 1968, 1974; (ii) the absence of comprehensive codified guidance for sections subjected to combined axial and bending actions, particularly in the context of cable-stayed bridges (Eurocode 4:2004); and (iii) the increasing demand in advanced applications such as composite girder decks and integral abutment bridges, where compressive forces are of primary importance. A critical concern is the susceptibility to local instabilities in the steel web and bottom flange when compressive axial forces interact with negative moments at internal supports (Vasdravellis et al. 2012). While extensive research has advanced the understanding of various aspects of composite behaviour - including post buckling performance (Gonçalves et al. 2016; Abdelhakim et al. 2018; Behnam & Denavit 2020; Denavit 2020; Oliveira & Reis 2010), shear response (Tavares 2009; Giaccu et al. 2025; Byers 1999; Chen et al. 2005; Muke & Bajad 2016; Zhai 2013; Wu et al. 2012), and post-tensioned systems (Chen & Zhang 2006; El-Zohairy et al. 2015; Moscoso et al. 2017; Cai & Aref 2017; Yuan et al. 2016; Reginato et al. 2018) - a significant knowledge gap persists in relation to the behaviour of individual composite beams subjected to combined axial and flexural actions (Wang et al. 2023; Zhang et al. 2023; Huang et al. 2025; She et al. 2024; Yu et al. 2024). Existing design standards, including Eurocode 4:2004, provide only limited treatment of such interactions, primarily addressing pure bending conditions. As a result, engineers are frequently compelled to adopt complex and resource-intensive Finite Element models to accurately capture the response under combined loading scenarios (Oliveira & Reis 2010; Tavares 2009). In response to these limitations, this study proposes a simplified design methodology for twin-girder composite decks subjected to combined axial and bending loads, applicable to both Ultimate and Serviceability Limit States. A key innovation of the proposed approach is the introduction of a non-dimensional Aspect Ratio parameter  = / ( /2 ), which enables generalization across a variety of cross-sectional configurations while incorporating the influence of shear connector deformability. The proposed methodology is validated through comprehensive linear and nonlinear Finite Element simulations and is specifically tailored to address the shear lag phenomenon in composite cable-stayed bridge decks under simultaneous axial and flexural effects. Building upon the Eurocode’s existing framework and incorporating recent contributions by the authors (Giaccu et al. 2022, 2025), the method extends current procedures to more accurately reflect the real structural response. By leveraging classical beam theory, this approach provides a practical and accessible alternative to full FE modeling, thereby offering structural designers a simplified yet robust tool for the design and verification of composite steel–concrete beams. The following section presents the proposed generalized calculation procedure. 2. Parametric investigation A comprehensive parametric investigation was carried out to assess the structural performance of composite twin girder decks under varying configurations. Emphasis was placed on accurately modeling the nonlinear material behavior to capture the interaction between bending and axial compression. In particular, the study compares two connection strategies between the concrete slab and steel girders - rigid and flexible - by evaluating their respective effects on global and local structural responses. As highlighted in previous studies (Kalibhat & Upadhyay 2020; Zeng et al. 2019; Baertschi et al. 2023), partial shear interaction can significantly influence the behavior of composite systems, both at the Ultimate and the Serviceability Limit State. The stress distribution and load-sharing mechanisms are strongly affected by interface slip, which in turn alters key design parameters such as the stress ratio ( , ) and the integral interaction ratio ( , ) , where = / and  = /( /2) , see Fig. 1. Accordingly, this study explicitly incorporates shear connector slip to investigate the influence of flexible connections on the overall deck response. The numerical analysis follows the provisions of Eurocode 4:2004, EN 1993-1-5:2006, and EN 1992-1-1:2004, using the Strand7 finite element software (Strand7 Software, 1999), see Fig. 2. Both linear elastic and bilinear constitutive laws are adopted to simulate the material response of the steel and concrete components. The steel flanges, web, and concrete slab are modeled using shell elements, while the shear connectors are represented as discrete link