Structural behaviour of cold-formed steel purlin-sheeting systems under uplift loading

This thesis provides an investigation into the structural behaviour of cold-formed steel zed- and channel-section purlins when subjected to uplift loading in purlin-sheeting systems. In pre-buckling, an analytical model is presented to describe the bending and twisting behaviour of partially restrained zed- and channel-section purlins when subjected to uplift loading. Formulae used to calculate the bending stresses of the roof purlins are derived by using the classical bending theory of thin-walled beams. Detailed comparisons are made between the present model and the simplified model proposed in Eurocode EN1993-1-3. In buckling, a numerical investigation is presented on the buckling behaviour of partially restrained cold formed steel zed- and channel-section purlins when subject to transverse distributed uplift loading. The buckling behaviour of zed- and channel-section purlins of different dimensions subjected to uplift loading under the influence of rotational spring stiffness applied on the middle line of the upper flange is examined. In the post-buckling, nonlinear finite element analysis models are created for the partially restrained cold-formed steel zed- and channel-section purlins subjected to transverse uniformly distributed uplift loading. The analyses are performed by considering both geometric and material nonlinearities, and corresponding design curves of zed- and channel-section purlins are established

On the Direct Strength Design of Continuous Cold-formed Steel Beams

The work reported in this paper concerns an ongoing investigation aimed at developing an efficient methodology to design continuous cold-formed steel beams failing in modes that combine local, distortional and global features. At this stage, it is intended to assess how accurately can the load-carrying capacity of lipped channel continuous (two and three-span) beams subjected to non-uniform bending be predicted by means of the current Direct Strength Method (DSM) design curves. “Exact” ultimate strength values yielded by geometrically and materially non-linear shell finite element analyses are compared with estimates provided by the DSM equations and, on the basis of this comparison, it is possible to identify some features that must be included in a DSM approach applicable to continuous cold-formed steel beams

Prestressed cold-formed steel beams

A novel concept, whereby prestressing techniques are utilised to enhance the load-carrying capacity and serviceability performance of cold-formed steel beams, is proposed. The prestressing is applied by means of a high-strength steel cable located within the bottom hollow flange of the cold-formed steel beam at an eccentric location with respect to its strong geometric axis. Since the initial stresses generated by the prestressing are opposite in sign to those induced during the subsequent imposed vertical loading stage, the development of instabilities is delayed and thus the capacity of the cold-formed steel beam is enhanced. Owing to the pre-camber that is induced along the member during the prestressing stage and the contribution of the cable to the system bending stiffness, the overall vertical deflections of the member are reduced significantly. Prestressed cold-formed steel beams can provide highly-efficient structural solutions. Owing to their enhanced structural performance, for a given span and imposed vertical load, a smaller cross-section is required and thus a more lightweight solution is achievable. Potentially, the proposed beams can open up new applications for cold-formed steel in construction, particularly in cases where increased load-carrying capacities, longer spans and reduced deflections are desired. The current thesis presents the conceptual development of prestressed cold-formed steel beams. The mechanical behaviour of the proposed structural system is explored using analytical and numerical models with the origin of the obtained structural benefits being demonstrated. Finite element modelling is employed to simulate the behaviour of the proposed beams during the prestressing and imposed vertical loading stages and parametric studies are conducted to investigate the influence of the key controlling parameters, such as the prestress level, cable size, section slenderness and centroid location. Finally, design recommendations and failure criteria, which define the permissible design zone for the prestressed system, are developed with their implementation being demonstrated through practical a worked example.Open Acces

Behavior of web elements with openings subjected to bending, shear and the combination of bending and shear

PREFACE The current design criteria for cold-formed steel members published by the American Iron and Steel Institute (1986,1991) do not provide sufficient information for the design of web elements with openings. The primary objective of this investigation was to study the structural behavior of cold-formed steel C-sections with web openings. This study deals with the structural behavior of beam webs subjected to bending moment, shear force and combined bending moment and shear force. This report presents a summary of experimental study and findings of analytical results. The test results indicate that the current AISI Specification does not account for the behavior of C-section members with web openings. Design recommendations are proposed to predict the strength of cold-formed steel beams with web openings. This report is based on a thesis presented to the Faculty of the Graduate School of the University of Missouri-Rolla in partial fulfillment of the requirements for the degree of Doctor of Philosophy. This investigation was sponsored by the American Iron and Steel Insti tute (AISI) and Metal/Lath Steel Framing Association (MLSFA). The technical guidance was provided by the MLSFA-AISI Joint Task Force: J.E. Sullivan (chairman), C. Bissey, R.L. Brockenbrough, C.R. Clauer, E.R. diGirolamo, S.J. Errera, E.R. Estes, Jr., L. Hernandez, A.L. Johnson, K.H. Klippstein, J.P. Matsen, W.R. Midgley, T.B. Pekoz, N. Peterson, G.S. Ralph, R.M. Schuster, T.W. Trestain, and R.A. LaBoube. Thanks are also extended to R.B. Haws and K.L. Cole, AISI staff, and A.L. Sisco, MLSFA Staff, for their assistance

Calculation of Critical Load for Pure Distortional Buckling of Lipped Channel Columns

This paper presents a method that allows calculating the elastic critical stress for the distortional buckling mode, based on the buckling mode classification of typical lipped channel columns. In our case, Cold-Formed Steel Lipped Channel Columns are subjected to compression. Moreover, in order to consolidate the important findings of this work, a comparative study was carried out to assess the reliability of various distortional buckling models that are provided by different design Standards. It was found that the American and Australian approaches, given in the codes of practice, are closer to the Finite Strip Method than to the European method. An analytical solution was proposed for the determination of the distortional buckling stress on the basis of a statistical method; it corresponds to lipped channel sections with a flange width to web width ratio b/h ranging from 0.1 to 1, and a lip width to web width ratio c/h between 0 and 0.5. After comparison with the results given by the finite strip method for pure distortional buckling, it turned out that the proposed approach provides a reasonable prediction for the elastic distortional buckling stress for lipped channel sections subjected to compression. In fact, this method gives better results than the American approach

Experimental and numerical investigation of a method for strengthening cold-formed steel profiles in bending

Perforated cold-formed steel (CFS) beams subjected to different bending scenarios should be able to deal with different buckling modes. There is almost no simple way to address this significant concern. This paper investigates the bending capacity and flexural behavior of a novel-designed system using bolt and nut reinforcing system through both experimental and numerical approaches. For the experiential program, a total of eighteen specimens of three types were manufactured: a non-reinforced section, and two sections reinforced along the upright length at 200 mm and 300 mm pitches. Then, monotonic loading was applied to both the minor and major axes of the specimens. The finite element models were also generated and proved the accuracy of the test results. Using the proposed reinforcing system the flexural capacity of the upright sections was improved around either the major axis or minor axis. The 200 mm reinforcement type provided the best performance of the three types. The proposed reinforcing pattern enhanced flexural behavior and constrained irregular buckling and deformation. Thus, the proposed reinforcements can be a very useful and cost-effective method for strengthening all open CFS sections under flexural loading, considering the trade-off between flexural performance and the cost of using the method

Progress on the Direct Strength Method

The Direct Strength method is a new design procedure for cold-formed steel members. The method employs elastic buckling solutions for the cross-section, instead of the element-by-element plate buckling solutions used in traditional design. The use of cross-section elastic buckling solutions insures inter-element compatibility and equilibrium. The Direct Strength method uses strength formulas on the gross section, similar to conventional column curves, for capacity prediction in local and distortional buckling. This avoids effective section calculations altogether. The reliability of the Direct Strength method is demonstrated for a broad selection of beams and columns by comparison with existing test data. Extension of the method to beam-columns is discussed and a solution proposed and demonstrated. Areas needing further research for final implementation are highlighted. The Appendix to the paper provides detailed specification style language appropriate for employing the Direct Strength method for the design of beams and columns