Issue 78

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

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his paper presents a numerical investigation on the use of closed flanges to enhance web resistance under patch loading, considering two different flange configurations as triangle and rectangle. The study covers wide range of parameters more than what is available in literature, mainly length of patch load, location of stiffener along the web depth, and different relative thicknesses between stiffener and web. Recommendation and results are concluded as follows:  Use stiffeners with compact plates rather than non-compact or slender plates. Compact classified stiffeners remarkably increase strength; however, slender stiffeners affect the strength gained.  Use stiffeners with thickness equal or more the web thickness. With a very small patch loading length, the more the stiffener thickness increases, the more strength is gained. The biggest stiffener effect is at the smallest web thickness. The higher the increase in stiffener thickness, the higher the gain in strength, and this is not affected by loaded length. A low stiffener-to-web thickness ratio results in thinner plates, which can cause local buckling at lower loads and limit the stiffener's stress transfer capability.  For concentrated load (patch load is almost zero), use stiffeners at least double the web thickness. For Slender stiffeners do not affect load length -to- stiffener depth aspect ratio. Non-compact stiffener has a positive effect on strength, especially when the loaded length is equal to the stiffener's depth. At a very small patch loading length, the more the triangular-shaped stiffener thickness increases, the more strength is gained.  Deeper stiffeners increase web resistance by lowering the effective clear height and establishing a rigid load-transfer system. This arrangement helps to rebalance the patch load, which reduces localized web out-of-plane deformation and delays instability, changing the failure mode to have a more distributed failure involving several structural components.  To maximize the structural efficiency of steel members under concentrated loads, designers should prioritize a steeper stiffener height in conjunction with an extended bearing length. For slender webs, the stiffener height acts as the critical safeguard against instability, whereas for stockier webs, the loaded length plays a more dominant role in governing the ultimate capacity.  The optimized section is to use triangular configuration for the stiffened flange, however, for specific thickness and if the steel quantity is not an issue, use rectangular closed flange which provides more web capacity. Both stiffening methods of triangular and rectangular flange configurations have the same effect at a small loaded length, no matter whether the stiffener thickness differs. Increasing the loaded length shows a higher capacity gain when using a rectangular shape, but a limited gain compared to the increased steel amount. Triangular shape strengthening provides higher strength for the same steel area than rectangular shape strengthening.  Use EN code to calculate the web capacity, using the actual stiffener rigidity not to the limit. In general, EN 1993-1-5 is conservative for both configurations, mainly due to the stiffener rigidity limit in the rules. Escaping this limit reduces the FE-to-code gap and better captures the geometry effect. The remaining discrepancy indicates that flexural rigidity alone cannot fully represent the behavior of closed flange, highlighting the need for geometry- and torsion-sensitive parameters. Finaly, for an I-section beam subject to a patch load, it is advised to use inclined plates to stiffen the web from local instability and convert the flat plate flange to a triangular closed section. The thickness of the stiffener plates is equal to or larger than the web thickness, conditioned that the classification of the stiffener plates is to be compact. The study is limited to a fixed span length of 500 mm and to a stiffener inclination angle of 59 degrees. The increase in the web capacity is expected to reach double the original web capacity of the unstiffened condition, triple in case of slender webs. For future studies, investigating variations in span lengths, different inclination angles of stiffener plates is suggested. Also, using stiffeners with material different than web needs study as a likely case for retrofitting using material different from the original element.

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he authors declare that they have no competing interests.

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