Cold-formed Steel Connections Research Articles

An experimental study of cold-formed steel connections with screwed clip angle under tension and shear loads

This paper aims to investigate the performance of typical cold-formed steel (CFS) beam-to-beam connections between floor bearers and floor joists through a series of full-scale experiments. The investigated connections consisted of a lipped channel (floor joist), an unlipped channel (floor bearer), a thin-walled clip angle (connector), and a self-drilling screw (fastener). In this study, customised T-shaped and H-shaped CFS connection specimens were tested to examine the performance under tension and shear loads, respectively. Two sizes of self-drilling screws, 10 G and 14 G, were used with the selected load bearing clip angle. The fabricated connection specimens were subjected to displacement-controlled tension and shear loads. The experiments uncovered the ultimate connection strengths, load-displacement relationships, and failure mechanisms of the designed CFS connections under the applied loads. Moreover, the experimental tensile and shear strengths of the connections were compared with the strengths predicted by the standard design equations. It was found that the screw pullout on the anchored leg of the clip angle was the main mode of failure under the tensile load. On the other hand, two different failure modes—fracturing of 10 G screws or shear buckling of the cantilevered leg of the clip angle with 14 G screws—were observed in the H-shaped specimen under the applied shear load. Among the designed connection specimens, the maximum tensile load capacity was 30.86 kN and the maximum shear load capacity 88.15 kN. The standard design calculations predicted screw pullout failure under tension, while the shear bucking failure mode of the clip angle under shear load was overlooked.

Simplified numerical model development for sophisticated analysis of lightweight-concrete encased cold-formed steel compressed column-end joints and panels

Since cold-formed steel (CFS) members are prone to complex buckling phenomena; that decrease their structural capability, many strengthening trends have been developed so far. One of these methods has been proposed as a new structural system comprising cold-formed steel (CFS) elements encased in polystyrene aggregate concrete (PAC). In this research, a simplified finite element model has been developed using ANSYS software, where a geometrically and materially nonlinear analysis with imperfections (GMNI) was used to solve the problem of PAC-encased CFS connections and panels subjected to compression. Each model was validated against available test results from literature. The full detailed volume-contact element-based model of concrete was replaced by spring elements perpendicular to the plate’s plane with equivalent properties, where a specific formula was set to define the stiffness of springs. A parametric study was performed to investigate in which range this stiffness has an impact on the ultimate load of the examined members. Additionally, different geometric imperfection amplitudes were implemented to test the real imperfection values. Finally, a failure mode-based equivalent geometrical imperfection amplitude was proposed to anticipate the design resistance of PAC-encased connections and panels.

Investigation of Parameters Affecting Rotational Behavior of Cold-Formed Steel Connection

This study aims to understand the behavior of connection, which holds an important key to efficient designs of the cold-formed steel structure. The focus is on examining parameters that affect the rotational stiffness and behavior of the cold-formed steel connection made of single-lipped channel sections with a bolt and gusset plate system. The numerical study has been conducted using component-based finite element analysis, with the studied parameters including bolt diameters, number of bolts in the group, size of the bolt group, thickness of the cold-formed steel cross-section, and thickness of the connecting plate. The effect of the variables is evaluated and explained by comparison. The result of the study reveals that the rotational behavior of the connection depends on the details of the assembly. For the given base connection, the parameters that give the greatest effect on the rotation stiffness and the moment capacity are the number of bolts and the bolt diameters, whereas the parameter that provides the least effect is the bolt spacing. On the other hand, the bolt group size is the most influential parameter for the member connection. For both types of connections, the thickness of the cold-formed steel section and the thickness of the gusset plate have shown limited effect on stiffness and strength. Using different variables in assembling the connection can also lead to different failure types, either bolt failure or excessive bolt hole deformation, which leads to plate failure. Doi: 10.28991/CEJ-2023-09-11-08 Full Text: PDF

A computational modeling approach for cold-formed steel bolted connections exhibiting bolt slip and bearing behavior

Cold-formed steel (CFS) is increasingly used for buildings and infrastructures, as it is lightweight and easy to install in the field when a bolted joint method is used to join elements. Unlike “rigid” welded joints, bolted joints allow for movement during load transfer, adding flexibility. Such flexibility is more prominent in thin-walled CFS bolted connections. The structural performance of a CFS structure with bolted joints is considerably governed by the mechanical behavior of the bolted joints. In many existing finite element (FE) simulations, an individual CFS bolted joint was modeled as a simplified linear or bilinear behavior, leading to less accurate results of both local and global behavior of the CFS structure. This study presents a novel FE-based method for modeling a bolted joint for CFS, which enables the simulation of nonlinear behavior of the bolted joint, characterized by the bolt slip and the progressive bearing. This approach utilizes the connector elements in series, defined with the friction and plasticity models, to model this bolt slip-bearing behavior. A well-known torque–friction force relationship is used to estimate the bolt friction slip load needed for the friction model definition. Rex equation is used to predict the bearing load–deformation response, which is inserted as the input into the plasticity model. The suitability and the capability of the approach is first validated using two of the CFS lap connections with a single bolted joint subjected to monotonic/cyclic loading. Then, the proposed approach is validated for the large-scale moment-resisting CFS connection beams with eight different configurations. It is expected that the application of the proposed approach can be extended to any type of thin-walled bolted connections exhibiting the bolt slip and bearing response.

Experimental studies of timber to cold-formed steel connections with self-drilling screws

This paper examines the behaviour of connections comprising self-drilling screws, cold-formed steel (CFS) and timber. Initially, a series of tensile coupon tests and shear tests were conducted to determine the material properties of the CFS material and the shear capacity of the screws that were used in the study. Subsequently, the ultimate load-carrying capacity of the connections was determined by undertaking a series of pull-out tests. Four connection configurations were investigated where the screws were driven perpendicular to the steel-timber interface. An assessment of the type of timber; plywood and Balau hardwood, and the thickness of the steel; 1.2 mm, 1.6 mm, and 2.0 mm, used in the connections was also undertaken. It was seen that as the thickness of the CFS material increased, the failure mode changed from screw withdrawal to failure in the screw. When the same CFS material was used with Balau, only failure in the screws was observed. The performance of current design guidance in the Australian standard (AS1720.1), North American Specification and Australian/New Zealand Standard (AISI S100 & AS/NZS 4600), and Eurocode 5 was assessed by comparing the shear strengths obtained from the experiments. From the comparison results, it was found that the Eurocode 5 is the most effective while predicting the performance of specimens with plywood and Balau hardwood, being 7 % conservative to the test results.

Shear connection performance of cold-formed steel and plywood composite flooring systems: Experimental and numerical investigation

This study carries out numerical and experimental investigation on shear connection performance of flooring systems comprising of cold-formed steel joists and engineered plywood panels. A 3-D finite element model has been developed using ANSYS software which considers the nonlinear behaviour and damage of plywood panels using a maximum stress-based criterion incorporated in the FE model. The yielding behaviour of steel joists and fasteners (screws and bolts) is defined as a multilinear plastic hardening material model defined in ANSYS software. The FE model was validated against the experimental results obtained from push-out tests conducted on specimens utilising different connection arrangements. The validated FE models showed a good agreement with the experimental results of the push-out tests in terms of load-slip behaviour, ultimate peak capacity, and failure modes. Finally, the validated FE models are utilised to perform a parametric study that investigates the effect of varying various parameters, including diameter and yield strength of self-drilling screws, the thickness of plywood panels, value of friction coefficient, and the ratio of fastener diameter to the joist thickness, on the initial stiffness, failure mode and ultimate load capacity of the cold-formed steel and plywood composite connections.

Research on the Simplified Method of Nonlinear Finite Element Analysis for CFS-SPR Connections

This study reviewed some simplified methods of finite element analysis (FEA) for connections in cold-formed steel (CFS) structure, and summarized eight simplified methods divided into three categories. Shear performance tests were performed for six groups of self-piercing riveted (SPR) connection in CFS. A constitutive model of shear behavior for SPR connections was proposed, which was simplified from the load–displacement curve of shear performance test results. The models of SPR connection were established in ABAQUS by the eight simplified methods, and then the FEA results and the test results were compared. The applicable scope of each simplified model was explored, and a simplified method of FEA that was most suitable for the shear behavior of the CFS-SPR connection was proposed. Moreover, the shear performance test of the CFS shear wall with SPR was conducted by considering the rivet spacing, and failure modes and load–deformation curves were obtained. On this basis, numerical models of the CFS-SPR connection shear wall were established. By comparing the test results and the FEA results for the CFS-SPR connection shear wall, the feasibility of a simplified method of FEA applied to the CFS-SPR connection was verified. The main failure modes of the CFS-SPR connection were that the rivet tail pulled out from the bottom sheet and the rivet head pulled over from the top sheet. The SPR connection of the CFS frame could be simplified with a pin or a fastener element, and the SPR connection between the steel frame and the sheathing could be simulated by a Cartesian connector or a Spring2 element. The FEA results were highly similar to the test results for the CFS-SPR shear wall.

Experimental and numerical study of a novel cold-formed steel T-Stub connection subjected to tension force

Cold-formed steel (CFS) is a popular form of construction in New Zealand and the developed world because it can be cost-effective, durable, sustainable, resilient to extreme environmental and seismic loads and easy to install. When used in domestic and medium-rise buildings, T-Stub connection between CFS channel sections is formed by using self-drilling screws which connect the flanges. In the literature, no prior investigation on the tension capacity of such connection has been reported. Therefore, this paper describes a novel test procedure for determining the tension capacity of such CFS T-Stub connection and the associated mechanism of failure. An experimental test program that comprises thirty experimental tests is reported in this paper. Three channel section thicknesses were examined: 0.55 mm, 0.75 mm, and 0.95 mm. All specimens failed as a result of tilting of the screws and bearing failure. Non-linear elasto-plastic finite element (FE) models were developed, and the predictions were validated against the experimental test results. It was adjudged that there was reasonable agreement in terms of connection strength and failure mode prediction. Finally, the design strengths derived from current design standards were compared with the experimental failure loads. It was found that the American Iron and Steel Institute (AISI 2016) and Australian and New Zealand Standard (AS/NZS 2018) are conservative by 6% and unconservative by 10% for the initial and final failure loads of T-Stub CFS connections, respectively.

FINITE ELEMENT ANALYSIS OF GUSSET PLATE CONNECTION DESIGN FOR COLD-FORMED STEEL FRAMES

The application of cold-formed steel sections has been extended from secondary members to primary structural members in recent years. This increases the use of gusset plate connections in cold-formed steel since it is a common connection used in steel structures. However, current design codes on connection design do not have a comprehensive method to consider the effects due to the buckling of the thin cold-formed steel sections. Therefore, it is important to develop a more accurate model to predict the capacity of cold-formed steel connections. This paper aims to propose an equation for gusset plate beam-to-column connection using finite element models. Finite element models have been developed and compared with existing test results. The failure mode and ultimate strength of the numerical models are similar to the experimental results. The validated finite element models are then used to study the effects of gusset plate thickness, effects of cold-formed steel section depth and thickness, and the effects of bolt size and spacings. Elastic and plastic stiffnesses are obtained from the developed models. The connection behavior followed a typical elastic-plastic curve according to the connection ductility and failure mode. An empirical model is developed from the finite element models to predict the joint behavior of gusset plate beam-to-column connection for cold-formed steel structures. AS 4600 may have underestimated the initial stiffness of the connection.

A practical numerical model for thin-walled steel connections and built-up members

The interaction between the cold-formed steel members and fasteners in a structural assembly often involves complex stress states, material plasticity, significant in-plane deformations and curling, rendering computational modelling of the resulting structural system challenging. In addition, the presence of initial geometric imperfections and the susceptibility of cold-formed steel members to local instabilities adds a further degree of complexity to the numerical modelling. The behaviour of bolted connections between cold-formed steel members has traditionally been assessed based on physical tests, though these can be time consuming and expensive. In this paper, a practical numerical approach to the simulation of bolted cold-formed steel connections, capturing all the key behavioural features including bolt preload, slippage and bearing, is introduced. Starting from a typical shell finite element (FE) model of a cold-formed steel plate, a small number of solid elements is introduced around the bolt holes, to allow accurate replication of the bolt-ply interaction, with contact pairs defined between the bolts and the surrounding material. An explicit dynamic solver is then employed to solve the geometrically and materially nonlinear problem, both with and without bolt slippage. Validation of the FE model is conducted using benchmark tests reported in the literature. It is shown that the proposed numerical method can accurately capture the structural behaviour of bolted cold-formed steel connections and cold-formed steel built-up sections, while being computationally efficient and numerically stable; the proposed approach is therefore recommended for the numerical modelling of cold-formed steel systems assembled using fasteners.

Effects of corrosion on the strength of self-drilling screw connections in cold-formed steel structures-experiments and finite element modeling

Cold-formed steel (CFS) self-drilling screw connections are popular in construction industry due to rapid fastening and ease of installation. However, the performance of the CFS connections is very critical when they are exposed to corrosive environments such as humid and acidic or chloride-rich environments. The corrosion damage of CFS structures when exposed to these environments can result in significant reduction of mechanical properties, which can affect the safety and durability of such structures. Very limited research is available in the literature to investigate the effect of corrosion on the behaviour of CFS connections. This issue is addressed herein through an experimental investigation. This paper presents a total of 36 new experiments on different types of CFS self-drilling screws (12 and 14 gauge) and steel sheet thickness (2.5 mm). Half of the tests were for corroded specimens and the remaining half were for non-corroded specimens. Further, one to two screws per arrangements connecting the steel sheets having yield strength of 450 MPa, were tested. Screws were immersed for 31 days and the CFS plates were immersed for 8 weeks in corrosion chamber before the tests were conducted. The experimental program was designed to investigate the effect of number of screws on shear strength of corroded specimens. Furthermore, a nonlinear explicit dynamic FE model was developed and validated against the experimental test results. The FE results were in reasonable agreement with the test results, both in terms of connection strength and failure modes.

Behavior of different bolted connection configurations in frames composed of cold-formed sections

Rigid bolted cold-formed steel connections have been very popular for their better distribution of straining actions and for better full-filling serviceability requirements. However, the behavior of every single element in the connection as well as the interaction between them is not clear due to its complex behavior. A nonlinear finite element parametric study was conducted on the apex knee connection of a portal frame constructed with cold-formed back-to-back double-lipped C sections for both the rafter and the column. These back-to-back lipped C sections, connected by gusset plates are used as beam and column members. Only the webs of lipped C sections are connected to the connection gusset plates with bolts. The section flanges are not connected to the columns for ease of construction. The study focused on investigating the effect of different parameters on the ultimate capacity and the failure mode of the eave connection. Studied parameters include different bolt configurations, bolt diameters, gusset plate thickness, the bolt pre-tensioning force, and the edge distance. The effect of bolt bearing on the C-section web was considered in the calculation of the capacity of the connection. Five modes of failure were identified namely; lateral out-of-plane buckling of the gusset plate, local buckling of the connected section, bolt bearing of C-section web and the gusset plate, tearing of C-section web. It was observed that bearing failure is the dominant mode of failure. However, in some cases, the lateral-out-of-plane buckling of the gusset plate takes place and causes the premature failure of the connections at low values of applied loads. The local failure of the connected sections occurs at the ultimate loading phase. Several observations were found and some useful recommendations for optimum connection configuration are suggested. A simple interaction design procedure for special configuration to calculate the resistance of the joint connection under moment, shear, and normal forces based on the bolt bearing failure for different bolt pitches is proposed.

An improved and robust finite element model for simulation of thin-walled steel bolted connections

Thin-walled cold-formed steel (CFS) structures are widely used in low-storey buildings and are seeing increased applications in multi-storey buildings as well as long-span structures. The bolted connection is one of the most common types of fasteners employed in CFS structures. The bolt slippage and load-bearing mechanism in CFS connections have a significant influence on the structural behaviour of cold-formed steel construction assemblies, which are typically numerically simulated using spring/connector elements. However, the determination of input parameters for the spring/connector elements generally requires load-deformation characteristics obtained from physical tests of lap joints, which are costly and time-consuming. Moreover, the use of spring/connector elements fails to explicitly capture the bolt bearing behaviour, resulting in inaccuracy in the simulation of CFS connections. In this paper, an accurate, efficient and robust numerical method that can fully replicate the behaviour of CFS connections considering both the effects of bolt slippage and bearing has been introduced. In the proposed numerical methodology, the cold-formed plates are modelled using the conventional shell elements, while a number of solid elements are introduced around the bolt holes, enabling the bolt-plate interaction to be explicitly accounted for, with contact pairs defined between the bolts and their surrounding solid element surfaces. The proposed numerical method could find a convergence solution using dynamic implicit solver in the general-purpose finite element (FE) software ABAQUS. Good agreement between the FE models using the proposed method and the available test results of CFS bolted connections were achieved, which sheds light on the promising application of the proposed method for simulating CFS assembled using bolted connections.

Experimental study of the screw connection of cold-formed steel applied in the same and opposite directions

Cold-formed steel has been used widely on buildings, especially for the roof truss-structures. This material and its typical section are beneficial in weight and strength. Despite the minimum total weight, the structure can fulfil the strength requirement. For the truss installations, screws are commonly used to join the truss members due to time consideration and convenience process. However, there is a potential failure using the screw because it does not have a nut to prevent it from detaching. In this research, experimental tests of the screw connection of the cold-formed steel were conducted. The specimens consist of two groups: 8 specimens using screws were applied in the same direction and the other 8 specimens in the opposite direction. A different application of screws was aimed to observe the effect of the screw’s head, which may play a role as a nut to prevent detaching. Typical conditions of tilted screw and indented hole on the joined channel section were found during the test. The experimental results showed that the ultimate load for two different directions of screw were slightly different within the range of 0.4 – 10.0 %. The effect of the screw’s head position was minor to the resistance of the connection.

Optimisation of cold-formed steel beams for best seismic performance in bolted moment connections

Typical thin-walled cold-formed steel (CFS) elements and connections used in portal frames are not generally suitable for moment-resisting frames in high seismic regions due to their low ductility and energy dissipation capacity and inherited susceptibility to local/global instabilities. This study aims to improve the seismic performance of CFS moment-resisting frames by developing a methodology to obtain more efficient bolted moment connections using optimised CFS beams with enhanced non-linear post-buckling behaviour. By taking into account material non-linearity and geometrical imperfection effects, the detailed Finite Element (FE) models of a typical CFS bolted moment connection is developed using ABAQUS software, and then validated based on experimental cyclic test results. Particle Swarm Optimisation (PSO) algorithm is linked to GMNIA ABAQUS FE analysis to optimise CFS bolted moment connections based on either energy dissipation capacity or ductility. To demonstrate the efficiency of the method, connections with five different beam cross-sectional shapes are optimised, and the results are compared with a standard CFS channel section used as a benchmark. The relative dimensions of the cross-sections, the inclination of the lip stiffeners, and the location of triangular intermediate stiffeners are selected as main design variables. To provide practical beam cross-sections, the plate slenderness limit values defined by Eurocode 3 (EC3) along with a range of manufacturing limitations are imposed as design constraints in the optimisation process. It is shown that, for a given plate width and thickness, the proposed optimisation framework results in a considerable (up to 195%) improvement in the energy dissipation capacity and ductility of the CFS bolted moment connections.

BEHAVIOR OF PARTIAL STRENGTH OF BEAM-TO-COLUMN CONNECTION WITH GUSSET PLATE FOR COLD-FORMED STEEL SECTIONS

The use of cold-formed steel (CFS) as part of a major construction has been rapidly increasing recently, as it is considered viable as an alternative to the conventional hot-rolled steel section. However, information about CFS as part of a non-composite system is still limited as the connection is designed as a pinned connection, only good for roof trusses and wall paneling. The aim of this research is to propose a connection for CFS sections that could enhance the strength of typical CFS connections, from pinned to partial strength connections. This paper presents the behavior of the partial strength of a non-composite beam-to-column connection for a cold-formed steel section where a gusset plate was used as a stiffener. This type of proposed connection for CFS is not limited only to the construction of roof trusses and wall cladding, but is also able to provide better flexural strength in semi-continuous construction. The proposed connections consisted of 6 specimens divided into two types, the first of which were stiffened by the gusset plate and the second of which were enhanced further with the top, seat and web angles. From the results of the experimental tests it was concluded that all the proposed connections can be categorized as partial strength connections, where the strength was more than 25% of the connected beam and suitable for use in semi-continuous construction. A relatively good agreement was recorded for the comparison between the experimental results and the predicted results from the EC3 component method super-imposed with a haunched gusset plate and top, seat and web angle components, which differed in the range of 1.31 to 1.5 times for the moment resistance and 1.07 to 1.37 times for the stiffness of the proposed connections.

Influence of seat angles on the behaviour of cold-formed steel concrete composite joints

This paper presents an experimental study on cold-formed steel (CFS) concrete composite joints, with and without seat angles, to investigate their structural behaviour and determine the influence of seat angle on this kind of connection. These CFS-concrete composite joints were made up of a cold-formed steel (CFS) connection consisting of two lipped CFS C-channels (bolted back-to-back as a beam and column with a trough plate or gusset plate), a composite slab (comprising a reinforced concrete slab with profiled CFS deck, and bolted shear connectors). In the literature review, the CFS-concrete composite beams and composite slabs were found to have been investigated experimentally and analytically in several studies but little attention has been paid to CFS-concrete composite joints. This is the reason for the current investigation on these kinds of composite joints. Six full-scale specimens of composite joints, three bare joints and three seat-angle joints, were evaluated experimentally by isolated joint tests, in which a one-point load was monotonically applied to the beams. The responses of the specimens were recorded by measuring instruments: i.e. five LVDTs, two inclinometers, and five strain gauges. The results obtained were used to provide moment-rotation relationships of the connections, which were necessary for quantifying the key characteristics of the connections. The three linear moment-rotation responses of the tested composite joints showed that adding the seat angles to the CFS-concrete composite joints increases both their moment resistance and rotational stiffness (by 8% and 21%, respectively), and reduces the rotation capacity (by 17%), on average. Furthermore, it was observed that the behaviour of these types of joints tends to transform from partial strength to full strength by adding a seat angle.

Local Buckling in Cold-Formed Steel Moment-Resisting Bolted Connections: Behavior, Capacity, and Design

AbstractThe research presented in this paper aimed to investigate local buckling failure occurring adjacent to moment-resisting bolted connections in cold-formed steel back-to-back channel beams co...

Beam-Column Connections Behavior of Cold-Formed Steel Members: New Experimental Configuration

The main purpose of this paper is to study the structural behavior of cold-formed steel connections. Five specimens of cold-formed beam to column connections are studied. The beams are connected to the columns by using two different shapes of gusset plates and self-drilling screws. Experimental tests on these cold-formed connections are carried out to obtain their ultimate capacities. A numerical analysis is performed on the same tested specimens and the finite element model is verified. The ultimate loads obtained from the numerical analysis as well as the load-strain curves and buckling modes are compared with those obtained from the experimental tests and are found to be in good agreement. The finite element model well predicts the stress and deflection values, as well as the buckling modes of the specimens. Some recommendations are concluded to help researchers and designers choose the optimum type of beam to column connection.

Experimental behaviour and modelling of screwed connections of high strength sheet steels in shear

Abstract Screws are commonly used as connectors in cold-formed steel structures as they can be drilled easily through thin sheet steels, allowing on-site assembly. However, in a connection undergoing shear and bending, while a screw may fail, other screws may be still in the elastic state. Therefore, investigation in the load-deflection response of screw connectors before failure is required for further understanding of complex screwed connections. Experiments and simulation are being carried out at the University of Sydney in order to investigate the behaviour of cold-formed steel connections with screw fasteners in shear. The screw model is built with solid deformable elements containing the screw threads. A plasticity model is applied to the steel sheets to represent different stress states of steel during failure. The model successfully simulates the different behaviour occurring simultaneously in the screwed connection tests: tearing and piling up of the steel sheets, end plate failure, tilting and fracture of the screws.