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

Effect of Stressed-Skin Action on the Behaviour of Cold-Formed Steel Portal Frames

This paper describes six full-scale laboratory tests conducted on cold-formed steel portal frames buildings in order to investigate the effects of joint flexibility and stressed-skin diaphragm action. The frames used for the laboratory tests were of span of 6 m, height of 3 m and pitch of 10⁰; the frame spacing was 3 m. The laboratory test setup represented buildings of length of 9 m, having two gable frames and two internal frames. Tests were conducted on frames having two joint sizes, both with and without roof cladding. It was shown that as a result of stressed-skin diaphragm action, under horizontal load the bending moment at the eaves was reduced by approximately a factor of three, relative to the bare frame. It was also shown that as a result of stressed-skin action, the deflection of the internal frame reduced by 90%, and that the stiffness was independent of joint flexibility. On the other hand, owing to redistribution of bending moment from the eaves to the apex, the effect of joint flexibility was shown not to be significant on the overall failure load of the frame

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

Stressed Skin Design of Steel Sheeting Panels – Part 1: Shear Resistance and Flexibility of Screw Lapped Joists

The shear resistance and flexibility of a steel roof diaphragm depend largely on shear resistance and slip flexibility of the single screw lap joint. In this paper, screw connections relevant to modern roof construction are investigated. The tests provided experimental values of shear/tearing resistance and joint flexibility of seam connections, cladding/purlin connections and purlin/rafter connections. The novel aspects of the experimental research include investigation of the behaviour of shear connections in 0.5mm thick sheeting and thick-to-thin connections in S550 high tensile steel. Overall, six series of tests were conducted and each test was repeated five times in order to demonstrate a scatter of test results. Test results were examined against existing semi-empirical formulas for predicting the shear resistance of screw joints. It was demonstrated that the design equation presented by Toma et al. (1993), without the additional condition included in Eurocode 3, offers the closest prediction in terms of joint shear resistance. In terms of joint flexibility, it was demonstrated that existing formulas developed for bolted connection (Zadanfarrokh and Bryan (1992) and Dubina and Zaharia (2006)) can be successfully used for screw connections. The flexibility reduction factor npf=0.4 was also proposed to take account of perfect fit screw connections

Stressed Skin Design of Steel Sheeting Panels – Part 2: Shear Panels with Sheeting Fixed on all 4 Sides

In this paper, the strength and stiffness of different roof panels were investigated, in order to establish their ability to act as in-plane diaphragms for stressed skin design of cold-formed steel portal frames. A total of 6 roof panels, approximately 3 x 3m, were examined by testing with sheeting profiles fixed on 4 sides. A variety of sheeting profiles in two industry standard thicknesses of 0.5 and 0.7mm were tested, all using top-hat shaped purlins fixed with self-drilling, self-tapping screws. The experimental strength and stiffness of each panel were then compared against existing design methods. The Finite Element Analysis (FEA) modelling techniques were also presented and validated against series of full-scale tests. The FEA results have shown that the ‘true’ level of loading transferred via shear connector screws was on average 13% lower than that assumed by standard design methods. On the contrary, seam connections failure, according to FEA results, have governed a design in all of the analysed cases and the analytical method overestimated shear resistances of the panels by 45% and 35% in case of 0.5mm and 0.7mm thick sheeting profiles respectively. It was demonstrated that FEA results have represented the upper bound of experimental shear stiffness, with a very close prediction for 0.5mm thick sheeting profiles. Overall all, the tested panels demonstrated an average 41% greater flexibility then this predicted using FEA models

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

Effect of stressed skin action on the behaviour of cold-formed steel portal frames

This paper describes a series of six full-scale laboratory tests 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 6 m span, 3 m height, 10° pitch and the frame spacing was 3. m. Vertical loading was applied in two tests, and horizontal loading was applied in another four tests. The laboratory test set-up represented a building having two gable frames and two internal frames. Tests were conducted on frames having two joint types, both with and without roof sheeting. It was shown that as a result of stressed skin action, the internal frame with roof sheeting resisted approximately three times more horizontal load than the bare frame and the deflection of the internal frame was reduced by 90% relative to the bare frame. When the difference in loads between 2D (bare frame model) and 3D (stressed skin model) were considered, it was shown that the joint flexibility of the frame has a significant effect on the load transfer between frames through the roof sheeting. It was found that the 'true' loads transferred to the gable frames are between three and seven times higher than the loads deriving from tributary area. By using stressed skin analysis, it is possible to assess the shear force in the roof sheeting so that damage to the fixings is prevented and a more economical design is possible

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 effect of semi-rigid joints on the design of cold-formed steel portal frame structures

This paper investigates the effect of semi-rigid joints and finite connection length on the design of cold-formed steel portal frames. The performance of frames sized using a rigid joint and full joint strength assumption is compared with frames having semi-rigid joints and partial strength. It goes on to describe whether it can offset the fact that the joints cannot sustain the full moment capacity of the sections. Experimental, analytical and finite element modelling techniques have been used. They demonstrate that frames of modest span sized using a rigid joint and full joint strength assumption, 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 into consideration when analysing cold-formed steel portal frames