Shear capacity of the cold-formed steel beam to column welded moment connection using clip-angle and flange-cleat

The current research endeavours to evaluate the shear performance of the cold-formed steel (CFS) welded moment connection between beam-to-column with 36 laboratory tests. The web portions of the beam and column were connected by CFS welded clip-angle to form a CFS welded shear connection. Subsequently, it is converted into a welded moment connection by including a flange cleat between the flange portions. The shear capacity of the welded shear connection increases by an average of 67% after the inclusion of flange cleats, which is quantified using a performance ratio variable. This research presents two shear equations for the CFS welded moment connection (i) a new empirical shear equation; (ii) a new shear equation representing the shear strength of the moment connection as a function of the shear strength of the shear connection. The variability of the shear performance of welded moment and welded shear connections is expressed with force versus displacement plots and failure modes of the clip angles. The failure modes observed in the clip-angle in both welded moment and welded shear connections are (i) Local buckling and (ii) Distortional buckling. The shift in failure modes of some of the clip-angle in the WM connection (Local buckling) and the WS connection (Distortional buckling) indicates the effectiveness of flange-cleat in resisting due to free twisting of the beam because of load offset from the shear center. The design factors were also determined for the LRFD, LSD, and ASD methods by performing reliability studies

Improved Design Shear Method for the Bolted Cold-Formed Steel Clip-Angle Connector

In this paper, the ultimate shear capacity of the 3-bolt cold-formed steel clip-angle between the cold-formed steel (CFS) beam and column is evaluated through 54 laboratory tests. A series of experiments were conducted by varying (1) thickness, and (2) aspect ratio (L/D) of clip-angles for different depths (D) and widths (A). The experimental program consists of three phases of tests: (1) Phase-I: direct shear load tests on clip-angle attached to a CFS column through 4.6-grade bolts; (2) Phase-II: CFS column replaced with a hot-rolled steel (HRS) column, since the CFS column experienced bearing failure in Phase-I; and (3) Phase-III: 10.9-grade bolts used instead of 4.6, as the 4.6-grade bolts subjected to bolt shear failure in Phase-II. Failure modes observed in the test specimens are (1) shear local buckling of clip-angle; (2) column bearing failure; (3) bolt shear failure; and (4) tear failure in clip-angle. Design shear equations from the literature, for the bolted clip-angle, were found to be inefficient for the high-grade steel (fy=375 MPa to 550 MPa), and conservative for the commonly available low-grade steel (fy=275 MPa). Hence a new shear strength equation is suggested for the clip-angle from the collated data of the present study and past research work. A comparative study between 2-bolt and 3-bolt clip-angle configurations was conducted to evaluate the increase in shear strength. Reliability studies were conducted, and corresponding resistance and safety factors were suggested for the design shear strength calculation corresponding to load and resistance factor design (LRFD), limit state design (LSD), and allowable strength design (ASD) methods

Splice Connections for Built-Up Column Assemblies in Cold-Formed Steel Construction

The development of simple connection methods is necessary for increasing cold-formed steel (CFS) construction activities. A new splice connection concept for the CFS built-up column-to-column connections is presented in this paper. This simple connection concept will use the same size and shape of the geometry as the CFS built-up column, which will enable a quick erection process. The new splice connection configurations, arrangement, and installation methods for cold-formed steel construction are demonstrated. Twenty-eight experiments, which include four actual columns, two disconnected columns, and 22 columns with splice connections, are carried out. This paper examines the influence of various parameters of splice connections such as length, thickness, and number of fastener rows. The design strength of the actual column is determined using the direct strength method (with modified global and local slenderness approaches) and is compared with the results of the splice connected columns for adequacy. The force transfer mechanism and failure modes of splice connection components are demonstrated in the form of detailed sketches. The smaller length splice connections led to localized failures, while the longer splice connections enabled the uniform force distribution between the built-up column cross-sections. Finally, it is recommended that the splice connection configuration should be a minimum of 300 mm in length, the splice thickness should be equal to the CFS built-up cross-section, and two rows of fasteners for attaining the required design strength of the built-up column members

Influence of Sheathing-Fastener Connection Stiffness on the Design Strength of Cold-Formed Steel Wall Panels

This paper investigates the bracing effect of sheathing fastener connection on point-symmetric shaped cold formed steel (CFS) studs through full-scale laboratory tests. The test parameters include slenderness of the CFS stud, fastener connection spacing, and sheathing thickness. The test results indicate that the bracing effect of the particle cement board has significantly enhanced the moment capacity of the CFS stud, nevertheless, this was not completely captured by the current design specifications of American Iron and Steel Institute (AISI). The detailed discussion on the unsuitability of the current AISI design method in determining the design moment capacity and failure modes of the sheathed CFS wall stud is presented. In addition, the PCB-fastener connection demand check shows that the provided sheathing configurations are adequate to brace the CFS studs from biaxial bending, which is in contrast compared to the experimental results. Therefore, a new test setup is proposed to accurately determine the stiffness of the sheathing fastener connections to formulate a reliable design method. © 2020 American Society of Civil Engineers

Recommendations for design of sheathing bracing systems for slender cold-formed steel structural members

The use of sheathing material as a structural component in the Cold-formed steel (CFS) construction holds great potential for stability of the structure and savings in the construction cost. However, there is no robust design method to account the structural contribution of sheathing boards. This paper presents the experimental results of 107 (including unsheathed and sheathed) CFS wall frame studs subjected to out-of-plane loading to study the feasibility of using gypsum board as a structural bracing sheathing material. The test parameters include various shapes and slendernesses of the CFS frame stud, thickness of the sheathing board and spacing between the sheathing bracing connections. The out-of-plane loading is applied as it causes lateral torsional buckling in the CFS structural members thereby creates pull-through failure at the sheathing bracing connections. Moreover, the suitability of the current AISI and Eurocode specification for the design of sheathing braced CFS structural member is studied. The experimental results indicate that the lateral buckling of the symmetric shaped (against the loading axis) CFS wall frame studs can be inhibited by gypsum sheathing. Whereas most of the singly symmetric and point symmetric CFS studs exhibit lateral torsional buckling and biaxial bending, respectively, due to the inadequate bracing effect of gypsum sheathing resulting in pull-through failure at the sheathing bracing connections. Therefore, a set of limitations for the use of gypsum sheathing as a structural bracing is suggested in the form of a generalized design parameter. Finally, the experimental results indicates that the design strength of the CFS wall frame stud can be increased in the range of 39% to 595% based on the shape and slenderness. © 2020 Elsevier Lt

Sheathing Braced Design of CFS Wall Panels: Review and Proposal

The objective of this investigation is to understand the existing design rules of sheathed cold-formed steel (CFS) members. A comprehensive analytical study on the stiffnesses offered by the sheathing to the CFS panel was conducted. Three different combinations of stiffnesses that are offered by the sheathing were examined based on the failure modes and ultimate moment capacities obtained from the experimental results. The experimental results were compared with the predicted design strengths using the existing design approaches available. The deficiency of the current design approaches are highlighted, and a modified approach for incorporating the stiffnesses that are offered by the sheathing in the elastic buckling analysis is presented. The design strengths predicted using the modified approach on stiffness selection showed good agreement with the ultimate moments obtained from the experiments. A reliability analysis performed indicates that the modified design approach presented in this work can be employed for the design of gypsum sheathed cold-formed steel members subjected to bending (out-of-plane)

Sheathing Braced Design of CFS Studs using Direct Stiffness-Strength Method Design

A new design procedure for considering the bracing effect of sheathing boards to the cold-formed steel (CFS) structural members is proposed. The previous investigations show that the current AISI design specification for sheathing bracing design of CFS wall panels is unconservative (i.e. design standard predict a larger failure load than the experimental test results) due to exaggerated sheathing stiffnesses calculated from ideal loading conditions rather than worst-case loading conditions. Therefore, a new design procedure is suggested based on the performance (strength and stiffness) of the individual sheathing fastener connections. A new and simplified test setup is also introduced to simulate the realistic failure modes of the sheathing fastener connections using a conventional universal testing machine. A total of 67 individual sheathing fastener connection tests were carried out, including parameters such as ten different dimensions of the CFS stud (to account for the slenderness) and seven various sheathing board types (to account for the performance of each sheathing type). Based on the individual sheathing fastener connection test results, new expressions are formulated to predict the stiffness and strength of the individual sheathing fastener connections. An application-oriented merit-based and statistical assessment indicated that the proposed expressions are appropriate for the design

Design of Cold-Formed Steel Built-Up Closed Section Columns - Modified Local Slenderness Equation

Structural behavior of Cold-formed Steel (CFS) face-to-face connected built-up closed cross-section columns is investigated. The CFS built-up columns are designed as long and locally slender to verify the influence of intermediate longitudinal connection spacing and check the appropriateness of the current Direct Strength Method (DSM) design and the intermediate connection spacing limitations of AISI. A total of 31 axial compression tests were carried out with fixed- fixed end conditions. The design parameters such as local slenderness, global slenderness, intermediate longitudinal fastener spacing, and length of the column are varied. The failure modes of the column are summarized and the reason for them is explained. The influence of intermediate longitudinal connection spacing was observed in the ultimate loading capacity and failure modes. The test results including ultimate load and failure modes were compared with the current direct strength method design predictions. A modified local slenderness expression is proposed to consider the influence of intermediate longitudinal fastener spacing in the design strength of cold-formed steel built-up closed cross-section columns

Design of Cold-Formed Steel Built-Up I Section Columns subjected to Interactive Buckling

An experimental investigation into the behaviour of cold-formed steel (CFS) built-up I column assembly compressed between fixed ends is presented. To study the interactive buckling mode of failures, the built-up column assembly is designed to be doubly symmetric and locally slender. A total of forty-one columns were tested including different cross- section dimensions, lengths, intermediate connection spacing, and slendernesses. It is experimentally shown that the local buckling deformations caused the built-up cross-section assembly columns to fail predominantly in interactive local and flexural-torsional buckling. The influence of intermediate fastener spacing was also prevalent in the failure modes. The appropriateness of the AISI’s maximum intermediate connection spacing limitation is verified to prevent global instability failures. The test and design results comparison indicated that the current AISI’s DSM design curve for interactive buckling is unconservative for the CFS built-up columns with predominant interactive local-global failure mode vulnerability. Therefore, a modified design curve for interactive local-global buckling is proposed

Behaviour and Design of Sheathed Cold-formed Steel Wall Studs subjected to Torsional Buckling

The beauty of Cold-Formed Steel (CFS) is that it allows need based optimization owing to its ease in manufacturing the required structural sections. The superstructure in a light-gage steel construction is fabricated from CFS structural components of which wall panels forms a major part. The wall panels in the light-gage steel construction is attached to external sheathing on both sides of the panel. The inherent sheathing can brace the wall studs from buckling to some extent. However, there is lack of design guidelines to account the structural contribution of sheathing that offers resistance against global buckling of the CFS studs. Therefore, this dissertation focuses on investigating the effect of various sheathing board materials on CFS structural members. The recommendations of the current design specifications for the sheathing braced design of CFS wall studs is explained in detail. The experimental investigation was carried out for various design parameters such as sheathing configuration [sheathing material type, fastener spacing (df), and thickness of the sheathing (tb)] and different shapes (singly, point and doubly symmetric) and slenderness (local, distortional and global) of the CFS studs. The experimental results show that the sheathing can be appropriately designed to bear the forces developed at the sheathing-fastener connections. A comparison of experimental results and design predictions indicates that the current AISI design specifications is unconservative. The further investigation revealed that the sheathing stiffnesses predicted by AISI for the axial compression loading case has been implicitly recommended for both axial compression and out-of-plane loading case. Therefore, a new test setup is developed to simulate the failure of the CFS stud subjected to out-of-plane loading case and appropriate sheathing stiffness in determined