Effect of stressed skin action on the behaviour of cold-formed steel portal frames with non-linear flexible joints and top-hat purlins

Previously held under moratorium from 1st December 2016 until 1st December 2021.A conceptual design of a cold-formed steel (CFS) portal frame system proposed by the industrial partner is to be investigated and improved in light of known design challenges. Unlike previous studies which focused on the behaviour of CFS bare frame, this thesis focuses on the design and analysis methods for clad portal frames. A wide range of design problems and industry practices have been investigated by testing, as follows:1) Resistance and stiffness of bolted moment-connections. The bearing resistance and stiffness of the threaded bolt shank in single and double shear lapped connections were tested and compared against design recommendations.The existing methods for deriving the moment resistance and the rotational stiffness of the moment-connections were updated for tested joint configurations 2) Shear resistance and stiffness of cladding panels. A total of eighteen 3 x 3 m cladding panels were tested including options in which sheeting is fixed on all four sides and fixed on two sides only. The design features not yet recognized by design codes such as top-hat purlins, sheeting profiles fixed in the crest, profile thickness less than 0.5mm and composite panels were investigated in the test programme. 3) The difference in the structural behaviour of the bare and clad portal frames. A series of six full-scale laboratory tests were conducted on cold-formed steel portal frame buildings in order to investigate the effects of joint flexibility and stressed skin diaphragm action. The frames used for the laboratory tests were of 6m span, 3m height, 10° pitch and the frame spacing was 3 m. When the difference in loads between 2D (bare frame model) and 3D (stressed skin model) were considered, it was shown that the resistances and flexibilities of frames and cladding should be calculated or established by testing so safe and economical design is possible.A conceptual design of a cold-formed steel (CFS) portal frame system proposed by the industrial partner is to be investigated and improved in light of known design challenges. Unlike previous studies which focused on the behaviour of CFS bare frame, this thesis focuses on the design and analysis methods for clad portal frames. A wide range of design problems and industry practices have been investigated by testing, as follows:1) Resistance and stiffness of bolted moment-connections. The bearing resistance and stiffness of the threaded bolt shank in single and double shear lapped connections were tested and compared against design recommendations.The existing methods for deriving the moment resistance and the rotational stiffness of the moment-connections were updated for tested joint configurations 2) Shear resistance and stiffness of cladding panels. A total of eighteen 3 x 3 m cladding panels were tested including options in which sheeting is fixed on all four sides and fixed on two sides only. The design features not yet recognized by design codes such as top-hat purlins, sheeting profiles fixed in the crest, profile thickness less than 0.5mm and composite panels were investigated in the test programme. 3) The difference in the structural behaviour of the bare and clad portal frames. A series of six full-scale laboratory tests were conducted on cold-formed steel portal frame buildings in order to investigate the effects of joint flexibility and stressed skin diaphragm action. The frames used for the laboratory tests were of 6m span, 3m height, 10° pitch and the frame spacing was 3 m. When the difference in loads between 2D (bare frame model) and 3D (stressed skin model) were considered, it was shown that the resistances and flexibilities of frames and cladding should be calculated or established by testing so safe and economical design is possible

Experimental axial-compressive behaviour of bare cold-formed-steel studs with semirigid-track and ideal-hinged boundary-conditions

Studs are the primary load-bearing components in cold-formed steel (CFS) wall panels, connected to tracks at both ends with self-tapping screws, forming a semirigid boundary condition (BCT). Most existing tests on the axial compressive behaviour of bare CFS studs are based on either theoretically-hinged (BCH) or fully-fixed boundary conditions. Previous researchers have employed BCT only on sheathed stud-wall panels. However, practicing engineers and current design codes, e.g., Eurocode 3, follow an all-steel design. Therefore, this research experimentally investigated bare-CFS-studs' axial compressive behaviour with BCT, considering, for the first time, the combined effect of the tracks' warping rigidity, stud-to-track gap, non-linear connection stiffness, and bare studs' various cross-sectional slenderness. Forty-two industry-standard lipped channel sections (studs) of five thicknesses (1.2-3 mm), three depths (75–125 mm), and two heights (1.2 & 1.5 m) were tested under static-concentric axial compressive loading with BCT. Another fourteen studs were tested with BCH, a comparator to BCT. Results demonstrated that the studs' global failure mechanisms were flexural-torsional in BCT instead of flexural in BCH. Studs' axial stiffness was two-phased in BCT due to the stud-to-track gap, compared to single-phased stiffness in BCH. >1.8 mm stud-to-track gap caused stud-to-track connections' failure and studs' sudden capacity reduction during gap closure. Studs achieved 1.22 times higher axial-compressive strength, 2.3 times more axial-shortening, 0.7 times lower axial stiffness, and 58% lower axial-compressive strain at the web-midheight under BCT-PhaseII than BCH. Tested strengths were compared with EC3 design strength, and an effective-length-factor of 0.65 was suggested for efficient design of studs with BCT

Effect of Stressed-Skin Action on Optimal Design of a Cold-Formed Steel Portal Framing System

Cold-formed steel portal frames can be a viable alternative to conventional hot-rolled steel portal frames. They are commonly used for low-rise commercial, light industrial and agricultural buildings. In this paper, the effect of semi-rigid joints and stressed-skin action are taken into account in the optimal design of cold-formed steel portal frames. A frame idealization is presented, the results of which are verified against full-scale. A real-coded niching genetic algorithm (RC-NGA) is then applied to search for the minimum cost for a building of span of 6 m, height-to-eaves of 3 m and length of 9 m, with a frame spacing of 3 m. It was shown that if stressed-skin action and joints effects are taken into account, that the wind load cases are no longer critical and that the serviceability limit state controls for the gravity load case with the apex deflection binding. It was also shown that frame costs are reduced by approximately 65%, when compared against a design that does not consider stressed-skin action, and 50% when compared against a design based on rigid joints

Design of top-hat purlins for cold-formed steel portal frames

This paper considers the use of cold-formed steel top-hat sections for purlins in the UK, as an alternative to conventional zed-sections. The use of such top-hat sections could be viable for cold-formed steel portal framing systems, where both the frame spacing and purlin span may be smaller than that of conventional hot-rolled steel portal frames. Furthermore, such sections are torsionally stiffer than zed-sections, and so have a greater resistance to lateral-torsional buckling. They also do not require the installation of anti-sag rods. The paper describes a combination of full-scale laboratory tests and non-linear elasto plastic finite element analyses. The results of twenty-seven tests on four different top-hat sections are presented. In terms of stiffness, good agreement between the experimental and finite element results is shown. The finite element model is then used for a parametric study to investigate the effect of different thicknesses and steel grades. Design recommendations are provided in the form of charts. The use of the finite element method in this way exploits modern computational techniques for an otherwise difficult structural design problem and reduces the need for an expensive and time consuming full laboratory study, whilst maintaining realistic and safe coverage of the important structural design issues

Numerical Investigation of Cold-Formed Steel Top Hat Purlins

This paper considers the use of cold-formed steel top-hat sections for purlins as an alternative to conventional zed-sections. The use of such top-hat sections may be viable for use in cold-formed steel portal framing systems, where both the frame spacing and purlin span may be smaller than in conventional hot-rolled steel portal frames. Furthermore, such sections are torsionally stiffer than zed-sections, and so have a greater resistance to lateral-torsional buckling. They also do not require the installation of anti-sag rods. The paper describes non-linear elasto plastic finite element analyses conducted on top-hat sections. The results of twenty-seven tests on four different top-hat sections are presented. Good agreement between experimental and finite element results is shown. The finite element model is then used for a parametric study to investigate the effect of different thicknesses and steel grades. Design recommendations are provided in the form of charts that can be used to assist designers when deciding which geometry of top-hat section to consider for further development. The use of the finite element method in this way exploits modern computational techniques for an otherwise difficult structural design problem and reduces the need for an expensive and time consuming full laboratory study, whilst maintaining realistic and safe coverage of the important structural design issues

The structural behaviour in fire of a cold-formed steel portal frame having semi-rigid joints

This paper describes a non-linear finite element study into the effects of elevated temperature on a cold-formed steel portal frame having semi-rigid joints. Numerical modelling was carried out using ABAQUS finite element analysis software with shell elements used to capture localised buckling effects. Results for the ambient shell models are compared against previous full-scale tests. Material properties are taken from the literature, in order to predict the behaviour of the frame at elevated temperature. The results of finite element beam models are compared against those of shell models to enable comparison. At elevated temperature, shell models are shown to detect failure much earlier within the fire. Therefore shell models are recommended for such studies, for a conservative approach

Effect of reduced joint strength and semi-rigid joints on cold-formed steel portal frames

This paper considers the effect of semi-rigid joints and finite connection length on the design of cold-formed steel portal frames sized using a rigid joint and full joint strength assumption, and whether it can offset the fact that the joints cannot sustain the full moment capacity of the sections. The paper shows that for frames of modest span (around 10 m), sized using a rigid joint and full joint strength assumption, that the frames are unsafe under gravity load and do not satisfy the ultimate limit state. Designers should therefore take the semi-rigidity and partial strength of the joints when analysing such frames, particularly when stressed skin action is also used in the design

Finite-Element Analysis of the Eaves Joint of Cold-Formed Steel Portal Frames having Single Channel-Sections

A finite element model is described for the eaves joint of a cold-formed steel portal frame that comprises a single channel section for the column and rafters eaves connections. The members are connected to the brackets through both screws and bolts. Such a joint detail is commonly used in practice in New Zealand and Australia, where the function of the screws is to prevent slip of the joint during frame erection since the bolt holes are detailed for nominal clearance. The results of the finite element model are compared against two experimental test results. In both, the critical mode of failure is a combination of torsion of the eaves joint and shear failure of screws. It is found that at ultimate load, the bolts have not engaged i.e. they have slipped. It is shown that the stiffness of the joints can be accurately predicted from the equations of bolt and screw stiffness of Zaharia and Dubina (2000). It is also shown that the finite element model can be used to determine both an upper and lower bound to the failure load