Behavior of Headed Studs in Composite Structures: An Overview

1  Introduction

Composite construction has been widely recognized as a central approach in structural engineering because it integrates steel and concrete in a manner that maximizes their respective strengths while minimizing weaknesses [1–3]. The increasing reliance on composite members in bridges, buildings, and prefabricated systems is tied to their capacity for improved stiffness, efficiency, and long-term durability when compared with conventional structural forms [4–6]. A key factor in the effective action of these systems lies in the performance of shear connectors, particularly steel-headed studs, which transfer longitudinal shear between the concrete slab and steel beam [7–10]. Headed studs have been extensively studied under a variety of mechanical and environmental conditions, revealing a strong dependence of their behavior on geometry, arrangement, and surrounding concrete properties [11–13]. Existing research has established that variables such as stud diameter, embedment depth, and material grade significantly influence shear capacity, slip characteristics, and fatigue resistance [14–16]. These findings have been supported through full-scale experiments, push-out tests, and advanced finite element simulations that provide detailed predictions of load-slip response and failure mechanisms [17–20].

Durability has emerged as another fundamental concern, as shear connectors are exposed to long-term environmental influences, cracking in surrounding concrete, and corrosive conditions that may reduce their capacity over time [19,21,22]. Investigations have demonstrated that freeze-thaw cycles, sustained load, and cyclic stress can alter stiffness degradation and residual strength of connectors embedded in conventional or advanced cementitious materials [23–25]. This dimension of research has been complemented by studies focused on extreme conditions such as fire and low temperatures, which have highlighted potential vulnerabilities in connectors when subjected to severe service environments [26–28]. Parallel to experimental developments, computational and analytical approaches have been crucial in advancing predictive capability for stud behavior. Early models concentrated on empirical formulations calibrated against test data, whereas recent advances have incorporated nonlinear finite element simulations and data-driven techniques such as machine learning to refine predictions of resistance and stiffness, reflecting wider efforts to adopt advanced computational tools in civil engineering [29–33]. These methods have made it possible to evaluate parameters difficult to capture experimentally, such as local stress concentration and slip distribution along the interface [34–36]. The application of novel materials has further expanded the scope of investigation, particularly with the use of ultra-high-performance concrete (UHPC), engineered cementitious composites, and hybrid concretes incorporating fibers or recycled elements [37–39]. Headed studs embedded in these advanced concretes have shown increased shear resistance and ductility, but concerns remain regarding cracking, anchorage efficiency, and constructability [40–42]. Comparative studies involving conventional connectors such as bolted or demountable systems have also pointed to differences in ease of assembly, reusability, and mechanical performance, highlighting the practical implications of stud selection for different structural contexts [43–45]. Despite the progress, design codes still face challenges in consistently incorporating these research outcomes into reliable provisions. Current recommendations vary across regions and often lag behind experimental evidence, leading to discrepancies between theoretical resistance predictions and observed structural behavior [46,47]. Some provisions address studs in profiled sheeting or under cyclic actions, yet the complexity of emerging materials and connection types has outpaced updates in standards [48]. In addition, uncertainties surrounding grouped arrangements, fatigue under mixed loading, and degradation in extreme service conditions remain partly unresolved [49–51]. The available literature demonstrates an extensive effort in experimental, analytical, and numerical study of headed studs across various structural applications. Nevertheless, the literature still lacks a recent paper that simultaneously consolidates the technical evidence across loading and environmental conditions and quantifies research trends and thematic evolution through a bibliometric mapping of the field. Accordingly, this study addresses this gap by conducting a systematic and bibliometric review on the behavior of steel-headed studs in composite structures. The novelty of the work lies in integrating (i) a systematic screening and synthesis of experimental, analytical, numerical, and data-driven studies that report strength, stiffness, slip, fatigue, and degradation mechanisms, with (ii) a bibliometric mapping of the field that identifies dominant research clusters and underrepresented topics, and then (iii) translating both components into design-facing implications and prioritized research gaps linked to code development. The aim is to present a structured evaluation of experimental, analytical, and computational findings while identifying persistent challenges such as long-term durability, seismic performance, and modeling accuracy. Through this effort, the work intends to provide researchers, engineers, and designers with a consolidated reference that supports more informed applications and future research directions in composite construction.

2  Review Methodology

In general, the literature sources for the systematic and bibliometric reviews were assembled from the Google Scholar and Scopus databases, targeting publications from 2000 to 2025 that address headed stud connectors in steel-concrete composite structures. The Scopus query was constructed using title, abstract, and keyword fields to capture both terminology and application context. The core search terms included headed stud, headed shear stud, stud shear connector, shear connector, composite beam, composite slab, steel–concrete composite, push-out, pull-out, fatigue, cyclic loading, and ultra-high-performance concrete, combined with Boolean operators to avoid irrelevant stud applications outside composite construction.

The screening workflow followed two stages. First, titles and abstracts were screened to confirm relevance to steel-headed studs used as shear connectors in steel-concrete composite systems. Second, full-text screening verified that each retained record contributed experimental, analytical, numerical, data-driven, or design-provision evidence that could be synthesized within the scope of this review. Studies were excluded when the work focused on unrelated fastening applications or when insufficient technical detail was available to support comparison.

The bibliometric component used the Scopus-exported database to quantify publication growth, outlets, keywords, and collaboration patterns. Keyword co-occurrence and source/country mapping were conducted using VOSViewer. Threshold settings were selected to emphasize terms and nodes with meaningful recurrence while retaining the ability to interpret emerging themes. The combined systematic and bibliometric workflow supports both technical synthesis and identification of research clusters and gaps.

The limitation of this review is the fact that the collected literature reflects only the Google Scholar and Scopus indexing and the selected query structure, so relevant studies indexed exclusively in other databases may not be captured. In addition, differences in test setups and reporting formats across laboratories limit direct quantitative aggregation, so the synthesis emphasizes comparative interpretation and design-relevant implications rather than meta-analytic pooling.

3  Bibliometric Analysis

This section reports the results of the bibliometric analysis performed in this study. In general, a total of 385 publications focusing on the behavior of headed studs in composite structures were identified in the Scopus database between 2000 and 2025. Fig. 1 illustrates the annual distribution of these studies. The early 2000s witnessed only limited activity, with fewer than ten papers each year. A gradual rise began after 2010, followed by a more consistent growth pattern, peaking around 2019 and 2023 with 33 publications recorded in each of these years. The data for 2025 remain incomplete, yet the continuing trend suggests sustained interest in this topic. The increase in research output reflects the growing relevance of composite structures and the ongoing refinement of headed stud behavior in design and performance assessment.

Figure 1: Annual number of Scopus-indexed publications on the behavior of headed studs in steel-concrete composite structures (search window 2000–2025; n = 385; 2025 reflects partial indexing at the time of database extraction).

Fig. 2 shows the distribution of publication types. The majority of contributions appeared as journal articles, totaling 307 documents, which indicates the dominance of peer-reviewed research in this area. Conference papers accounted for 76 records, showing that ongoing experiments and model validations are frequently presented at professional meetings before journal submission. Only two book chapters were identified, implying that the subject remains primarily research-driven rather than summarized in broader academic compilations.

Figure 2: Document types within the Scopus dataset on headed stud behavior in steel–concrete composite structures (2000–2025; n = 385), showing the relative share of journal articles, conference papers, and book chapters.

Keyword co-occurrence mapping in Fig. 3 provides an overview of the conceptual directions followed by researchers. The most frequent keywords include “studs (fasteners)”, “studs (structural members)”, “headed stud”, and “concrete slabs”. Other recurring terms such as “composite beams and girders”, “push-out tests”, “finite element method”, and “shear connector” suggest that the literature has largely concentrated on load transfer mechanisms, structural performance, and computational modeling. The strong clustering of these keywords indicates that mechanical behavior, interface shear resistance, and structural optimization continue to anchor most investigations. This concentration around experimental and analytical modeling reflects a technical rather than theoretical focus in the field. Citation analysis of journals, presented in Fig. 4, reveals that Engineering Structures has been the most influential source, with 47 publications and more than 2100 citations. The Journal of Constructional Steel Research ranks closely with 34 papers and comparable citation counts. Other active journals include Steel and Composite Structures, Structures, and Construction and Building Materials. Their citation link strengths highlight a stable network of publications dedicated to the interface between steel and concrete performance. The prominence of these journals confirms that research on headed studs remains closely tied to mainstream civil and structural engineering outlets, reinforcing the technical orientation observed in the keyword map.

Figure 3: Keyword co-occurrence network for the Scopus dataset on headed stud behavior in composite structures (2000–2025; n = 385), highlighting dominant clusters related to push-out testing, finite element modeling, and composite beam/slab applications.

Figure 4: Source/journal citation influence within the Scopus dataset on headed stud behavior in composite structures (2000–2025; n = 385), reporting the most influential publication venues by output and citation impact.

The distribution of contributing countries in Fig. 5 demonstrates that China leads the field with 120 publications and the highest citation and link strength values. The United Kingdom and the United States follow, showing considerable collaboration links across continents. Germany, Japan, and Australia also appear as strong contributors, while several emerging participants such as Serbia, Singapore, and Brazil signal growing regional interest. The visual clustering of countries in the network diagram points toward active international cooperation, with China and the United States acting as central hubs in shared authorship and citation networks.

Figure 5: Country contribution and collaboration network for Scopus-indexed studies on headed stud behavior in composite structures (2000–2025; n = 385), illustrating the main hubs and collaboration link strengths.

A technical interpretation of the bibliometric patterns helps link research activity to engineering themes addressed in the remainder of the manuscript. The dominant keywords and their co-occurrence around push-out tests, shear transfer, finite element method, and composite beams indicate that experimental characterization and mechanics-based modeling form the principal knowledge backbone, which is reviewed in Sections 3 and 4. The repeated appearance of terms associated with fatigue, cyclic behavior, and durability aligns with the growing body of work on bridge and infrastructure demands, which is synthesized in Section 3 and revisited in the Discussion as a code-relevant gap. The increasing presence of UHPC and other advanced cementitious systems in the keyword space is consistent with the recent concentration of highly cited studies on thin high-performance slabs and modified failure modes, which is integrated into Sections 3 and 5. Finally, the emergence of data-driven and machine learning terminology in the literature signals a newer research cluster that complements analytical and numerical approaches, which is reviewed in Section 4 and discussed as a pathway for interpretable design-oriented tools.

Overall, the bibliometric analysis outlines a research field that has developed steadily over the past two decades, characterized by a concentration of studies in a few leading journals and active global collaboration. The results collectively show that investigations into headed studs in composite structures have transitioned from isolated experimental studies to an interconnected body of work grounded in analytical modeling, large-scale testing, and international research partnerships.

4  Experimental Evidence on Headed Stud Behavior

Experimental investigations have provided the foundation for understanding the role of headed studs in composite structures, Table 1. Over the decades of research, push-out tests, pull-out tests, fatigue studies, and large-scale structural experiments have been performed to characterize their strength, stiffness, ductility, and long-term durability under diverse conditions [1–3]. These studies have collectively defined the mechanisms through which headed studs transfer shear between steel and concrete, while highlighting how geometry, material properties, environmental exposure, and construction detailing influence their behavior [4–6].

4.1 Impact of Cementitious Systems

The development of UHPC and engineered cementitious composites has motivated a large body of experimental research on headed studs. Static and fatigue push-out studies confirmed the distinctive behavior of studs embedded in thin UHPC layers, where higher strength concretes modify load transfer and reduce slip demand [4,17]. Further studies on steel–UHPC composite sections examined the combined action of studs and UHPC slabs, demonstrating altered mechanical interaction compared with conventional concretes [37,38]. Push-out experiments on studs embedded in steel-engineered cementitious composite (steel–ECC) slabs also highlighted the effect of strain-hardening cementitious composites, showing differences in shear transfer compared with ordinary concretes [13]. Parallel investigations assessed headed studs in steel fiber reinforced systems, extending experimental focus to hybrid concretes with fibers or recycled components [39,41,52]. Such results illustrate the consistent experimental effort devoted to quantifying how novel cementitious materials affect bond, slip, and ultimate shear strength of studs. Studies involving engineered cementitious composites also confirmed that advanced binders provide distinct responses in pull-out testing of studs, further diversifying the knowledge of material–stud interaction [53].

4.2 Push-Out Behavior

Push-out tests have long been employed to examine shear transfer mechanisms of headed studs, producing direct data on shear capacity, load-slip response, and failure modes. One of the most cited comparisons involves the relative performance of bolted connectors and welded headed studs, where push-out testing revealed distinctions in stiffness and failure characteristics that underpin design recommendations [2]. Further comparisons between headed studs and other connector types in prefabricated systems have also reinforced the significance of push-out testing in assessing alternative solutions for composite decks [54]. Large-scale push-out studies have explored the influence of grouped studs, demonstrating how close spacing modifies slip distribution and changes resistance [48,49,55]. Such work has consistently emphasized that the collective behavior of stud groups cannot be simplified to the sum of individual studs, an observation reinforced in later finite element validation studies [48]. Investigations involving thin plates and UHPC further confirmed that stud geometry, plate thickness, and material grade all play defining roles in the shear capacity observed in push-out setups [9,10,56]. Several researchers have extended static testing to large-headed studs embedded in UHPC, highlighting differences in slip stiffness and resistance compared with conventional studs [40]. Other work emphasized how reinforcement detailing within surrounding concrete can alter the overall structural performance of connections, underlining the interdependence of reinforcement design and connector action [16]. Early full-scale beam tests also remain central, demonstrating fundamental patterns in shear transfer of headed studs that continue to influence interpretation of push-out results today [57].

4.3 Pull-Out Behavior

Pull-out testing complements push-out experiments by isolating tensile resistance of headed studs. Investigations on tensile capacity have demonstrated the influence of stud head diameter and embedment length on pull-out strength [35]. Experimental work on handcrafted studs assessed variability in performance and quality compared with industrially manufactured connectors, offering insight into construction practices in resource-limited contexts [63]. More recent studies on thin panels reinforced with void formers have extended tensile investigations to non-standard geometries, revealing the adaptability of pull-out testing for specialized configurations [70]. The pull-out behavior of headed studs has also been tested under UHPC and ECC conditions, confirming differences in anchorage and stiffness relative to normal concretes [12,53]. Studies investigating concrete consolidation at stud roots demonstrated how incomplete compaction significantly reduces stud performance, emphasizing construction quality as a determining factor [71]. Similarly, pull-out experiments on innovative ribbed stud geometries in slab–beam connections offered further evidence on the influence of shape modifications on anchorage behavior [72].

4.4 Fatigue and Cyclic Behavior

Fatigue performance has been the focus of several experimental campaigns, Table 2, as long-term cyclic loading is critical in bridges and heavily trafficked structures. Foundational research investigated fatigue resistance of welded headed studs under rotating shear, emphasizing the role of welding details and ferrule quality [24]. Subsequent large programs examined fatigue loading in detail, generating data that continues to inform resistance models [3]. More recent fatigue studies examined studs in UHPC and ultra-high toughness cementitious composites, where superior fatigue resistance has been reported compared with conventional concretes [4,25]. Experimental research on grouped arrangements under cyclic shear demonstrated how load distribution shifts across stud groups, influencing long-term stiffness degradation [73]. Work on large-headed studs and demountable connections has also confirmed that cyclic demands produce different stiffness degradation paths than monotonic loading [45,74]. Investigations on fatigue of studs in precast connections or with improved reinforcement have added further experimental knowledge, supporting design recommendations in complex structural systems [14]. Complementary studies considered repair-welded studs and their fatigue strength, contributing insight into maintenance and retrofitting practices [75]. When studs are used in groups, their collective performance differs substantially from isolated behavior. Push-out experiments confirmed that spacing, arrangement, and load sharing modify slip and shear resistance [49,55]. Finite element validations supported equivalent diameter calculation models designed to represent groups more accurately, and experimental evidence continues to serve as the foundation for these models [48]. Investigations into large-headed studs in grouped configurations further expanded knowledge, particularly when UHPC was used as the embedding material [40]. Studies considering grouped studs under cyclic loading in infill walls highlighted the complexity of the combined effects of reinforcement and connector distribution [50,73]. Research into stiffness degradation of studs under low-cycle fatigue confirmed that grouped connectors experience distinctive degradation paths [23].

4.5 Environmental Effects

Experimental studies have extended to severe service conditions where temperature or environmental exposure alters stud performance. Investigations into low-temperature conditions in Arctic infrastructure revealed the distinctive mechanical response of studs in cold environments [5,27]. Further pull-out and stress–strain model development at subzero temperatures supported the reliability of design under such conditions [28]. Fire has been another focal condition, with experiments on studs in composite beams at elevated temperature demonstrating changes in shear strength and ductility [26]. Recent studies on studs in modern profiled sheeting under high temperature expanded this line of research, considering contemporary composite systems under fire loading [36]. Research on repair methods involving UHPC also considered stud performance under severe conditions, highlighting connections to rehabilitation practice [9,10]. Corrosion remains a long-term concern, with experimental work confirming how cracks in surrounding concrete accelerate deterioration and reduce stud performance [21]. Durability studies on studs embedded in UHPC provided comparative evidence of improved resistance against electrochemical corrosion relative to conventional concretes [19].

4.6 Summary

Across the push-out and pull-out evidence summarized in Table 1, several comparative patterns emerge that support design-relevant interpretation. In monotonic push-out configurations, stud diameter and aspect ratio repeatedly control the governing failure transition between stud shank fracture, weld-line failure, and concrete-related cracking, which explains why some datasets report strong diameter effects on resistance while others emphasize ductility and post-peak slip as the primary differentiator. When advanced concretes are used, the evidence indicates that increased matrix strength often raises peak resistance and reduces slip demand, yet it can also shift failure toward the stud or the weld region, which is consistent with studies reporting weld-line sensitivity and different post-peak stages in UHPC systems [17,37]. In pull-out configurations, the governing failure mode largely determines the apparent influence of geometry: embedment length and bearing area become dominant when concrete cone breakout governs, while the influence of stud diameter becomes less pronounced when failure shifts to the steel shank, which is consistent with the observation that diameter can be negligible in some pull-out datasets but influential when boundary conditions and slab thickness promote breakout [8,27,64]. These comparative observations motivate the later discussion on why unified design equations remain challenging across normal concrete, UHPC, and ECC, and why code extensions require explicit recognition of failure-mode transitions rather than a single resistance expression.

Finally, beyond conventional welded studs, experimental work has tested alternative connector arrangements. Comparisons between headed studs and high-strength bolts in prefabricated composite decks demonstrated performance differences that guide choice in prefabricated systems [54]. Demountable connections with bolts and studs were also examined in detail, illustrating performance trends under shear loading and long-term behavior [45,76]. Investigations on post-installed connectors compared spring pins and welded studs, highlighting differences in push-out resistance and ductility [44]. Further studies tested innovative ribbed studs with triangular or spherical geometry, extending the experimental knowledge of connector alternatives in slab–beam junctions [72]. Research involving I-shape connectors compared with headed studs also provided comparative results on shear resistance and stiffness [66]. Several experimental programs have been closely tied to design development. Studies on studs in profiled sheeting evaluated their shear resistance and ductility, generating data for Eurocode 4 provisions and related design guidelines [47,77]. Later research advanced these efforts by testing studs with profiled sheeting transverse to beams, informing newer design approaches [7,34]. Grouped stud tests under different reinforcement detailing also contributed to recommendations for design adjustments [55]. Other tests on studs in railway bridges and steel–concrete joints supported the evolution of code provisions specific to infrastructure applications [42,78].

5  Numerical, Analytical, and Data-Driven Modeling

The study of steel-headed studs in composite structures has long been supported by numerical and analytical investigations, Table 3. Experimental work has established fundamental behavior, yet the complexity of material interaction and load transfer processes has driven the need for computational and theoretical models. Numerical analysis, finite element simulation, analytical formulations, and more recently data-driven modeling have been widely used to estimate stud resistance, slip development, and failure modes. Collectively, these methods have expanded the understanding of stud performance under diverse conditions, complementing the breadth of experimental research [17,18,20].

5.1 Numerical Approaches

Finite element analysis has been indispensable in investigating the detailed behavior of headed studs. The ability to capture local stress concentrations, bond conditions, and slip mechanisms has provided a powerful tool to complement experiments. Finite element studies of studs in UHPC have examined how material strength and stud geometry influence shear resistance, producing simulations consistent with push-out observations [17,19]. Numerical studies on grouped large-headed studs in UHPC also extended this line of work, offering computational evidence on stiffness, slip progression, and resistance under different embedment conditions [40].

Other finite element research addressed low-temperature conditions, with models of stress–strain behavior at subzero temperatures calibrated to laboratory measurements [28]. These simulations have contributed to a better representation of performance in Arctic infrastructure [5,27]. The role of head diameter has been considered in numerical investigations of tensile capacity, with simulations confirming the influence of geometry on pull-out resistance [35].

Computational modeling has also been applied to profiled steel sheeting, where the shear resistance of studs with ribs transverse to supporting beams was addressed through advanced finite element models [34]. Recent work further examined profiled steel sheeting under fire and elevated temperature through combined testing and numerical evaluation [36]. Finite element studies have been extended to eccentric loading, inclined shoulders, and complex composite systems, demonstrating the adaptability of numerical modeling to a wide variety of structural contexts [84,85]. Numerical models have also supported design recommendations for grouted joints in precast elements [85].

While monotonic loading has been widely analyzed, numerical models have also addressed cyclic and fatigue demands. Simulations of low-cycle fatigue behavior have provided estimates of stiffness degradation in studs [23]. Other numerical studies assessed cyclic slip and strength degradation in grouped studs under repeated shear [48,73], as well as studs embedded in composite frames with reinforced infill walls [50]. Fatigue behavior under bridge loading conditions was also examined through computational studies [51].

Numerical modeling has extended to seismic and bridge applications, including double skin composite shear walls with overlapped studs [15], steel–concrete joints under combined loading [78], demountable composite beams [86], and adaptive systems with bolts and studs [43,45]. Other studies addressed UHPC bridge repairs [9,10] and railway bridge decks [42].

Advanced numerical investigations also considered hybrid and innovative systems, including steel tubular stub columns stiffened by studs [87], composite slabs with varied rib geometries [88], and I-shape connectors compared with studs [66]. Durability-related simulations evaluated corrosion behavior in UHPC and normal concrete [18,19], while numerical studies on insufficient concrete consolidation at stud roots highlighted construction-related effects on performance [71].

5.2 Analytical Approaches

The earliest analytical formulations addressed the shear capacity of headed studs through simplified resistance models linking stud strength to shear area and embedment properties [1,89]. These formulations provided foundational equations for design codes and served as a starting point for subsequent refinements. Analytical predictions were commonly calibrated against push-out and pull-out results, with models designed to capture monotonic deformation under shear [20]. Analytical studies also addressed fatigue resistance, translating experimental patterns into predictive resistance models for repeated loading [3,80]. Later efforts examined the effect of reinforcement detailing, proposing analytical descriptions of how reinforcement influences slip distribution and shear transfer across the steel–concrete interface [16]. Some approaches extended to grouped studs, suggesting equivalent diameter models and analytical simplifications for collective arrangements [14,48,55]. These developments reflect the progressive incorporation of experimentally observed complexities into analytical formulations. Analytical models developed for fatigue loading were often used alongside numerical simulations to enable long-term resistance predictions [3,24,80].

5.3 Data-Driven Approaches

The expansion of data-driven methods has introduced a new dimension to predictive modeling of headed studs. Machine learning algorithms have been trained on experimental datasets to estimate shear resistance, offering alternatives to purely analytical formulations. Ensemble modeling strategies were applied to predict shear resistance under varied material and geometric inputs [22], while interpretable machine learning approaches aimed to provide transparent predictions for precast steel–concrete structures [30]. Other studies emphasized the potential of computational intelligence in predicting stud resistance [29]. Grouped connectors have also been examined using soft computing, where analytical predictions were complemented by machine learning-based formulations [31]. Artificial intelligence applications in related domains, such as gene expression programming for shear capacity of corrugated web steel beams, indicate transferable methodologies for shear connectors [90]. Machine learning-based stiffness estimation in reinforced concrete members further demonstrates cross-disciplinary relevance [33,91]. Reviews of physics-informed neural networks highlight the expanding role of AI in structural mechanics [92]. Several studies explicitly combined numerical, analytical, and data-driven approaches. Experimental-numerical comparisons on studs in UHPC validated finite element predictions while providing benchmarks for analytical models [11,18]. Hybrid approaches integrating machine learning with analytical features sought to balance interpretability and predictive accuracy [22,30]. Design recommendations for grouped studs were supported by numerical validation of analytical simplifications [14,48].

6  Design Provisions, Applications, and Special Topics

The development of design provisions for headed studs has drawn extensively from decades of experimental, analytical, and numerical studies, Table 3. As composite structures became standard in modern construction, the need for codified approaches to predict shear resistance, stiffness, and fatigue life of studs became evident. National and international standards have been shaped by comparative studies, large-scale testing, and reliability assessments that addressed different loading regimes, concrete types, and construction practices [47,77]. More recent contributions have incorporated data from advanced cementitious composites, novel stud geometries, and demountable systems, reflecting the expansion of applications for headed studs in both infrastructure and building sectors [7,45,72,86]. Early design models concentrated on the shear resistance of studs welded into conventional concrete slabs, drawing from push-out and beam tests. Full-scale experiments confirmed the adequacy of simplified shear resistance formulations, which formed the basis for early Eurocode provisions and related national standards [57,89,103]. Refinements followed as research extended to lightweight concrete, profiled steel sheeting, and fatigue conditions, with adjustments introduced into code equations to capture ductility and load-slip characteristics [34,46,104]. Special focus has been given to profiled sheeting, where the rib orientation and depth alter load transfer. Investigations provided evidence that guided Eurocode 4 updates and subsequent recommendations in international guidelines [47,76,77]. Further studies introduced design methods specific to railway bridges and composite decks, acknowledging the distinct load environments and geometric detailing in such applications [42,78]. Work on grouped studs has also influenced design guidelines, with recommendations made to account for collective effects and equivalent diameter approaches validated through finite element comparisons [14,48,49]. Composite bridges rely heavily on stud connectors to maintain composite action under heavy and repeated loading. Early studies demonstrated the fatigue capacity of studs in bridge girders, shaping design approaches that address cyclic performance [3,51,75]. More recent experimental and numerical efforts considered studs embedded in UHPC decks, reflecting the adoption of advanced concretes in bridge rehabilitation and new construction [9,19,37]. Research on post-installed connectors has supported bridge retrofitting practices, providing comparative data on coiled spring pins and welded studs for in-service strengthening [44]. Studies that examined the effect of external pressure on push-out behavior also aligned closely with bridge contexts, where environmental and traffic-induced effects can modify shear transfer [105]. Furthermore, machine learning–based predictive models for stud resistance have been tested with bridge applications in mind, offering supplementary approaches for design reliability in complex structures [22,29,30]. Composite floors in buildings often employ studs in profiled sheeting, where their placement governs overall slab–beam performance. Research into trapezoidal sheeting and rib orientation provided experimental data for design provisions specific to these systems [34,47,88]. Studies comparing studs with high-strength bolts in prefabricated decks extended the scope of applications, offering evidence for alternative connectors in modular construction [43,54]. Precast systems represent another important application, with research addressing short studs in grouted joints of composite girders [85]. These investigations highlighted the role of connector detailing in achieving effective load transfer in modular construction. Applications in seismic regions have received growing attention, particularly for double skin composite shear walls that incorporate overlapping studs [15,106]. Experimental evaluations of studs in infill walls and their monotonic shear performance further confirmed the relevance of these connectors in lateral load–resisting systems [50,73,107]. Such applications underline the adaptability of headed studs across building systems subjected to combined axial, shear, and seismic demands. Design provisions have expanded to address the adoption of UHPC and other high-performance materials. Static and fatigue testing of studs in thin UHPC slabs provided results that informed resistance models distinct from those used for normal concrete [4,17,38]. Further testing in steel–UHPC systems refined understanding of load transfer, allowing recommendations for shear strength and stiffness in emerging structural systems [11,108]. Push-out tests on studs in engineered cementitious composites demonstrated the impact of strain-hardening properties on shear resistance, supporting design considerations for such materials [13,53]. Comparative research on studs embedded in geopolymer concretes reinforced with steel fibers also contributed experimental evidence for specialized concrete types [109]. Additional investigations into UHPC containing different aggregate sizes have continued to inform design adjustments [110]. Group arrangements of studs have long been a challenge in design practice, as close spacing alters shear distribution. Experiments on grouped connectors led to recommendations for design methods incorporating group effects [14,49]. Finite element verification of equivalent diameter concepts provided further validation, highlighting that group behavior differs markedly from isolated stud assumptions [48]. Research into grouped large-headed studs embedded in UHPC offered evidence for specialized design adjustments, particularly in applications requiring concentrated load transfer [40]. Collective behavior under cyclic loads in infill walls demonstrated the significance of group interaction in seismic applications [73]. The inclusion of such findings in guidelines reflects the progression from isolated stud models toward more comprehensive provisions. Design provisions have also expanded to include thermal conditions. Studies on studs in composite beams subjected to fire loading highlighted the reduction in shear capacity at elevated temperatures, shaping guidance for fire-resistant design [26]. Experimental and numerical work on studs embedded in profiled sheeting under elevated temperature extended this focus to contemporary composite systems [36]. Cold regions present contrasting challenges, with studies on stud pull-out and mechanical performance at low temperatures providing data for design under Arctic conditions [5,27,28]. These contributions support provisions that consider environmental extremes in the design of infrastructure systems. Alternative connectors have been studied to address demands for demountability and reusability in modern construction. Research on bolted and stud combinations provided comparative evidence for connections that can be dismantled and reused [86,88]. Additional work on demountable beams highlighted differences in static and fatigue behavior compared with traditional welded studs [74]. Post-installed connectors were assessed in detail, where coiled spring pins were compared with welded studs through push-out tests, contributing to recommendations for retrofit and strengthening solutions [44]. Further investigations into I-shape connectors and ribbed studs extended the experimental evidence for alternatives, supporting applications where traditional studs may be limited [66,72]. Several experimental and numerical studies have examined conditions beyond conventional applications. Investigations on lightweight aggregate concrete provided evidence on stud performance in specialized concretes used for weight reduction [103]. Research on inclined stud geometries investigated through numerical studies expanded potential design variations [84]. Comparative studies between headed studs and other connectors, such as channels and bolts, provided design-related evidence relevant to both prefabrication and retrofitting contexts [43,54,66].

7  Discussion and Future Directions

The accumulated body of research on headed studs demonstrates both the progress achieved in understanding their behavior and the gaps that remain in fully codifying this knowledge into universally consistent design provisions. Experimental campaigns have covered a wide range of conditions, from static push-out tests on single connectors to fatigue investigations in bridges, grouped stud arrangements, and applications in ultra-high-performance concrete. These experimental efforts have formed the backbone of design models and code provisions, yet their results are dispersed across varying structural systems, material types, and test conditions. This dispersion has created a challenge for developing generalizable design equations that are consistently conservative but not overly restrictive. The variability across regional standards illustrates this ongoing difficulty, with Eurocode provisions differing in important respects from other international guidelines, particularly in their treatment of profiled sheeting, grouped connectors, and fatigue resistance. A recurrent theme across studies is the sensitivity of stud performance to surrounding concrete properties. Research on ultra-high-performance concrete, engineered cementitious composites, and fiber-reinforced matrices has shown that advanced materials modify load transfer and slip response in ways that traditional models do not fully capture. While some studies propose adjusted resistance models, incorporation of these findings into mainstream codes remains limited. The integration of new materials into design provisions will require not only additional experimental testing but also coordinated efforts to reconcile findings across different laboratories and loading conditions. Without such synthesis, design practice risks lagging behind construction practice, especially as UHPC and other advanced concretes gain wider adoption in bridge decks and prefabricated elements. Fatigue and long-term durability are also areas where progress has been made but uncertainties remain. Early fatigue research provided essential data for bridges, and more recent investigations on studs in UHPC and ultra high toughness cementitious composite (UHTCC) have expanded this knowledge. However, questions persist regarding performance under combined loading conditions, such as when cyclic shear is superimposed on sustained loads or environmental degradation. Durability studies on corrosion, freeze-thaw exposure, and fire conditions have highlighted vulnerabilities in stud performance, particularly when cracks form in the surrounding concrete and accelerate deterioration. Incorporating these effects into design remains an open challenge, as codes typically treat durability through safety factors rather than direct modeling of degradation. The rise of computational and data-driven modeling presents opportunities for addressing these complexities. Finite element models have advanced to the point where local slip distribution, stress concentration, and group effects can be studied in detail. Machine learning applications have begun to provide predictive models trained on large experimental datasets, offering the potential for more adaptable design tools. However, the adoption of such methods into practice will require transparent and interpretable approaches that engineers can apply with confidence. While interpretable machine learning methods have been proposed, their role in official design provisions is still at an early stage. Integration of analytical, numerical, and data-driven methods appears to be the most promising pathway for future design frameworks, enabling the strengths of each approach to compensate for the weaknesses of the others. Practical applications in bridges, prefabricated systems, and seismic design continue to broaden the contexts in which headed studs are deployed. Research on demountable systems and alternative connectors illustrates how the design of composite structures is shifting toward adaptability and reuse, rather than permanent connections alone. This shift introduces new challenges in design provisions, as demountable systems must balance mechanical performance with reusability, introducing failure mechanisms not present in welded studs. The comparative studies between studs and alternative connectors provide useful data, but their codification remains incomplete. The effect of grouped studs remains one of the more complex design issues. Collective behavior deviates significantly from isolated assumptions, and despite decades of work, the prediction of resistance in dense stud groups remains less precise than desired. Equivalent diameter methods and finite element validations have improved predictive capability, yet questions persist about their applicability across different concrete types and reinforcement conditions. This suggests the need for further systematic research that consolidates group behavior into reliable design models. Special topics, including stud performance under fire and at low temperatures, have been the focus of targeted research. While fire testing has provided essential data on resistance reduction, and Arctic studies have examined low-temperature mechanical properties, these conditions are often treated as exceptions in design codes rather than systematically integrated. As climate-related hazards and environmental extremes increasingly influence structural demand and deterioration, design models for headed studs will benefit from more explicit treatment of temperature effects, freeze-thaw exposure, corrosion progression, and their interaction with cyclic loading, rather than relying mainly on global safety margins.

Looking ahead, the evidence synthesized in this review supports several prioritized research directions that can be translated into actionable studies and code-facing outcomes. Harmonization efforts should begin with consistent definitions of key response metrics across laboratories, particularly shear stiffness definitions, slip measurement conventions, and fatigue damage indicators, because inconsistent definitions remain a major driver of apparent discrepancies when results are compared across studies. Advanced concretes such as UHPC, ECC, and UHTCC require resistance models that explicitly account for changed failure tendencies and weld-region effects observed in several experimental programs, and coordinated multi-laboratory datasets are needed to separate material effects from test-setup effects. Grouped stud behavior remains a design-critical gap; future studies should clarify how group interaction varies across concrete types and reinforcement layouts and should establish ranges where equivalent-diameter approaches remain reliable. Durability under mixed actions is another priority; research should quantify how corrosion and freeze-thaw exposure alter fatigue resistance and stiffness degradation when cyclic shear is combined with sustained service loading. Data-driven prediction has demonstrated high accuracy in recent studies, yet design adoption depends on external validation, uncertainty quantification, and interpretability; future work should therefore report transparent feature definitions, provide open benchmarking against code equations, and evaluate model robustness under out-of-distribution cases such as thin UHPC layers, dense stud groups, and extreme temperatures. Finally, the growing push toward modularity and reuse motivates focused work on demountable and hybrid connector systems, where mechanical performance must be assessed together with constructability, inspectability, and reliable long-term behavior.

8  Conclusion

This study addressed the gap in the literature regarding a comprehensive review of headed stud behavior in steel-concrete composite structures that combines systematic technical synthesis with bibliometric mapping of research trends. The review consolidated experimental evidence on monotonic, cyclic, fatigue, and environmentally influenced behavior together with analytical, numerical, and data-driven modeling approaches, and it connected these findings to design provisions and practical applications discussed in bridges, buildings, prefabricated systems, seismic components, and emerging demountable solutions.

The synthesized evidence indicates that stud geometry, detailing, and surrounding concrete type remain primary drivers of strength, stiffness, slip response, and governing failure mechanisms, and that grouped stud interaction and boundary conditions can produce deviations from isolated-stud assumptions that are design-relevant. Advanced concretes such as UHPC and ECC can increase stiffness and alter failure tendencies, yet these effects are not consistently reflected in standard design approaches across regions. Modeling capability has advanced substantially through nonlinear finite element simulations and data-driven prediction; however, practical adoption depends on transparent definitions, external validation, and robust treatment of uncertainty, especially for dense stud groups, thin UHPC layers, and environmental extremes.

The limitations of this study lie primarily in reliance on published literature without access to unpublished experimental programs and proprietary test databases, which may limit the completeness of reported datasets. In addition, heterogeneity in test protocols and response definitions across laboratories limits direct quantitative aggregation, particularly for stiffness measures and post-peak behavior. From an application perspective, the findings support more informed connector selection and detailing for composite members, highlight where durability and mixed-action demands require explicit consideration, and indicate where design checks may be sensitive when advanced concretes or demountable systems are adopted. Future research should prioritize harmonized experimental reporting, coordinated multi-laboratory studies on grouped studs and advanced concretes, explicit modeling of degradation under mixed actions, and interpretable predictive tools that can be benchmarked against and ultimately support updates to design provisions.

Acknowledgement: Not applicable.

Funding Statement: The authors received no specific funding.

Author Contributions: The authors confirm their contribution to the paper as follows: study conception and design: Mahmoud Alhashash, Abdullah Alariyan, Favzi Ghreivati, Ahed Habib, and Maan Habib; analysis and interpretation of results: Mahmoud Alhashash, Abdullah Alariyan, Favzi Ghreivati, Ahed Habib, and Maan Habib; draft manuscript preparation: Mahmoud Alhashash, Abdullah Alariyan, and Favzi Ghreivati; manuscript review & editing: Ahed Habib, and Maan Habib. All authors reviewed and approved the final version of the manuscript.

Availability of Data and Materials: The data supporting this study are available from the corresponding author on reasonable request.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest.

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