Issue 78

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

for selecting stiffener dimensions and closed-flange configurations to improve web resistance under patch-loading. K EYWORDS . Closed flange, Finite-element, Patch loading, Web stiffening, Crippling.

I NTRODUCTION

I

-sections are widely used across all steel construction fields due to their ease of fabrication and high resistance to various loading conditions. The top flange is commonly under compression and subject to lateral torsional buckling, resulting in a reduction of the bending moment capacity. This drop is treated by using a closed flange instead of the flat plate one, which enhances the girder bending capacity significantly (Ibrahim et al., 2012 [1], Hassanein and Kharoob, 2013 [2], Shaat et al., 2022 [3], Matloub et al., 2023 & 2024 [4,5]). This closed section of the flange can be fabricated either from the beginning using a hollow tubular section welded to the web, or later by welding inclined steel plates or angles between the flange and web. This closed flange also benefits the web by stiffening its out-of-plane stiffness, which increases the web capacity from local stresses caused by in-plane concentrated loads. Previous research focused on the gain in the flexural capacity of the tubular flange section, while limited research has investigated the web capacity locally under the load. To fill this gap, this paper provides a numerical study on an I-section with a closed flange subject to patch loading to investigate the enhancement of the web capacity under the load. The top conventional flange plate is stiffened in two different ways, one with plates, while the other uses angles. In the first method, two inclined stiffener plates are placed under the loaded flange, connecting the flange edges to the web, forming a triangular shape. In the second method, angles are used to form a rectangular flange. Both methods are shown in Fig. 1. The present investigation examines a more extensive parametric range than has been considered in existing literature, with particular emphasis on the patch load length, the longitudinal position of the stiffener along the web depth, and the relative thickness ratio between the stiffener and the web.

Figure 1: I-section with stiffened top flange. A substantial amount of research was conducted to identify various web failure modes in conventional I-sections under patch loading. Carden et al. (2007) [6] performed studies on I-beams under patch loading. They identified several interacting limit states for the flange and web, such as flange local bending, post-compression, and web yielding. Chacón et al. (2009) [7] and then Markovic and Kovacevic (2019) [8] proved that under localized edge patch loading, the ultimate resistance to web local buckling/crippling increases more with the increase in the patch loading length. Web yielding was determined to be the critical limit state under patch loading, rather than buckling, in thick webs. Chen et al. (2016) [9] performed experiments on hot-rolled channel webs under different conditions of concentrated loads, and found web crippling failure, marked by out-of-plane web buckling and flange distortion. This was accompanied by plastic hinges forming and the plastic zone shifting from the web's mid-height toward the loaded flange as the load increased. Li and Young (2017) [10] investigated concentrated bearing in the end and interior positions of cold-formed and stainless-steel tubular sections. Web crippling was the predominant failure mode with a single peak load, and the resistance to web crippling was significantly increased with the bearing length increase. Stepped I-girders are prone to local buckling at depth discontinuities, which can severely limit their buckling capability. However, suitable step placement and the application of stiffeners can effectively restore, or even improve, the girder's initial buckling resistance (Almasri & Akhtar, 2025) [11].

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