Issue 78

S. M. Khalil et alii, Fracture and Structural Integrity, 78 (2026) 346-366; DOI: 10.3221/IGF-ESIS.78.20

Many studies investigated different methods to strengthen the web from buckling under the load and increase the web capacity. Previous research on web strengthening under patch loading mainly focused on adding longitudinal and transverse stiffeners and determining the best stiffener position and configuration. Wide experimental and numerical research on longitudinally stiffened webs showed that even weak open-section stiffeners can prevent global web buckling. Kövesdi et al. (2018) [12] tested longitudinally stiffened slender girders and then conducted a numerical investigation on eight large scale specimens. The unstiffened web failed by global buckling, whereas all stiffened specimens failed by local buckling of the web under the loaded flange, with failure mode strongly affected by stiffener position. Even the smallest stiffener was sufficient to avoid global buckling. Patch loading resistance depended strongly on stiffener location. Markovic and Kovacevic (2019) [8] have proven that by limiting the local buckling deformation to the upper web sub-panel, the strengthening effect of adding a single longitudinal web stiffener increases significantly under longer patch loading length, although it is negligible at shorter load lengths. Menkulasi and Farzana (2019) [13] found that web compression buckling is the main governing failure mechanism of unstiffened wide-flange I-sections under opposing patch loads. Chacón et al. (2019) [14] investigated the sensitivity of the imperfection of transversally and longitudinally stiffened girders under patch loading. Loaiza et al. (2018) [15] performed design-oriented nonlinear analyses on girders with a stiffener at one-fifth depth. They proposed patch loading design recommendations that account for the interaction of web thickness, panel aspect ratio, stiffener rigidity, as well as load length, and document different failure modes and out-of-plane displacements for stiffened versus unstiffened webs. Loaiza et al. (2019) [16] continued the study and refined the definition of web slenderness for stiffened girders by fully restricting the stiffener’s vertical displacement and out-of-plane direction at ultimate load. Demari et al. (2020) [17] conducted numerical studies on slender I - girders with a single longitudinal stiffener. They confirmed that failure was a bending buckling of the full web panel (or sub - panels) combined with web crippling. Local buckling of the two-web sub - panels, with the optimal stiffener position for patch loading governed mainly by crippling of the upper sub - panel, was also observed. Properly placed stiffeners could reduce lateral displacement, transfer the failure mode to local web buckling for both web sub-panels, and increase the ultimate capacity. For certain imperfection shapes and stiffener positions, the effect of stiffeners, on the contrary, gave capacities lower than the corresponding unstiffened girders. Ć erani ć et al. (2022) [18] quantified a strengthening effect as the ratio of the ultimate strength of stiffened to unstiffened plate girders. They found that this effect increased with patch load length and decreased with web slenderness, and that it was governed more by key geometric ratios than by absolute girder size. An advanced numerical model and tests for slender plate girders with multiple longitudinal stiffeners under patch loads show that stiffener stiffness and spacing govern failure mode and resistance. Current EN 1993-1-5-based methods can underestimate resistance by 20-60%, motivating an improved design equation for locally buckling webs. (Kövesdi & Dunai, 2023). [19], Chacón et al. (2010) [20] used numerical simulations to study the behavior and resistance of hybrid steel plate girders under patch loading. Their findings revealed that the flange yield strength did not affect the ultimate patch-loading resistance, whereas the web-to-flange yield-strength ratio was the key factor in the structural response. Kövesdi et al. (2010) [21] investigated the patch-loading resistance of steel girders with corrugated webs using numerical and experimental methods. The findings revealed that raising the loading length greatly increases the load-carrying capacity, while increasing the corrugation angle improves patch-loading resistance. Beyond traditional stiffeners, studies were performed on closed flanges as a stiffening method for both the flanges and web. Retrofitting beams with long lateral unsupported compression flanges, such as crane runway girders, is performed by adding angles and converting the flat flange to a rectangular closed flange. This method increases the lateral stiffness of the compression flange and, at the same time, the web resistance to local stresses. Using a closed flange shows a significant increase in web capacity compared to beams with a conventional flange plate. According to Arabzadeh and Varmazyari (2009) [22], delta-stiffened I-girders have a 95% higher critical patch load capacity than other stiffened girders, as proven by finite element models and experimental data. Numerical and experimental studies on triangular cell flanges were performed by Casanova et al. (2022) [23] and Graciano et al. (2021) [24], introducing a flange-based strengthening concept that significantly enhanced web resistance to patch loading. The study showed that triangular cell flanges increased web plate resistance, and new design expressions were introduced for buckling coefficients and ultimate. It was also concluded that a low stiffener-to-web thickness ratio, caused by using thin stiffeners, causes local buckling at lower loads and limits the stiffener's stress transfer capability. Mohamed et al.'s 2020 [25] investigated traditional tubular beams under concentrated loads that experience premature local flange and web deformation. This can be lessened by extending the web through the top hollow flange to serve as an internal strengthening element. Measurements by Kettler et al. (2020) [26] showed that additional local bending stresses inside the closed section could still be significant and should not be neglected in fatigue design, leading to a new stress calculation concept. Studies by Jamil et al. (2025) [27] showed that steel closed stiffeners with optimized internal and external stiffeners could achieve over 95% of the maximum for intensive solutions while improving seismic behavior. Stiffening flanges with longitudinal stiffeners reduced the effective width ratio but could increase shear lag effects at the web-flange junction, which required careful design to balance flange stiffness and shear flow

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