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The use of back-to-back built-up channel beams in cold-formed steel (CFS) structures is steadily rising. The growing demand for CFS sections as a cost-effective design solution has driven the development of these CFS built-up sections. Despite this, there has been limited research on the performance of mild steel (MS) based CFS at high temperatures, particularly regarding its flexural behavior. This study thoroughly explores the behavior of MS-based CFS beams with different spans under high temperatures, followed by cooling with air or water. It assesses the impact of thermal loading and evaluates the effectiveness of these cooling methods. Experimental findings are validated and analyzed in conjunction with Finite Element Modeling (FEM) using ABAQUS and the Direct Strength Method (DSM). The study also conducts a parametric analysis to determine how the varying span that affects flexural capacity of beam. Among beams heated to the same temperature, those cooled with water exhibit slightly lower load capacities than those cooled with air. The maximum load observed is 91.21 kN for the reference specimen, while the minimum load is 39.82 kN for the specimen heated for 90 min and cooled with water, resulting in a 78.45% difference between these values. Additionally, as heating duration increases, ductility of beam also increases. Various failure modes are observed based on different heating and cooling conditions across different beam spans. This study offers valuable insights into the performance of MS-based CFS beams under thermal stress and different cooling conditions, providing important data for structural design and safety in construction. [ABSTRACT FROM AUTHOR]

The use of back-to-back built-up channel beams in cold-formed steel (CFS) structures is steadily rising. The growing demand for CFS sections as a cost-effective design solution has driven the development of these CFS built-up sections. Despite this, there has been limited research on the performance of mild steel (MS) based CFS at high temperatures, particularly regarding its flexural behavior. This study thoroughly explores the behavior of MS-based CFS beams with different spans under high temperatures, followed by cooling with air or water. It assesses the impact of thermal loading and evaluates the effectiveness of these cooling methods. Experimental findings are validated and analyzed in conjunction with Finite Element Modeling (FEM) using ABAQUS and the Direct Strength Method (DSM). The study also conducts a parametric analysis to determine how the varying span that affects flexural capacity of beam. Among beams heated to the same temperature, those cooled with water exhibit slightly lower load capacities than those cooled with air. The maximum load observed is 91.21 kN for the reference specimen, while the minimum load is 39.82 kN for the specimen heated for 90 min and cooled with water, resulting in a 78.45% difference between these values. Additionally, as heating duration increases, ductility of beam also increases. Various failure modes are observed based on different heating and cooling conditions across different beam spans. This study offers valuable insights into the performance of MS-based CFS beams under thermal stress and different cooling conditions, providing important data for structural design and safety in construction.

This paper investigates the influence of commonly used screw connections on the integrity of built-up beams formed by connecting two lipped channel sections in a back-to-back configuration. An experimental investigation on the behaviour of 6 built-up CFS beams connected by self-drilling screws is presented in this paper. The screw spacing was varied along the length as well as the depth of the beams. The test results indicate that the screw spacing has some influence on the capacity of the built-up beams. An increase in screw spacing leads to an undesirable separation of channels and thus has a negative impact on the composite action between the members. Finite element models were developed and after successful validation, a parametric study involving 124 models was carried out, to investigate the impact of larger longitudinal screw spacings on the integral behaviour of the beams. It was observed that the flexural capacity decreased by 11%, 14%, 17% and 19% when the longitudinal distance between the screws increased from 50 to 1050 mm for beams with web depths 150 mm, 200 mm, 250 mm and 300 mm respectively. It can be concluded that flexural capacity is directly proportional to the decrease in the screw spacing and increase in the number of screw rows. The results also indicate that the percentage decrease in strength is greater for beams with larger web depths. An idealized cross section assuming complete composite action was simulated for each web depth using FE modelling and the capacities of all the screw-connected "semi-rigid" beams were compared against it. The capacity of screw connected built-up beams with web depths 150 mm, 200 mm, 250 mm and 300 mm was found to be 0.71, 0.64, 0.61 and 0.59 times that of the idealized cross-section on average, respectively. The moment capacities of screw-connected beams were also compared against the theoretical values obtained from AISI S-100. It was observed that the design guidelines provided in AISI S-100 were unable to account for the reduction in the moment capacity due to the screw connection in built-up members. AISI S-100 is conservative by as much as 34%, 42%, 46% and 48% for beams with web depths 150 mm, 200 mm, 250 mm and 300 mm respectively. [ABSTRACT FROM AUTHOR]

In this study, analytical methods were used to investigate the performance of built-up timber (BT) beams with different designs. A simplified analytical approach based on the gamma and capacity methods was used to evaluate the initial stiffness, allowable strength, and ultimate strength of BT beams. Current analytical methods cannot be used to estimate the ductility ratio, thus hampering the development of a complete analytical model for BT beams. In this study, a complete analytical model was established for BT beams. In this model, the ductility ratio is determined according to experimental results. The ductility ratios of specimens were determined using the equivalent energy method. The developed complete analytical model for BT beams can accurately model the behavior of these beams. The estimated ductility ratios suggest that specimens with one web and two webs are suitably ductile and marginally brittle, respectively. The developed model can be used to predict the flexural performance of BT beams with different design parameters, and the ductility ratio can be applied in structural design. [ABSTRACT FROM AUTHOR]

The study deals with the effect of using Slurry infiltrated fiber concrete (SIFCON) with the reinforced concrete beams to explore its enhancement to the flexural capacity. The experimental work consists of the casting of six beams, two beams were fully cast by conventional concrete (CC) and SIFCON, as references. While the remaining was made by contributing a layer of SIFCON diverse in-depth and position, towards complete the overall depths of the built-up beam with conventional concrete CC. Also, an investigation was done through the control specimens testing about the mechanical properties of SIFCON. The results showed a stiffer behavior with a significant increase in load-carrying capacity when SIFCON used in tension zones. Otherwise high ductility and energy dissipation appeared when SIFCON placed in compression zones with a slight increment in ultimate load. The high volumetric ratio of steel fibers enabled SIFCON to magnificent tensile properties.

Although previous attempts for enhancing the buckling resistance of cold-formed steel (CFS) built-up beams through intermittent stiffening, external stiffening and adoption of concrete as composite sections were successful, they may compromise the lightweight constructional feature of these elements. This paper presents a test program aimed at developing CFS composite built-up beams with superior buckling performance and low self-weight for ensuring lightweight construction. Unlike concrete, lightweight stiff boards of timber and glass fibre reinforced polymer (GFRP) were sandwiched between the CFS channels in the web and flange region to construct the lightweight composite beams. All of the beam specimens were subjected to four-point bending with simply supported boundary conditions. Peak strength, load-deformation curves, initial stiffness, strength-to-weight ratios, failure modes and the normalized yield and plastic moment strengths were compared. This study established the structural feasibility of developing lightweight CFS composite built-up beams from timber and GFRP boards, indicating that the flexural strength and stiffness are significantly improved by 150% and 80%, respectively, as compared to the unstiffened CFS built-up beams. Moreover, the studied composite beams were shown to attain their full yield moment capacities, and in some cases more than 90% of their plastic moment capacity, resulting in built-up members with a higher structural efficiency. • A new CFS GFRP-timber composite lightweight beam system was developed. • The efficiency of different combinations of GFRP and timber packing were explored. • Feasibility to adopt GFRP & timber to form CFS composite beams is demonstrated. • Large improvement in strength, stiffness & strength-to-weight ratio was achieved. • Proposed system exhibited around 100% & 90% of its nominal yield & plastic capacity. [ABSTRACT FROM AUTHOR]

The flexural behavior of partially welded flush end-plate connections incorporating built-up hybrid beams and columns is analytically and numerically investigated. An experimentally obeying three-dimensional finite element model is first constructed. To circumvent the laborious effort of three-dimensional simulation and experimental work, a new experimentally and numerically complying equation approach is introduced for the construction of a continuous moment–rotation (M–θ) description. For the proposed equation, two essential terms are required: the rotational stiffness, Sj,ini, obtained by employing the component method and the maximum moment, Mmax, produced using the proposed linearly distributed multi-parameter fitting technique. To demonstrate the applicability of the proposed equation, a variation in the geometric configuration of connections within the practical range is considered. Excellent agreement is noted when comparing all M–θ relationships produced by the proposed equation to those by the finite element method and experiments. In addition, the stress distribution and main deformation modes are numerically obtained, where the ranking of stress criticality is offered for all structural parts. The depth, width, flange, and web thicknesses, as well as the yield stress of the beam, have a major influence on Mmax, as predicted by the proposed equation. Also, bolts have been identified as the most critically stressed component. [ABSTRACT FROM AUTHOR]