Cold-formed Steel Buildings Research Articles

Seismic design rules for lightweight steel shear walls with wood panel sheathing within the Eurocode format

Within the 2nd-generation of Eurocodes several new lateral force resisting systems (LFRS) typically used in lightweight cold-formed steel (CFS) buildings have been introduced, including shear walls with wood sheathing (SWWS). Therefore, extensive studies have been carried out to define the seismic design parameters according to the format of European code for seismic design, i.e. Eurocode 8. In current version of the 2nd-generation Eurocode 8 (prEN1998-1-2, CEN 2022), in case of SWWS the behaviour factor has been defined based on the FEMA P695 (FEMA, 2009) [3] methodology, whereas the other seismic design parameters have been set based on assumption validated with analyses on limited case studies. To overcome this lack, specific research devoted to defining the main seismic design parameters for SWWS belonging to Ductility class 3 (DC3) according to prEN1998-1-2 (CEN 2022) has been carried out at the University of Naples “Federico II”. The research shows that the Easley et al. (1982) method gives consistent in-plane horizontal wall resistance predictions if the shear resistance of wood panel-to-steel sheet screw connections (sheathing connections) is evaluated with reliable approaches and formulas for the evaluation of the shear resistance of sheathing connections have been calibrated. Based on the results obtained from this research, a partial safety factor for resistance evaluation of SWWS according to limit state design (LSD) equal to 1.3 is proposed and an overstrength (expected resistance) factor for the design of non-ductile elements according to capacity design equal to 1.7 is recommended.

Comparative Study of an Industrial Building Made of Cold formed steel versus Hot Rolled Steel Elements

Industrial buildings based on Cold Formed Steel (CFS) members have gained an extent of construction all over the world. It becomes an alternative process of construction to the Hot Rolled Steel (HRS) elements, allowing it to respond to the requirements in a short time. The aim of this work is to present a technical and economic comparative study of an industrial building case, made of CFS versus HRS elements, and designed according to the European and American codes. Relying on numerical modeling, the design according to Eurocode 3 demonstrates that the industrial building made of CFS is more economical compared to the HRS members building by 43% in terms of weight and 28% in terms of cost. In addition, based on the Average Capacity Design (ACD) ratio of the elements of a CFS, the design according to Eurocode 3 part 1-3 is less conservative than the same building designed according to American Iron and Steel Institute (AISI) code. Moreover, the American code privileges the safety aspect of the designed CFS building with the ASD method over the LRFD and LSD methods. Otherwise, a CFS building designed according to Eurocode 3 part 1-3 with truss columns and beams reveals a weight saving of 25% and 14%, respectively, compared to a building with a full web of beams and columns, and a building with a full web of columns and steel trusses.

Seismic performance and risk mitigation of CFS shear walls with CFRP-reinforced screw connections and special-shape L-section end studs

Cracking damage of screw connections and buckling of end studs always resulted in performance degradation of cold-formed steel (CFS) shear walls. To improve the seismic performance and to avoid the brittle failure of CFS shear walls, two methods for strengthening CFS walls are proposed in this paper: carbon fiber reinforced plastic (CFRP) toughened screw connections (CFRP-SC) and special-shape built-up end studs with L-shaped sections (SBC-L). A total of 5 full-scale strengthened CFS walls with CFRP-SC or SBC-L were tested to examine the performance, and simplified numerical models of strengthened CFS walls and 5-story CFS buildings were developed and validated. Nonlinear dynamic analyses are performed to investigate the effects of those two methods on seismic responses and risks of CFS buildings, and the risk mitigation of CFS buildings is then discussed. The results show that the CFS shear walls with CFRP-SC can mitigate the damage to screw connections and wallboards nearby those connections, especially for the connections at the corners of the walls. The connections could continually dissipate the earthquake energy due to large loading and deformation capacities. The SBC-L can effectively delay the buckling failure of end studs, thus enhancing the shear resistance of CFS walls. Due to the two methods, the collapse probability of the CFS building with CFRP-SC in service life is reduced by almost 40.8%, and that of the building with CFRP-SC and SBC-L is reduced by almost 59.7%.

Numerical modelling and investigation of CFS built-up stud walls in fire

Cold-formed steel (CFS) stud walls with the studs made of BC (built-up back-to-back CFS channel) and NC (built-up nested CFS channel) members are likely to be utilised in many applications in CFS building structures. However, the knowledge and understanding related to the thermal and structural behaviour of these improved CFS stud walls under fire exposure and their FRLs (fire resistance levels) are still limited. Although standard fire tests of real-sized walls could show the behaviour and FRL of some BC and NC stud walls, their results are limited because of their high cost and time-consuming nature, and are applicable to only the tested configurations. In this study, thermal and sequentially coupled structural finite element models were established. The validation of these models was achieved using the standard fire test results of real-sized BC, NC and channel stud walls. Later, parametric investigations based on the established models were undertaken to investigate the behaviour and FRLs of load-bearing BC, NC and channel stud walls with or without cavity insulation under varying load ratios. The outcomes from these investigations clarified the superiority of cavity insulated NC stud walls. Details of this study and its outcomes are presented in this paper.

Pull-out failure capacities of cold-formed steel batten screw connections at sub-zero temperatures

Cold-formed steel (CFS) roof and wall cladding systems are used worldwide, and are constructed using high-strength, thin steel battens and claddings. In North American countries, which are frequently subjected to blizzards (winter storms), these cladding systems are particularly vulnerable to pull-out failures during combined windstorm and snowy conditions. Despite extensive research studies investigating the pull-out failure mechanism and the critical parameters of screw connections between cladding and battens at ambient temperature, the behaviour of these connections under blizzard conditions has not been experimentally investigated. To fill this knowledge gap, an experimental study was undertaken that included 144 laboratory tests of screw connections in battens of varying grades and thicknesses, and three distinct types of screws. The pull-out failure capacities of screw connections in battens were evaluated under wind suction loading at four different temperatures, including ambient and sub-zero temperatures. The results of the study provided insights into the pull-out failure mechanisms of screw connections in battens and enabled the development of suitable design rules for combined wind action and sub-zero temperature conditions. The findings of this study are expected to be of significant value to engineers and building designers who are involved in the construction of lightweight cold-formed steel buildings.

Research on seismic resilience evaluation index of mid-rise CFS structures

This paper focuses on the deficiencies of Chinese standard GB/T 38591–2020, and combines the mechanical characteristics, post-earthquake damage characteristics, and repair methods of mid-rise cold-formed steel (CFS) structures to define the damage grade of structural components and revise various index parameters in the “Standard”. Based on the calculation results of 120 sets of 6-story CFS composite shear wall structures, the multi-parameter seismic resilience evaluation index and grading criteria applicable to mid-rise CFS structures are fitted, and the index values (R values) for dividing the 5 resilience grades are 0.5, 1.0, 1.25, and 1.7, respectively. By considering different building usage functions and seismic effects, the 5-story CFS structure shaking table test models and the irregular eccentric CFS structure numerical model are used to jointly verify the reasonableness and universality of the seismic resilience evaluation index and grade classification criteria. The multi-parameter resilience evaluation index system lays the foundation for the study of seismic resilience of mid-rise CFS buildings and provides new ideas for establishing quantitative resilience evaluation indexes for different building systems.

Seismic design of cold-formed steel beams based on flexural capacity - ductility – Energy dissipation

In the past decade, cold-formed steel (CFS) has been widely used in building structures due to its excellent structural performance. In particular, due to its remarkable light weight, CFS is ideally suited as a major component of buildings in seismic zones. However, CFS has an inherent characteristic of a large width-to-thickness ratio, which does provide high flexural stiffness, but at the same time weakens the seismic capacity of the member to some extent. Specifically, under seismic loading, members with large width-to-thickness ratios are more prone to local buckling, thus unable to fully consume seismic wave energy. Apparently, the direct application of CFS to buildings in seismic zones is challenging due to the lack of design specifications. Accordingly, for the safety of CFS buildings in seismic zones, the maximum width-to-thickness ratio of CFS members must be specified to ensure that their energy dissipation capacity, bearing capacity, and ductility are adequate. Therefore, in this study, the effect of the web width-to-thickness ratio and flange width-to-thickness ratio of CFS beams on flexural capacity, ductility, and energy dissipation capacity under monotonic and seismic loads is investigated, and a design metric of width-to-thickness ratio threshold (Tw/t) is proposed. Under seismic loading, for CFS members of different cross-section types, when the width-to-thickness ratio is within the specified range of Tw/t, it shall be regarded as fulfilling the requirements of bearing capacity, ductility and energy dissipation capacity. To introduce Tw/t in detail, in this study, a numerical model of the CFS beam is first constructed and its accuracy is verified by comparison with experiments and other studies; Then, the effects of prototype, wall thickness, and section size on bearing capacity, ductility, and energy dissipation capacity under monotonic and seismic loading are analyzed, respectively; Subsequently, the damage modes of CFS beams are discussed and a buckling energy dissipation factor is proposed as an indicator of the energy dissipation capacity of CFS members; Finally, based on the above analyses, the Tw/t of each prototypical CFS beam is determined. The results show that all CFS members with a width-to-thickness ratio within Tw/t can fulfill the seismic design requirements in terms of flexural capacity, ductility and energy dissipation.

FEMA P695 seismic performance evaluation of Cold-formed steel buildings with different shear wall strengths and stiffnesses

Lateral load capacity and stiffness of CFS framed OSB sheathed shear walls can be improved by increasing the number of fasteners between sheathing panel and framing members or by providing sheathing on both sides of panel. Both of these methods could be a viable option to meet high seismic demands when the number of walls is limited by architectural or other concerns. With the increasing use of CFS building systems in high seismic regions, such applications are expected to be more widespread in the future. The present study aims to investigate the seismic response of CFS framed archetype buildings utilizing OSB sheathed wall panels with different levels of stiffness and shear resistance. Nonlinear time history analyses were conducted on 28 archetype buildings and collapse performance evaluation was performed according to FEMA P695 methodology. Load-displacement hysteresis behaviors of wall panels measured in a recent experimental study were adopted to simulate the shear wall response in the numerical models. Buildings utilizing shear walls that are sheathed with OSB panels on one side with 300 mm fastener spacing possess limited overstrength and fail to satisfy the performance criteria regardless of the response modification coefficient used for design. With the use of shear walls that are constructed with 50 mm fastener spacing and single or double sided sheathing configurations, the building design is governed by drift limitation instead of strength requirement. Irregular drift profiles occur with lateral displacements localizing at the first floor when shear walls with single sided sheathing and loose fastener layout is utilized. Using shear walls with a dense fastener layout results in a uniform distribution of lateral displacements among floors.

Testing, modelling and analysis of full-scale cold-formed steel center-sheathed shear walls in fire

Cold-formed steel center-sheathed shear wall (CFSCSSW) is an innovative shear wall applied to multi-story cold-formed steel buildings. In this study, six tests were carried out on CFSCSSWs, including one room temperature test to obtain load-bearing capacity and five fire tests to investigate the influence of the vertical load ratio, lateral load ratio, interior stud and aspect ratio on the fire behavior of the CFSCSSWs. Subsequently, finite element models were developed to investigate the influence of various combinations of vertical and lateral loads and the sheathing thickness on the fire behavior of the wall. The research results showed that the temperature distribution along the thickness of the wall section is quite non-uniform and the maximum difference reaches 65% from the hot flange to the cold flange. The effect of the vertical load ratio on the fire resistance of the wall is greater than that of the lateral load ratio. The interior stud plays a positive role in improving the fire resistance of the wall. The aspect ratio has little influence on the fire resistance of the wall. The failure model of the wall mainly depends on vertical or lateral loads as well as the aspect ratio. With the increase in the vertical load ratio, the failure location of the wall moves down. Based on the parametric studies, a sheathing thickness of 0.8 mm was recommended for CFSCSSWs.

Experiment and Design Method of Cold-Formed Thin-Walled Steel Double-Lipped Equal-Leg Angle under Axial Compression

The cold-formed steel (CFS) double-lipped equal-leg angle is widely used in modular container houses and cold-formed steel buildings. To study the buckling behavior and bearing capacity design method of the cold-formed steel (CFS) double-lipped equal-leg angle under axial compression, 24 CFS double-lipped equal-leg angles with different sections and slenderness ratios the axial compression were conducted. The test results showed that the distortional buckling occurs for specimens with a small width-to-thickness ratio and small slenderness ratio. The buckling interactive with distortional and global flexural buckling was observed for the specimens with small width-to-thickness ratios and large slenderness ratios. The specimens with large width-to-thickness ratios and small slenderness ratios showed interactive buckling with local and distortion buckling. The specimens with large width-to-thickness ratios and large slenderness ratio developed interactive buckling with local, distortional, and global flexural buckling. The finite element model established by ABAQUS software was used to simulate and analyze the test. The buckling modes and the load-carrying capacities analyzed by the finite element model agreed with the test results, which showed that the developed finite element model was feasible to analyze the buckling and bearing capacity of the CFS double-lipped equal-leg angles. The experimental results were compared with those calculated by the direct strength method in the North American standard and the effective width method in the Chinese standard. The comparisons indicated that the calculated results are very conservative with maximum value 36% and 51% for direct strength method and effective width method, respectively. The coefficient of variation was 0.276 and 0.397, respectively. Finally, the modified direct strength method and the modified effective width method were proposed based on the experimental results. The comparison on the ultimate strength between test results and calculated results by using the modified method showed a good agreement. The modified method can be as a proposed desigh method for the ultimate strength of the CFS double-lipped equal-leg angles under axial compression.

Seismic Damage Assessment and Shaking-Table Test Validation of Midrise Cold-Formed Steel Composite Shear Wall Buildings

This paper proposes damage index–based limit-state criteria for seismic design of midrise cold-formed steel (CFS) buildings instead of the drift-based ones, and the Park-Ang damage model was used to calculate the damage indexes. Two 5-story CFS buildings with different wall configurations were tested, and a test-validated numerical model was developed. An incremental dynamic analysis was conducted on a typical midrise CFS building to obtain the degradation parameter β of different CFS walls. The damage indexes computed from numerical analyses were compared with the damage indexes estimated from test observations and the proposed damage index–based limit-state criteria. The results showed that the calculated degradation parameter as well as the proposed damage index–based limit-state criteria are effective for seismic damage assessment and damage level classification of midrise CFS buildings according to the validations by shaking-table tests on two CFS buildings. The Park-Ang damage index used in this paper and the proposed limit-state criteria could be used for performance-based seismic design of midrise CFS buildings.

Shear bearing capacity of Self-drilling screw group connections of CFS sheets

Self-drilling screws are commonly used as connectors in cold-formed steel (CFS) buildings as they can be drilled easily through thin sheet steels, allowing on-site assembly. The screw connections in the project all appear in the form of screw group connections. Studies have shown that the shear bearing capacity of a screw group connection of CFS sheets is not simply the shear bearing capacity of a single screw connection multiplied by the number of screws. But instead, there is a reduction effect of the screw group connection, called the Group Effect. However, there is no design provision for the shear bearing capacity of the screw group connection in the current design specifications, AISI S100-16w/S1-18, AS/NZS-4600:2018, EN1993-1–3, and GB50018-2002. Based on the authors' and other researchers' tests collected by authors, this paper verifies and analyzes the four existing design equations for the connection strength of the screw group connections that consider the Group Effect. The results show that the four current design equations cannot cover the test data that list in the paper because the existing four design equations all have their respective scopes and limitations, such as the screw spacing and the strength of steel sheetings, etc. For that, the paper proposed a new design equation that can calculate the nominal shear capacity of self-drilling screw group connections, including high-strength steel such as G550 CFS sheets and extensive screw spacing connections. The authors' new test of twelve screw group connections and the other 237 tests verified the new equation. The unsafe rate for the new design equation is 4.42% among the 249 connections test, and the new design equation has high reliability and broader application scope.

Numerical study of cold-formed steel frame walls under compression loading

Bearing walls in cold-formed steel buildings act as a transmitter of vertical loads of the building and as a retainer of the exterior of the building and also absorb the lateral loads of the building such as wind and earthquake loads. In this article, the behaviour of cold-formed steel frame walls under compressive axial load has been investigated numerically. First, the finite element model of the cold-formed steel wall was developed using ABAQUS software, and then, structural analysis was performed, taking into account the effects of geometric and materials nonlinearities. The results of the numerical model were compared with the experimental results to confirm the accuracy of the numerical model. In the following, parametric studies have been carried out considering some parameters affecting the behavior and the ultimate load of cold-formed steel frame walls such as type of sheathing, stud web depth, stud flange width and stud cross-section shape. The results of the structural analysis showed that the type of wall sheathing did not have a significant effect on the ultimate load of the wall, but increasing the web depth of the stud, has led to a decrease in the load-bearing capacity of the steel wall. The wall with hollow flange studs has a higher ultimate load than other sections, and also, the use of web stiffened stud leads to an increase in the final bearing capacity of the wall compared to typical sections.

More efficient design of CFS strap-braced frames under vertical and seismic loading

Strap-braced stud walls are one of the primary lateral-force resisting systems for conventional cold-formed steel (CFS) buildings. While CFS wall-panels in general exhibit a satisfactory seismic performance, previous studies showed that they may experience a premature brittle failure in the presence of vertical loading. However, current design codes do not make any provisions for calculating the lateral load capacity and ductility of strap-braced stud wall frames under vertical loading. This study aims to develop, for the first time, a practical design methodology for seismic design of CFS strap-braced stud wall frames under such conditions. To this end, a comprehensive parametric study was carried out using experimentally validated detailed numerical models in ABAQUS, accounting for material nonlinearities, initial geometric imperfections, and secondary moments due to P-Δ effects. Parameters of the investigations were the strap thickness and the intensity of the vertical loading. Design formulae were derived, and a preliminary design methodology was proposed for predicting the lateral load capacity and ductility of single strap-braced stud walls under a range of vertical loading ratios. The efficiency of the proposed method compared to Eurocode 8 design was then demonstrated for a 6-storey CFS frame, which highlighted the importance of considering the effects of vertical loads in the seismic design of these systems. It was demonstrated that ignoring those factors can lead to a brittle lateral response even if all other code requirements are satisfied, while the proposed design methodology was shown to be efficient to reach the target lateral load and ductility capacities.

Predicting shear strength of CFS channels with slotted webs by machine learning models

Staggered rectangular perforations (slots) are provided in the webs of cold-formed steel (CFS) beams and columns to reduce their thermal conductivity and improve the energy efficiency of CFS buildings. The perforations adversely affect the structural characteristics of the members, especially those governed by the web parameters, such as the shear strength and shear buckling. This paper presents machine learning (ML) models to predict the elastic shear buckling load and the ultimate shear strength of CFS channels with slotted webs. Support vector machine (SVM), decision tree (DT), random forest (RF), and k-nearest neighbor (KNN) regressors were trained using a large dataset of numerical results with 3512 samples. An extensive search was conducted to find optimal hyperparameters of the models that result in the best predictions and prevent overfitting. The models’ performances were evaluated employing the ten-fold cross-validation method to make more data available for training and reduce bias and variance. The SVR, DT, and RF models demonstrate good prediction accuracy, which exceeds the accuracy of the existing descriptive equations. Relative feature importance was evaluated using the permutation and SHAP methods for each model. Partial dependence of the buckling load and the shear strength from the channel features was assessed. The predictions of the developed ML models were also compared with the predictions of previously developed artificial neural networks (ANNs). The comparisons demonstrated that ANNs showed higher accuracy in predicting the elastic buckling load, whereas the SVR model provided the most accurate shear strength predictions.

Influencing factors analysis on shear capacity of cold-formed steel light frame shear walls

The Cold-formed steel (CFS) light frame shear wall is the most important lateral force-resisting element in CFS building systems. Extensive researches have been completed on the shear performance of CFS shear walls with various kinds of panels. It’s found that the shear capacity of the CFS shear walls is affected by many aspects. Therefore, this paper consolidates a large number of test results world-wide and establishes a database on the lateral strength of CFS light frame shear walls. The influencing factors of the shear wall’s shear capacity are analyzed systematically, which includes material properties, stud sections and spacing, framing thickness, sheathing thickness, aspect ratios, openings, screw types and spacing, loading methods, etc. The logic behind these factors is found out through the analyses of the test data and design suggestions are made accordingly.

Effect of Boundary Condition on Buckling Behavior of Built-up Cold-Formed Steel Members Under Bending

Built-up member constituted by lipped channel sections is often used in cold-formed steel building. But, it has not been clarified yet whether the connection interval of chord members affects the buckling behavior of built-up member. In addition, there are not enough considerations on the end boundary conditions where stress is transmitted only to the web. This study clarifies the influence of the boundary condition and spacing of connection on the local and flexural buckling of builtup cold-formed steel members under bending.

Experimental response of cold-formed steel stud shear wall with hardboard sheathing under seismic loading

In the design of cold-formed steel (CFS) buildings, sheathings are used to provide lateral resistance to seismic or wind load. Many researchers have thoroughly studied the basic behavior of different sheathing (including walls sheathed with plywood, oriented strand board, gypsum wallboard, gypsum sheathing board, steel sheet sheathing, and fiberboard) with different thicknesses. However, despite many studies, the examination of existing experimental studies demonstrates that the lateral bracing and stiffness provided by the sheathing are generally ignored, and sheathing is provided as a non-structural component. This study aims at that neglected aspect. In this study, cold-formed steel shear wall (CFSSW) response has been investigated with varied thickness of hardboard sheathing used as a structural element. The main aim was to determine the contribution of sheathing in resisting lateral forces. For this purpose, three full-scale specimens (i) frame without sheathing, (ii) frame sheathed with 4 mm thick hardboard, (iii) frame sheathed with 10 mm thick hardboard were tested using a uni-directional shake table. The sheathing was screw-fastened to the cold-formed studs and tracked for the development of shear stiffness and strength in the wall system. The test results revealed that in addition to increasing the lateral stiffness of the structure, the hardboard sheathing also acts as an efficient bracing system. However, some minor but recoverable damages were also noticed. In addition, stud local buckling failure mode was observed, and it could be caused by the low anchor stiffness of using cleats instead of hold-downs. It is also important to mention that the number of tests performed in the current study was limited, and further similar research needs to be carried out before generalizing the results.

Performance-based assessment of CFS strap-braced stud walls under seismic loading

The use of cold-formed steel (CFS) systems has significantly increased in the past few decades, especially in the construction of low to mid-rise buildings. Compared to hot-rolled sections, CFS members are often more economical and efficient due to their low weight, ease and speed of construction and greater flexibility in manufacture. In most conventional CFS buildings, diagonally strap-braced stud walls provide the primary lateral force-resisting system. This study aimed to develop a better understanding of the structural behaviour of CFS strap-braced stud wall systems under seismic loading. To achieve this, a detailed numerical model was developed, accounting for material nonlinearity, initial geometric imperfections, nonlinear behaviour of the connections and secondary moments due to P-Δ effects. This model was validated against previous experimental results on full-scale wall systems. A comprehensive parametric study was then conducted using the validated model to investigate the effect of key design parameters, namely the number of studs, the presence and intensity of vertical loading, the thickness of the structural elements and the steel grade of the straps, on the seismic performance of the system. The lateral load-resisting capacity, deformation capacity, ductility and energy dissipation under lateral loading were investigated and are here discussed. An efficiency index was proposed for each of these variables, allowing design solutions to be rated in terms of their ability to improve the material efficiency of the system, and design recommendations were derived for performance-based design.

Lateral Force Resisting Systems Made of Cold-Formed Steel Material: Proposal of Seismic Design Criteria for 2nd Generation of Eurocode 8

The current edition of Eurocode 8 does not cover the design of the Cold-Formed steel (CFS) building structures under the seismic design condition. As part of the revision process of Euro-code 8 to reflect the outcomes of extensive research carried out in the past decade, University of Naples “Federico II” is involved in the validation of existing seismic design criteria and development of new rules for the design of CFS systems. In particular, different types of Lateral Force Resisting System (LFRS) are analyzed that can be listed in the second generation of Eurocode 8. The investigated LFRS’s include CFS strap braced walls and CFS shear walls with steel sheets, wood, or gypsum sheathing. This paper provides the background information on the research works and the reference design standards, already being used in some parts of the world, which formed the basis of design criteria for these LFRS systems. The design criteria for the LFRS-s common to CFS buildings would include rules necessary for ensuring the dissipative behavior, appropriate values of the behavior factor, guidelines to predict the design strength, geometrical and mechanical limitations.